[tsalhus]

Hi All,
My 2014 L2H2 is undergoing transformation to a Camper for two.
I purchased a 100AH lithium battery kit from Electriccarparts.com and plan to run it mostly by solar (2x100watt) and mount it as an auxillary with isolation for alternator charge when required. The MPPT controller has user programmable parameters for charging so I hope the battery charge will not be a problem.
When I research the info on the lithium battery management system (BMS) I note that it has a built in safety program that will activate a disconnect of the battery from the rest of the electrical system (solenoid relay) if there is a critical event of overcharge or discharge of any of the cells. One presumes this will only rarely occur in normal operation but should it occur in a charging situation it would likely have the potential to create a transient voltage spike or voltage dump - I also gather this has the potential to be unkind to alternators, charge controllers and other electronic bits.
Sterling Power USA and Electriccarparts sell alternator protection devices http://www.sterling-power-usa.com/alternatorprotectiondevice.aspx and I have read that TVS diodes (avalanche diodes) are an inexpensive fix for elsewhere.
This not an area where I have any experience and I wonder if any members of the forum have advice or experience.
Thanks,
tsalhus

 

[mikemarmar]

I asked about this a few weeks ago on sprinter source: http://sprinter-source.com/forum/showthread.php?t=46486

The conclusion was, as long as the starter battery is always connected to the alternator it won't be a problem as the starter battery can absorb the transient. As for the MPPT controller, it probably depends on which one you have. Some have better built in protection than others. I would ask the manufacturer.

Edit: I also asked about disconnecting the batteries from my MPPT controller while leaving the panels connected here: http://sprinter-source.com/forum/showthread.php?t=46893
For my controller (Victron BlueSolar MPPT 75/50) it is not a problem as long as the open-circuit voltage of the solar array doesn't exceed 75v. Note however that when you disconnect the batteries a spark will be generated at the point of disconnect, which in your case is probably the solenoid. The solenoid should have arc suppression built in to handle this.

 

[Zoomyn]

I'm in the same situation, I have LiFePo cells and want to add them in as house battery.

A little dinky-doo 12V relay coil can produce a 200V spike opening let alone what an industrial heavy duty relay could spike... Solenoids usually only protect its own coils control circuit with a diode to make sure when its own coil circuit opens it will dump its magnetic fields reverse-force into a loop back into itself instead of surging it back through the control circuits (other components may not have high enough 'Reverse Standoff Voltage' protection, etc.) with about the only penalty for using this is a slightly slower relay opening time.

I'd like to use a solid-state relay for engine/house battery switching (to replace solenoids) but finding pure DC ones above 100A gets expensive. eBay china-made units are likely over-rated against what we actually get too, and require active cooling to approach their ratings so erasing any reduced power gains.

Maybe to isolate the battery manager or protector systems, the controller that actually operates the solenoid would use a DC SSR as a good buffer just for long term reliability.

When protecting the switched circuit just remember things like Varistors (MOVs) do fatigue over time, it'd be easy enough to put MOVs across to ground to soak up the worst switching spikes but use fat (high current) ones so they just don't vanish one day w/o being noticed.

I'm thinking of using a Midnite Solar SPD MNSPD-115 (link) as an alternative to DIY battery protection - it is very robust with a 5-year warranty and has pilot lights that indicate it's still functioning.

Also - a fully charged battery rejects accepting current & voltage, there is no/low energy soak to be found there and the voltage can easily rise way above the battery potential. An example might be a renewable energy wind turbine charging a battery, when the battery is charged the wind turbine alternator/generator sees no load so has no work to slow its rotation and overspeeds, runs away & destroys the blades from high RPMs...

Where our RV systems could see something similar is if the rooftop solar has done its work and all batteries are charged 99.5%, there just isn't enough draw to be counted on to absorb spikes. Remember lithium and AGM's have low self-discharge so when we return to the van after a string of sunny days and start fumbling around having the engine battery in circuit won't be much if any help...

 

[mikemarmar]

Zoomyn, I am using LiFePO4 for a house battery as well. I have a solid state isolator (Victron ArgoFET 200) to isolate the starter battery, and it works very nicely. It is somewhat expensive...I think I paid $135. The big benefit to using a solid state isolator instead of a relay is that the two batteries are never connected to each other. If you go this route you will have to run a new cable directly from the alternator. I drilled a new hole in the bottom right of the battery box.

