I've found an old lead-acid battery and decided to check if it still is in a decent condition. I've opened all rubber caps and checked that the water level was still OK. It measured 10.5 V before I started charging.

I programmed a controllable power source to provide 13.15 V under a 1.4 A current limit. Differently from what I expected, charging current started at about 100 mA (constant voltage) and it's slowly increasing for 2 hours. It has just passed 600 mA.

The battery is not heating or noticeably releasing gases.

Is this expected? It's different from the behavior we normally see on deeply discharged batteries (current limited first, than constant voltage).

  • \$\begingroup\$ Dont know about the specific details but if the battery's is holding onto the an ever increasing voltage after the charger is removed I would say it looks good. Seems plausible the internal resistance might decrease as the battery is revived \$\endgroup\$
    – DKNguyen
    Commented May 30, 2019 at 13:01

2 Answers 2


Lead sulphate oxidizes the plates to increase battery ESR. THis can be recovered if not left too long for days or months.


(13.15-10.5V )/ 0.1A = 26.5 Ohms initially then
(13.15-10.5V )/ 0.6A = 4.33 Ohms

Good SLA Battery < 0.1 Ohm charged Good battery but discharged < 11V > 20 Ohms

As you can see a good battery drops in resistance when in useful charge range > 10% SoC. If the resistance to voltage change does not drop , then battery is too worn out, but can be used for less demanding jobs.


simulate this circuit – Schematic created using CircuitLab


Sounds normal to me.

The electrolyte in a deeply-discharged battery is very weak — almost pure water, because the sulfur is bound up as lead sulfate in the plates. This makes the internal resistance very high, resulting in a low charging current initially.

Charging the battery releases some of the sulfate into the electrolyte as sulfuric acid, gradually increasing its conductivity and the charging current.

It sounds like this battery may still be usable. If the enough of the sulfate crystallizes and becomes insoluble, then the battery is generally unrecoverable.

  • \$\begingroup\$ Thanks! I'll give it more time to see if the current starts reducing. \$\endgroup\$
    – devnull
    Commented May 30, 2019 at 13:11
  • 1
    \$\begingroup\$ There is not any lead sulphide involved. The normal discharge state of a lead-acid battery is with both plates having significant amounts of lead sulphate and as you say the electrolyte being reduced in strength. This is normally only a problem with starved electrolyte batteries such as in sealed versions with flooded batteries normally having an excess of electrolyte. \$\endgroup\$ Commented May 30, 2019 at 14:18
  • \$\begingroup\$ @KevinWhite: Thanks, fixed. I was misremembering the details. \$\endgroup\$
    – Dave Tweed
    Commented May 30, 2019 at 14:28

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