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Are you damaging your 12V batteries?

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! There’s lots of ways you can save money when camping and 4WDing, and this is just one of them. For some more, check out 12 ways to wear your 4WD out faster.

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. If you are wondering how low you can discharge a deep cycle battery, read on.

Amaron starting motor

Our Amaron Dmax starting motor

When is a 12V 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 (or cycles) it lasts for.

Sure, some fridges have adjustable voltage cut offs, but at the end of the day if you aren’t aware of the state of charge of your battery you are running a risk of shortening the battery life.

Deep cycle battery state of charge

Deep cycle battery State of Charge. This is a guide only, and will vary from battery to battery (refers to floating voltage)

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 deep cycle battery 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.

Please remember that this refers to the floating voltage after nothing has been drawn or added for at least half an hour. If you draw a lot of power the voltage will drop down significantly and then re-bound in time.

Also, you need to make sure that you are charging your batteries properly, to get the maximum life out of them. AGM battery voltages can be different to a traditional lead acid, or calcium battery and they are most certainly very different to Lithium battery voltages (and these also have a very different amount of usable power).

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 over 24 hours, 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.

Who’s guilty?

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?!

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13 comments… add one
  • Ziggy February 22, 2017, 3:12 AM

    Getting them properly charged from an alternator can be a problem. One industry guy says most never get fully charged.

    Bulk charging requires 14.6 v and more. How many alternators can deliver that? Let alone the volts and time to go beyond absorption.

    We struggled for two years with a camper trailer system that didn’t deliver – relying on the alternator and a smart isolator. It yielded 19 amps at the Anderson (good) and 13.7 v at the batteries (not good). Finally sorted it by installing a DC-DC charger that could actually bulk charge.

  • Aaron at 4WDing Australia February 22, 2017, 5:43 PM

    Hey Ziggy,

    You make a great point; another one that many people don’t know. The alternative of course, is to have solar panels and a decent regulator; should do a similar job I believe?


  • Ziggy February 22, 2017, 10:59 PM

    That’s right.
    Again, you’ve gotta do the numbers to make sure the amps in match the amps out.

  • Crocodile July 9, 2020, 1:59 PM

    The article is a little bit misleading. Talk in energy delivered rather than number of cycles. Discharging a battery to 40% rather than 80% simply requires double the number of recharge cycles so you’re no better off. Discharge a battery by 40% 1000 times or 80% 500 times delivers the same amount of energy over the life of the battery.

  • Aaron Schubert July 13, 2020, 6:23 PM

    Hey Crocodile,

    Interesting comment, and its not without merit. I guess its all about balancing the duration the battery lasts for against its cycles. Halving the discharge will make the battery last for a longer period of time before they naturally die. In your example discharging to 40% once a day gives you 3 years of life, and 80% gives you half of that. Each to their own

    All the best

  • Joe September 2, 2020, 8:35 AM

    I’m trying to find information on battery size vs solar size vs amount used.

    I figured I’d go bigger batteries, and set up an inverter to cut off power when batteries hit 70% discharge.

    I’m unclear on solar size vs battery size.

    Like if I have 800w of solar panel. And a 520ah 12v battery set up, obviously the solar isn’t enough to recharge the batteries on a big discharge. But it is enough if I’m only discharging to 70% per night?

    My idea is to get bigger batteries, so there is less discharge and therefore longer life? Am I completely misunderstanding something?


  • Aaron Schubert September 2, 2020, 7:33 PM

    Hi Joe,

    Ideally you want your solar to top up your batteries in terms of how much you’ve used by about 11 – 12PM (so bank on 3 – 4 hours of good sunlight). On top of this, if you can go two to three full days without charge and still have a healthy battery (IE over 50% state of charge) you have a pretty good system.

    The less you discharge the batteries the longer they will live, but bigger batteries are also heavier and take up more space. At those sort of figures, I’d be inclined to look at Lithium options, which will give you the best of both worlds (and are actually pretty economical in terms of overall lifespan).

    All the best

  • Deb October 18, 2020, 4:50 PM

    Hi, I’m pretty new to all this and have just found that my second 120amp agm battery is flat – not driving my car much due to Covid has resulted in inadequate daily charging I’m

    What are my options to charge my second battery better? Adding solar to my van roof? ( have 200 watt moveable panels when camping) trickle charger???


  • Aaron Schubert October 18, 2020, 6:41 PM

    Hi Deb,

    Solar is a great option, but I would start with keeping an eye on the voltages, and if it gets low top them up with a portable panel.

    Do you have a way of isolating the power completely? If so, you should be able to put the batteries into a state where they hardly discharge – they will take months to flatten normally, so something must be consuming power.

    You can get 240V smart chargers that can stay connected 24/7 too, and they will just top it up as required, or some people fit very small solar panels (15 – 30W) which will do the same thing.

    Either way, start with the battery voltages, where they are isolated and then speak to an auto electrician about a solar panel or get a 240V smart charger

    All the best

  • Crocodile November 10, 2020, 9:28 AM

    Thanks Aaron,

    Re your response,

    “Interesting comment, and its not without merit. I guess its all about balancing the duration the battery lasts for against its cycles. Halving the discharge will make the battery last for a longer period of time before they naturally die. In your example discharging to 40% once a day gives you 3 years of life, and 80% gives you half of that. Each to their own”

    I think you missed my point. What you say is valid only if one doubles their load size. My point is that under the same load conditions it doesn’t matter if the cycle is 30, 40 or even 80%. Discharging to 80% rather than 40% takes twice the amount of time under equivalent loads. Discharging to 40% and charging every day makes no difference to discharging to 80% and charging every second day. That much is evident by observing an agm DoD / cycle chart. This one from Fullriver.

  • Aaron Schubert November 10, 2020, 6:49 PM

    Hi Crocodile,

    I understand what you are saying, but don’t agree with it. The graph you posted, and that of many others from various battery manufacturers are not linear. If you discharge 40%, you will not get exactly half of that by discharging 80%. It’s usually less than that, by up to 30% or so, depending on how far you discharge them. 40 to 80% is about 13% less value, and 20% to 80% is about 20% less value. Obviously this will vary depending on what battery is being investigated.

    Absolutely there is some correlation, and for the minor difference perhaps its not worth worrying about, but if you want the best value out of your batteries, you shouldn’t discharge them heavily, and most certainly not down below 20% state of charge or risk a dramatic lifetime reduction.

    There is a reason that most battery manufacturers give a recommended depth of discharge.

    All the best

  • Peter March 27, 2021, 5:15 PM

    I think manufacturers are being misleading when they advertise a 120 Amp hour battery which can only safely deliver 60 Amp hours – that would be like an airline saying yes we can fly you to Cairns but you have to get off the plane in Brisbane otherwise it shortens the life of the plane…. a better system would be to state the number of full discharge cycles and the number of cycles the battery is capable of at full discharge. That way you could categorically known how long a battery will last – the current system is a joke.

  • Aaron Schubert March 27, 2021, 7:14 PM

    Hey Peter,

    If you look up the technical specifications for each battery, many of them do exactly this. The lower you discharge them, the less cycles you get. In theory though, you could use the full 120 amp hours from a 120aH battery, but it’d not be good for the battery. I’m not sure at what point you call safe though, as they still work, just have a shorter life. It is a bit of smoke and mirrors though, like roof rack load ratings, and towing capacities, and heaps of other things in the 4WD world.

    All the best

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