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The various ways to charge our coach batteries has come up in many threads across the forum, often out of place or as a response to someones’ post on their build thread. This is a good topic to have a thread for everyone to share their thoughts and the results of their work. My disclaimer here is that I have no experience in systems that manage the charging of coach batteries other than from the starter battery circuit or from a shore power plug in as in a campground or at your home.
Here is how I see the choices. If one has any of the various lead acid batteries they need to be maintained above about a minimum of 50% charge and less discharge is better, in fact if maintained at higher levels less sulfation occurs and battery life is extended. (http://batteryuniversity.com/learn/article/sulfation_and_how_to_prevent_it) Long periods of slow charging at a higher level called Float will revers and prevent this condition. Sulfation is probably the single most common factor in a Pb-acid life. Other failures do occur due to low electrolyte and physical failure of course. (http://batteryuniversity.com/learn/article/sulfation_and_how_to_prevent_it)
On this forum there have been lots of posts on rapid charging. Most battery makers suggest limiting the charge rate to 1/5 of the battery’s capacity. Solar does this perfectly and all modern solar controller have settings to properly charge the battery and idle days gets it to float and works wonderfully.
Lithium batteries are a different breed and are not being considered here. They will rule in high discharge and high recharge situations like electric vehicles. Does that make them ideal for our coach battery- no as we don’t normally have that need. (Flame me if you want.)
Solar is almost the perfect choice. Simple three stage plug in chargers are good as well. The van’s charger is probably the poorest choice as most do not have a float charge at all, aim to rapid charge the battery for the next start and are designed to charge ONE battery not TWO or three.
I recommend you do not use the van’s charger at all, but in reality no harm will be done in an emergency when low discharge might be worse. I have a simple battery plug in charger in my WFCO RV converter/fuse/110v breaker unit. I seldom use that either but it may be necessary. I connect my golf cart batteries to the van when I must with a simple solenoid that I have switched and keep turned off so the van does not normally interfere with my solar charging. This interconnect is not a good place to spend money. ([ame]https://www.amazon.com/Stinger-SGP38-80-AMP-Battery-Isolator/dp/B001HC6UJ0/ref=sr_1_14?s=automotive&ie=UTF8&qid=1505569958&sr=1-14&keywords=80+amp+Solenoid[/ame])
 
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Good idea!

I think a place for the various designs that forum members have implemented would be a useful reference for others...

My non-solar setup (carport issue) is pretty simple...

2 AGM 100Ah batteries.... I used Advance Auto platinum AGMs because they have a 3 year non-prorated warranty

Ways to charge:

Battery Doctor from van battery, which has worked well for charging the aux batteries for the last 2 1/2 years
45A 4 stage converter/charger that charges aux batteries when plugged into shore power

I use circuit breakers to protect all the wiring at the usual points. Batteries are chassis grounded, but I use 2 wires for most 12V connections.

Both charging sources have been very good at keeping my campervan powered up. As long as my TV, fridge and fans are working, I'm a happy camper!

I don't go on long off-the-grid trips, and surely would add solar if I did! There's something about FREE electricity that's appealing...

12V Loads: Inverter for Microwave & Keurig, TV, Engel fridge, Maxxair 4500 fan, Fantastic 12V box fan, Cellular Internet wifi hub, Ham Radio, Scanner, USB charging, lighting.
 

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The van’s charger is probably the poorest choice as most do not have a float charge at all, aim to rapid charge the battery for the next start and are designed to charge ONE battery not TWO or three.
I assume "van's charger" means alternator. Alternator charging is poorest for Absorption because the voltage isn't high enough [and will likely not be running for a sufficient duration]. Alternator charging is likely best for Bulk because of the relatively high current compared to solar (Winston notwithstanding!) and sanely-priced shore power devices.

As far as charging one battery vs. three; the commonly-recommended Battery Doctor isolator used by many here ensures the starter battery [is "charged"] before engaging the circuit to the house battery.


T
I connect my golf cart batteries to the van when I must with a simple solenoid that I have switched and keep turned off so the van does not normally interfere with my solar charging. This interconnect is not a good place to spend money. (https://www.amazon.com/Stinger-SGP3...d=1505569958&sr=1-14&keywords=80+amp+Solenoid)

IMO the alternator will not interfere with solar charging. At worst it will augment charging up to the alternator's output voltage then make no difference at all.

