Will a small 12v battery work?

[MADMAX2]

I have built a circuit in the car for an accessory and need the smallest 12v battery to power it I don't want it powered from the car battery ( too much hassle long story!!!!) I have found the small aa type 12v battery but shorter which would be perfect size but would it power the circuit if I connected a pos and neg to it? It's only for a light in a very small space I tried it on a small 12v motorcycle size battery and it worked I need it to be as small as the aa style 12v battery as there is no space


Will the small 12v battery work?

[BritishRacinGrin]

How are you going to charge the battery?

[andyiley]

MADMAX2 said:

Will the small 12v battery work?

12 volts is 12 volts is 12 volts, where they come from is irrelevant, 12 volts will light a 12 volt light bulb.

The other issue is for how long.

If you have a 1 amp hour 12 volt battery powering a 1 amp circuit then it will only do so for 1 hour.

[Mr2Mike]

How much current does your circuit draw? The 12v batteries used in car alarm keyfobs are just a bunch of tiny watch batteries stacked up inside. This means that they have very little capacity (only 30 to 40 milliamp hours) and the have fairly high internal resistance so they can't deliver much in the way of current either.

If you circuit draws only a few milliamps, then this might be practical but they are expensive.

andyiley said:

12 volts is 12 volts is 12 volts, where they come from is irrelevant, 12 volts will light a 12 volt light bulb.

Not true I'm afraid, the internal resistance of tiny batteries greatly limits the current they can push out. You wouldn't be able to light e.g. a 55Watt headlamp bulb with an A23 battery (the one the OP is referring to), since the voltage would just collapse under the load.

Edited by Mr2Mike on Tuesday 19th August 14:33

[andyiley]

andyiley said:

12 volts is 12 volts is 12 volts, where they come from is irrelevant, 12 volts will light a 12 volt light bulb.

The other issue is for how long.

If you have a 1 amp hour 12 volt battery powering a 1 amp circuit then it will only do so for 1 hour.

Mr2Mike,

Now add in the second part of my posting & this explains it as you say, take it to the extreme & you have the circumstances you explain.

QED.

[Mr2Mike]

andyiley said:

Mr2Mike,

Now add in the second part of my posting & this explains it as you say, take it to the extreme & you have the circumstances you explain.

QED.

Not quite. Capacity and internal resistance are separate issues. You could make a battery with high capacity but high internal resistance - it would deliver a small current for a long time but isn't capable of delivering a large current for even a short time.

[andyiley]

Quite correct, which is covered in the bit where I said Ah rating.

Which will take into account internal resistance.