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I'm working on a solar project that involves using 4 100AH LFE (LiFePo4) cells to supply 12 volts. The LFE cells indicate the charge rate should be between 0.3C and 2C, which is a lot of current.

The average 300 Watt solar cell only provides 8 Amps, which is less than 0.1C. Will it do any harm to the LFE cells to just use 8 amps to charge them or is it important to give it at least 30 amps?

The other problems is that I can't find a CC/CV board that does more than 16 amps, but that a question for tomorrow.

Thanks in advance,

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    \$\begingroup\$ No, you can charge them as slow as you want. \$\endgroup\$ – winny Oct 19 '16 at 6:48
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    \$\begingroup\$ If a 300W cell is providing 8A, it's also providing 37.5V. By the time your charge controller steps that down to 12V it'll be just shy of 25A. \$\endgroup\$ – Brian Drummond Oct 19 '16 at 10:43
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I've only done a lot of reading, but have perceived a consensus on this issue. Supplying lower currents than recommended over longer time periods is in itself unlikely to affect longevity.

However assuming these are "native" LFP4 cells, perhaps with balancing and UVP/OVP circuitry but not "lead drop-in replacement" style, the key to longevity is NOT pushing voltage up high, and NOT trying to get top level SoC.

With short charges from a high-output alternator, best practice for longevity is 13.9-14.0V; .3C to .5C current is actually ideal if you can manage it, and then STOP when current drops to .02C.

With lower-amp inputs, over long periods as from solar, the CV max should be lowered to 3.45Vpc or 13.8. Resting voltage should settle to around 13.6, which will be within .1% SoC from what you would get charging harmfully over 14V, but much gentler, and require less frequent balancing.

Ideally​ you don't float at all for longevity, but if you do "have to", then bring it down to 13.2v (3.3vpc)

Never draw them below 20% SoC.

When not being cycled regularly, let the bank rest at 40-60% SoC, never "full" (resting >= 13.6V)

Never allow any charge input when cells are under 40°F.

With this sort of coddling they will likely last at 80%+ capacity for well over 2000 50% cycles, maybe 20 years. But of course no one knows yet.

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Generally charging cells slowly means they stay at high charge levels for very, very long, because they have internal leakage that increases with their voltage. Or they may even never reach completely full.

A completely full cell degrades much quicker.

However, LiFePO4 is a bit special, it deteriorates much less differentiated over state of charge. (It doesn't go bad three times as quick when completely full, like normal LiIon, only a bit faster). And it also is capable of handling levels of over charge, and has quite low internal leakage.

Now, of course, one LiFePO4 battery isn't the next, so I cannot tell you your stack's leakage currents. I would be surprised, however if they came close to dozens of mA, leave alone Ampere.

The only important thing that traditional Lithium Ion and LiFePO4 share is the wisdom in keeping cells balanced on voltage. So if you charge and discharge often and don't know for sure they will be completely topped-off cycles, it's wise to get an active balancer for the cells.

Just try it, if the last leg of the charging cycle doesn't take more than twice what you would expect based on linear maths, it's extremely likely it'll be fine.

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You can't calculate the current like this. You need a charge controller which adjusts the loading voltage to that your battery stack needs. If your stack is 48V, obviously the current is less than 8A.

But I would vote to have two stacks of 24V and two charge controllers so you can replace one stack while the other one is in use. You also need another converter which does 12V from 24V.

Don't ever try to go without charge controllers and Lithium batteries, they burn your place down when overcharged. Even at low charging currents. There is no "safe overloading" where the excess charge is put into heat, as with NiCd, NiMH, Pb cells.

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    \$\begingroup\$ These are LiFePO4. \$\endgroup\$ – mkeith Oct 19 '16 at 7:30
  • \$\begingroup\$ What's all this about voltages? \$\endgroup\$ – Asmyldof Oct 19 '16 at 7:35
  • \$\begingroup\$ The OP said he had a 12V circuit. And four 100Ah batteries to supply it. Unclear if he meant four 12V/100Ah or four 3.7V/100Ah. His panel is 300W/8A, that means it has a peak voltage of 37.5V -> NO MATCH. When you match the voltages, the currents change. So this is all about voltages, too. \$\endgroup\$ – Janka Oct 19 '16 at 7:43
  • \$\begingroup\$ @LiFePO4 can burn down, too. The only difference to LiCoO2 is the oxygen isn't released easily, which avoids rapid inflammation. But the Li dendrites which provoke internal shorting also grow when loading with much too high voltage. \$\endgroup\$ – Janka Oct 19 '16 at 7:54
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    \$\begingroup\$ My 12 volt cell is really made of 4 3.6 volt LFE cells in series. I am using a "shunt" circuit to balance each 3.6 volt cell to make sure that it doesn't overcharge. I also have a BMS circuit to manage the cells for over discharge and current limiting. The BMS does not do much useful on the charging cycle. \$\endgroup\$ – keithwilke Oct 20 '16 at 22:23

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