is it possible to Accidentally Reverse Charge a dead Lead Acid Battery - Page 1
Forum section: electronics / general

author: jonovid
author_rank: Super Contributor
timestamp: February 23, 2025, 10:26:58 am
content: from an odd mechanics tale about a Volkswagen Beetle that ended up with a reverse polarity electrical system.the customer complained that the battery was not charging & radio was not working-the battery had reversed polarity!

author: tom66
author_rank: Super Contributor
timestamp: February 23, 2025, 11:45:55 am
content: Lead acid batteries (at least conventional ones, don't know about AGM/gel cells) are mostly symmetrical in construction. So it's not that surprising that you can get them to charge up with reversed polarity. To do so you'll have to pull them all the way through zero volts though, so it probably degrades the battery pretty badly. But an interesting curiosity. I wonder if the capacity is optimal in the 'correct' polarity, and if it is, what causes that.

author: Gyro
author_rank: Super Contributor
timestamp: February 23, 2025, 12:13:03 pm
content: Quote from: jonovid on February 23, 2025, 10:26:58 amThat would have been in the days of dynamos rather than alternators. They relied on remnant magnetic field to determine the polarity at startup. It was fairly common, with the advent of transistor radios, to convert positive earth vehicles to negative earth. This simply involved reversing the battery connections and 'sparking' the field coil connection on the dynamo to flip its remnant field polarity before starting the engine. Nothing else (including the electromechanical regulator) cared back then.I never heard of such an (accidental?) conversion overwhelming the battery though (not without smoke anyway!).

author: Xena E
author_rank: Frequent Contributor
timestamp: February 23, 2025, 02:36:12 pm
content: Not really possible to reverse charge them as even though they may seem to be symetrical in their construction, the chemical composition of the plates is changed by the charging process and to try to reverse that would pretty much destroy it. Also the positive plates of the batteries are now pre made with lead oxide coating.An old fashioned deep discharge solid plate batterymayhave been possible to charge either wayfrom first charge.X

author: Gregg
author_rank: Super Contributor
timestamp: February 24, 2025, 01:22:33 am
content: It is absolutely possible to reverse charge a flooded lead acid battery. The capacity and life span will be vastly reduced; although ill advised, it is possible.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2025, 01:58:20 am
content: Yes, I can tell you from experience that it is not only possible to charge an automotive battery backwards well enough to operate stuff (and blow some fuses in the process) but it is additionally possible to discharge the reverse-charged battery, charge it correctly and still have it work well enough to start the car and drive away.

author: antenna
author_rank: Frequent Contributor
timestamp: February 24, 2025, 02:12:44 am
content: Just adding another yes report. Neighbor called me over a couple years ago to see what was wrong with his lawn mower. Battery had full charge but mower would not do anything. After a lot of head scratching, I noticed the meter was reading -12.6v. He used an old trolling motor to bring it back to 0v and charged it the right way. The battery survived AFAIK.

author: BradC
author_rank: Super Contributor
timestamp: February 24, 2025, 02:21:21 am
content: Quote from: Gyro on February 23, 2025, 12:13:03 pmMy dad had/has (still have the car but not him) a Triumph 1967 TR4A with a generator. My brother jump started it with reverse polarity. The battery survived but the generator didn't. At the time I assumed the large quantity of smoke was due to the oil I'd spilled on the exhaust manifold. So yeah, smoke and lots of it. The generator looked "pretty second hand" when I tore it down after replacing it. The key with this one was the battery was actually in very good condition. Just too flat to crank the high compression tractor motor after a long session trying to get it started. Once it ran up, the battery fought the generator and the battery won.His jump leads had one side of the rubbers on each end reversed and he wasn't clever enough to follow the "pretty coloured wires". I bought him a new set for his birthday on the condition I got his old set to cut to pieces.It's the second car he's done that to. Mums Landrover Discovery 1 never recovered from the first time.

author: floobydust
author_rank: Super Contributor
timestamp: February 24, 2025, 03:15:51 am
content: Having trouble believing all this.The field winding is reverse-polarity, the voltage regulator is a few relays - so who is polarity sensitive then? No semiconductors involved on old cars.Does the starter motor turn backwards? I think they are a series-shunt motor in an old VW.I've reverse-charged lead-acid batteries and all that is good for is making hydrogen really. The plate's lead types are different.Did nothing for sulphation.

author: bdunham7
author_rank: Super Contributor
timestamp: February 24, 2025, 04:22:47 am
content: Quote from: floobydust on February 24, 2025, 03:15:51 amIn the case I saw it was a bit newer car (90s) and it blew a lot of fuses including the main alternator fuse so it didn't start or even crank so I have no idea what capacity the battery would have in reverse. It would have had a PM starter motor so it would have gone backwards or disengaged. What tipped me off is that the power window motors were going the wrong way...I have seen old cars with reversed polarity simply due to misconnection, especially older 6V systems. This probably was a common issue for a decade or so after cars went from positive to negative ground. Most of those old starter motors didn't care about polarity and some generators would instantly repolarize themselves while others would not work or burn up. Radios obviously would be no bueno.

