The new article on
Battery Power is a great start to explain the basics of planning your DC power, but hopefully some improvements will be made so that it doesn’t perpetuate some of the common misunderstandings about DC power.
http://www.exploroz.com/Vehicle/Electrics/Batteries.asp
“the fridge is on, the camera and laptop chargers run off a 600 watt inverter for 3 hours and the 12 volt lead light is used for 5 hours. Total amp draw is 60 amps.”
- no, the total capacity taken from the battery when these loads are used for this time is 60 amphours. Battery capacity is correctly described in amphours in the earlier paragraph in this article. If you subtract 60 amphours from the battery’s 90 amphour capacity, you know you have 30 amphours left in the battery. Amps is used only to indicate the instantaneous load and doesn’t help you understand how much capacity it takes from the battery. If a starter draws 300 amps for 6 seconds how much has it discharged the battery ? You need to calculate amphours - 6 seconds is 1/600th of an hour - so it has taken 300 x 1/600 = 1/2 amphour.
“------Device------------------------------- Usage (amps per hour)”
“ 12 volt fridge (compressor type)----------2.5-3.0 “
- A fridge draws 5 amps when the compressor is running and 0 amps when it is not. So to give an indication of how much power it draws on average, you could call it 2.5 amps average or 2.5 amphours per hour.
“Wet Cell / Marine Battery / Deep Cycle and Gel Batteries / AGM”
- this suggests you need to choose between Wet Cell OR Marine OR Deep Cycle OR Gel OR AGM.
- the electrolyte storage can be wet OR gel OR flooded and the plate construction can be Starting OR Marine OR Deep Cycle. So if you decide to buy a Wetcell, you can still choose between Starting OR Marine OR Deep Cycle plate construction. AGMs are available as Starting OR Deep Cycle.
“The minimum cable thickness that should be used is 100 amp 8 B&S cable (B&S stands for battery and starting cable).”
- no, 8B&S wire is rated closer to 56 amps. Keep in mind that the current rating for a wire only tells you much current you can put through it - without the insulation melting. If you are concerned about voltage drop, you need to consider the length of wire, as
well as its size.
- no, B&S stands for Brown & Sharp, the name before it became the American standard and was then called AWG. 30B&S wire is .25mm in diameter - only for starting very small motors.
Mike