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Actually 2/0 is a touch overkill. 4AGW will carry 100 amps 10 feet.
 

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Actually 2/0 is a touch overkill. 4AGW will carry 100 amps 10 feet.
For me, I have the 15 foot run fused at 200a and do see north of 150amps, so I think I could have gotten away with 0 gauge with a 3% voltage drop but certainly not 4awg.

For anyone looking at wire gauge, the below is a great resource.

http://circuitwizard.bluesea.com
 

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The BMS monitors the individual cell voltage and switches to a resistive load after 3.52v has been reached on a per cell basis.

Do you think there are issues with this approach?
Our BMS is similar, it 'shunts' a resistance across any cell that has reached 3.55v. This 'shunting', however, is intended to aid in 'balancing' the cells - - it does not protect the cell or battery from overcharging.

Individual cell voltages of 3.55 correspond to a pack voltage of 14.2 volts. At this voltage the pack is essentially 100% charged. When new, we initially charged our pack to 14.2 to assure full charge and to confirm/force balancing. The question is - - is there ever a reason to thereafter charge the pack to that high 14.2v level again?

We can think of two reasons to again charge our pack to 14.2 volts, first, to rebalance the pack; and, second, to bring the pack back up to 100%/full charge.

Two pieces of 'wisdom' we've gleaned from our readings are: 1) once balanced, lithium batteries don't require rebalancing; and, 2) it is not a good practice to maintain lithium at 100% charge - - that to do so may stress the battery. Commentators suggest 90% or less as a target.

We've decided to maintain our pack at 80% charge (unless we know we're going to be off-grid without sufficient solar support), then we'll take it back to 100%. Our tests have established that 80% charge occurs at a pack voltage (resting) of 13.36 volts. Thus we have programmed our solar controller (Midnite Solar) and shore power charger (Magnum) to 13.3 or 13.4 volts.

So the ‘issue’ with alternator charging is that most vehicle alternators output a voltage closer to 14.5 volts which voltage is more than a volt higher than our target pack voltage of 13.4 volts, but is also above the 14.2 volt level necessary to fully charge the lithium pack. In short, if maintained, alternator charging will overcharge a lithium pack. And certainly, it will ‘blow past’ our target 80% SoC/13.4 volt target charging point.

This all said, we see no problem with running a higher charging voltage, such as 14.5 volts, if you’re manually controlling and monitoring your State of Charge, terminating alternator charging when you reach your target SoC, whether 80%, 90%, or 100%.
 

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We've decided to maintain our pack at 80% charge (unless we know we're going to be off-grid without sufficient solar support), then we'll take it back to 100%. Our tests have established that 80% charge occurs at a pack voltage (resting) of 13.36 volts. Thus we have programmed our solar controller (Midnite Solar) and shore power charger (Magnum) to 13.3 or 13.4 volts.

So the ‘issue’ with alternator charging is that most vehicle alternators output a voltage closer to 14.5 volts which voltage is more than a volt higher than our target pack voltage of 13.4 volts, but is also above the 14.2 volt level necessary to fully charge the lithium pack. In short, if maintained, alternator charging will overcharge a lithium pack. And certainly, it will ‘blow past’ our target 80% SoC/13.4 volt target charging point.

This all said, we see no problem with running a higher charging voltage, such as 14.5 volts, if you’re manually controlling and monitoring your State of Charge, terminating alternator charging when you reach your target SoC, whether 80%, 90%, or 100%.
Thanks. So your midnite and magnum are set up with a bulk/absorb charge to X volts, where X corresponds to the charging pack voltage at 13.36 resting voltage? And you have the batteries floating at 13.36v? Ie 13.7v bulk and 13.36v float, for example.

How did you test the 80% SOC when charging and come up with X. Did you just use the diaplayed SOC when charging and then remove the charge source to get the resting voltage? The charging voltage will depend on the amperage that is charging the battery right?

I'm set up with 14.2 bulk/absorb and 13.6 float currently for charging, which was what was recommended to me by the distributor. I guess this is optimized for 100% SOC.

What brand/size of batteries do you have?

