You may have some experience already with running out of power while you're out camping, and if that's the case then you probably know how long you can usually go before that happens. But when your battery or batteries are so low that your lights are dim or your water pump won't pump water, you've already gone beyond where you should have stopped draining them.
Draining your lead acid batteries below 50% of their capacity has a negative effect on their lifespan. The farther you go below 50% on a routine basis, the more often you can expect to be buying new batteries. This applies to deep cycle batteries, which are designed to be drawn down quite a bit before being recharged. But if you're using typical starting batteries like that found in your car, you can expect to destroy a battery much, much faster using it as an RV house battery. They aren't designed for this type of use.
Another thing to consider in answering this question is whether or not you have a means of charging your batteries while you're out camping. If you don't, then you need your batteries to last for the duration of the trip, depleting them no more than 50% or so. If you plan to boondock for relatively short periods of time, you might be able to achieve that by merely adding one or more batteries to your existing bank of batteries.
You can figure out how many amp hours of battery storage capacity you need in your bank of batteries by figuring out your typical daily electrical usage (as discussed in Part 2) multiplied by the number of days you want to camp, and doubling that number to keep from depleting below 50%.
Battery bank capacity needed = (typical daily amp hour usage) x (number of days needed) x 2
The above formula applies even if you have a means of recharging your batteries while out camping. In that case your "number of days needed" might be lower. For example, with my system I like to be able to go about 5 days before running down to 50% of battery capacity, assuming no charging during that time. This way if a 5-day storm moves in where there is little sun, I am still ok. I could run into trouble if there's no sun for longer than that, but that's never happened yet.
If I didn't have any way to charge the batteries, I'd want them to last longer - at least a week or maybe two. But that would be difficult to achieve without lugging along a whole slew of batteries. I could probably fit four batteries in the areas of my truck bed not occupied by the camper, but I prefer to use most of that for storage of other items. So by having a solar charging system on the roof, not only can I stay out as long as I want without regard to running low on power, I also have a little extra storage space since I only need two batteries.
I typically use about 25 amp hours daily and want the batteries to last for 5 days. So using the battery bank capacity formula above, 25 x 5 x 2 = 250 amp hours. Well, I only have 220 amp hours in my battery bank, so that really works out to being able to stay out 4.4 days before being depleted to 50% without charging. So I'm a bit short of my 5-day goal, but in reality even on cloudy days there is still some charging taking place. This system has worked great for me as it is, and if I ever do run into a situation where the batteries are getting depleted due to inadequate sun, I wouldn't have a problem conserving electricity until conditions improve.
Before we move on, a warning: Beware, some manufacturers are trying to fool you.
While looking for a new battery recently I discovered that some list the amp hour rating at the 1 amp discharge rate. This is highly deceptive. A deep cycle battery discharged at a 1 amp rate will take longer to discharge (and thereby appear to have a higher amp hour capacity) than if it had been discharged at a higher, more realistic rate. It used to be that amp hour ratings on batteries were always listed at the 20-hour rate, meaning the number of amps of constant discharge to enable that 12v battery to last 20 hours before being discharged to 10.5v. But apparently at least some manufacturers are playing with those numbers now so they can advertise a deceptively higher amp hour rating. The higher the load on a battery, the faster it discharges (disproportionately faster, a phenomenon called the Peukert Effect).
The battery I was looking at recently said it had a 109 amp hour capacity calculated at the 1 amp rate. Theoretically it would last 109 hours at that discharge rate before being completely dead. I decided to plug these numbers into a Peukert Effect calculator to see how many amp hours it would be rated at if it were tested like its competition is - at the 20-hour rate. It turns out that this is really about a 93 amp hour battery when tested that way (with a 4.67 amp load, which takes 20 hours to discharge the battery). The higher the discharge rate, the less efficient the battery becomes because of the Peukert Effect.
If you really planned to run a 1 amp load with this battery, then yes you could call it a 109 amp hour battery. But I don't know of anyone who does that. Most people with RVs probably discharge their batteries at an average rate that is many times higher than that. Regardless, the playing field for battery ratings should be a level one, and clearly it's not when these kinds of shenanigans are going on.
To learn more about the Peukert Effect, see this page. A Peukert Effect calculator is at this page at csgnetwork.com.
Now that you know how long your batteries need to last and how much amp hour capacity they need to have, what kind should you get? That's up next in Part 4, What Type of Batteries Should I Use?
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