# LiFePO4 for camper batteries and a design review.



## dougingraham (Jul 26, 2011)

This will mostly work as you think.

The C/20 cutoff current when you parallel four cells is 20 amps (400ah/20). Unless you can charge at 20 amps you will already be fully charged when the voltage reaches 3.6.

For top balance what you propose will get you 95% of the way there. You will have to hand finish the last little bit on the cells individually because you would need to leave them paralleled for weeks. This will get you close enough however to continue. I would just measure the cell voltages after 12 hours of rest after a charge and bleed off a tiny bit from the highest cell. When you get to a difference of less then 0.001 it is close enough.

A safe float voltage of a well top balanced 4S pack would be 13.6 or a little less.

You can charge just fine down to 32F (0C) per the specs. I have had no trouble charging down to 20F at low currents (5A). What happens is the cells simply look like they are charged to the electronics. The voltage goes up and they refuse to take a charge. You might be able to feed it in very slowly but that doesn't do any good. You have similar problems when it is cold with discharging. The voltage sags excessively when it is cold. If this turns out to be a problem you might want to insulate the battery box and put a small heating element inside that you can turn on to raise the battery temp. Or move the battery inside the vehicle.

Keep an eye on the solid state relays. When they fail they invariably fail turned on.

Best Wishes!


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## skooler (Mar 26, 2011)

All looks more or less ok to me.

As has been mentioned. SSRs fail closed. Have you considered a winch relay instead?

Also be aware that by top balancing (which IS the right thing to do in this application) you will have no protection from overdischarge. If this goes flat the chances are you will damage the weaker cells.


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## OLand (Oct 19, 2013)

Thanks Doug and Skooler for the input. 

Good point about C/20 for the batteries in parallel. When charged to 3.6V, should I stop the charge as soon as I get to 3.6, or should it be allowed to sit a 3.6 for some time?

That is good news about the temperature effects on the batteries. I can deal with reduced capacity and poor charging performance at low temperature as long as there is no damage. I have at least a factor of 2 margin in my capacity, so I can withstand significant decrease in capacity. Secondly the biggest draw is the compressor fridge, which only runs when it is warm out anyway. The pack will be mounted under the hood, so it will be warmed quite quickly when charging from the alternator. Similarly, the peak charge from the solar panels obviously occurs in the middle of the day when I can expect warmer temperatures. Any one have a guess at the specific heat capacity of LiFePO4? How is self heating while they charge? Would it be enough to insulate them and allow them to warm on their own as they charge?

As for the SSR, I am not wedded to these, but it does seem like to best option for my application. I have considered a winch or continuous duty starter solenoid and even 70A automotive style relays, however the major issue with these is the significant coil current that these draw. For safety sake I need to configure these in NO configuration (ie coil energized to close contacts) so in case the JLD404 loses power, the relays open and isolate the pack. This means that both relays would be energized 95% of the time. The coil currents range from ~200 - 1000ma (x2 for both relays) which is a significant parasitic load (10 - 20 Ah/day). The other option (that my uC controlled isolation relay uses) is a latching relay, however the JLD404 can't drive these directly. The SSRs draw almost no 'coil' current at < 3mA. They are also majorly oversized - the charge current from the alternator is generally peaks at ~50A and is typically more like 30A. The load currents peak at about 10A, so hopefully this would extend the life of the SSRs. Other suggestions on how to do this would be welcomed. 

Is a low voltage cutoff insufficient to prevent over-discharge? I could set this to trigger on the Ah reading of the JLD404, but that seems less reliable as charge accounting tends to drift over time, particularly with low loads. 

Thanks again for the feedback!


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## Red Neck (Feb 1, 2013)

Your plan is solid but for floating and for hot environments (above 40C), for longevity reasons, it is best to charge them to about 80% capacity, instead of 95%. I would guess charging with a voltage of 3.4v per. Floating seems OK.

LiFePo4 batteries do not last as long when at higher SOC and high temperatures.

Also, top balance them at 3.9v or even 4v. Slowly. Beyond 3.6v only voltage should grow, without meaningful current. You should leave them there for 2-3 hours then stop. Then they will be well balanced. Just make sure the voltage is kept with minimal current. You will see that practically no current will go in to maintain that voltage. Then put them in parallel and deplete them a bit, so they do not stay full when you store it. Storing them at full charge degrades their
life.


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