Can you fix a lead-acid battery that's charged backwards?

Generally no, since you can't access the individual cells inside the battery separately you'll normally end up with some cells charging ok and others reversed, it's easier to just buy new batteries.

The speed controller is probably pretty simple, crack it open and have a look.

ROTFPML!!!!! Is this post serious?

I actually have some experience with backwards batteries. I grew up in my fathers auto repair shop in the sixties. Back then if you hooked up the jumper cables to a generator equipped car you could start the car and the generator would reverse polarity and you could go on your merry way with a positive ground American car. We would just leave the lights on over night and then charge the battery the correct way and polarize the generator (jump the positive to the generator output lead for a second) in the morning. If the battery was good to start with it would recover fine.

Jim.........

I don't see how, as the cutout wouldn't pull in. That only happened when the generator started producing more than battery volts which it couldn't when reverse connected.

One way to instantly polarize the dynamo was to flick the cutout in manually.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

I think you should have a good shot at this. Try just discharging them first, use a light bulb that lights normally across the battery to discharge them, and let it go a couple days past the time when the bulb is no longer glowing.

Then connect the charger properly and try to charge them, slowly. If they take too much current, insert the light bulb again as a current limiter, but you should be able to take the bulb out of the circuit after a day or 2.

Let us know how this progresses.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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I guess I'll have to think about the WHY of this, but I've seen it happen. I agree that it's odd. The time I saw it, the owner had installed the battery backwards, then realized his mistake and put it in correctly. When I got to it the battery was correct, but I eventually determined that the generator was polarized backwards.

Now I have to admit that it never occurred to me to wonder why putting the battery in backwards would reverse polarize the generator while putting the generator in forwards wouldn't correct it.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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I think the key to an explanation is that the cut out relay senses current direction as determined by referencing the magnetic field of the current to the magnetic field of a voltage fed electromagnet.

Since both were reversed the cut out relay didn't have a clue as to the polarity.

I hope I explained my thinking clear enough.

Bill K7NOM

In the UK, when I were a lad, most cars were positive ground. This of course made no sense to the average punter, and it was very common for the clueless to fit a new battery the wrong way round. And the symptoms were the battery didn't charge. As electronics became more popular, the change was made back to negative ground for new vehicles. And if the owner of an older vehicle wanted to fit a modern stereo, the easy answer was to convert it to negative earth, and I've done dozens. And always had to re-polarise the dynamo before it would cut in and charge. If the cut in had operated with the battery the 'wrong' way round, that would have re-polarised the dynamo and made this unnecessary.

Of course, this was with the infamous Lucas electrics. Could be that other makes work differently.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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...the Prince of Darkness....

jak

I had this happen (by accident) with a NiCad pack. I just flattened it and re-charged it the correct way no preoblems.

JD

In technical terms, you probably have a "sulfated" battery. In car sizes, 5 amps for 24 hours should get it charging in the right direction. Smaller battery, less current. It will take a while. You will be forming the battery, giving it its initial charge.

Go for it.

Jim

mike writes:

: I bought a pair of electric scooters. They'd "fixed" the charger : and managed to get the batteries charged backwards. Can I discharge : them and charge 'em up forwards? Any special techniques? Fast/slow/ : charging? : May be a moot point as I expect the speed controller box is fried too. : mike

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Thanks for the advice. I fixed the charger and re-reversed the batteries. One scooter seems to be working fine. The batteries seem to be ok on the other one, but somebody ran it with insufficient air in the rear tire and hurt the inner tube. Won't hold air long enough to give it a good test. mike

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If the battery is sulphated, most modern chargers won't deliver enough volts to get anywhere near 5 amps due to the high cell resistance. Many are voltage limited to perhaps 14 volts or so, and might need several days to get any result - if ever.

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    Dave Plowman        dave@davenoise.co.uk           London SW
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That's probably for the best anyway. A sulfated battery should never be charged at a high rate. That will just hydrolize water and can damage the plates. IF recovery is possible, and it sometimes is, then your best bet is with a very small charging current over a number of days. I find that a week is typical for a car battery, but I had one that took 2 weeks.

If you insist on instant gratification, you should just buy a new battery.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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Yup. I was just worried someone might try using a variable bench supply to push through the 'required' 5 amps if their charger didn't do this.

Think you'll end up doing this anyway. ;-)

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    Dave Plowman        dave@davenoise.co.uk           London SW
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It's generally pretty easy to tell whether you have a good chance or not. If you just measure the voltage while trickle charging it, the voltage should rather quickly come up to 2 V or more per cell. If your battery seems to be low by some multiple of 2V, then you probably have a shorted cell, which generally can't be fixed. If you can get to the

2V per cell threshold, on a trickle charge, then you have a pretty good chance of recovery.

Make sure the water is covering the plates.

If there is a broken connection in the battery the charging voltage will be high, but it will drop quickly as soon as you stop charging and put a very small load on it. You won't be able to fix a broken connection, either.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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