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Battery Charging.

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Penguin45
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Posts: 4,559
Attribute: Compulsive rust poker.

Battery Charging. Feb 10, 2022 18:32:09 GMT

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Post by Penguin45 on Feb 10, 2022 18:32:09 GMT

What a dull subject. This was quite interesting though. Honest.

I seem to spend a fair amount of time sorting the local kids scooters, bikes, motorbikes, quad bikes and the like. Current project is re-wiring a little quad bike. Well, wiring it up from scratch as there isn't anything there. The yoof provided me a "fully charged" gel battery. It actually had 5.8vDC in it when I checked. The intelligent battery charger said "No" when presented with the problem and, much to surprise, the vicious little Silverline "dumb" charger wouldn't even begin to charge it. I ended up wiring it in parallel with a known good 12vDC lead acid battery from the alarm system and it did start sucking up some charge.

Battery charger by Penguin 45, on Flickr

After 3 hours, the gel battery read 12.6v. Unfortunately, it immediately dropped to 11 when taken off charge, and by this morning it had fallen back to 5.8vDC again. So, that's knackered then.

What was interesting is that with a little bit of subterfuge, it could be persuaded to charge at all.

Chris.

What would you like welding up?

dave1800
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Posts: 3,586
Attribute: Technical Guru.

Battery Charging. Feb 11, 2022 2:07:11 GMT

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Post by dave1800 on Feb 11, 2022 2:07:11 GMT

The worst culprits are the lithium ion / polymer batteries. If they fall just a few tens of millivolts below the cut-off voltage after being stored for example, the chargers refuse to start. They can be "encouraged" by charging them from a higher voltage source via a current limiting resistor keeping the current to just a few milli-amps until the voltage rises sufficiently for the charger to work. I cannot recommend this however as the battery chemistry may have degraded when it is over discharged and this leads to, at best a reduction in capacity, or at worst FIRE!

I have found these batteries are best stored out of the device if left for any length of time as many modern items don't have a hard on-off switch and can continue to draw a tiny stand-by current.

It does seem strange that your dumb charger wouldn't work. A word of warning though if you connect two lead acid batteries (SLAs in this case) in parallel then as the flat one charges its internal resistance may suddenly fall and cause a high current to pass and damage both. It can be quite spectacular with car batteries and is not unknown with vehicles that use two batteries in parallel.

David

Last Edit: Feb 11, 2022 2:12:19 GMT by dave1800