I have a lantern with two 6V 4AH batteries, which I guess makes 12V and 8AH.
It has a 100W car bulb* and I was trying to work out how long it should last on a full charge.
I think its: 100W / 12V = 8.3A therefore the torch should give a bit less than an hour's light (which it does)
Is that the right formula to use?
The message from "Mark W" contains these words:
Sorry, it only makes 12V and 4Ah. You don't get double the duration just 'cos they're in series - only double the voltage.
yes your working is right
watts equal amps times volts
So you wire them in parallel, then.
And get 6V and 8Ah
True. I was thinking more of duration than outright voltage.
The message from snipped-for-privacy@italiancar.co.uk (SteveH) contains these words:
That'll get you 6V @ 8Ah.
Yes but not quite as simple as that see below. Also a 12V battery is more like 13.2V when charged. (2.2V per cell)
Except that the battery has only 4Ah of capacity.
To get 12v you are in series, so the current is drawn through one 6V battery then the other. Hence you will only get 4 Ah of capacity.
However, that capacity is stated as the 20 hour rate, which means that you can only expect 4Ah of capacity if you draw it out over 20 hours, so you would be able to get 0.2A for 20 hours. (0.2x20=4). (it might also be a ten hour rate, in whcih case alter the numbers)
The actual 1 hour discharge capacity will be considerably less than that. If you chose the discharge rate such that the battery lasted for an hour you may find that you could only draw say 3 amps. Conversely if you took 4 amps out of it then you may find you only got 40 minutes instead of the expected hour.
If you are getting an hour out of two 4Ah batteries in series then you are probably using a 55W bulb.
Then they'd only last longer with a lower light output. It's the watts that matter, and they're the product of volts and amps.
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