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Old 28th Jan 2010, 10:32 pm   #1
Crowner John
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Default overheating rechargeable battery

I have just renewed the batteries in my handheld marine transceiver, about 20 years old, original batteries failed to take charge after long disuse. The batteries are eight AA size, bought seperately have metal tags to solder in series for 12v. Joined them together, put on charge, set works fine, but a few days later opend the battery compartment and felt the fourth battery in the row was almost too hot to handle! Not sure if the set was even switched on at the time. Have taken them out, in case the set is damaged by heat or rupture of the battery.
Any ideas about the cause - was it just one dud new battery?
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Old 30th Jan 2010, 6:24 pm   #2
G8HQP Dave
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Default Re: overheating rechargeable battery

Were these Ni-Cads? They can sometimes develop an internal short through growing fine metal whiskers. The short will discharge the cell quickly. If it was fully charged at the time this could generate a fair amount of heat. If you are lucky the discharge current will blow the whisker and the cell will be OK again.

Check the voltage of each cell. Even a 'discharged' cell should show a bit of voltage (maybe 0.5-1V). A shorted cell will show 0V.
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Old 31st Jan 2010, 2:37 am   #3
BGmidsUK
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Default Re: overheating rechargeable battery

When connecting cells in series it's important that they not only have the same capacity (new cells from the same batch should be close enough) but MUST also be at the same charge state. If one is fully charged and the others are not, it will overcharge and overheat. Same goes for one which holds less charge, it will be forced into negative charge whilst the others are still supplying power.

The best thing to do is to separate them and perform a complete charge/discharge/charge cycle, then test them for capacity. You will be better doing this with an electronic charger which senses when the cells are fully charged (Delta-V).


Hope this helps


Brian
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Old 31st Jan 2010, 6:23 pm   #4
McMurdo
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Default Re: overheating rechargeable battery

This reminds me of a fault I had a few weeks ago on a handheld rechargeable instrument (a surface hadness tester, but it doesnt matter).
The NiCd batteries were shot and it wouldnt hold a charge. One had leaked out. I replaced them and put it on charge, after a couple of hours there was a click or snap from the unit and when I went over the batteries were red hot and one had bulged out.
It turned out that chemical leakage from the original batteries had leached up onto the circuit board and rotted through the track that provides voltage feedback to the charging circuit, and caused it to overcharge and split.

NiCd and NiMh battery charging circuits for portable stuff are often chip-controlled and use either DV/DT or DI/DT sensing to determine when the cells are charged, and when to reduce it to a trickel charge...but of course they need feedback.

Just a thought!
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Old 1st Feb 2010, 9:37 pm   #5
Crowner John
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Default Re: overheating rechargeable battery

Thanks to three respondents to my query about a hot NiCad! I have unsoldered all the connecting tabs and checked that each one seems to be delivering its proper voltage. Now joined them all up again and have it on charge once more -so far so good.
I'll let y'all know if the house burns down,.
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Old 2nd Feb 2010, 2:46 am   #6
MichaelR
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Default Re: overheating rechargeable battery

Just a tip , you have to check batteries under load to have any indication of their charge state.
Also remember that 0.1V difference can be a fairly significant difference in charge state.


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