72V system - mixing batteries okay?

Question

I'm a noob but want to begin designing an EV motorcycle. I know the very basics at this stage.

To get better range, I want to use some larger 12V batteries along with lighter ones. The voltage will be regulated with a suitable controller (Alltrax 7245).

So I'm proposing:
12V 27Ah x4
12V 51Ah x2

But I've heard that batteries should be matched and have the same rating. Aside from recharging hassles, are there any other reasons?
The batteries I'd like to use are Hawker SBS30 and SBS60 absorbed glass mat types.

Thanks!

Answer 1

if it is a 72V system and you have 6 X 12 V batteries
that means that you are running them in series.

you should only use the same capacity batteries when you do that, they should be of the same age and never used outside that set.

the reason is that,
when you discharge a battery set the higher capacity batteries will have a substantial charge when the smaller ones are completely dead, potentially causing charge to reverse on the smaller batteries, and this makes them very hard to fix, and not really possible if you are just using a standard charger.
also it is better to know when your batteries are losing charge because taking a battery down to 10V is fine, taking one to 0 V will damage to it. and if some of your batteries are larger than the others, then you might not notice what you are doing to the smaller ones before you have messed them up.

also if you have mis matched batteries you can not charge them on series, and having to rewire the batteries to charge them would be really annoying.

use a very well matched set of batteries when using them in series.

what were you hoping to gain by having different sizes ?

Answer 2

It depends how much current you intend to draw. The 51 Ah batteries will be able to supply twice as much current as the smaller ones, but you must ensure that the current is limited to the supply capacity of the 27 Ah batteries. As for charging, are you using a switch to throw all the batteries in parallel and charging at 12 V, or charging in series at 72V? If the latter, there may be issues regarding different cell capacities.

It sounds an exciting project. Good luck with it!

Answer 3

The way you have planned all the batteries need to be in series. And in such a case, it is better to choose batteries of the same type (with similar range and internal resistance) so that you don't have problems of some getting overcharged and some getting over-discharged and thus affecting the others.

Answer 4

Batteries should be the same ratings and characteristics as their internal resistance will vary otherwise.