Solar + alternator charging is likely the best possible scenario. In effect alt handles Bulk and solar can handle the high-voltage Absorption and long duration Float duties. If the vehicle is driven regularly the solar install can likely be downsized considerably.

The problem as I see it with solar + alt is the possibility of feeding too high a voltage from the solar to the chassis when the circuits are combined. I am pondering that issue. Right now I am running the ground lead of the Battery doctor though a solenoid that disconnects the circuit when the house bank >= 13.7V. I think that setting will also serve me when I eventually upgrade to LiFePO4.

Currently (ha ha!) my charging setup is this:

  1. solar (570W mono bought cheap locally) on the roof
  2. alternator charging via the isolator arrangement described above
  3. DIY converter for shore power made from a 110v -> 24V power supply feeding a spare 10A MPPT controller. Works great: fully configurable and cost $100 total, including MT-50 monitor.
I also use the "converter" LOAD output to control an opportunity circuit.
 

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We built our system to use commercially, through a Cobra 2500 modified sine inverter, using a blue top Optima SC31DM Model 31 battery. We isolated the Optima from the vehicle's electrics intentionally, so when we're using the van for service work, we only draw from the Optima battery. This has proven to work VERY well, the Optima can deliver for a 16G shop vac for 30min+ or a 10" Mitre saw for 2 dozen cuts+ in heavy aluminum storefront framing. Smaller power tools, small air compressors, and cordless tool battery chargers have proven to be no challenge at all for the Optima battery so far, we have yet to drop it beneath 12 volts after useage (I only put in voltage monitors, not amperage monitors, since the inverter cuts out automatically @10volts, and I want to be sure I would see that instead of having the mitre saw shut down in the middle of a stick of aluminum tube...a disaster, I'm sure). While we did this strictly to protect the vehicles computer(s) from our secondary electrical system, it has proven to be much better than other vehicles we've used the same, but older, Cobra 2500 inverters, hooked direct to the vehicle's battery. IF I had the space and the need, I'd add an Optima battery in the older vehicles. If I had a motor home, I'd go for two of the blue top optima's, but really have no idea what a microwave, fridge, or AC unit draw, only our tools mentioned above.

The solenoid uses vehicle (key on) power to close it, allowing the inverter to only draw from the 2nd battery, not the vehicle battery, and therefore only charges battery 2 when the vehicle it running. I would have to call that, relatively, a "fast" charge, coming through the OEM smaller 120 amp alternator.

Since the two batteries are isolated when the vehicle is off, we also added a momentary switch on the dash to use the power from battery 2 to close the solenoid so we could effectively start the van from battery 2 in the event of a dead main vehicle battery.

Hope that's helpful info to the group, in some way.
 

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Long periods of slow charging at a higher level called Float will revers and prevent this condition [sulfation].

The van’s charger is probably the poorest choice as most do not have a float charge at all . . . are designed to charge ONE battery not TWO or three.
RD, it is probably correct to say that manufacturers didn't expect that multiple batteries would be tied to their alternator so, technically, your comment is accurate. But your comment implies that there is something wrong with having multiple batteries tied to the alternator. Unless the states-of-discharge of those several batteries are such that the alternator is being asked to provide more current than its rating (which we think is unlikely). We see nothing inappropriate in tying multiple batteries to the vehicle alternator. And a further point on this - - we don't understand the 'philosophy' of those chargers (e.g. Battery Doctor) that isolate (do not charge) the house battery until the vehicle battery has reached some magic (full charge?) level. After a night of off-grid camping it is the house battery that most requires (immediate) charging - - the vehicle battery is 'sitting pretty' as it's been automatically disconnected (by that solenoid of which you speak) the moment the engine was silenced.

And we're not in agreement with your comment that alternators don't have 'float' charge capability. We've discussed - - and we believe agreed - - that constant voltage sources (which an alternator is) are problematic chargers for any type battery. When the battery is discharged, its low terminal voltage and comparatively low internal resistance result in a very substantial initial charging current. As many seem to like the term, we'll call this "bulk" charging. The 'problematic' aspect of this is, of course, that this high initial charge current drop rather quickly, and continues to drop, as the battery charges - - leaving us scratching our heads "we have all this 'amperage' capability, but very little actual current is flowing". We would contend that the 'constant voltage' to which the alternator is forcing the battery represents two things: first, that voltage where the battery is considered fully charged; and, second, the "float" voltage - - that so-called 'float stage'. Yes, the current to the battery will be very low, but it's exactly the correct amount - - any more and the battery will overcharge, any less, and the battery is discharging from its 100% full charge state.

