These days, 12V batteries are the heart of many camping setups. Whether you are using a dual battery system in the back of a 4WD, or you have batteries in your camper trailer, caravan or RV, they are used everywhere for running a heap of different appliances. A good 12V system makes camping much more comfortable, but they aren't cheap to setup and install.
I'm always on a mission to help people travel easier, cheaper and better, and one of my pet frustrations is seeing people damaging their 12V batteries through a lack of understanding. Batteries cost a lot of money, and if more people understood what they were doing was expensive and a waste, they'd think twice!
I'm a long way off knowing everything there is to know about 12V systems, but I do know people run their 12V batteries too flat, and damage them in the process.
When is a battery flat?
When would you call a battery flat? When your fridge stops running? When your lights turn off? By the time your fridge or lights turn off your battery is pretty much dead. The problem with this is there's a direct link between how much you flatten your battery, and how many years it lasts for.
Battery state of charge
Your 12V batteries have a state of charge; when they are completely full they are at 100%, and when they are dead as a doornail, they are at 0%. This is where many people go wrong. For most deep cycle batteries, if you want them to last a long time, you should never go below 50% state of charge. Say what?!
Yep - most deep cycle batteries have a lifespan of about 1500 - 2000 cycles when you discharge them only 30% (so 70% state of charge). If you cycle it down to 50% state of charge, you get roughly half as many cycles before the battery needs replacing. Go lower than that and your lifespan drops even further.
Ideally then, you don't run your battery below 70% state of charge, but this is impractical. If you had a 200 amp hour battery system, you could only use 60 amp hours, which is ridiculous. It becomes a balance between having enough battery capacity and not wrecking what you have. It's not practical to carry a tonne of batteries, but at the same time not economical to quarter the life of your batteries. You've got to find the balance for yourself.
What does this mean in terms of usable power?
If you have a 100 amp hour battery, it means you should only be using 30 - 50 amp hours before charging the battery again. Most fridge/freezers use in between 20 and 120 amp hours a day. Next time someone tells you their battery runs a fridge and lights for 3 days without charging it, let them know they are damaging their batteries (unless they have a massive power bank). It's simply not possible any other way.
Our 80 Series has a 105 amp hour battery, and 66 amp hour cranking battery (with probably 15 usable amp hours before the isolator kicks in). That gives us a maximum capacity of 52 + 15 amp hours. Our Evakool draws around 25 - 30 amp hours a day, giving us just under two days of 'off the grid' camping. We run a number of solar panels, and are often driving anyway, but having a voltage gauge has been a great addition.
State of charge and safe discharge rates vary
Not every battery is the same; every one will have a different number of cycle rates for various state of charges. Take a minute to find out what your safe discharge rates are, and how many cycles you will get out of it. AGM vs lead acid and calcium deep cycles are not the same.
When does a 12V fridge cut out?
12V Fridges will cut out anywhere from 12.5 volts to 10.1 volts, depending on the fridge model. Some of them are adjustable, but in general if you are allowing your fridge to cut out you are reducing the batteries lifespan.
Running different batteries in the same system
Another very easy way to damage your batteries is to run different batteries in the same system. Lead acid deep cycle batteries, AGM, Gel and Calcium all require different charging rates, and if you are charging several different batteries at the same voltage, you will do damage to them very quickly. It's not a good idea to mix and match different battery types, but many people do.
This is just a basic guide; look into it further.
The bottom line is this; run your 12V system however you want to, as long as you understand the consequences. If you still choose to run your batteries down to 20% state of charge that's fine; its your call and I'm not going to get in the way of it. The purpose of this post is to help those who don't understand what they are doing is damaging their batteries, and give them the knowledge to change things.
I've made this quite basic too; realistically its more complicated when you look at Watts, cable sizes, current draw as the battery voltage drops etc. However, for the purpose of a basic guide, I will leave it at that; its easy to get confused with too much information too quickly.
Righto, who's run their battery system down too far? I'll put my hand up first; on our 5 weeks in the Kimberley our second battery started playing up, and unknown to me it was a normal lead acid battery with a very low amp hour rating. Every morning our fridge was dead, until we got back into Broome and replaced it with a new deep cycle.
So, have you run your battery flatter than you should have?!