# Battery reverse polarity?



## Downunder Bob (Apr 28, 2017)

Hi guys, I recently set up a LED lamp above my lathe, it required 12v. I had a spare 12v gel cell battery looking for a purpose, so I hooked it up. Nice bright light with it's own on/off switch. I added a small plug pack charger, 1 amp, and capable of floating when fully charged. I also intend to run  a few more low power items off this setup.

After a couple of hours running I switched it off and retired for the night. The next day I went to switch it on and the light didn't work, put multimeter on battery = 12.5v ?  assuming the new light fitting was faulty I took it down and pulled it apart, Another similar battery was on the workbench so I touched the light wires to it and presto it worked. Went back to the original battery, wouldn't work? checked battery again 12.5v? take a second look at meter  -12.5v  switched the meter leads around and got+12.5v. hooked up the led light wrong way around, yes it worked. ????

The two batteries are identical 12v gel cells about 2 years old, now one of them suddenly decides to switch polarity, overnight with every thing turned off. 

Any clues gentlemen, and ladies? Can I get the polarity to switch back, or can it be used like this.????


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## JimDawson (Apr 28, 2017)

OK, that is a new one.  Is it possible that you had the charger connected backwards? I really don't understand how a battery can switch polarity.  Hopefully someone here can explain how this is possible.

This is just a guess, but I would think that the battery is done.


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## CluelessNewB (Apr 28, 2017)

A gel cell is a type of lead acid battery.  Both + and - plates are lead.  If you discharge it enough and then hook it up to a charger backwards you could conceivably reverse the polarity.   

https://en.wikipedia.org/wiki/Lead–acid_battery

https://www.batterystuff.com/blog/battery-myth-can-a-battery-reverse-polarity.html


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## Silverbullet (Apr 28, 2017)

Wouldn't that help clean the plates ?


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## markba633csi (Apr 28, 2017)

My guess is the first battery had only a light "surface charge" and you had the trickle charger hooked backwards.  Make sure of your meter readings' polarity too.
Mark S.


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## RandyWilson (Apr 28, 2017)

Yes, you  can put a reverse surface charge on a lead/acid battery with the charger reversed. Just drain it down and charge it the correct way around.


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## 4GSR (Apr 28, 2017)

My luck would be, reverse the charging leads = BOOM!!!  on the battery.


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## jim18655 (Apr 28, 2017)

Take the battery outside and connect the charger. Plug the charger into a long extension cord and plug that in far away. Maybe a shield between you and the battery or a bucket over the battery to contain the flying parts and acid should something bad happen.


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## darkzero (Apr 28, 2017)

Wow, learned something new today. I used to work in automotive too, never did I see a car battery reverse polarity. I would have never thought that was even possible!


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## 4GSR (Apr 28, 2017)

darkzero said:


> Wow, learned something new today. I used to work in automotive too, never did I see a car battery reverse polarity. I would have never thought that was even possible!


I blew up a car battery by reversing the jumper cables when I was young.  I was very lucky, I turned my head away from the car when it blew.  40 years later, my ears still hurt thinking about it and all of the acid that got on me, too. I assure you, that was the first time and the last time I did that too!


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## RandyWilson (Apr 28, 2017)

I grew up working on  positive ground British iron. Yes, you can reverse charge a battery. Not well, but it will show -12 on a meter. Drain the current charge (light or some such load), then charge it normally.


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## FOMOGO (Apr 28, 2017)

That's how Frankenstein's  monster got such a bad attitude, they hooked him up backwards. Always have to watch out for that human error syndrome. Some early US stuff was + ground also. Mike


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## tq60 (Apr 29, 2017)

A deep discharge can cause reverse polarity in a cell so it is possible for a small capacity battery it be possible.

