Batteries

There are a lot of ways to recharge your batteries, and how you choose to do that is affected by how you want to live and it's practicality. Most RVs are designed to be plugged in at a campsite which lets them run most of their equipment off "shore power" instead of their batteries - with that shore power constantly trickle charging their batteries. If you don't want to depend on shore power, and you want to be as off-grid as possible, then you need your own way to constantly recharge your batteries to replace what amps you've used.

Firstly, the amount of energy stored in a battery is rated in Amps or Amp Hours. An amp hour is simply a measure of how long a battery can provide one amp of power per hour. Now most devices we use don't draw exactly one amp of power, but you can think of one amp as one "unit", and the amp hour is how many units of energy it holds. Amps and amp hours are essentially the same; A 70Amp battery holds 70 amps worth of electricity, or can provide one amp of electricity for 70 hours.

There is a rating on the battery called "reserve capacity", which is the number of minutes a fully charged battery can sustain a designated constant load before it is fully discharged. Here is a picture of a AGM battery, rated at 70Amp Hours, and it's reserve capacity is 140 minutes at 25amps. So if I was drawing a constant 25amps, the battery could sustain that for 140 minutes.

 The most important things to know when you're shopping for batteries are it's Amp Hour rating, and it's voltage. Voltage is like the flow rate of electricity/current inside the battery, and it's "flow rate" if combined with another battery. The battery above is a 70Amp battery at 12Volts. Now if one battery is not large enough to supply your electrical needs, then you can wire it together to another battery (or multiple other batteries). How you wire them together could either increase the voltage or increase the amperage, but not both (we'll get to this later). If you wire batteries together, then they need to be the same type of battery - as in AGM to AGM, Lead-Acid to Lead-Acid, etc. You cannot have different types of batteries together because they charge at different rates, their lifetime usage is at different rates, some need to be vented and some don't, and their daily care requirements are different. A lead-acid battery is the same type as what's in your car, and are the cheapest type of battery, but they release gas as they charge and need to be vented outside and require you to add distilled water to the cells inside as the water in the acid mixture evaporates over time. AGM (absorbed glass mat) batteries require no additional water to be added as they utilize a gel like substance and also last longer than lead-acid batteries, but they're more expensive.

Now this battery above at 70Amps may contain 70Amps, but you never want to discharge a battery to 0% because it damages it's ability to store current. When you're discharging the battery past a reasonable amount, the acid mixture in the battery will heat up and the voltage and stored amperage will lower, and it will start to corrode the plates inside the battery because it's voltage has lowered. This hurts the battery's ability to conduct current because the plates are now covered/oxidized, and it essentially lowers it's amperage capabilities.

Different types of batteries each have their own rating for how far you can discharge them - and usually the more expensive battery can be discharged farther than others. AGM batteries can usually be discharged down to 20%, Lead-Acid down to 60%, and Lithium-Ion batteries to almost 0%. The absolute best batteries you can buy are Lithium batteries because of their discharge rate and very long lifetime rate (at least 10 years), but they're almost x3 the price of an AGM battery at less storage capacity. Most people use AGM batteries because they're an ideal "middle ground", but Lead-Acid are cheap to replace and with a little care can last very long as well (or can even be re-conditioned). Some people even make their own, if you're courageous enough!

 

Wiring Batteries in Series vs Parallel:

 So let's say that one battery's capacity isn't large enough for your daily usage, so you want to add another battery to increase your storage capacity. Or perhaps you have equipment that runs on 24Volts and you only have a 12V battery; How you wire them together will either increase the storage capacity (parallel wiring adds the amp hours for each battery together and keeps the voltage the same) or it will increase the voltage (series wiring adds the voltage together but keeps the amperage amount the same). Here on the left is a diagram of series wiring and parallel wiring on the right:

The reason that series wiring does not also increase the amp capacity, even though they're wired together, is because of that dual polarity wiring - the positive to the negative; In a battery the current flows through the direction of the positive terminal, so each battery is essentially isolated from each other in storage capacity because they can't equalize the amperage (positive to negative stops that flow) but they equalize the voltage between them - doubling your voltage ability. This design is used for people who wire 6V golf cart batteries together to get a 12V system. Now you can actually have a system that utilizes both series and parallel, where each set of batteries is wired parallel to mimic "one large battery", and then wire those together;

 

For most circumstances, people buy at least two 12V batteries and wire them together in parallel for higher capacity - but you have your options depending on what type of system you want.

