HOW MUCH BATTERY?

[Ron Yason]

I can only get 4x of 150w 12v solar panel at the moment. The question is what is the best size of battery I can put into it?

[Acercanto]

Welcome! Typically you'll want to charge the batteries at a "low C rate". One C is whatever the capacity of your battery is, so if  you have a hypothetical 100 Amp hour battery, you would want to charge it at about .25C, which is somewhere in the 25 amp range.

So with that, we can work backwards from your solar capacity and get a ballpark size for the battery.

With 600 Watts of solar at 12 Volts, that's a maximum of 50 Amps. So if you want to charge your future battery at 50 (max) Amps, you would want something in the 200 Amp hour range. If you went smaller, like 100 Ah, then you'd be charging at .5C, and if you went larger, like 300 Ah, then you'd be charging at .16C

This all depends on your loads, too. If you need 500 Ah to run your heater through the night or whatever, then that's what you need, and your 600 Watts of solar panels may just not be able to fully charge every day.

Hope that helps!

Acercanto

[Dacian Todea]

Ron,

Those panels will put out each 8 to 11A depending on conditions so 32 to 44A thus you will want an absolute minimum battery capacity of 140Ah ideally closer to 200Ah and above that is not a problem is just not economical to have a battery with to large of a capacity.

[Casey]

How important is the "low C rate", and what is the ideal target - 0.3C or less?  My current battery cells are rated for a max constant charge of 1C, but would it deteriorate the batteries prematurely if I actually charged that fast?  I assumed that up to 1C was okay, and I've seen other brands of LFP cells rated for 2-3C or even 0.5C so it doesn't seem universal.

[Acercanto]

The slower you can charge or discharge any battery, the longer it will last. By sizing the battery so that it only charges at <.5C rates, you can double or more the life of the battery.

An example could be those small Lipo batteries that they use in racing drones. They discharge them at like 50C and up! The tradeoff is that they start degrading at like 20 cycles or something.

Given that your solar panels are more or less a fixed charge rate, they need to be able to refill whatever was used during the night, so it's important to have a large enough array.

Depth of Discharge is another factor that greatly affects the life of a battery. If you discharge all the way down to whatever the cutoff is for your chemistry, that's called 100% DoD. Most batteries have a cycle life rating at 100% DoD. The red Headway cells are rated for 1500 cycles at 100% DoD, but if you only discharge them 80% of the way down, that cycle life goes up to 2000! That's a 50% increased lifespan just by leaving 20% "in the tank".

I'm sure I could ramble on and probably not make much more sense, so I'll stop there.

Take care,

Acercanto

[Dacian Todea]

Casey,

For most cases 0.3C should be absolute max but you will likely try to be around 0.2 to 0.25C as max.

There are multiple reasons for my 0.2 to 0.3C max charge rate including practical solar applications where when charging from solar and you have 5+ hours of sun it will not be useful to have a charge rate outside this range but is also to have a long battery life and not very high voltage drops on the cells affecting charge parameters.

Many cells spec 0.5C or 1C as max charge rate for the large cells but those are sort of extreme and to the limit thus the reason they recommend 3.65V max charge voltage to deal with the extra voltage drop due to high charge rate but default SBMS0 parameters are 3.55V as I consider a typical solar system with 0.2 to 0.3C max charge rate.

If you care about battery life 0.3C should be absolute max and if you care more about high speed charging the  you can go to 0.5C or even 1C if you have quality battery capable of that in real life and very well done cell connections.

Discharge rate should also be around 0.6C max continues with short seconds peaks up to 1C if needed.

[Casey]

Thank you, that is excellent information.  I will definitely get my battery bank size increased before scaling up charging & load too much.