Likely best to leave as is and remark posts or if feeling lucky clip a light bulb to it and discharge it then change it.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk


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## Ulma Doctor (Apr 29, 2017)

i learned something today, batteries can be reversed charged - i never even considered the thought.

when i was in my late 20's , i got asked by a friend to do some work on an antique john deere , 2 cylinder- gas and/or kerosene fueled tractor, to use on his ranch.
it had cloth wrapped cables that were falling apart.
i replaced the old cables and all the simple wiring of the tractor with modern wire, cable and lugs.
one thing i do remember about working on the tractor, was that it had 6V + ground electrical system. that has been the only + ground i have seen
the Ol' Deere was really cool to work on and even cooler to drive!


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## Downunder Bob (Apr 29, 2017)

JimDawson said:


> OK, that is a new one.  Is it possible that you had the charger connected backwards? I really don't understand how a battery can switch polarity.  Hopefully someone here can explain how this is possible.
> 
> This is just a guess, but I would think that the battery is done.



 G'day Jim, yes I guess it is possible that I connect the charger back to front, yet it continued to work for a couple of hours after that, but anything is possible. I'm going to experiment  a bit. Even 24 hrs after the battery is still showing 12.5v but neg polarity on the + red terminal.

Oddly enough in the back of my mind where the memory cells are a bit lost. I seem to recall hearing about this once before, quite some time ago, but I don't recall the circumstances or the results.

If connecting the charger wrong way around caused this to happen, I wonder if I connect it right way round will it fix the battery.


bobshobby said:


> Hi guys, I recently set up a LED lamp above my lathe, it required 12v. I had a spare 12v gel cell battery looking for a purpose, so I hooked it up. Nice bright light with it's own on/off switch. I added a small plug pack charger, 1 amp, and capable of floating when fully charged. I also intend to run  a few more low power items off this setup.
> 
> After a couple of hours running I switched it off and retired for the night. The next day I went to switch it on and the light didn't work, put multimeter on battery = 12.5v ?  assuming the new light fitting was faulty I took it down and pulled it apart, Another similar battery was on the workbench so I touched the light wires to it and presto it worked. Went back to the original battery, wouldn't work? checked battery again 12.5v? take a second look at meter  -12.5v  switched the meter leads around and got+12.5v. hooked up the led light wrong way around, yes it worked. ????
> 
> ...



Ok so I had to go out into the garage and take some photos. this is the new led light above the lathe on the left, the right pic is  a veiw of the lateh squeezed into its corner,with some shelves i made above it filling up with tools. And the last pic is of the battery with multimeter connected show
	

		
			
		

		
	



	

		
			
		

		
	
ing 12.1
	

		
			
		

		
	














	

		
			
		

		
	
 v with neg sign 
	

		
			
		

		
	




	

		
			
		

		
	
The battery has been stable at 12.1 v for 3 days now. so maybe 
	

		
			
		

		
	





	

		
			
		

		
	
 I should hook it up to something and drain it down the recharge it teh right way around and see what happens.


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## RandyWilson (Apr 29, 2017)

Since it worked for a while, I suspect you started with a hot battery, and the puny 1A charger has a current limiting circuit in it. This would be why it didn't catch fire the first time. I wouldn't temp the luck gods any more. Don't use the charger to drain it down, use a standard inductive load that doesn't care about polarity.


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## cathead (Apr 29, 2017)

I would discharge the battery and then charge it with some kind of limiting resistor in series with the charge leads.  A car
tail light works well for this( #1157 comes to mind).    That way it can reverse it's polarity without much danger.  I often use
this method on smaller batteries that would be cooked with a regular car charger.


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## Downunder Bob (Apr 29, 2017)

RandyWilson said:


> Since it worked for a while, I suspect you started with a hot battery, and the puny 1A charger has a current limiting circuit in it. This would be why it didn't catch fire the first time. I wouldn't temp the luck gods any more. Don't use the charger to drain it down, use a standard inductive load that doesn't care about polarity.



Thanks Randy, not sure what you mean by "hot"battery the battery was at ambient temperature about 18c  on the day, and it was not flat as I checked the voltage before hooking it up, it was 12.4v and polarity was correct otherwise the led light would not work it was connected to the light fitting for a while before I connected it to the 1A charger. 

