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I'm trying to charge a 6-year-old OEM lead-acid battery in a 2008 Volkswagen Jetta with an external charger, whilst it's still connected within the vehicle. (The battery was working fine, until it got discharged to perhaps 25% due to a prolonged episode of charging the mobile phone through the car without the engine running, and wasn't recharged fully since then; I've now added distilled water, and am trying to recharge it after a delay of only a couple of weeks since the last proper charge.)

When tested under slight load (key in the ON position without the engine running, and with the lights off), it would appear that the voltage of the circuit being charged is only about 12.6V or so; when testing under no load, it's maybe around 13.5V (with the charger being in maintenance mode).

Is there any point in trying to recharge a sulphated battery with a charger that appears to only be capable of producing merely 13V in its maintenance mode, for a 12V battery? Would I have a much better luck with 14V for trying to de-sulphate my battery?

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    \$\begingroup\$ If sulphation could be reversed simply by charging, there wouldn't exists a wide range of snake-oil remedies for it. In my experience, the remedy for a slowly discharged battery is to slowly charge it. \$\endgroup\$
    – gbarry
    Commented Nov 30, 2014 at 8:22
  • \$\begingroup\$ @gbarry, how do you slowly charge it? what voltage would be appropriate for such slow charging? \$\endgroup\$
    – cnst
    Commented Dec 1, 2014 at 3:09
  • \$\begingroup\$ It's not by voltage, as the battery will hold the voltage to its own charge level. Current is more relevant. A trickle charge of 1 to 5 amps would suffice. If the battery is bad, it isn't going to accept much more than that...and that's the point. Just leave it charging for a week or more. That's what I meant by slow. \$\endgroup\$
    – gbarry
    Commented Dec 2, 2014 at 0:26
  • \$\begingroup\$ @gbarry, I'm kinda confused -- there is a lot of literature that suggests that the minimum voltage the batteries should be charged with is like 13,8 V, but that it also shouldn't go above 14,4V or so -- are you suggesting that's not the case? If the voltage of the charger is lower than the voltage of the battery, wouldn't it be discharging the battery? The literature seems to be suggesting that if you don't give 14V to a battery, then it'll become sulfated. \$\endgroup\$
    – cnst
    Commented Dec 3, 2014 at 22:45
  • \$\begingroup\$ Minimum voltage would be anything higher than what the battery is showing at the moment...and naturally that goes up as the battery charges. Usually when applying a higher voltage, the battery load will drag that voltage back down, so the measured voltage doesn't reveal as much as measuring the current. I don't have any modern equipment, just stuff that's 30 to 50 years old. Somewhat crudely made, but still quite effective. It's still all about pushing electrons backward through the battery. \$\endgroup\$
    – gbarry
    Commented Dec 5, 2014 at 6:00

4 Answers 4

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13V read on your multimeter is probably RMS value. There are probably relatively high voltage pulses.

I would just charge that battery and try to use it. If it does not hold charge - you should replace it.

I was experimenting a lot with desulphation and... for car battery it does not make sense. If there are big sulphur crystals - you have to use chemical method (instead of electrical method with fancy charger). If there is just small sulphur dust on electrodes - desulphating will not help a lot. It is waste of time.

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The best way to damage a charger, is to start your car while the charger is connected to the battery! Hopefully you have not damaged your charger.

You are right, you need more that 12v to charge the battery to 12v. However, even though the battery might show 12v with no load, as soon as you start the car, its voltage will drop to 10v or less. If the charger is "current limited," it will charge the battery without trouble. If it is not, then initially there will be a high current rate, which will taper off, as the battery's voltage increases. If the current is too high, the battery and/or the charger could be damaged.

To charge you battery, disconnect the cable from the positive terminal. Connect the 13v charger to the battery terminals and leave it connected continuously for at least two days.

Disconnect charger, reconnect the positive cable and try to start the car. If it does not start, you have a dead battery or a damaged charger. If you are sure the charger is good, then replace the battery.

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TL; DR: Please replace the battery and avoid injuring yourself.

