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Two way charging batteries?! New to me,.. Please help

Hi,… This weeks results to last weeks puzzler seems AMAZING to me. You can REALLY charge a battery up backwords??? What kind of batteries will do this? It is my understanding that most Marine batteries (like most auto batteries) are of the general Lead-Acid variety. Anyone able to give me more info on this amazing battery with a two way charging ability,… and the electro-chemical science in it’s unique ability? I am really interested!!

If you have a completely dead Lead Acid Battery, and put a charger on it with the leads reversed you will reverse the polarity of the battery. I believe you can do the same with NiCads and NIMh batteries or any other batteries as well.

Just because you can do something doesn’t mean you should, its dangerous and will probably will shorten the life of the battery. This is very dangerous and could cause the battery to blow up when someone goes to connect it, it can also damage your car, it can wreak havoc on your alternator and whatnot if you accidentally hook up the battery with the polarities reversed.


There are some very intelligent people on here that can probably explain the chemistry behind it, I have never read a proper account of what actually happens to the chemistry of the battery.

I have seen this happen once, I had an elderly neighbor that had a riding mower. He left the key on and ran the battery dead. He hooked up the charger backwards and when he went to start the mower the starter operated but would not engage the ring gear because it was turning reverse of normal operation. It took me a bit to figure it out and I have never seen it in person up until that point.
We went and got a new battery and all was well, and out of curiosity I took the old battery and discharged it with a lamp and recharged it properly and all was well. I used the battery for a bit but it kept discharging and I don’t know if the battery was damaged by the reverse charge or if it was already weak. ( I gave him the money for the core, and kept the battery to see how long it would last).

I have been thinking about this and you seem really excited by the concept of reversing a batteries polarity. Again, please do not do this, batteries are not designed with this in mind and it wouldn’t take much for a battery to blow up on you. So while we can all learn and explore the theory here on these forums, please do not experiment with this at home. There is no reason to ever purposely reverse the polarity on a battery.

Promise us now… Please…

Popular Science magazine used to have a monthly series about the Model Garage with its proprietor, Gus Wilson. In one episode of the early 1960s, a person did have that problem with a 1957 Plymouth. Somehow, due to corroded connections, the generator (we didn’t have alternators in those days) reversed charged the battery. I don’t remember the entire story.
I had a battery connected backward in a 1948 Dodge that I bought. The ammeter read backward. I assume that the battery was reverse charged. When I got the car home I put a load on the battery and completely discharged it, then recharged it over a period of a couple of days with a trickle charger. I repolarized the generator and all was fine. In my case, the woman’s son had put in a new battery. This son was in his 50s, but had probably forgotten that some cars had a positive ground and installed the battery backward.

“There is no reason to purposely reverse the polarity on a battery.” Amen to that.

Years ago I worked with a guy that said they’d recharged a truck battery backwards at his other job. He said they hooked a pair of headlights to it to discharge it and recharged it properly the next day after it drained overnight. I thought he was BS-ing me at the time, but reading this, I guess it’s true.

Lead-acid is probably the only cell chemistry this will work for, and not very efficiently as others have said. I remember working on some pro video equipment batteries which had individual lead-acid cells in series. They warned us not to over-discharge the batteries because a weak cell could become depleted and would be reversed charged by the other cells. When rebuilding battery packs, we had to be sure to use cells from the same lot number, and never replace just one bad cell.


The Toyota Prius Synergy drive is very careful not to let any of the cells get to a discharged state, they say if a cell goes dead its polarity can be reversed causing battery pack failure.

You Asked For It (I’m old enough to remember that TV show):

Here’s the overall reaction in a lead-acid battery:

PbO2(positive plate) + Pb(negative plate) + 2H2SO4 = 2PbSO4(both plates) + 2H2O

[Discharging goes to the right; charging goes to the left.]

A totally discharged battery is chemically the same on both plates and can be charged either way. I believe there are significant physical differences in the plates such that “reverse charging” produces a much inferior battery.

When a lead-acid battery is in a charged state one electrode is metallic lead and the other is (coated with) lead peroxide).
When a lead-acid battery is completely discharged both electrodes are coated with lead sulfate.
They are then chemically identical.
From this state it is possible to charge in reverse; coating the plate that was designed for peroxide with more metallic lead and the other plate with peroxide.
The physical layout of the electrodes are different on each side so this backwards arrangement doesn’t work so well, but it does work.

Did you know if you put a backwards battery in an old diesel truck the engine would start up and run backwards? In real life this usually happens when attempting to climb a really steep hill with a very heavy load and stalling the engine. If you can’t get the clutch in fast enough the load will roll back in gear and the engine will run backwards with the exhaust coming out the air cleaner.