After seeing that someone had calculated the time it takes for the battery to drain on the Punto with the parasitic current draw can someone work this one out for me please:
If you have a 24v electrical system powered by two 12v batteries that are 120ah each, how long will it take a 2.6amp current draw to flatten the batteries. I'm guessing that it is just over 46 hours. Is this right?
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I make it 92 hours. Two 12V 120ah batteries connected in series gives you double the voltage (24V) and double the capacity (240ah).
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However, that's the max figure. Knock off upwards of 15% for a battery that's not new, in cold weather etc.
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Ah but this is what I didn't know. If it was the two batteries powering a 12v system then yes I agree that it would give you 240ah. But seeing as they are in series powering a 24v system - does the available amount of amp/hours still double?
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does the available amount of amp/hours still double?
No, the energy capacity of the system is limited / governed by the weaker battery. In your idealised case, you would have 120Ah. As the batteries are in series, the current is common.
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Sorry bagpuss I agree with Simon. Putting the batteries in parallel will double the capacity, putting them in series will double the voltage.
Edit: Beaten to the draw (just) by NC.
Edited by bathtub tom on 04/01/2009 at 17:58
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You're right, of course. Too much apres ski, I think:-)
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At what voltage does the equipemt drawing the 2.6 amps stop working. Regards Peter
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I don't know but it is low enough to draw the batteries really really flat. My question has been answered though, the 46 hours sounds about right.
Thankyou gentlemen.
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Of course two 12v lead acid type batteries properly connected in series become one 24v battery, made up of 12 cells. If that makes sense. Overall Capacity is dependant on the Ah of each cell.
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My old Landcruiser has 2 batteries. They're connected in parallel for normal running, but connect in series for 24v starting. So if each has a CCA of 660 amps, does that mean when starting it has 660 for longer, or 1320 for the same time as if I only had one.
What is CCA anyway, and for how long can you pull that current?
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CCA = Cold Cranking Amperage.
It defines the current that may be drawn from a stone cold battery for about 30(?) seconds without a significant drop in voltage (about 2V?)
I don't have a definition for 'cold' - but would guess freezing point - zero celsius.
It's in series for starting so CCA = 660 Amps (at 24V with perhaps an allowed drop by 4V) for about 30 seconds.
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What is CCA anyway and for how long can you pull that current?
According to this page:-
www.chemie.de/lexikon/e/Car_battery/
"the amount of current a battery can provide at 0F (minus18C). The rating is defined as the amperage a lead-acid battery at that temperature can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery). "
Not sure how much of a modern car's electronics will still function at 7.2V....
Edited by SpamCan61 {P} on 05/01/2009 at 15:45
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Not sure how much of a modern car's electronics will still function at 7.2V....
Quite a lot, most of the automotive stuff I've looked at has very onerous design specs, having to cope with low voltages, 100V spikes both pos and negative as well as a wide input operating range.
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