I believe that a 12 V LiFePO4 battery is quite safe if used with care.
It does not have a thermal runaway mode, but will burn, not explode, if
mistreated to an internal temperature of about 800 C (well above the
melting point of aluminium).
By the way, LiFePO4 is not for low temperatures; one example increases
internal resistance (4 mOhm) below 10 C as follows: 20 mOhm at 0 C, 40
mOhm at -10 C etc.
How I intend to use with care:
1. Charge with a CC/CV regime, just like a lead-acid battery.
CC: small alternator, charging current guaranteed less than 2C
CV: maximum charging voltage 14.5 V; less is ok but the maximum SOC will
be less than 100%
2. Cell balancing is much slower and worse than with a lead-acid battery
(because a fully charged cell largely stops passing current).
Mitigation:
- long stays at CV interrupted by shallow discharges - normal starter
battery use actually
- apply an external shunt (3.70 V) across each cell to speed up
balancing if/when needed
Monitor:
- all cell voltages must remain below 4.00 V; remedy: stop charging
3. Replace if ever discharged to below 20% SOC
Monitor:
- battery voltage must remain > 12.8 V; remedy: if needed carry on
discharging, but do not recharge, replace
4. Have 2
I designed a "monitor & top equalizer" for attachment to a Shorai
LiFePO4 battery. It has just two indications: 1. a cell has a shunt
operating, 2. a cell has a shunt operating but even so its voltage
exceeds 4.00 V.
According to the battery suppliers no equalizing or monitoring is needed
- which may be right ofcourse. Neither of the indications may ever show.
On the other hand - they do sell balancing chargers...
Attached a typical charge curve for a LiFePO4 cell (author unknown). See
the current drop when a SOC of 100% is approached.
Jan de Jong
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