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

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.

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

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.

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.

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:

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.

Thanks guys. I really appreciate the help.

Found a calculator that confirms it too.

http://www.batterystuff.com/kb/tools/calculator-for-load-specific-run-time-b.html

did you ever come up with a solution for this? i'm trying to do the same thing at my cabin...