As for transients, I have tested a worst case scenario with no ill effects. My house battery was drained down to 50% and was pulling 175A from the alternator. The starter battery was totally charged. I cut the current to the house battery with a disconnect switch (also tested by opening a heavy duty relay). No load dump, no alternator damage, nothing. Even if the starter battery is "100%" charged, it can absorb the transient spike from disconnecting 175A.

 

[Zoomyn]

I see an oops, sorry. I wrote: "I'm thinking of using a Midnite Solar SPD MNSPD-115 (link) as an alternative to DIY battery protection" and that should read ". ..as an alternative to DIY circuit protection". With campground power ALWAYS being suspect, as well as summer thunderstorms and winter dry-air static hazards I think the SPD device will be cheap insurance.

Actually, looking at it again, there are hardened milspec quality circuit filters on most/all the vehicles modules, especially on the computers, that eat noise like candy otherwise all ell would break loose often. The details above concern raw battery/batteries and how it's easy the internet in general overlooks basics unless we've got an oscilloscope handy...

mikemarmar: Have you narrowed down the Management & Protection brand/model you're going to go with? I see 'Management' usually means real time cell balancing, and a separate board does 'Protection' meaning charge/output over & under voltage monitoring.. then temperature limiting is also tossed in the mix also.

Finding LiFePo battery devices that don't push the chemistries limits is proving difficult, I'd be very happy to only take 80% to leave wider margins at the high/low ends to extend battery life.

 

[jostalli]

Zoomyn, I am using LiFePO4 for a house battery as well. I have a solid state isolator (Victron ArgoFET 200) to isolate the starter battery, and it works very nicely. It is somewhat expensive...I think I paid $135. The big benefit to using a solid state isolator instead of a relay is that the two batteries are never connected to each other. If you go this route you will have to run a new cable directly from the alternator. I drilled a new hole in the bottom right of the battery box. .

I also have LiFePO4 battery for my house bank. What is the benefit of the Victron Argo FET 200 over simply hard wiring directly from start battery to house battery for alternator charging?

 

[mikemarmar]

I have 12 cells, arranged in 3 parallel strings of 4 cells. Each string has one of these boards http://www.batteryspace.com/pcmwith...cells128vlifepo4batterypackat100alimited.aspx

That gives me 300A charge/discharge (way overkill for me), under/over charge protection and cell balancing. The over charge protection will cut out at 15.6v, which is quite high. However, it would only come into play if one of my charging devices (solar, alternator, 120v charger) malfunctions. This is unlikely, and a one time event at 15.6v for a brief period is not going to do much damage. The undercharge protection is also quite low at 8v. I intend to be careful about never going below 10v, and it hasn't been an issue so far. My BMV-702 monitor has been very useful for monitoring charge and not taking them too far down.

The other protection I have is a 200A contactor relay that will cut charge current if the temperature drops below 0C. There is a small relay on the BMV-702 that can be programmed to open below a set temperature. This small relay is controlling the contactor. I also have a 40A relay on the solar panels that will open when the main contactor opens. Apparently this isn't required, but I didn't like the idea of the solar charger being hot when I want everything else turned off. The relays are also wired to open when the master switch is opened.

 

[mikemarmar]

I also have LiFePO4 battery for my house bank. What is the benefit of the Victron Argo FET 200 over simply hard wiring directly from start battery to house battery for alternator charging?

If you hard wire directly from the starter battery there are two issues. First, you risk getting stranded if you drain down your batteries. Second, connecting batteries of different chemistries and capacities in parallel is not a good idea. The batteries have different charge and discharge profiles, but by wiring them in parallel you are forcing them to be at the same voltage at all times which might not be optimal.

Using a relay isolator is better than hard wiring, as it prevents you from draining your starter battery. It still wires the batteries in parallel when the relay is closed, which is not ideal. In addition to the problem of wiring different chemistries in parallel, if you close the relay when the lithium batteries are depleted, they will pull a huge current from the starter battery. Not sure if this will cause any issues, but something to be aware of.

The solid state isolators keep the two batteries isolated from each other at all times. The only downsides are cost (make sure to get a FET based isolator, not a cheap diode based one) and having to run a new wire from the alternator (and disconnect the existing one)

 

[jostalli]

No I mean for alternator charging only. I simply flip a switch when I want my alternator to charge my house battery and flip it off when I wish to stop charging. The different battery chemistries does not matter since the alternator is only going to supply a voltage of around 14V which is perfectly fine for these batteries.

 

[mikemarmar]

Ah yes a manual switch works fine. Just be careful not to close the switch when the engine is off. Or if you do, be aware that a very large current can flow between the batteries if they are at different states of charge.