How to stop killing your batteries!

Sometimes the biggest and angriest charger you can get your hands on isn’t always the best, and this simple mistake is killing your batteries by pushing too many amps into them… without you even realising!

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 is the newer-tech whizz-bang lithium iron phosphate (LiFePo4) batteries.

Century Deep Cycle

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.

So what is a safe charging amperage?


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 five per cent of total amp-hour capacity as your minimum charging rate, and 30 per cent as your maximum, with somewhere around 10-20 per cent 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.

For example, if you have a 100Ah battery, a Redarc BCDC1225D (25amp DC-DC charger) would be just about perfect. It will charge up at 25 amps maximum, which is well above the minimum the battery needs to charge at, and five amps below the rule-of-thumb maximum. This capacity charger is safe, will do the job perfectly and keep your batteries in good nick by not jamming too many amps down their throats.

And if you decide to add a second 100Ah battery down the track – making a 200Ah battery bank – the BCDC1225D 25 amp charger will still deliver enough charge (12.5 amps) to be above the minimum charging rate of 10 amps.

Bear in mind, the maths is different for lithium batteries; you’ll see how further down.

So what kills batteries?

One of the biggest killers of batteries is running a whopping big charger and jamming way too many amps into your battery bank. Let’s just say I absolutely need the biggest, angriest, most expensive charger I can get my hands on, right? And so I go and get myself a you-beaut Redarc BCDC1250D to charge my single 100Ah lead-acid based battery. By throwing 50 amps at it, it will charge to full in just two hours (give or take), but it’ll also get hot and most likely damage the plates in the battery. It’s honestly all bad.

Redarc Bcdc1250d

Instead of going and getting the biggest and angriest DC-DC charger you can get your hands on, it makes a lot more sense to get a charger specifically matched to what you need; work on the rule of thumb outlined above. Or, even better, speak to the manufacturer of your battery and to find out exactly what charging current will maximise its life.

If you have already gone and bought a 1250D, and you’ve only got the one 100Ah battery, tell the better half you’ve got Wes’ permission to go buy another two 100Ah batteries to make a 300Ah battery bank. It’ll save the batteries in the long run, and it’ll keep the beers cold for longer! Now that I’ve given you a ‘get-out-of-jail-free-card’, we need to talk about the new kid on the block… lithium.

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 (my Redarc BCDC1240D has, anyway). But how many amps can you push into a lithium battery without killing it? That comes down to each specific manufacturer. For example, the Revolution Power 100Ah LiFePo4 battery will take anywhere from 5-60 amps. And that is one of the greatest things about lithium batteries. 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.

Screen Shot 2020 07 13 At 1.40.27 Pm

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.

Thanks for sticking around to the end; hopefully you’ve now got enough information to stop you killing your batteries!


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