Now it is true that we Lithites don't want to continuously hold our batteries at this fully charged state, but as you noted in your thread opening comments, this is precisely where the lead-acid people want to be to minimize sulfation.

I assume "van's charger" means alternator. Alternator charging is poorest for Absorption because the voltage isn't high enough . . . Alternator charging is likely best for Bulk because of the relatively high current compared to solar and sanely-priced shore power devices.

IMO the alternator will not interfere with solar charging. At worst it will augment charging up to the alternator's output voltage then make no difference at all.

Solar + alternator charging is likely the best possible scenario. In effect alt handles Bulk and solar can handle the high-voltage Absorption and long duration Float duties. If the vehicle is driven regularly the solar install can likely be downsized considerably.

The problem as I see it with solar + alt is the possibility of feeding too high a voltage from the solar to the chassis when the circuits are combined.

. . . my charging setup is . . .
3. DIY converter for shore power made from a 110v -> 24V power supply feeding a spare 10A MPPT controller. Works great: fully configurable and cost $100 total, including MT-50 monitor.
FS, seems we should start with a definition of Bulk, Absorption, and Float and the purposes of each. It is our understanding that they represent progressively lower charging (voltage) states which, if correct, we don’t understand why an alternator would be said to be good for Bulk while having insufficient voltage for the Absorption stage.

As noted above, we don’t believe that any of the low-voltage charger sources are good Bulk chargers (e.g. alternators and many/most shore power chargers). We really like your DIY shore power approach as it adopts the solar panel ‘high voltage’ approach that can maintain a much higher charging current profile than, say, an alternator.

You raise an interesting question which hasn’t been adequately explored on this or any other forum: “What is the consequence of connecting multiple charging sources in parallel - - all operational and connected to the same battery?”

We think the answer is . . . for most devices . . . “not a problem”. If there is a problem, it lies in the fact that each of the charging sources is ‘doing its own thing’ and setting its own ‘voltage’ that it wants to apply to the battery. But when connected in parallel there can be but one voltage. As far as we’ve been able to determine, the “highest voltage” wins . . . and any other charge source that is seeking to charge the batteries to a lower voltage will be ‘overridden’ . . . and that lower voltage charge source will, in essence, be ‘idle’. It doesn’t ‘have’ to be this way . . . see what happens, for example, when you connect one of those 6 volt golf cart batteries to a 12 volt lead-acid battery . . . the 6 volt battery will surely not be idle! But happily, it appears that electronic chargers have designed for the likelihood of being connected to a higher voltage source - - which allows us to ‘tie ‘em all together’.

With that conclusion said, we see little likelihood of having a problem with paralleling your solar controller with other charge sources. At least our MidNite Solar MPPT controller is happy . . . it goes into a “Resting” mode.

And since we’re a “Lithite”, we’ll end this missive with the comment that we love MPPT controllers as they allow for ‘throwing everything the charger has” at the battery (we’ll call this Bulk mode) but, really, it’s a Constant Current charging profile. That current just ‘sits there’ and doesn’t budge as the battery charges. Then we go to what most would call a Float mode. This point is determined when the battery reaches a predetermined Percentage of Charge - - generally less than 100%. We’re currently shooting for 80% PoC. At this point the charger becomes a Constant Voltage source with very little current flowing into or from the battery. If a load is applied, the charger ‘picks-up’ that load - - the battery still sees very little current either direction.
 

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FS, seems we should start with a definition of Bulk, Absorption, and Float and the purposes of each. It is our understanding that they represent progressively lower charging (voltage) states which, if correct, we don’t understand why an alternator would be said to be good for Bulk while having insufficient voltage for the Absorption stage.

As noted above, we don’t believe that any of the low-voltage charger sources are good Bulk chargers (e.g. alternators and many/most shore power chargers). We really like your DIY shore power approach as it adopts the solar panel ‘high voltage’ approach that can maintain a much higher charging current profile than, say, an alternator.
This is not quite right. Bulk stage is typically lower voltage than Absorption. It's a current limited stage where the current is limited by the capacity of the charging source or a specified max current to protect the battery. In the case of battery that can take a lot of current (lifeline AGM) the van charging system is great for bulk in a deeply discharged battery since it can put out a lot of current
 

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papab's description of Bulk is my understanding also.

