Say I have a 12V battery discharged to 11V and I connect it in parallel with an AC adapter whose final output is 12V DC.

In ideal case any current flow towards battery should stop as soon as battery is charged to the same voltage as the charging voltage (due to 0 net potential difference between charger and the battery.)

Is it safe to leave the battery connected to the AC adapter or it will start overcharging?

In case of overcharging what is the mechanism when there is no potential difference (for any chemistry)?

  • 3
    \$\begingroup\$ Rectified (with a diode "ofc" - whatever one of those is) 12 AC will peak at \$ 12 \sqrt 2 ~V \$. That will most likely overcharge your battery. \$\endgroup\$
    – Transistor
    Jun 18, 2016 at 18:00
  • \$\begingroup\$ Not a rectifier but a protection against reverse current discharge in case ac adapter is turned off while battery is still connected \$\endgroup\$
    – Allahjane
    Jun 18, 2016 at 18:13
  • 1
    \$\begingroup\$ If you mean charging the battery with a 12 V DC adaptor then you need to edit your question. Explain what a "diode ofc" is. Add a schematic using the built-in editor so we all understand what you are proposing. \$\endgroup\$
    – Transistor
    Jun 18, 2016 at 18:16
  • \$\begingroup\$ Edited: an AC Adapter whose final output is 12v dc, its just an example case really what i mean is if the charging source is an ideal emf source with same voltage as the battery's rated output voltage at full charge \$\endgroup\$
    – Allahjane
    Jun 18, 2016 at 18:25
  • \$\begingroup\$ @transistor ofc is an Internet-ism meaning "of course". \$\endgroup\$ Jun 19, 2016 at 10:48

2 Answers 2


A 12V lead-acid battery will not be damaged by overcharge if the voltage is kept low enough to avoid electrolysis, and the charging current is kept below 0.2C (5 times less than the Ah capacity).

Some types of lead-acid battery can handle higher voltage that others. SLA batteries must not be allowed to gas or they will lose water (which cannot be replaced) so they should be kept below 13.5V (at 20°C, lower at high temperatures). AGM batteries have some ability to recombine gas, so they can be 'floated' at up to 13.8V. Flooded lead-acid batteries can be charged to 14.4V or higher, so long as they are kept topped up with deionized water (but this is not recommended because hydrogen/oxygen gas is explosive!).

However if your AC adapter puts out exactly 12VDC then the battery won't be charged either, so you will damage it by undercharge. Note that just because your adapter says '12VDC' does not mean that it always puts out exactly 12V. An unregulated supply may go up to 18V or higher at low current.

The other problem is that the AC adapter is probably not designed to limit current. If it can put out more current than the battery can take then the battery may overheat. If the battery can take it then the power supply may not (either overheating, shutting down or blowing a fuse). A simple way to fix this is wire a 6V or 12V incandescent light bulb in series with the battery. The light bulb will limit current and also glow weakly to show that the battery is charging.

If your power supply voltage is too low then it just won't work. If it is too high then it will overcharge the battery, but you might be able to add a voltage regulator to lower the voltage and limit the current to suit your battery.


I'm assuming you're referring to lead acid chemistry. If the voltage (potential) is not greater then no current will flow, therefore it is impossible to overcharge. Look at battery university plot of charging phases (constant current, topping charge and float).

battery university

  • \$\begingroup\$ I think you missed the part that says there is only one battery and a battery eliminator (AC Adapter) \$\endgroup\$
    – Allahjane
    Jun 18, 2016 at 17:30
  • \$\begingroup\$ Oh thanks. You should check the voltage of the AC adaptor. If it's not greater than the battery it will not overcharge it. You should check it's specs and be sure it's up for the task \$\endgroup\$
    – jmaturner
    Jun 18, 2016 at 17:53
  • \$\begingroup\$ Any battery charger without current limiting can cause problems. Should the battery, (any chemistry) not be in good condition, the voltage will never reach 12V, causing a charger without current limiting to continue to supply current until the battery overheats, as well as the charger. \$\endgroup\$
    – Sara Heart
    May 24, 2018 at 4:56

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