I'm not sure what you mean by a "standard inductive load", but I think I'll settle for a resistive load aka a light bulb. drain the battery down and then try to recharge it with correct polarity. 

I also know from past experience that when you connect a battery charger reverse polarity you get big sparks, did not notice any big sparks, but yes, I guess it is possible that is what I did.


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## pdentrem (Apr 29, 2017)

Just put a light across to discharge it and then when flat charge normally. This happens more often than one would think. The only thing that one can run into is an automatic charger may not start if the battery is completely flat. Either use a manual charger or hook a charged battery in parallel to give the sensor a voltage to start, once there is a small charge you can remove the charged battery or leave them both hooked up until both are fully charged.
Pierre


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## Downunder Bob (Apr 29, 2017)

pdentrem said:


> Just put a light across to discharge it and then when flat charge normally. This happens more often than one would think. The only thing that one can run into is an automatic charger may not start if the battery is completely flat. Either use a manual charger or hook a charged battery in parallel to give the sensor a voltage to start, once there is a small charge you can remove the charged battery or leave them both hooked up until both are fully charged.
> Pierre


Thanks Pierre, that is more or less what I intend to do. surprised that it is somewhat common.


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## Downunder Bob (Apr 29, 2017)

cathead said:


> I would discharge the battery and then charge it with some kind of limiting resistor in series with the charge leads.  A car
> tail light works well for this( #1157 comes to mind).    That way it can reverse it's polarity without much danger.  I often use
> this method on smaller batteries that would be cooked with a regular car charger.



Thanks I have two small chargers the 1A one mentioned and another that I can set at two Amps, I think either of them will suffice I'll probably use the 1A one, but yes it won't charge a completely flat battery so I'll interrupt the discharge before it's completely flat.


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## rgray (Apr 29, 2017)

tq60 said:


> A deep discharge can cause reverse polarity in a cell so it is possible for a small capacity battery it be possible.
> 
> Likely best to leave as is and remark posts



Remarking the posts gets my vote... Lead acid batteries do not like to be fully discharged.  They will only survive it a very limited number of times. Even if they do survive it they are usually damaged (lessened capacity)


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## cathead (Apr 29, 2017)

bobshobby said:


> Thanks I have two small chargers the 1A one mentioned and another that I can set at two Amps, I think either of them will suffice I'll probably use the 1A one, but yes it won't charge a completely flat battery so I'll interrupt the discharge before it's completely flat.



I think you need to drain the battery completely.  At that point you can charge it as it was intended.  The positive and negative
battery plates are not the same so best to get the pos to be pos and the neg marked termianl to be neg.  Also, it's not a bad
idea to know how many ampere-hours the battery is supposed to hold so you can charge it accordingly.  For example: 
a 10 ampere-hour battery could be charged at 1 ampere for 10 hours plus maybe 20% or so.  This is self limiting to some
extent since as the battery voltage climbs, the difference between the battery voltage and the charge voltage decreases. 
This probably won't happen but approaches it since if both battery voltage and charge voltage were the same, no current would flow.


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## Scruffy (Dec 19, 2017)

I know this is a old post 30 years ago I left the key on of a jd riding mower. Put the charger on backwards. Got it all charged up and the starter would just spin. Scratch head.  The starter was spinning backwards and the bendex wouldnot engage.
Left key and lights on for several days recharged battery, then threw it in the junk and replaced battery. All well.
Thanks ron


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## Tim9 (Dec 22, 2017)

Best bet is to TOTALLY DISCHARGE BATTERY and then properly charge it ....Positive to positive....Neg to neg. 

  I've had this happen a couple of times back when I had a service station. Sometimes new employees would screw up and hook the charger backwards. When the battery is totally dead, you don't even get a big spark when hooking up cables backwards. If the battery is partially charged, it's hard to make that mistake because it will really arch if you hook up backwards. In most cases, after proper charging all is good providing the battery isn't too old. But...discharge it first, then recharge.


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