I really should have been more precise in my alternator bulk charging claim:

Alternator charging is likely best for [early] Bulk because of the relatively high current...
I add this [bracketed] qualification because the alternator will probably not be able to complete Bulk (ie, achieve the Vabs setpoint).
 

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If I would have done my electrical system my self, I would have gone with the CTEK D250S DUAL:

https://smartercharger.com/battery-chargers/#CTEK D250S DUAL

Connect your solar, connect your van battery and it just handles it. Will use what ever source is better at the time and properly does multi-stage, temperature controlled charging from either source. Once the coach battery is full, it will even put a maintenance charge back on the starter. These are what Hein uses and he gives them a thumbs up.
 

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So right now I have a chance to go lithium for my batteries. Only issue is I'm having trouble finding charge controllers that will charge Li ion. The ones I have found are for LiFePO. Can I get the LiFe models and change my voltage thresholds for that of Li ion? The model I was specifically looking at was the Zamp 30a.
 

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We Still Hate the Terms: Bulk, Absorption & Float

This is not quite right. Bulk stage is typically lower voltage than Absorption. It's a current limited stage where the current is limited by the capacity of the charging source or a specified max current to protect the battery.
We continue to believe that these terms require further refinement or, possibly, the problem is that different manufacturers treat them differently.

Our MidNite Solar "Classic" controller, for example, does not allow for the setting of a Bulk voltage, instead, one sets the Absorb voltage. If the battery is below that voltage, then, the controller places itself in Bulk mode which MidNite defines as the maximum current available from the solar panels taking into account their wattage and sun conditions. (The most current we've observed is 57 amperes which not only is well below the theoretical 3C maximum charge current [of 1500 amperes] for our lithium batteries but within the allowable charge rate for most lead-acid systems). It is in this mode, the Bulk mode, where the MPPT controller shines (notwithstanding MsNomer's suggestion that her PWM controller is the equal - - which it may be). The MPPT controller picks a voltage (in the Bulk mode) that maximizes the charge rate - - giving us all that the solar panels are capable of outputting. While we've never measured this voltage (and it changes according to sun and battery conditions), it is surely above the the preset Absorb voltage level. When the battery reaches the Absorb voltage, the MidNite Solar controller becomes a constant voltage source at this lower voltage which implies that the charging current will also drop.

Hopefully you will understand why we feel that Bulk, Absorb, and Float are progressively lesser charge regimes and - - due to this continuing ambiguity - - why we wish we could just discontinue use of these terms relying, instead, on definitions of the actual desired overall charge profile for a given house battery configuration.
 

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Lithium

So right now I have a chance to go lithium for my batteries. Only issue is I'm having trouble finding charge controllers that will charge Li ion. The ones I have found are for LiFePO. Can I get the LiFe models and change my voltage thresholds for that of Li ion? The model I was specifically looking at was the Zamp 30a.
Lithium batteries come in various forms. The Li-Ion batteries in your cell phone are different (higher energy density and more likely to explode upon overcharging) than the lithium batteries used as RV house batteries. The Lithium batteries we use in RV's are, in fact, LiFePO(4) so the charge controllers you're looking at should be fine. Most chargers - - including solar controllers - - can be 'programmed' to set charge voltage levels, sometime current levels, and sometimes durations. We have three charging systems: 1) a MidNite solar controller, 2) a Magnum shore power charger; and, 3) Balmar external voltage regulator for our 2nd engine alternator. There are huge differences in the way each of these is programmed/adjusted but, in the end, they all can be 'forced' to more-or-less define an acceptable lithium battery charging profile. Pick a controller, get the manual and read it.

Lithium batteries are curious creatures that require a bit more knowledge and care than lead-acid. If you're serious about lithium, you will find our build-thread instructive at: http://www.promasterforum.com/forum/showpost.php?p=405113&postcount=50 (with photos of the installed panels at Post 43).
 

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We have 2 100 ah Stark Power LiFePo4 batteries located inside our Promaster.

