Heat from Charging VS ThermoCouples

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ArcticFox
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Would it do batteries any good to charge them cold?

If one was to put a few thermocouples on a battery, and connect those when the charger is on to keep the batteries cooler during charging, would that help the charging process or battery lifespan?

Just throwing around some ideas...

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andrew
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Re: Heat from Charging VS ThermoCouples

It might help life a little. Battery life in a standby application is most effected by temperature, but I haven't been able to find any direct comparisons between cycle life and operating temperature. In a cycle application the time lifespan in years is so much shorter.

I don't think there may be much of a relationship, but I'll continue to look for information.

In any case, charging the batteries when they are hot can lead to overcharging and thermal runaway. That can certainly decrease battery life. If it is cheap to implement than it could really help. But thermal compensation of the charge voltage might be much easier to implement.

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reikiman
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Re: Heat from Charging VS ThermoCouples

I've seen recommendations to not let the batteries get too hot while charging, and to not charge them when the ambient temperature is hot. This recommendation was especially for NiMH packs. I haven't verified whether that advice was correct, but it sure seems like it ought to be correct.

- David Herron, http://davidherron.com/

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ArcticFox
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Re: Heat from Charging VS ThermoCouples

Is the main problem because of out gassing, case melting or something else?

Would setting the batteries in a container of transformer oil do anything for it?

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reikiman
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Re: Heat from Charging VS ThermoCouples

Yes there is a danger of outgassing.. e.g. SLA batteries can dry out from being overcharged. But I don't know if forcing the batteries to cool would help.

- David Herron, http://davidherron.com/

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andrew
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Re: Heat from Charging VS ThermoCouples

Quote:

Is the main problem because of out gassing, case melting or something else?

The main problem is with out gassing to my knowledge. As the temperature rises the gasification voltage drops so the safe voltage to charge at also drops. This can lead to thermal runaway which can result in the charger not switching off correctly at end of charge. It might go something like this:

1. You stick the batteries on the charger when they are hot.

2. The batteries charge ok most of the charge cycle, but the high current heats them up some. Heat is not normally a problem, but this heat is causing them to gas before they can come high enough in voltage for the charger to switch to CV (constant voltage) and allow the current to gradually taper back. Instead it keeps pumping maximum power at CC (constant current mode). Because the gasification voltage has dropped the batteries now resist a rise in voltage, instead they convert the energy into gas. Some of the hydrogen and oxygen gets converted back into water before being vented from the battery. This conversion causes additional heating.

3. The process continues to heat the batteries up and soon they can't accept any more charge and gas even more. VRLA batteries are not designed very well to vent this much gas the way flooded batteries might be, so they might balloon, and in some cases crack.

4. The charger may not have a default shutoff time parameter so it continues to remain in CC for hours and hours.

5. "hmm my charger is still in the red and I've had it on for 8 hrs OMG!"

6. "hmm my scooter cutout after 2 miles WTH!"

Yea I think an oil bath might help keep the temperature down.

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