How many 12v car batteries or which batteries do I need to run a 150w LED light 18 hours a day?


Joined Jan 8, 2017
Is that on the assumption that they get fully charged in the remaining 6 hours ?
You would need 225 amper hours capacity in theory but it is bad practice to fully discharge a lead acid battery. So I would aim for about twice that rating. Leasure batteries are probably a better choice for that kind of service.



Joined Feb 8, 2018
You need 2700 watt·hours, deliverable without over-discharging the battery. At 12 volts nominal, that is 225 ampere hours, which is a big battery. If the actual LEDs are 150 watts, then the input power to deliver that power will probably be at least 10% higher unless a very efficient driver circuit is used. 95% efficiency is not totally unrealistic, but would require a very good design.

Unless the LEDs are specifically made for 12 volt input, some means of power conversion will be required. Efficiency of power conversion is likely to be higher for higher input voltage. Lots of industrial battery systems are nominally 24 volts. The currents are half what they would be at 12 volts, which can sometimes result in substantial savings just in wiring costs. 48 volts is getting up into the realm of somewhat dangerous from a shock hazard point of view (you aren't likely to be killed by 48 VDC, but you can be burned or injured).

You need to consider operating temperature. If the batteries will be very cold, their capacity will be greatly reduced. It is even worse if the charge voltage is not temperature compensated. In cold environments, a discharged battery can freeze and be completely destroyed.

Batteries for industrial use are usually well documented with performance curves at various temperatures and the like. You may find it difficult to get much that is helpful for "consumer" batteries.