Lead acid batteries in modern vehicles are typically maintenance free, which makes me wonder how you added distilled water - assuming you didn't do anything bad to the case, please read on.

Sulphation is the normal mode of lead acid battery discharge. When the discharge is really slow, the layer is built very tightly into all the nooks and crannies on the plate. Rapid discharges are much looser in nature and therefore easier to recover from.

To clean the plates without dismantling the battery (which you could do, with great effort and care), you have to perform an equalizing battery charge. The problem is that commercial lead acid batteries for vehicles are not built to withstand this.

The basics of equalizing are simply to continue charging so as to maintain a fixed, higher than normal voltage for a specified period of time after the normal charge is complete. Don't do this to a car battery! The industrial cells this is done on are large, well ventilated things with flash arrestors and hydrogen monitoring.

The elevated voltage at the end of the cycle is intentionally held to promote off-gassing to mechanically disturb the sulphation on the plate and encourage it to mix into the electrolyte - note that these batteries also could have the benefit of an agitation system to help circulate the electrolyte. The fully charged battery won't absorb any more power so it simply performs more electrolysis than usual.

There are chargers out there with so called super charge settings that could do this, but i don't recommend them and neither would the battery manufacturer. Please do not attempt to equalize a sealed lead acid battery.

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  • \$\begingroup\$ Yes, I have a maintenance-free battery. It has a sticker/label on top, underneath of which six flat plastic caps are present, which can be unscrewed with a flat hex screwdriver or flat key. Most maintenance-free batteries require some water to be added to them. \$\endgroup\$
    – cnst
    Commented Feb 26, 2015 at 23:13
  • \$\begingroup\$ Excellent. Not all batteries still have those. Still, the rest of the post applies as written. \$\endgroup\$
    – user39962
    Commented Feb 26, 2015 at 23:26
  • \$\begingroup\$ Well, most of the maintenance-free batteries do require water to be added, which is a funny concept, but it's true. :-) \$\endgroup\$
    – cnst
    Commented Feb 26, 2015 at 23:55
  • \$\begingroup\$ Under most circumstances, in most climates, and with proper system operation, SLA batteries require no watering. You are right that it can be necessary, but the idea that most require it is incorrect. \$\endgroup\$
    – user39962
    Commented Feb 27, 2015 at 0:28
  • \$\begingroup\$ It is my understanding that most maintenance-free batteries are not sealed, yet they're still called "maintenance-free" and still require the watering. I wish I knew about the watering sooner rather than later, I think my battery might have been in better condition today had I watered it earlier! \$\endgroup\$
    – cnst
    Commented Feb 27, 2015 at 2:22
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I have build a 12v lead acid desulfator I revived my dead 6sm battery from being used for 5 years from having a charge of 4.6volts what just I do is replace the acid with strong battery solution then created a desulfator after I used this desulfator my battery reads 13.86v and that was good

YOU NEED THESE 1.) 100uf 375v (for 10amps) 2.) bridge rectifier (10 amps) 3.) 0.68 uf

STEP1 Connec the 100uf at 'one line' in the mains in series STEP2 CONNECT THE MAINS IN THE AL LINE IN THE RECTIFIER REMEMBER THAT THE RECTIFIER IS IN SIRIES WITH THE 100UF CAPACITOR

STEP3 CONNEC THE 0.68UF CAPACITOR AT THE OUTPUT DC OF THE RECTIFIER THE CONNECT IT TO YOU BATTERYIN OPEN CIRCUIT IT WOULD READ 300V DC THE IF YOU CONNECT IT WITH THE BATTERYIN THESE WILL READ 8 TO 12V IF THAT RISE UP TO 16V THAT WAS FULL CHARGE

*TRY THESE BUT I'M NOT RESPONSIBLE FOR ANY DAMAGES. *JUST TRUST MY ANSWER I'VE TESTED IT WORKS PROPERLY

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    \$\begingroup\$ -1 for bad formatting / circuit description instead of schematic / advice that may be dangerous \$\endgroup\$
    – Kamil
    Commented Dec 28, 2014 at 14:19

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