For reference, I have 5 GBS 100ah nominal packs.

The balancing threshold is actually 3.55v per cell and the SOC reset is at 14.08v (avg of 3.52 per cell)

It looks like for my batteries (based on published charge/discharge curves) ~13.85v while charging at 0.5C corresponds to a 80% SOC. I'm more likely to charge closer to 0.15 to 0.25C though, which should put this charge voltage lower.

Do you charge with the alternator at all?

For me, the alternator is a sort of bulk charger that would get me from 20% to 70 or 80% quickly when solar wouldn't keep up in prior days.

Some things to ponder. Thank you.
 

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How did you test the 80% SOC when charging and come up with X.

What brand/size of batteries do you have?

Do you charge with the alternator at all?

For me, the alternator is a sort of bulk charger that would get me from 20% to 70 or 80% quickly when solar wouldn't keep up in prior days.
Ahh, MsNomer, please have mercy on us for drifting from 'alternator outputs' to lithium. It's a disease.

Copa, do we have the same system: 500ah of GBS lithium with an Elite/Starlight BMS? In any event, your quoted ‘numbers’ sound the same as ours.

We do not rely on the BMS State of Charge ‘meter’ when running our tests. Our tests are conducted at a total battery current of 10 amperes either in 5 or 10 hour increments (5/10 hours corresponding to 10/20% discharges). We turn the system off after each discharge ‘segment’ and let the battery rest for a minimum of 6 hours, but usually overnight before taking our resting voltage reading. We’ve conducted three complete discharge cycles (obtaining a total of 495, 515, and 518 ahs, respectively) and have confirmed the ‘13.36 volt equals 80% SoC’ on each occasion both when discharging in 10% and 20% increments.

But our battery pack voltage is extremely flat in the 80% charge region and we see virtually NO change in voltage between 90% and 80% SoC. In our last two tests, the 90% SoC also rested at 13.36 volts.

It is said that lithiums should not be ‘floated’. We reasoned that if we connect a charger of voltage equal to the resting voltage, then the battery is not being ‘floated’ as, in fact, no current will flow into or out of, the battery. When a load is applied to the system, as a practical matter it will be the charger that picks up ‘the tab’ and supplies that load current.

It is also said that it stresses lithiums to be maintained at 100% charge. We chose 80/90% (i.e. 13.36 volts) to be our ‘go to’ target for normal operations. Of course current technology doesn’t does permit such precision as both the Magnum and MidNite Solar permit setting voltage only in 0.1 volt increments - - so this all is an approximation. But the ‘take-away’ from this is that we don’t want to operate our system at 14.5 or 14.2 volts.

To the extent possible, we operate our chargers in the Constant Voltage mode - - targeting the noted voltage level.

We do charge from the alternator. In fact, we have a 2nd alternator with an external, programmable Balmar regulator. We treat this charge source like you do - - it’s rarely ‘on’ and, when it is, it’s programmed to a higher voltage (13.9) and is used to more quickly restore the battery SoC.
 

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Copa, do we have the same system: 500ah of GBS lithium with an Elite/Starlight BMS? In any event, your quoted ‘numbers’ sound the same as ours.
Thanks for the detailed information. We do have the same system.

I agree with your description of floating lithiums.

For reference, what are you using for your bull/absorb/float profile in your midnite/magnum?

It seems like it may be a good idea to adjust my charging parameters down a bit for my midnite and gopower inverter.
 

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For reference, what are you using for your bull/absorb/float profile in your midnite/magnum?
Both the Magnum and Midnite Solar are set to Constant Voltage mode of 13.4 volts. If we need a more aggressive initial charge we either manually reprogram one of these chargers to a higher voltage (e.g. 13.8 or 13.9) or run our 2nd alternator that is programmed, similarly, to a Constant Voltage, but a higher voltage, 13.8 until we get the state of charge back to the 80-90% level.
 

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I did a little test while driving around today. I was getting sustained 125 - 150 amps while driving around at 1.5 to 2.5k rpms. When stopped at lights and idling it went down to closer to 80-85 amps.
Copa, do you any type of DC to DC charger?
 
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