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I have read many discussions about this, but I have been unable to find a definitive answer.

Many people say that it's fine to use a standard household (cheap/dumb) 12V 4A battery charger to charge a car battery while it is still connected to the car. Others say that it's safer to disconnect the negative battery terminal and charge like that. Some quote car manuals which say you can charge in-situ it as long as your battery charger can't put out more than 7A (though I suspect that this may be a specific recommendation for those cars). Yet others say charging in-situ is fine for a healthy battery, but dangerous to the car electronics if the battery is dead or deeply discharged. Some also say that charging in-situ can 'spike' the electronics of a modern car, while others say modern electronics are more robust and there is more danger from the uninitiated dis/reconnecting a battery incorrectly.

I know from electrical engineering that no matter what the battery charger does, the voltage across the battery will ultimately be determined by it's internal resistance. I know that dead or deeply discharged batteries can have a very high resistance, and that as the battery gets fully charged the resistance rises, so some of the above statements seem likely. What I don't know is if the voltage across the battery would ever get high enough to damage other devices still connected to the car battery such as the ECU or devices plugged into an un-switched cigarette lighter.

It seems generally accepted that C/20 charging shouldn't cause long term damage to a lead acid battery, so an 80Ah battery should never be in danger from a 4A charger (This could be where the 7A max recommendation came from if the car had a 140Ah battery, or they though C/10 on a 70Ah battery was fine), however this appears to depend on whether the charger fixes the voltage and lets the current change according to the battery charge state, or whether the charger runs with a constant current and varies the voltage.

So, under what conditions, if any, should I consider it safe to charge a car battery in-situ, without disconnecting it from the car?

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  • What does the manual for the charger recommend?
    – CharlieRB
    Feb 14, 2018 at 17:02
  • Find the manual @CharlieRB? After several house moves? You must be kidding. *8') Also, I was attempting to elicit answers which don't make assumptions about a specific charger and provide the info needed to work out for yourself if a given charger would be appropriate, hence why I talk about C/20 and battery physics.
    – Mark Booth
    Feb 14, 2018 at 17:07
  • How foolish of me to assume you had the manual. LOL
    – CharlieRB
    Feb 14, 2018 at 17:40
  • Just make sure the clamping voltage doesn't go above 14.4V to prevent hydrogen formation in the battery. Some chargers reach realitvely high voltages if the current draw is low. Cheaper chargers or chargers not made for car appliances may have that issue.
    – Bart
    Feb 14, 2018 at 18:42
  • I found the manual @CharlieRB it was non-committal. It recommended you disconnect, and that not doing so 'could damage the alternator' but didn't suggest under what circumstances it is or isn't recommended.
    – Mark Booth
    Feb 15, 2018 at 10:52

1 Answer 1

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You can charge the battery on the car with all chargers that don’t have the “fast-charge” facility - so those tend to be professional and they know what they are doing. if your charger does have the fast charge - check the instructions carefully...

I charge mine often and you don’t have to disconnect the battery either. If you do you will loose all the codes for the radio etc

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  • Have you ever done this on a car with a dead (sulphated) or deep discharged battery?
    – Mark Booth
    Feb 14, 2018 at 17:26
  • Incidentally, I have an Android head unit, so on a battery disconnect it doesn't lose any settings as they are written to non-volatile flash memory. The only difference is that the HU needs to do a full boot (20 seconds) rather than waking from sleep (2 seconds) the first time after a battery disconnect.
    – Mark Booth
    Feb 14, 2018 at 17:28
  • @MarkBooth I have charged a battery that got so hot the electrolyte was boiling... didn’t work before ... or after :)
    – Solar Mike
    Feb 14, 2018 at 17:30
  • @MarkBooth - The term "boiling" when describing batteries is specific terminology meaning there is excess charging going on, which forms bubbles at a quick rate, causing battery acid to be discharged from the battery. Whether the battery acid is at 113°C+ is sort of irrelevant.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 14, 2018 at 18:07
  • @MarkBooth i have charged, or tried to charge, batteries in every state possible : discharged, dead flat, sulphated, even ones that have been incorrectly charged the wrong way : so the positive was actually the negative (was dead flat and someone connected it backwards : it took a charge but obviously couldn’t be used...) so we tried discharging it and then re-charging correctly but no luck...
    – Solar Mike
    Feb 14, 2018 at 18:15

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