Charged by the vehicle alternator through 600amp Yandina voltage sensing relay. The voltages for combining and separating are those typical of lead acid batteries so, based on advice from Yandina, we used a small relay connected to the Promaster upfitters connection in the passenger B post to combine and separate the house and vehicle battery. The relay is connected to the ignition on and the engine running pins. We haven't yet found anyone who makes an automatic relay that works with typical LiFePo4 voltages,

We also have a SPDT switch connected in series with the small relay that allows us to manually separate or combine the batteries. Even if I forget to separate the batteries the alternator's maximum voltage of 14.2 - 14.3 can't charge the batteries more than 95%, which Stark power thinks is just fine.

The alternator charges the batteries quite quickly, typically in 1 or 2 hours of driving depending on how much the Espar furnace was used at night. If we are staying put for a couple of days, 15 - 30 minutes of idling the engine will replace most of a night's usage as well as heat water for showers in the Isotherm hot water heater connected to the engine cooling system. I've seen charge rates as high as 80 - 90 amps.

The high charge rates we get from the alternator seems to eliminate the need for solar panels for us. We seldom sit in one spot longer than 2 or 3 days without a short drive for groceries, Laundromat, sight seeing, bakery, etc.

We also have a LiFePo4 specific 10 amp battery charger that we use if plugged in for a long period of time. In a recent 2 1/2 month tour of the west we used it 2 nights.

Our loads are a 12v compressor refrigerator, Espar D2 furnace, Wallis diesel cooktop, water pump, LED lights, Maxxair fan, 2 laptop chargers and 2 cell phone chargers.

One benefit of the LiFePo4 batteries is the high voltage during discharge. The diesel stove needs a solid 12v to light.

During our visit to Sequoia NP we stopped in Ontario, Ca and purchased a 1000w Renogy pure sine inverter to run a small 750w induction cook unit and a 750w toaster for those days (summer in southern in the southwest ) rather than start the diesel cooktop.
 

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I'm considering this unit for simplicity in wiring, control, and optimization in charging between solar and van alternator:
http://http://www.kisaepower.com/products/battery-chargers/model-dmt-1230/

I plan to use two 125AH Vmax AGM batteries with 200W of solar on roof. The only shortcoming I see is that it only outputs 30A which is probably ok but would be nice to have higher output for quicker initial charge from alternator if/when batteries are low.

Any opinions or experience with this device?
 

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We continue to believe that these terms require further refinement or, possibly, the problem is that different manufacturers treat them differently.

Our MidNite Solar "Classic" controller, for example, does not allow for the setting of a Bulk voltage, instead, one sets the Absorb voltage. If the battery is below that voltage, then, the controller places itself in Bulk mode which MidNite defines as the maximum current available from the solar panels taking into account their wattage and sun conditions. (The most current we've observed is 57 amperes which not only is well below the theoretical 3C maximum charge current [of 1500 amperes] for our lithium batteries but within the allowable charge rate for most lead-acid systems). It is in this mode, the Bulk mode, where the MPPT controller shines (notwithstanding MsNomer's suggestion that her PWM controller is the equal - - which it may be). The MPPT controller picks a voltage (in the Bulk mode) that maximizes the charge rate - - giving us all that the solar panels are capable of outputting. While we've never measured this voltage (and it changes according to sun and battery conditions), it is surely above the the preset Absorb voltage level. When the battery reaches the Absorb voltage, the MidNite Solar controller becomes a constant voltage source at this lower voltage which implies that the charging current will also drop.

Hopefully you will understand why we feel that Bulk, Absorb, and Float are progressively lesser charge regimes and - - due to this continuing ambiguity - - why we wish we could just discontinue use of these terms relying, instead, on definitions of the actual desired overall charge profile for a given house battery configuration.
I'm sure I didn't explain it very well. This is better, except they call it the constant-current phase.
http://batteryuniversity.com/learn/article/charging_the_lead_acid_battery
"The constant-current charge applies the bulk of the charge"
During bulk phase the charger is putting out as much current as it can (or is set to), when at that current it reaches the absorption voltage it's in absorption phase.

I think most of what you said is correct, I think this may be where you're confused.
"While we've never measured this voltage (and it changes according to sun and battery conditions), it is surely above the the preset Absorb voltage level."
I think you should measure it, you'll find that it is below the absorb level.

Msnomer is right, during bulk mode pwm & mppt are roughly equivalent.

