Math and Physics, love them, or hate them they make the world go ‘round, things in the Overlanding world are no different. In today’s modern world power needs are more important than ever and a basic understanding of a few terms and some basic math will help anyone with designing a power solution that will work best for them.
Terms: First off there are a few terms you will see as related to power on modern electronics. For the basis of simplicity this article is meant only to “get your feet wet” and will cover the broad strokes and equations to help you fulfill your power needs.
A (Amps) = Unit of electrical current
Ah (Amp Hours) = Amount of energy charge in a battery that will allow one Amp of current to flow for one hour
V (Volts) = Unit of electromotive force (this article will focus on 12V D/C only)
W (Watts) = Simply put that amount of A (Amps) X V (Volts) = W (Watts)
Now let’s dive into some actual examples of power usage for someone looking to travel by vehicle, these figures are simply based off the manufacture’s listed specifications but should give us a good starting point for your power needs. I’m arbitrarily picking a 12v Overlanding Fridge that is quite common today but you should tailor a list to your specific equipment and power consumption needs. Amp draws are listed by most every manufacturer for anything that consumes electricity.
ARB fridge 50qt. 0.87 A/hr
That means if you’re planning to run your 50qt fridge for 2 days without any added power you will need:
0.87 A/hr X 48 Hrs = 41.76 Ah of battery power
What does that mean for a battery? First off it means you should NOT use your standard “starting” car battery since a starting battery is not made to draw down deeply, basically it should always remain close to full capacity failure to heed this warning will leave your at best needing a jump start, at worst needing a whole new battery. A deep cycle battery, one that is designed to be deeply discharged, will not be harmed by discharging to 50% as that is its designed usage. A second and more modern option would be a LiFePo battery which can be discharged down to 0% without undue harm to the battery. Quite simply this means a 40 Ah LiFePo battery has the same usable capacity as an 80 Ah deep cycle. Simple enough!
So your two basic choices for batteries are something like a Dometic PLB-40 (LiFePo 40 Ah), or a standard deep cycle battery like an 80 Ah marine battery.
Either choice will power your fridge for the same amount of time given the same parameters. A couple of facts about the batteries, the Dometic PLB-40 has built in DC/DC charging capability as well as a MPPT solar charge controller and on board power monitoring, a standard deep cycle marine battery will require you add those chargers and monitoring to it. Also a LiFePo battery will be about 75% lighter in weight than a comparable AGM or Lead Acid battery.
So how long will your 50 Qt. fridge run with that batteries we discussed?
40 Ah of usable battery / 0.87 Ah battery consumption = ~ 46 hrs under ideal circumstances.
So what if you want to run for longer than 46 hours? Well you’ll need to either start your vehicle and allow DC current to flow from the Alternator of your vehicle, through your charger into your battery to replenish what you’ve depleted, fire up a gas generator that can output Voltage through a charger into your battery, or finally and really the best option, add some solar capacity to your vehicle. Time for more math…
In order to simply keep up with the average current (A) draw of your 50 Qt. fridge you would need to add the 0.87 Ah back into the system, since solar doesn’t work at night keep in mind that you can’t do this 24 hours a day perhaps 8hrs/ day is more reasonable for our calculations.
0.87 Ah X 24 Hrs = 20.88 Ah Per Day at 12V or 250W of power per day / 8 hrs = 31.32W
You need to add 31.32W per hour for 8 hours per day to keep your battery from ever draining down during ideal conditions. A 50W or greater solar panel should more than cover those needs. Obviously if you don’t have any sunlight or your panel is producing less than ideal output you will need to upsize. Batteries and chargers also have an Input Max Amperage, in the case of the Dometic that is 8A. 8A X 12V = 96W which is approximately the Maximum Solar panel size you will be able to take advantage of so adding 300W of solar panels would only hurt your wallet not keep your battery any more full as the battery system can only accept so much current at any given time. This can vary between batteries and charging systems.
8A Max input also means that if your battery is at 0% and you want to bring it to 100% you would need to add 40Ah of power at 8A for 5 hours (40Ah / 8A = 5hrs) from either solar or by running your vehicle.
Everything discussed is simply an example to get you started, if you’re planning to run lights, a fan, charge cell phones, or use any other type of electricity while camping figure all those into your calculations. This is meant as a broad overview to get you thinking about power consumption and replenishment in a vehicle based system.