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How to stop killing your batteries

Charging your auxiliary batteries is a pretty straightforward exercise, right? The bigger the DC-DC charger you’ve got means your batteries will charge faster every morning once you break camp and start driving, because it gives more charge resulting in less time to charge up, right? What if I told you that was actually killing your batteries? It all comes down to bigger isn’t always better.


What’s of greatest importance is your batteries’ ability to take a charge, or more specifically, your batteries’ technology.



First off, let’s consider starting batteries. They’re built to be able to supply high current; whether starting your engine or powering your winch, starting batteries are designed to handle a high-current draw over a short period of time. This works in reverse, too; starting batteries can take a high-current charge quickly and happily. Your alternator is probably able to deliver in the vicinity of 70-100 amps to your starting battery, and as long as you don’t run it flat, and you keep the water in it topped up, it’ll last you more than a few years.


Auxiliary batteries are different. Most auxiliary batteries are a ‘deep cycle’ design, regardless of the technology inside. These technologies include Absorbed Glass Matt (AGM), Gel, or flooded wet-cell (just like your starting battery) designs. The difference between these and your starting battery is that they’re designed to discharge low power (say to run a fridge) over a long period of time, instead of discharge high power (say to turn an engine over) over a short period of time. And then, of course, there are the newer-tech whizz-bang lithium iron phosphate batteries.


Where this gets interesting is that replacing the power you’ve used (i.e. charging) needs to happen at a similar rate to discharging, so while your starting battery will take just about all the amps you can throw at it, your deep-cycle battery needs to be charged at a slower rate if you don’t want to kill it.


The correct charging rate absolutely depends on what batteries you’ve got. There’s a pretty safe rule of thumb for most lead-acid deep-cycle batteries; work on around 5% of total amp-hour capacity as your minimum charging rate, and 30% as your maximum, with somewhere around 10-20% being about spot on. Using this will help stop you from killing your batteries.


A bit of beer coaster maths will look like this:

100Ah battery = 5A minimum, 30A maximum and 10-20A ‘perfect’ charge current.

200Ah battery = 10A minimum, 60A maximum and 20-40A ‘perfect’ charge current.



What about charging lithium?


Lithium batteries need a specific voltage profile to charge, so some batteries have an included charger, and any good-quality DC-DC charger will also have that charging profile in it too. But how many amps can you push into a lithium battery without killing it? That comes down to each specific manufacturer. One of the greatest things about lithium batteries is that you’re able to draw and charge them as if they weren’t deep cycle, but you can still use them for prolonged deep-cycle duties.



There are some lithium batteries that are claimed to be able to handle a charge rate of up to 100 amps; as quick as you want, really. Any decent lithium battery should come with a tech-spec sheet that will list its maximum charge and discharge current, among other things.


This is an edited version of an article published on

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