Electronic – Charging a lead acid battery with a lower bulk voltage

Lead-acid batteries are fairly forgiving. Unless the battery is quite low, it can be charged by holding its voltage at the "float charge" level. That is usually 13.6 to 14.0 V for a 12 V (6 cell) battery. This is basically what is done in automobiles. When the engine runs, it turns the alternator, which has a regulator around it so that the "12 V" line is really kept at the float charge voltage.

For a really depleated lead-acid battery, you have to be careful to limit its charge current. Just applying the float charge voltage would cause too much charging current until the battery charge level gets higher. Since you have your own power supply in there managing things, adding this protection is actually quite easy. What you want is a supply that is both voltage and current limited. Make sure that is only puts out the float charge voltage, or the maximum allowed charge current, whichever comes out to less output power.

Sealed Lead-Acid batteries are usually arranged so that they only have just enough electrolyte (dilute sulfuric acid) to operate (this is often referred to as starved electrolyte). This allows the battery to operate in any position, even up-side down. Normal car batteries have much more liquid (flooded cells) and can spill if inverted.

If sealed electrolyte batteries are left for a long time to completely discharge all of the sulphate ions in the electrolyte will combine with the lead plates to form lead-sulphate so that the electrolyte changes into just water.

Pure water does not conduct electricity very well so that when you attempt to charge the battery the voltage will rise very rapidly and the battery will not charge.

This state is often called "sulfation" where the plates contain lead-sulphate that cannot be easily converted back to lead or lead dioxide as normally happens during charging.

Sometimes a small current will flow and the battery will eventually charge (days or weeks) but almost certainly the battery will never regain its original capacity. I have never succeeded in recovering a battery in this state.