I have one question that been bothering me for a while. How many personalities is Winston? :)
 

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We continue to believe that these terms require further refinement or, possibly, the problem is that different manufacturers treat them differently.
Those terms are fairly standard. Some of the Chinese tracer variants use "boost" for "absorption", and some others say "acceptance" for "absorption" but the terms above probably cover 90% of usage.


Our MidNite Solar "Classic" controller, for example, does not allow for the setting of a Bulk voltage, instead, one sets the Absorb voltage.
Bulk, by the usual definition, does not have a voltage setpoint. It ends when the absorption voltage setpoint (Vabs) is attained. There are some esoteric situations where controllers have a setting for bulk voltage but I don't know if it is a semantic variation, germane to nonstandard chemistries, or what. If anyone knows, please chime in.


It is in this mode, the Bulk mode, where the MPPT controller shines (notwithstanding MsNomer's suggestion that her PWM controller is the equal - - which it may be).
Agreed, bulk mode is where most folks will see the most obvious benefit from MPPT, but power point tracking happens all the time to adjust for loads and charging stage.

During bulk the PWM is running wide open, effectiviely tying the panels to the battery. If the battery bank is sitting at 12.2v in the morning the panels are running at 12.2v. This is a long way from the panels' max power voltage (Vmp), typically ~17v on a nominal 12v panel.

So if we are floating at, say, 13.2v and add a big load the best the PWM can do is whatever the panel's output is at 13.2v in present conditions. Again, a good distance from ~17v Vmp.
The PPT will run the panels at whatever voltage the meets the power output needs, up to the panel's max power in present conditions.

One practical effect is that one can squeeze a bit more power from a PWM by configuring Vabs and Vfloat setpoints on the higher side. This runs the panels at closer to Vmp and extracts more (wastes less) power.


The MPPT controller picks a voltage (in the Bulk mode) that maximizes the charge rate - - giving us all that the solar panels are capable of outputting. While we've never measured this voltage (and it changes according to sun and battery conditions), it is surely above the the preset Absorb voltage level.
Yes the PPT controller will run the panels at Vmp during bulk, ~17v for nominal 12v, ~34v for nominal 24v, etc. Exception: current limit configured in controller.

I think this statement was misread by a subsequent poster.

When the battery reaches the Absorb voltage, the MidNite Solar controller becomes a constant voltage source at this lower voltage which implies that the charging current will also drop.
Yes, because the battery accepts less current as absorption progresses. The controller isn't directing that behavior.

Hopefully you will understand why we feel that Bulk, Absorb, and Float are progressively lesser charge regimes and - - due to this continuing ambiguity - - why we wish we could just discontinue use of these terms relying, instead, on definitions of the actual desired overall charge profile for a given house battery configuration.
I guess if one is thinking in terms of charging current then with no loads Bulk > Absorption > Float. There are two problems associated with that approach:

  • it ignores loads - adding a load increases the need for more current, not more voltage at the battery bank. This means that [in] Float and Absorption stages the controller can be providing the same amount of current as Bulk, and in Float the controller can be providing more current than Absorption. How many of us have zero loads during charging?
  • charging setpoints in the controller are generally defined as voltages.
 

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A Re-evaluation of Bulk/Absorption with a Concession?

I have one question that been bothering me for a while. How many personalities is Winston? :)
A curious question that we'll temporarily sidestep in favor of one last stab at the Bulk/Absorption conundrum.

We think the Battery University article, in particular Figure 1, is accurate and useful and not horribly inconsistent with our understanding. We have disparaged the over-used terms: Bulk, Absorption, Float, preferring to define an appropriate "charging profile" for each battery type. Then, with this profile, we ask 'how do we design a charger to meet it?

Figure 1 provides that profile . . . it's the current curve. How do we achieve that profile? The "cell voltage" curve of Figure 1 gives us the answer. The charger voltage must continually rise during the Constant Current "Bulk" phase. This is why alternators make poor Bulk chargers. As alternators are constant voltage sources, they initially impress with their high charging currents, but quickly lose their high current character as the internal battery voltage rises to meet the fixed voltage of the alternator.

So what happens with a CC/CV (constant current/constant voltage) charger (of the type spoken of by Battery University and shown in Figure 1's current profile), at the instant of switching from Constant Current (Bulk?) to Constant Voltage (Absorb?)? Really nothing. The charger voltage has been rising throughout the Bulk phase in order to maintain the Constant Current. At the instant of Absorption mode switching, the current is the same as it was the instant before switching . . . thus the Absorption mode Constant Voltage is that voltage that existed at the precise moment of switching, Vabs.