author: ace1903
author_rank: Regular Contributor
timestamp: February 24, 2025, 06:16:38 am
content: It happened to my father. Nephew turned on high beam lights while playing in car and discharged battery to 0V (old halogen lamps when cold have low resistance).My father disconnected battery from car (Toyota avensis 2008 diesel) and connected to lidl car battery charger but in reverse. Diodes in alternator were directly polarized and main 140A fuse blew.Beside fuse and car radio no other harm was done. It was tricky to find where this main fuse is, and it was specific form that was difficult to find in stores.I discharged battery to 0V again using headlight bulb and it took several hours, I am guessing capacity was near declared on package.When connecting charger again voltage was going up and down and I decided it is time battery to go to recycling center.It was not obvious to me what happened at the beginning and I researched a lot and found out that in factory both electrodes are made same and anode and cathode are formed on first charge cycle.I also found some russian YT chennel where they are testing this, just search name of the channel and name of the video:Garage 54Can you revive a dead car battery by inverting its polarity?

author: tom66
author_rank: Super Contributor
timestamp: February 24, 2025, 10:00:53 am
content: The requirements for most car electronics include tolerance to single reversed battery (e.g. -14V) and double series battery (up to 28V) for aggressive jump start attempts, as well as normal load dump events. Perhaps reverse-connected batteries are more common if manufacturers find it necessary to include these precautions.

author: AVGresponding
author_rank: Super Contributor
timestamp: February 24, 2025, 06:12:33 pm
content: I unsubscribed from Garage 54 when Vlad started wearing a "Z" t-shirt.

author: tom66
author_rank: Super Contributor
timestamp: February 24, 2025, 06:16:48 pm
content: Quote from: AVGresponding on February 24, 2025, 06:12:33 pmSame, they also deleted comments on the war that are pro-Ukraine. Bye bye.

author: johansen
author_rank: Super Contributor
timestamp: February 24, 2025, 06:49:58 pm
content: I have run several reverse polearity batteries in cars to get more life out of them.you have to do this before plate damage sets in but in theory you could get maybe 30% more life out of the battery if you reverse charge it when it reaches about 25% of its nameplate capacity.so for real terms, a 100 amp hour car battery might have a 20 amp reverse polarity capacity, and last for several years in a car as a starting battery (it can be a challenge to make the battery fit backwards so the cables can reach the backwards posts. oh, and they aren't the same size so that's fun.so anyhow, the most capacity i ever got was 83 amp hours out of a 100 amp hour, completely open circuit agm GNB backup battery designed for UPS standby float, not cycling. most lead acid batteries need 4x nameplate amp hours to fully reverse charge, the GBN battery required 600 to 800 i can't remember. as recieved, the battery was completely open circuit, the first backwards discharge delivered 83, the second 40, the third maybe 25 amp hours, it quickly went back to near zero capacity.other batteries faired far better, a dozen or so charge and discharge cycles at 20% nameplate.keep in mind we were doing these tests on already sulphated batteries.what i think works much beter is to dump out the electrolyte, replace with water, and float charge at 14 volts for a couple months. then add acid if needed (you probably won't need to) and the battery will be desulphated.the reason this works is because the lower voltage potential of the dilute weak acid encourages the conversion of sulphate into acid and lead.

author: Sassy Taste
author_rank: Contributor
timestamp: February 27, 2025, 08:32:53 pm
content: Probably not. Eye balling anyone would see the negative battery leads are connected to a chassis ground. Unless they took it apart and connected the positive battery leads to the ground. Convention flow is + to -. Highly not likely to reverse polarity.

author: Gregg
author_rank: Super Contributor
timestamp: February 27, 2025, 11:11:46 pm
content: Quote from: Sassy Taste on February 27, 2025, 08:32:53 pmThis seems AI generated to me. After reading some other posts by the same poster, they all seem to be irrelevant.

author: CaptDon
author_rank: Super Contributor
timestamp: March 01, 2025, 01:48:57 pm
content: Gyro, you are very correct in your statement about the old car generators!! Polarizing was MANDATORY on the installation of a new generator!! I had a long argument with a NAPA auto parts dealer about buying a replacement generator for my positive ground 1948 truck. His book listed both a positive ground part number and a negative ground part number which were different. He had the negative ground Delco Remy in stock. I said "I'll take it". He said "I can't sell it to you for use on your positive ground truck, it will void the warranty". I said "Just sell it to me and you'll never see it again". I installed it and with the belt off opened the voltage regulator and pressed down on the cut out relay and ran the generator like a motor. Put the cover back on the regulator and the belt on the generator and it worked perfect!!! I do know of at least twice when working on some older generator equipped cars the residual magnetism seemed to reverse polarity if the car sat for months unused. When ever the generator tried to start charging the amp gauge would bury off the negative side, then flick back and forth a few times and eventually start charging correctly. I saw the weird things happen on old farm tractors. Then there were the guys who put 8 volt batteries in their tractors but never modified the regulator and wondered why the 8 volt batteries were always dead!!!