Lithium-ion 12v Battery Replacement

[RedDevil]

Seems they need constant, or at least very regular balancing.
Top balancing does not cut it, unless you charge very frequently. Mini solar panel?

[Ecky]

Originally Posted by RedDevil

Seems they need constant, or at least very regular balancing.
Top balancing does not cut it, unless you charge very frequently. Mini solar panel?

I think top balancing is ok so long as the balancing Is aggressive enough, and occurs in the range they typically operate in. The LEDs I chose only balance when the caps are above a certain voltage, and while sitting I'm sure they were below that for much of it. The longer they can sit near full without cycling, the more they'll stay in balance. A higher current LED would help.

[redpoint5]

I have LED/diode balancing on the caps in my motorcycle for like 4 years now. No problems with balance, and I probably don't even need any balancing.

Caps that get a little bit more voltage will also bleed off that voltage faster. In that way, they self balance.

Try it yourself. Charge 1 cap to 2.7v, and another to 2v, and chart the daily voltage drop.

[teoman]

Originally Posted by RedDevil

FWIW, LiFePO4 buddy pack, split in 3 12 cell units each with their own BMS, has been waiting further developments for a couple of years now.
Once a year I check on them, and they charge for a minute or so before they max out.
There is virtually no draw whatsoever from their BMS.

My 12Ah cells are good for 10C (120 Ampere) during 30 seconds.
The BMS limits output to 55 Ampere, and completely cuts off power when the current rises above 55 Amp, the temp sensor on the BMS exceeds 90 Celsius or any cell gets out of range (above 3.85 Volt, below 2.something).
It also has passive top bleeding to 3.65 Volt.

So far I'm very pleased with how it works out.

You should store them at %40 charge level.

My underwater scooter with a 1kWh battery has a best practice of:
Use it, sore it charge before use and use it.

[Ecky]

Update: This morning my car was only barely able to start. I'm guessing the caps were somewhere around 6-8v as it took a good 30-40 seconds of charging before the cluster had enough voltage to boot up.

Unsure what happened. I didn't drive my car yesterday, and overnight temperatures rose from around 0F to 39F. Any ideas? Maybe the battery unfroze and drained the caps?

I might try removing the resistor between battery and caps.

[redpoint5]

If the caps are connected to the battery, they should be the same voltage. What was the battery voltage?

As I said before, if the caps are not holding balance, they aren't in good health.

[RedDevil]

Originally Posted by teoman

You should store them at %40 charge level.

My underwater scooter with a 1kWh battery has a best practice of:
Use it, sore it charge before use and use it.

Good advice for most Lithium batteries.
The LiFePO4 chemistry is different in that it does not deteriorate on storage.
There's conflicting information on how to store them, I fear much of it is confusion with the other chemistries.

When I bought my LiFePO4 cells they had been untouched for several months since their production. They came fully charged, so I guess they can handle that.
When I did my first charge the chargers had them topped off in a minute.
So I think they would not deteriorate in their current 99% charged state.
Nor if stored at 40% for that matter, that's always a safe bet.

[Ecky]

I wasn't able to test voltage this morning - I started the car and went straight to work. However last time I had an issue, the battery was at ~8v. Last time I just pulled the battery out, charged it to ~14v, and things have been fine since then.

I have not yet checked the balance of the Maxwell 3000F cap pack I'm using in my car. I may not be able to until this weekend. I'll investigate this.

I can think of a few possibilities:

1) There's an intermittent parasitic draw which happened this weekend.

2) The caps are draining the battery.

3) The battery is draining the caps.

4) The battery is gradually losing charge over time due to the resistor.

5) Temperature change is causing voltage changes.

I'm quite surprised a 20Ah battery emptied out over a 36 hour period.

[hayden55]

Originally Posted by Ecky

I wasn't able to test voltage this morning -
I'm quite surprised a 20Ah battery emptied out over a 36 hour period.

Time to start clamping wires and see where the fuel is flowing. With that much flow that could be an easy one to find. Unless its weather related?

[redpoint5]

My first guess is parasitic drain. Second guess is a malfunctioning cap. A resistor in series between the battery and cap isn't going to cause any measurable drain.