12V charger Q about resistor for custom 12V lithium pack

[spinningmagnets]

Sorry for the newbie question, but I am experimenting with 4S lithium 18650's and I hope to charge them from an unregulated 12V automobile accessory socket.

4.2V x 4S = 16.8V, so the 14V socket output will never overcharge the pack. The pack will end up with a "half charge" when using a car socket (roughly 3.5V per cell) but I do have an RC charger that can provide 16.8V when desired. There is no BMS for this particular experiment.

I understand that 4S LiFePO4 is common for 12V, and also 3S 18650-cells, but that will be for another discussion elsewhere. This pack is lithium-NCM in the 18650 format of cells, 4S

I assume I must add a resistor to limit current. Should the resistor be located on the positive/red lead, or the negative/black?

Should I put a resistor in both?

 

[john61ct]

unregulated 12V automobile accessory socket.

Do you mean a standard ciggie sockets?

These are a dangerous abortion, avoid like the plague for anything important, or that you use regularly. Never more than 5-6A and only for short periods, as in a few minutes. Even then they are risky, an inherently poor design!

Blue Sea has a nice socket design that twist-locks with the matching plug, but will also accept standard ciggie plugs for smaller (<10A) loads.

Also the BMW/ Hella/ Merit/ Powerlet "Euro-style DIN" (ISO 4165) style is very robust.

Anderson plugs for high amps, for me my standard, for almost all power connections.

If you standardize on one of the last two types, there are adapters for guests, temporary use of devices with standard ciggie plugs.

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Also, LiFePO4 should never go above their mfg rated max voltage. 4.2V is crazy high, don't even think about that!

For longevity just get to 3.33 - 3.35Vpc resting, Full as you need to get, any higher is just surface charge, very little actual stored energy.

At a low charging C-rate like below 0.2C, 3.45Vpc is a good setpoint. Up to say 0.4C, 3.50V is enough.

At 13.5V for 4S, 3.38V yes charging at the high SoC mark will get very slow

but do not leave them on too long, that just makes it harder to tell when they're full, so *more* likely to overcharge.

I would just use an HVC without a proper charger CC-only will get very close to "working Full".

But if you must go into Absorb / CV stage, stop well before amps trail down below 0.1C

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If you are talking about charging while driving, 13.5V seems very low for alternator output, are you sure it doesn't ever go higher?

I would monitor that!

> I do have an RC charger that can provide 16.8V when desired.

Get a proper DC-DC charger with a user-custom adjustable setpoint, would really be better.

That should also do a much better job of limiting current.

I would not use the ciggie port for this type of job, at all.

Whatever the max current your proper charger pulls, or at the top end

for bank longevity, stick to 0.4C especially in cooler weather.

If hot weather maybe 0.6C is OK.

size a full round trip pair of good tinned boat wire for low voltage drop at double that current, size a fuse on hot close to source, ideally the starter / alternator circuit itself north of the firewall.

The resistors idea seems like a real kluge to me, current limit will vary with voltage, will mess with your setpoint depending on current rate, can't help you there.

Safety first!

 

[spinningmagnets]

Thanks!