Could charging a fairly modern car with lights on (or any other electronics on) damage it? My car battery drained to a voltage of 5.5V (very low I know) because I left the interior light on for 2 days. I left it to charge with the light still on (didn't realise it was on since it had gone out by that time and I forgot to check the switch), only realised after a few hours when I came back that the light had now turned on. If this, or any other electronics were on while the car battery was charging, could this cause problems? Also, would it prevent the battery from charging, since the battery didn't have any charge in it when I got back? Would the load from this interior light be enough to stop the battery charging, and is it dangerous to have done this - could I have damaged any other electronics or components in the car like the alternator. Sorry, this is my first car and so I am quite new to this!

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    Aside, you may have shortened the life of your starter battery by some amount. There's nothing you can do about this now. – Criggie Jan 1 at 3:13

It didn't damage your battery or the charging at all. Because:

  • All batteries in automotive created for simultaneously charging and discharging otherwise your car will not have an alternator in the first place.
  • Your charger (SMPS type or Transformer type) has an output diode to maintain the current flow in a single direction, so the current from your battery would not affect the charger.
  • Almost all chargers would work with a limited current. Even without a current limiter, the charge current will reduce constantly with the voltage increase. (SO don't worry about blowing up your charger).
  • And all charger basically using voltage limiter

And it wouldn't damage your electronic component in your car nor the alternator. Because the alternator has the diode to block the reverse current.

The main concerns are:

  • Your battery didn't charge at full capacity if your battery charger current is lower than your lamps.
  • Your battery charger would be hot or warmer because it took longer than it should.
  • Your lamps would be hot enough to melt some plastic near them, check if something out of order near your lamp.
  • If your cable to the lamps using small wire could be hot enough to melt too, check it again.
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    I see, I didn’t smell any burning or melting, so hopefully it’s fine. I switched off the lights and the charger is now outputting 2A at the low setting so I hope this means it’s fine. I could also up it to 4A on the high setting, but not sure if that’s a good idea. I’m thinking I’ll try charging for another 3-4 hours then try starting it. – Resquiens Dec 31 '20 at 19:03
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    that's a good idea to let it charge at full. good job – Cubic273.15 Dec 31 '20 at 19:07
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    Well, looks like it’s all up and running now, and hopefully the electronics haven’t failed. Thanks, and happy new year! – Resquiens Jan 1 at 2:47
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    It would have just taken longer to bring the battery up to fully charged, and may never have charged the battery if the light was consuming more current than the charger could supply (not the case here, but maybe if you'd left the headlights on while charging) – Criggie Jan 1 at 3:12
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    @Resquiens happy new year to you and good luck with your car! – Cubic273.15 Jan 1 at 10:15

You probably did no damage. Most chargers have built-in protection for excessive amperage drain. Courtesy lights do not draw very many amps and the battery probably was taking a charge. Turn off all the accessories and charge your battery according to the instructions per charger and you should be good to go.


Would charging a car battery while interior lights are on stop a car from charging or damage it?

It depends. A filament lamp is essentially a resistor, and the interior light in your car is probably 5 watts. When driven by 12v it will consume about 0.42 amps, or when in a system that is more like 14v (i.e. when it's being charged) 0.36A. This applies for each bulb that is on.

In order for your car battery to charge, your battery charger needs to output enough current to supply what the lamp demands plus an excess that will be absorbed by the battery during the charging process. If your charger is a trickle type, and charges using some low current such as 0.8 amps, with a voltage of 14v (ish) your 5W bulb is consuming about 0.4 amps then you have around 0.4 amps for charging the battery..

Your car battery has a capacity rating, measured in amp-hours or minutes-to-reserve. It represents a theoretical ability to supply some amount of amps for some amount of time. Minutes-to-reserve describes the amount of time it takes to deplete a battery at a certain current. Amp hours describes the amount of time an amount of amps can be delivered. Mathematically the two are typically related by dividing Ah by 0.42 to get reserve minutes, or multiplying reserve minutes by 0.42 to get Ah.

An 80Ah battery (quite a high capacity) could (theoretically, if it was possible to usefully get all the charge out of it without losing energy to wasted heating) supply 80 amps for an hour, or 1 amp for 80 hours, or 2 amps for 40 hours, or... You get the idea. In a real world context a higher supply current generates heat and wastes energy so you wouldn't get 80 amps for an hour but for sake of simplicity let's assume perfect linearity

So if your battery was diminished to 50% of capacity, and you wanted to charge it to full then it's down 40 Ah. You need to supply 1 amp for 40 hours, or 40 amps for 1 hour, or... You get the idea.

