Along with the rooftop solar panel array, our 12v battery bank is going to be our primary source of power. Because of this we need to seriously consider how much energy we, not only consume in an average day, but how much energy we can realistically harness with our solar panels in that same day.
Our main energy needs come from lighting in the evening, our laptops, cell phones, tablets, and watching a movie on the tv before bed. The batteries are also going to supply power to water pumps, fans, and whatever other needs may arise. Because we want our skoolie to be as self-sufficient as possible, we want to oversize our solar array and battery bank, at least as far as our needs are concerned.
The conciseness is that you should never use more than 50% of your battery banks capacity, therefore your battery banks total capacity should be 2x your desired capacity. If you need 100 Amp Hours (AH), than at minimum, your battery bank should be 200AH.
Deep Cycle batteries are rated in AH, for example, an 80AH battery would be able to supply 1 Amp for 80 Hours without charge. Or, 4 Amps for 20 Hours, 4 x 20 = 80.
Lead-Acid type batteries are the most cost efficient. The need to add water periodically is minimal compared to the advantages. (NEVER use anything but distilled water) You want genuine “Deep Cycle” batteries, not the automotive type designed to start a car.
AGM Deep-Cycle Sealed batteries typically last 4-10 Years, while marine deep cycle batteries usually only last 1-6 years on average. Your typical 6v golf cart battery lasts anywhere from 2-8 years, for comparison.
Good batteries are expensive, but are worth the investment if you can afford them. I think we’ll be going with 6 of these guys to start off with, 12V 100AH AGM Deep Cycle Battery. They aren’t the most technologically advanced, but they are time tested. We expect our entire electrical system to be the single most expensive aspect of getting off the grid. But once in place, the system will provide more energy than we need.
Do NOT mix battery types. It’s best if your batteries are all the same size and type, by the same manufacturer, and even the same age if you are able to.
With such a high amount of energy flowing through these wires, it is important to use the correct size of wire. For example, we will be using 2 Gauge Wire in Red, and Black, for our positive and negative lines, respectfully.
When using 6v batteries, it is important to remember to wire them in Series, NOT Parallel, as you would for 12v batteries
Or if you wanted to go BIGGER! (For increased Capacity, NOT VOLTAGE)
The life of your batteries will be longer and happier if you charge them correctly. The best chargers on the market are 3-stage chargers. Use of a good quality 3 stage charger will significantly improve your battery’s performance and lifespan. These chargers can be purchased separately or are included as part of many of the better quality inverters. When using a 3 stage charger, battery charging takes place in 3 basic stages: Bulk, Absorption, and Float.
Bulk Charge – The first stage of 3-stage battery charging. Current is sent to batteries at the maximum safe rate they will accept until voltage rises to near (80-90%) full charge level. Voltages at this stage typically range from 10.5 volts to 15 volts. There is no “correct” voltage for bulk charging, but there may be limits on the maximum current that the battery and/or wiring can take.
Absorption Charge – The 2nd stage of 3-stage battery charging. Voltage remains constant and current gradually tapers off as internal resistance increases during charging. It is during this stage that the charger puts out maximum voltage. Voltages at this stage are typically around 14.2 to 15.5 volts.
Float Charge – The 3rd stage of 3-stage battery charging. After batteries reach full charge, charging voltage is reduced to a lower level (typically 12.8 to 13.2 volts) to reduce gassing and prolong battery life. This is often referred to as a maintenance or trickle charge, since it’s main purpose is to keep an already charged battery from discharging.
Be sure to check out the 12 Volt Electrical post to learn more about how to setup your 12 volt accessories.