Understanding how much power electrical appliances use, and how long your battery will last can be quite frustrating if you’ve never been shown before. Like everything though, once its been explained its really simple.
In this post, we take a look at how long a 100aH battery will last, depending on what you are running, what type of battery you have and how flat you run the battery.
How big is a 100aH battery?
100 amp hours is a fairly typical battery size when it comes to the camping, travel and RV industries. If you haven’t got a 100 amp hour battery, you’ll have something close, or a multiple of it. 200 amp hour and 300 amp hour battery sizes are extremely common.
100 amp hours on a 12V battery is generally around 1250 – 1400 watt hours. You can either do your calculations in amp hours, or in watt hours; both work, and some people prefer one way or the other.
If you want to know how long will a 100ah battery run a fridge, simply work out its usable capacity, and divide the fridge consumption by it, and you’ll get your answer (but remember cycle times, as we go into below).
What is the battery type?
Battery type is one of the biggest influencers of how long a battery will last. Traditionally, lead acid or deep cycle batteries (often known as AGM batteries) were the go to for running fridges, lights, water pumps and so on and so forth.
They are still incredibly popular, but there is a big shift towards 12v Lithium Battery technology.
Lead acid batteries are able to be discharged, but the more you discharge them each time, the less number of cycles that you will get.
For example, if you discharge it 30% (leaving 70% state of charge, or 70 amp hours remaining) you might get 1500 cycles. If you discharge it 50% (leaving 50% SOC) you might get 700 cycles. If you discharge it 70% (leaving 30% SOC) you might only get 350 cycles.
At some point in time you have to pick the right balance for yourself; but don’t discharge them too much, too often or you’ll do permanent damage.
Lithium batteries on the other hand can be discharged much more, and you can use the full 100%. Again, by limiting how much you discharge them their lifespan will increase, and the ideal range is generally between 10% and 90% state of charge. This gives you 80 amp hours of usable capacity for a 100 amp hour lithium battery.
A good guideline for lead acid batteries is for every 100 amp hours of battery, your usable capacity is somewhere between 50 and 70. If you have a 100 amp hour lithium though, you’ll get at least 80 amp hours of use, and even more if you choose.
How much are you consuming?
From there, its just a simple maths equation. If you are drawing 5 amps, you divide the usable amp hours by the consumption, and you have the number of hours that your battery will last.
Drawing 5 amps, you get the following
50 amp hours = 10 hours
60 amp hours = 12 hours
70 amp hours = 14 hours
80 amp hours = 16 hours
What you need to know though, is appliances don’t generally draw their rate of consumption all the time. They will cycle on and off, and a 12V fridge is a typical example of this. It might spent 1/3 or 2/3 of its time not running, and then your calculations change again.
If a fridge drew 3 amp hours 2/3 of the time, you’d get just over 25 hours run time from 50 hours of usable capacity. This is an important consideration for things like water pumps and lights, that you might not use often and for long, but they still draw power!
Ideally, your usable battery capacity is 2 – 3 days without any charge. That means if you used 50 amp hours a day, you’d need 100 – 150 amp hours of usable battery capacity.
This will cover you for times when you aren’t driving and charging batteries from the alternator, or when its cloudy and your solar does not perform well at all.
How much power can you draw?
If you had 50 amp hours of usable capacity, you could effectively draw 50 amps, for one hour. That might be an induction cooktop running through an inverter at 600W.
However, batteries also have a rated discharge capacity, and you’d be wise not to exceed this. Drawing high current (30 amp plus) from a lead acid battery can often lead to its early demise, whereas lithium batteries will take much higher current draw all day long. Simply look at the ratings for the battery you have, and stick to them!
What size solar panel to charge 100aH Battery?
You can make a 100 amp hour battery last almost indefinitely if its sized correctly (until it dies of old age) if you have a suitable power draw, and the right solar array to re-charge it during the day.
If you want to be completely safe, fit 2 watts of solar for every amp hour of battery that you have. That means for a 100 amp hour battery, you should fit 200W of solar.
Realistically you can get away with less than this, and somewhere around the 1.3 – 1.6 watts per amp hour is generally enough, and it does depend on your battery type, and usable capacity.
To make it even more simple, you just need to replenish your consumption each day within about 4 hours. I say 4, to give you some tolerance for when its cloudy, or you use a bit more.
For example, if you consume 30 amp hours, you need to be able to replenish 7.5 amps per hour for 4 hours to top it back up again. That’s roughly a 200W panel, and having some additional solar is never a bad thing!
For more information, check out our post on Solar Panels for Camping.
It all needs to match together
When you design a 12V electrical system, you need to make sure your battery bank size matches your power requirements, and that your charging setup and solar power integrates in well too.
Get your battery size wrong, your power consumption wrong, or your solar panel size wrong and you will have issues keeping things running, and that can be seriously frustrating.
In our experience oversizing things is well worth doing for the peace of mind that you get from it.
A 120aH battery is a pretty common size, and suits a lot of different applications. What do you run from your batteries, and how do you charge them?