Thread: How long would it take for an interior light to flatten a battery?

Not sure how much/how long an interior light would drain, but when you reconnect your aux battery the poor battery will suck your good one down til they roughly equalise, and of course the worse your main is the lower the aux will be pulled down.
On that note if you were in the whoop whoop and you knew ahead of time that the main was very flat, the best course of action would be to remove it and fit the (presumeably good) 2nd battery in it's place rather than reconnect the two (whether via solenoid systems etc or jumper leads).

The interior light usually takes about 24hrs to drain the battery enough to have enough umph the turn the stater over (especially diesels).

******WARNING SCIENCE CONTENT ********

Ok so your typical interior light is what, 10 Watts ?
I = P/V
So at arbitary 12.5 volts it draws 10/12.5 = 0.8 amps - so lets say an amp with safety factor. If you have say, a 55 Amp/Hour battery, it should technically burn for 55 hours. ( 1 amp for 55 hours )

However your battery will probably drop below the threshhold voltage/capacity needed to turn the starter motor in about half that time, so lets say the interior light should be able to be left on for 27 hours without compromising your starting ability. This tallies with the practical experience mentioned above.

This is assuming of course the bulb/battery ratings as quoted above, ( you can work out the numbers for your situation ), you have a good battery, and there's no other load on the battery ( eg fridges, phone chargers etc )

Originally Posted by 100I

Not sure how much/how long an interior light would drain, but when you reconnect your aux battery the poor battery will suck your good one down til they roughly equalise, and of course the worse your main is the lower the aux will be pulled down.
On that note if you were in the whoop whoop and you knew ahead of time that the main was very flat, the best course of action would be to remove it and fit the (presumeably good) 2nd battery in it's place rather than reconnect the two (whether via solenoid systems etc or jumper leads).

Thanks Dan. I had been told about this but I thought it would not be an issue if the car had been started and was barreling down the road. The batteries were equalised/ing but if the alternator was doing its job they'd both charge up in time. I also understood that the equalisation was not an instantaneous thing.

But I know little about the matters.

Originally Posted by JamesH

Thanks Dan. I had been told about this but I thought it would not be an issue if the car had been started and was barreling down the road. The batteries were equalised/ing but if the alternator was doing its job they'd both charge up in time. I also understood that the equalisation was not an instantaneous thing.

But I know little about the matters.

Not if it's been started, I'm saying if you know you have a flat start battery, it's something to be wary of.
Say if you are at the bottom of a gully and you have only one shot at starting it, if you hook the batteries up in paralell, you now have two 1/2 flat batteries, neither strong enough to crank.
Know wha ah mean?

The interior light is probably 6w not 10, and may even be less. How long you can leave it on will depend on how good your battery is, and how easy it is to start your engine. I know from experience that in my Citroen, it won't last a long weekend, but will a normal weekend. The County, I don't know, but I can tell you it won't run the UHF for a week. Could be a good argument for fitting an LED bulb in your interior light?

John

Originally Posted by JDNSW

Could be a good argument for fitting an LED bulb in your interior light?

John

Yep you can buy them in the same form factor ( festoon ) as the incandescent. They would draw maybe less than 1/10th the current of the traditional bulb.

Originally Posted by waynep

Yep you can buy them in the same form factor ( festoon ) as the incandescent. They would draw maybe less than 1/10th the current of the traditional bulb.

And I do beleive that Inc sells them on this forum via the markets section !