Our conclusion, then, is that the charger's voltage will rise during the Bulk stage until reaching Vabs, at which point it will switch to a Constant Voltage, at Vabs - - the Absorption stage? So, we concede, most of the Bulk stage is at a voltage below the Absorption stage voltage - - at least for those chargers following the CC/CV model. But maybe it's a draw? While the Bulk voltage is not higher than the Absorption, in the CC/CV model, neither is the Absorption voltage above the Bulk.

And we have some doubt whether most chargers follow CC/CV model. In the hope of clarifying this uncertainty, we'll take our CTEK multistage charger (on our AGM house battery in the "old CaRV") and see what we can discover.

Now what did you mean by "multiple personalities"? :~)
 

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We have 2 100 ah Stark Power LiFePo4 batteries located inside our Promaster.

Even if I forget to separate the batteries the alternator's maximum voltage of 14.2 - 14.3 can't charge the batteries more than 95%, which Stark power thinks is just fine.
You are one of the few (if not only) forum members using one of the lithium so-called Lead-Acid 'drop-ins'. We went a different direction, that being: we assembled an overall battery 'pack' according to the desired AH capacity and added a corresponding Battery Management System [BMS] that covered the overall pack. Our conversations with Stark were less than revealing . . . we remained uncertain how their BMS was configured/worked and, further, were uncertain how separate battery pack BMS systems would 'play together' when connecting multiple Stark 100AH batteries together. With this said, we'd love to hear more about your implementation . . . specifically, what State/Percentage Of Charge metering you have, if any, and/or how you monitor those batteries and assess their state of charge/need for charge.

Another concern we had with Stark and the other drop-ins is that lithium voltage levels are different enough from lead-acid (lower than lead-acid) that dropping a typical lithium battery into a system charging to lead-acid standards would likely be harmful to the lithium. We were particularly surprised by your comment that it's ok to charge them to 14.2-14.3 volts and that that level represent 95% charge.

By coincidence we are presently running progressive discharge tests to confirm the AH capacity of our pack as well as to determine what cell/terminal voltages to expect at various levels of discharge. We are taking measurements at various load levels and after extended periods of 'resting' so we can finally program our various chargers to bring the batteries to, and maintain them at, a known Percentage of Charge.

But it's clear that 14.2-14.3 is way too high for our batteries. Indeed, our batteries are only charged (and rarely so) to 14.2 volts to 'balance' the cells. At 14.2 volts (which corresponds to 3.55 volts/cell) our BMS 'shunts' a resistive load across the cell to by-pass some of the charging of that cell while it waits for the other cells to 'catch-up'. During normal operation we're finding that 13.4 volts (0.2 volts below the manufacturers recommended 'float' voltage) represents about 90% charge. With the 'conventional wisdom' being that it is best not to maintain lithiums for extended periods of time at 100%, or even above 90%, we're contemplating setting 13.4 volts as our standard charge level.

We would recommend that you confirm the chemistry/technology of your particular batteries to make certain that you aren't shortening their life charging to 14.2-14.3 volts.
 

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I guess if one is thinking in terms of charging current then with no loads Bulk > Absorption > Float. There are two problems associated with that approach:

  • it ignores loads - adding a load increases the need for more current, not more voltage at the battery bank. This means that [in] Float and Absorption stages the controller can be providing the same amount of current as Bulk, and in Float the controller can be providing more current than Absorption. How many of us have zero loads during charging?
  • charging setpoints in the controller are generally defined as voltages.
Exactly . . . and that's why we want a two mode charging system: 1. Balls-Out, give us everything you've got until a set charge level/voltage level is reached; then, 2. Hold this voltage regardless of load.

When the 'assigned' charge level is reached, we want our charger to become a power supply (constant voltage source). In that mode, a very nominal, nearly zero current will flow into/from the battery. Any other loads on the system will, in essence, be supplied by the power supply/charger.

We are currently measuring battery voltages, both under load/charging and 'resting' as a function of State of Charge. We will pick a battery resting voltage corresponding to our desired State of Charge and set our charging sources to that "Constant Voltage". This will be in the order of 80%, maybe 90%, of SoC.
 
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