Reversing Battery Polarity

I'd dissconnect the batteries and measure them individually, ( ideally on some sort of load, say a headlight bulb) before trying to reach any conclusion.

Double check everything including the meter. Don't swap anything around. unless you are absolutely certain ^2.

No , I told him he should go get a new battery to replace the bad one, it was only putting out 5.7 volts ,actually -5.7volts? I tested them individually, one had 11.8 V. and the other was -5.7V. I didn't get a chance to put a load on the weird one, he has already taken it to town to get a new one, but a light would have come on no matter the polarity. Maybe a small 12V. motor, and see if it runs in opposite directions when attached to the different battery's. I have just never heard or seen a battery change it's polarity. I've seen a Pontiac distributer mechanical advance work backwards though.

Yes a battery will change polarity if completely dischargered and charged improperly

In your case you had two batteries in series. If one goes dead the dead cell or battery will see an opposite or reversed charge on its poles and its polarity will get reversed.

It is true that a 12-volt lead-acid battery has different plates. One is lead and the other lead oxide. Even though they normally work in a single polarity fashion, it is possible to get a reversed charge on them. It is also possible to overheat the battery in this configuration and actually have it explode.

The smart thing to do is replace both batteries if the remaining good one is about the same age as the bad one. It probably does not have that much life left in it.

On those big diesel truck batteries 11.8 volts the battery is for all intents and purposes dead. the -5.7 is junk.

Yes, a battery can reverse charge, but I think that a new battery is in order now.

I agree with you. If for some reason the batt went totally dead and it was charged in reverse polarity -- it would take this new charge and guess what.

REVERSE CHARGING is a common occurence. (not recommended the way your friend did it). IT happens all the time in dry cells. WHen you leave for example a torch (flashlight) on forever it actually reverse charges at least one cell after the light goes out. Check it out. But after turning off the device the cells actually usually 'regenerate ' to a positive voltage in dry cells at least (not to full voltage, after all you have really run the thing past zero volts and beyond).

Yes it happens.

I asked the same question several months ago, and got the same information.

yes ,it does , if battery is completly drained & recharged with reverse polarity it will charge in that order , i have seen such examples perticularly with ni-cd batteries in cordless phones , they perform well but need replacement , even polarity can be corrected , simply drain down battery completely , check battery temperature periodically, apply apulse of high voltage approx 18vdc with high current (in your case) for split second than again discharge , repeat second time with resistor added 10 ohms (wire wound ) in series couple of seconds , now recharge with proper charger for full 10 hr its up again , i have no clear idea for different battery configuration like ni-mh , li-io , lead acid etc

TO VIKAS: What battery type are you talking about here . I would have presumed lead acid flooded type? I've heard others say that double voltage actually causes sulfation buildup on the plates inside the battery to de-sulfate . but they said to use lower amps for much longer times,... I tried it on one lead acid trojan t-105 type.. no effect.. maybe I had a beyond repair battery.

You have been given a lot of correct answers already, what you have not been told is :- "go and buy 2 new batteries".......that is good advice and will save you from possible problems that could happen if you only replace one battery!!

Make sure that the new batteries are completely identical, even to the manufacturer and stock number. Pick ones with very close manufacturing dates, same day best. make sure that you measure the boxes they must go in accurately for height and the position of + & -, sometimes it is very specific as to what side they will go on......

Best of luck.

Lead-acid batteries are manufactured with different types of plates for the positive and the negative electrodes. Although you can charge it backwards and reverse the polarity, the chemistry and physical structure of the plates has been degraded and I expect you will have very poor lifetime or capacity for the battery that had been reversed.

--JMM

Most automotive parts stores will have a handheld device that tests a battery under load. Even if one battery is close to putting out the correct voltage, it may not be able to put out the amps needed for heavy-duty operation. A quick test will tell, but I'm sure, as Andy Germany and others have said, you need to replace both batteries to be safe for the long run.

Having mismatched batteries will shorten the life of the stronger battery. That's why you replace all the batteries at the same time in a battery-powered device.

Well said!

I hope he takes our (good!) advice!

Just a little further background on Batteries (and certainly not complete as Batteries is a science of its own!!) for those interested.....

The "weaker" battery (cell) or Batteries (depending upon type and of course number) can get "Reversed Polarised" by the stronger one when the battery(s) are discharged too far, this accelerates the damage and can also lead to overheating with some battery types...

In Lead Acid Batteries, each cell is a battery (bad description really as the name Battery implies more than one cell, my apologies for using it in this manner, but several Dictionaries I checked seem to allow such usage!!) in its own right which many forget and once one cell is "down" you are already in a bad situation......so to say......which is why there is a minimum recommended cell voltage that should not be gone under for any battery/cell types.

Some makers of small rechargeable batteries (more than one cell in a package), actually place a special diode across each cell, to allow the discharge current to bypass that cell without damaging it when it is discharged below a certain value!! But most do not, because they would be reducing their possible sales (Batteries would last too long!) as well as increasing production costs!!