Imagine that you were using a trickle charger of 0.8A and no light is on - it'd take 50 hours (2 days straight) to recharge your 50% depleted 80Ah battery. If the bulb is using half the current from the charger, the battery takes twice as long to charge

If you left two lights on you're probably in deficit - your trickle charger can't supply enough to run the bulbs let alone charge the battery

Will it stop charging?

We can't really answer, because you didn't say what the output current rating of your charger is, or how many bulbs of what wattages were left on. You can though; you can read labels or measure the amount of current that is leaving the battery when your lights are on - the most exact science will be to disconnect one of the battery leads an put a multimeter in amps mode between the battery and the lead so you can measure how much current is flowing into or out of the battery. If it's flowing in, the battery is charging. If out, then the charger can't run the bulbs so the battery is helping, and suffering in the process.

Will it damage it?

It depends, but the answer is either "no" or "it'll actually help"

If your car battery is at 5.5v then it's already well on the way to being ruined. Standard lead acid cells lose ability to hold charge if they're depleted lower than about 80%. Batteries intended for leisure use (golf carts, mobility scooters etc) or marine use are designed differently and can withstand being depleted lower, but no lead acid cell can withstand deep discharge even if immediately recharged.

Your battery will never recover its former capacity. Whether this ultimately makes a difference probably largely depends on the environment. If it was in great condition before this, and you live in a region that is persistently warm year round, you routinely drive for long times between engine-off stops and there are no earth faults or other electrical maladies with the car then you might be able to recharge it and carry on with the diminished capacity. If you live in an area that frequently sees sub zero temperatures, or there is some fault that is a permanent parasitic drain, or you do a lot of stop start driving, you may well need to replace the battery because it will reach a point where it is no longer able to start the car

The best chance the battery has of recovering as best it can is to be recharged slowly. Fast recharging generates heat inside the battery, and excessive temperatures cause warping of the internals and internal shorts which further diminishes capacity. To this end, if you have a high power charger and no option to get a lower power (trickle) charger, applying some electrical load (leaving lights on) can help the battery survive an aggressive charging regimen because it isn't absorbing all of the output of the charger

I'd charge your battery on a sub-1-amp charger, and see how it recovers. If it does and is able to start the car, great. You might find that the combination of driving style and environment means you have to periodically recharge your battery overnight, and if e.g. is gets really cold you might have to replace it. If having a working car is vital, head this problem off before it begins and replace the battery; 5.5v is really low

  • Thanks for this! I didn’t realise that the battery might be in very bad condition, so I’ll have to inspect it and see if it drains its charge again. It’s the first time this has happened, although the battery is over 5 years old, so it may also be past its sell by date. – Resquiens Jan 1 at 13:45
  • Lead acid cells are great for longevity if they maintain a high level of charge - in one car I have one that is 16 years old, but has been run low (10v) a few times and starting to show, especially with lockdown - I use the car once a month and i need to use a mains charger on it after 2 weeks; in the other car the battery is 10 years old, still working well and it has to suffer a greater demand because that car has auto stop start/aggressive charging etc - it has monitoring circuitry that'd disable the stop start if the battery was in poor condition. 5 years age wouldn't itself be a problem.. – Caius Jard Jan 1 at 14:19
  • ..it's just deep discharging (and rapid recharging) that really kills them. See how it goes with charging - it's cheap and easy; and you'll know pretty quickly if it's really shot – Caius Jard Jan 1 at 14:24
  • Saw on another comment that your charger is 2a - I'd say to look on your battery for the Ah or reserve minutes rating, if it's reserve minutes at 25A, divide the minutes by 0.42 to get Ah. The Ah rating of the battery, in your case, probably represents the minimum hours I'd charge for before trying to start, given that the batt is well below it's reserve capacity. If you come out with a number like 60, that's 3 days charging.. your "I'll put it on charge for another 3-4 hours" won't touch it - 2A is really low charge rate and it'll take a loooong time – Caius Jard Jan 1 at 14:43
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    (You should appreciate that your alternator might well be whacking charge back into the battery at 70amps if the battery is in good condition and slightly depleted from just having started the car. You're deliberately wanting to charge it tens of times slower than that otherwise you'll ruin it) – Caius Jard Jan 1 at 14:46

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