Battery Bank Size:

Determining how many amps you think you would use per day is the most useful step to sizing your battery bank. Although batteries are rated in Amps, most appliances that draw a small amount of power are rated in Watts: Watts is more of a universal unit of energy for small amounts, similar to milliampere(1/1000 of an amp), that could be applied to different voltage systems. Wattage is almost always rated per hour, so for example my hairdryer uses 1875Watts per hour. To convert Watts back to amps, so you can better see how much power it uses, is this equation:

Amps X Volts = Watts

The opposite / inverse action of multiplication is division, which is what we do to convert to Amps if you're starting with a Wattage rating. If amps x volts determines watts, then we can divide the Watts by the voltage and it will convert that wattage to amps.

Watts / Volts = Amps

1875Watts / 120V(your common household voltage) = 15.625 Amps per hour

Now usually we don't use a hairdryer for an hour, probably. Whenever you're totaling numbers together or dividing etc, make sure you always go two places past the decimal point. For ease of totaling, you could use 15.62, but don't use 15.6 because that will mess up your numbers! That's important - take it from me.

So we know it's 15.62 an hour, but I probably would only use it for 15 minutes, so let's divide it by 60 (60 minutes in an hour) to find out how much it uses per minute:

15.62 / 60 = 0.26amps per min

0.26 x 15minutes = 3.9Amps for 15 minutes.

So my hairdryer could double as a space heater with how much it draws, but you get the idea. Having your appliances before hand is certainly helpful, but I realize that's difficult for a lot of people to store your items long term while you build. The first thing I did was make a list of any electrical thing I use everyday. Include all the items that you want and separate them into groups; What do I absolutely need, what would I really like to take to keep my lifestyle of choice, and what are extra creature comforts that I could do without if I had to. 

Any piece of equipment that has a heating element, where most of the electricity is used to heat up metal coils like a hairdryer for example, uses a lot of power and should be heavily reconsidered if you need it or not. As mentioned above, my hairdryer is 1875Watts per hour - that's more than a microwave! This was important to me because I'm 100% off-grid/boondocking, but if you have items like this that are part of your lifestyle then you'll just have to compensate for this amount of draw. I like to drink coffee, so the heating element used in a coffee maker is something I'm willing to spend energy on verses the time and techniques needed for a french press. I have shorter hair, so I'm probably not going to bring a hairdryer, etc. An electric cooktop pulls a huge amount of electricity with it's heating elements, so I chose to use a propane gas cooktop in case I didn't have enough sunlight to charge the batteries, etc. Your design can be anything you want, but remember the equipment's power draw and how you plan to realistically re-charge your bank on a daily basis.

I always think "It may be nice to have this, but how often am I going to use this? Can I afford that type of electrical draw?" and it's a downer sometimes but efficiency is key here.

Once you have a general idea of how much power you think you'll be using, then you can better see how big your battery bank needs to be.

Charging Batteries:

The most common ways to recharge your batteries that I'm aware are:

• Connecting your battery bank to your truck's alternator system to charge them while you drive
• A gasoline, diesel, or propane powered generator
• Solar Panels
• Connecting to shore power and running your loads on shore while it charges your batteries
  
• A combination of any of those.

I personally chose to isolate my electrical system from my engine and installed solar panels, and have a backup gas generator if needed. The generator is probably the most common choice, but they're loud and you have to carry gasoline somehow. Some people have a connection from their fuel tank to their generator compartment for this, or even have a propane generator with it's own line to their tanks - it's up to you and mostly based on cost and practicality. If you're planning to do a stealth build then a generator isn't a realistic way to charge them when you're in the city.

Whatever source you choose to provide power will have to go through some type of regulating device to make sure it charges the batteries at the correct amperage and voltage. A charge controller is used for solar, a DC to DC converter is used for alternator systems, some inverters allow you to connect an extension cord from a generator or normal 120V home power straight to your batteries and converts it, etc.