Good NIMH or NICAD chargers allow single cells (AAA or AA or C & D Cells) to be discharged and recharged on an individual basis, bad ones need two cells or more at a time to work!!

Sadly it is difficult in modern lead acid both to measure the cell voltages of each cell as often the inter cell connections are completely hidden in plastic.....I have actually bored holes for meter leads, but that could go completely wrong!! It is a very corrosive acid and the dangers for your health are there....not to be recommended without taking full safety precautions first.......

On my Leisure Battery in my Caravan, the charger is set by me to stop at 13.6 volts (does not start gassing, but Battery is probably only about 80% full) and to restart at 12.6 volts. The Battery "cycles" between these two values. "Cycling" keeps cells fit (as it does humans too!!)

12.6 volts is not "empty" but it is the lowest voltage allowed on normal Lead Acid Batteries before sulphuration (spelling is a problem, CR4 does not know this or similar words, sorry!) effects start.....Leisure Batteries are far less sensitive, but expensive and I don't take chances. Mine is now over 5 years old and has still not needed any distilled water!!! This voltage is not agreed to by all concerned, but each to his own.....it is certainly a safe value!! and my Battery shows no signs of ageing up to now!

There is a massive amount of good helpful information on the web about Lead Acid Batteries if needed.

My son reverse charged his marine 12v lead acid battery. I think he hooked the charger up backwards...

Anyway, he completely drained the battery, thinking he could then properly charge it. Well, when we put the trickle charger on it two things happen. First, nothing. THen the ampmeter on the charger goes off-scale for a second, with a loud click, then back to nada.

Is he completely screwed? Can this battery be salvaged?

Thanks!

David Cowan

I think he needs both a new pair of glasses and a new battery......and he needs some grey cells too.....

Lucky for him, he does not (yet) need a new pair of eyes, but if he continues in this manner.......who knows....he might achieve the jackpot soon.....!

Your Son should go to the Darwin awards pages, read them and then take MORE care !!!

Try:- http://darwinawards.com/

Have a great day......sorry to be so negative......

Andy,

Thanks for the smart-assed reply. That was very helpful. Now, if you've finished chuckling, do you have the answer to the question? Is the battery toast?

Thanks,

David

I forgot the answer, sorry about that, yes I sincerely believe it is Toast.

I have heard of such damaged batteries recovering (seemingly), to go tits up a relatively short time later.......at a very inconvenient point in time for the owner usually.....nobody can say exactly what will happen, go for safety first.

If it runs in something like a boat for starting the engine, he should get rid of it and buy as large an "leisure" battery that the boat can take. You may ask why so big and why Leisure.

Its because that "leisure" batteries of a similar size to a car (engine starting) battery, cannot produce the "cranking amps" needed to turn over the motor so well, so you need to get some good advice as to just how big it should be, but a good starting point would be about twice the size.....(Bigger is better in such cases!)

Leisure batteries can take deep discharge much better and have a far far longer working life....with minimal maintenance and usage.

An SLA battery is also a good choice for a boat and has better cranking amps generally.....

By the way, I thought my original reply was a good mix between truth (nobody likes it do they?) and good manners, with a certain dash of humour!!! But there is no accounting for taste!

Does your son fully realize what could have happened???? I have the impression neither of you appreciate his luck fully.....amongst other things!

Have a really great day (in spite of my sense of humour!!)

Andy,

Thanks (really, this time) for your input. My son does not recall charging the battery and possibly doing it backwards (with 3 boats between us we are often recharging batteries and lose track) but thinks the reverse charge may have occurred when he was installing a new ignition switch and some other stuff and may have wired something wrong. The boat is now in the shop to get the wiring sorted out. In any case, the battery was reverse charged and as a result we will all be more aware of the potential for this in the future. Live and learn.

Yes, we are both very appreciative that we were not blown up or spattered with acid. This is the first time anything like this has occurred to us or to anyone we know, so it was all new to us. Note that once the problem was identified we did not force the issue (i.e., leave it on the charger to see what would happen). I have gotten similar advice from the fishing website I frequent. The battery will be replaced.

It is a marine battery and is a compromise between cold cranking amps and deep cycle, and we always buy the largest one that will fit into the battery box. We do not run a trolling motor or other high-amp drains.

Thanks again.

David

PS. I was US Army in Landstuhl 1980-1983.

Many thanks for your answer, you are doing the right thing and you clearly know it too, which is just perfect!

Did you learn German in your time here by any chance?

Andy,

I responded earlier but don't see my post anywhere. Must have reverse-posted it...

I learned enough German to get directions, buy train tickets, get dinner and a room, trash talk with German soldiers, etc. Not enough to discuss international financial policy.

Many fond memories.

David

Well done, that is a lot more than most visiting military do!!

Stay in touch and have a great day.