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Author Topic: MimH Baterries in series.  (Read 1598 times)

Tug-Kenny RIP

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MimH Baterries in series.
« on: February 25, 2013, 09:08:54 pm »


Just a quick one for all you bright modellers out there.

I have two 7.2 volt batteries wired in series to produce 14.4 volts. One is 1600 milliamp hours and the other is 3000 milliamp hours.

I left them feeding a speed controller overnight by forgetting to switch it off. The next day, the speed controller light had gone orf, showing that the supply had dried up.

Upon testing it was found that the 1600 was dead flat and cannot be recharged.   <:( .  The 3000 was able to be recharged.

My question is what happened internally to these batteries during the discharging process ?. Did the one battery have any bearing on the (alarmingly) flat battery by treating it as a dead short and continuing to feed a reduced voltage to the speed controller ?  or did an open circuit occur and stop the process of current drain. ?

I shall, of course, always fit matching batteries in future, but it was an expensive mistake to make, me-thinks.  <:(


As a PS, can the flat battery be re started with a direct feed of (say) a 6 volt source from a lead acid type, as it will not charge using the  sophisticated charging unit I have.

Over to you.

Ken
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grendel

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Re: MimH Baterries in series.
« Reply #1 on: February 25, 2013, 09:18:14 pm »

effectively after the smaller battery was discharged the bigger was trying to reverse charge it. now some of these batteries have a cutout that operates if the battery voltage goes too low, this can sometimes be reversed by applying a higher voltage across the battery terminals (only for a few seconds at most), I have managed to revive a couple of phone batteries like this, but its risky as if any moisture has got in they can explode. if it hasnt got a inbuilt cutout its probably toast.
Grendel
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malcolmfrary

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Re: MimH Baterries in series.
« Reply #2 on: February 26, 2013, 10:37:18 am »

effectively after the smaller battery was discharged the bigger was trying to reverse charge it. now some of these batteries have a cutout that operates if the battery voltage goes too low, this can sometimes be reversed by applying a higher voltage across the battery terminals (only for a few seconds at most), I have managed to revive a couple of phone batteries like this, but its risky as if any moisture has got in they can explode. if it hasnt got a inbuilt cutout its probably toast.
Grendel
What he said, but read the "few seconds" as "not more than a couple of seconds".  Dab the power on and off, take a reading.  If no voltage, give it another go.  If you get a reading, it could be able to take a charge, but I wouldn't really trust it.  If there is no sign of life after a half dozen dabs, its ballast.  The connector might be useful sometime in the future.
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Tug-Kenny RIP

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Re: MimH Baterries in series.
« Reply #3 on: February 26, 2013, 10:51:04 am »


Thank you both for the info. I'm glad I asked. O0

I had a feeling there was more to this  'new technology' when the green light on my charger didn't come on.  I shall invest in a new one as I won't get far on low amp hours. It was a test run to drive a 12 volt motor using lighter batteries on my Fairmile D, so it might have stopped in the middle of the lake  !!

Cheers

ken

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