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# How long can a 85ah 12v battery power five 8w led light bulbs?

Mon 26 Aug, 2013 11:55 am
If I have a deep cycle battery that is rated at 85ah (24dc group) and have five 8w 12v LED light bulbs, how long could I keep them running ?

Based on my calculations so far it works out like this.
Battery life = Battery Ah / Amps drawn
Amps = Watts / Volts

So in this case each bulb is 8w / 12v = 0.42 amps each
Total of five bulbs would make that 2.1 amps drawn

So battery life being Ah/Amps would be 85 ah / 2.1 amps , comes out to 40.5 hours until completely drained.

Is this correct?
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contrex

1
Mon 26 Aug, 2013 12:50 pm
8/12 is not the same as 0.42. (Can you not see that 8/12 is 2/3?)

Also, if this is a schoolbook exercise, then you can pretend you can discharge the battery to 100%, but if you are planning an installation you should know that real deep cycle batteries are designed to be discharged to no more than 50% to 80%. Their life is shortened by repeated full discharge.

contrex

1
Mon 26 Aug, 2013 01:08 pm
@contrex,
contrex wrote:
that real deep cycle batteries are designed to be discharged to no more than 50% to 80%.

Put another way, you should not go below 20% charge remaining and ideally you should recharge when there is 50% charge left.
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ghostdog662

1
Mon 26 Aug, 2013 01:20 pm
@contrex,
My mistake I was calculating with 5w ones earlier , that's how I arrived at the 0.42. Using 8w would actually be 0.66 amps per bulb (3.3 amps total).

The scenario I am calculating this for is while camping with family. We normally run lights in the evening with propane tanks/lanterns. This is costly to refill, a pain to bring the tanks home/back etc.

What I was proposing was adding fixtures that would be permanent at the camp site and just add the deep cycle battery/lights once we get there.

Normally we are there for 2 nights, operating lanterns from 7pm until 2am maximum, so about 7 hours of use per night.

Using 3.3 amps on an average night would drain about 27% of the battery. I arrived at that number by 85 ah / 3.3 amps drawn = 25.76 hours total life. 7 hours is 27% of 25.76 hours.
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contrex

1
Mon 26 Aug, 2013 02:13 pm
You have 40 W total load; at 12 volts the current is 40/12 amps This is 3.33 amps; time to 100% discharge 85/3.33 = 25.5 hours approx.

If you use 80% of the charge, that gives 0.8 x 25.5 hours which is 20.4 hours.

1 night of 7 hours would use 7/25 (27%) of the charge, and 2 nights would use 54% of the charge, leaving the battery 46% charged (around half, in round numbers)

This is well within duty cycle recommendations for a deep cycle battery.
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timur

2
Mon 26 Aug, 2013 02:35 pm
Good calculations but still theoretical as the discharge ratio is not linear.

In addition, the graph changes depending on the number of charge/discharge cycles, even on deep cycle batteries.

It also depends on temperature.

Example of discharge:

contrex

1
Mon 26 Aug, 2013 03:37 pm
@timur,
timur wrote:
theoretical as the discharge ratio is not linear.

Agreed, but the approximation is good enough to show that the battery will be (around) 50% discharged after (around) 14 hours use with the intended load.
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ghostdog662

1
Tue 27 Aug, 2013 10:52 am
Thanks guys. I really appreciate the help.
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ghostdog662

1
Tue 27 Aug, 2013 04:16 pm
Found a calculator that confirms it too.

phunknown

1
Thu 6 Apr, 2017 05:01 pm
@ghostdog662,
did you ever come up with a solution for this? i'm trying to do the same thing at my cabin...
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