All In One 24v battery configuration

[Russ]

On a 24 volt system can you only have 2 batteries? I know the batteries are put in series but can you add more since the 2 batteries are in series is 24v?
can you have 3 batteries or would you have 4 batteries with 2 in parallel for 12 and the other set in parallel the combine both sets in series to equal 24v
Being able to expand is what I was wondering about?

Also, if I decided to sell a 12v or 24v unit would it make much of a difference in resale if they had a PWM instead of MPPT. Reason being with a small sytem I have learned here that a MPPT will not really do much more than a PWM?

Thanks again guys

 

[gnubie]

When you put batteries in series its really important that all the batteries are very similar in paper and actual measured specifications and in a similar state of charge. If one battery has significantly different capacity or charge to another in the series set you can find yourself in the situation where, if you don't have a BMS to watch over things, one battery is grossly over discharged or worse pushed into reverse charge. If you have a BMS it will prevent batteries from being over discharged and damaged, but at the loss of accessible capacity. The BMS could also watch the voltages on the batteries to ensure they keep in sync. I have seen mismatched 12v batteries under charge where one battery had about 12v across it and the other 15. Definitely not desirable.

If you had 3 identical batteries and put them in series of (2 x parrallel)(3rd battery) that would be bad.

You can do up various series and parallel combinations to expand your battery capacity. If you have 4 identical batteries for a 24v system you arrange as 2 parallel sets of 2 batteries in series.

MPPT vs PWM comes down to how many panels you have, the array voltage compared to the battery voltage and the overall battery capacity. If you have a lot of battery capacity a MPPT charger + array with higher output voltage would be the direction I'd rather go in.

 

[Russ]

On a 24 volt system can you only have 2 batteries? I know the batteries are put in series but can you add more since the 2 batteries are in series is 24v?
can you have 3 batteries or would you have 4 batteries with 2 in parallel for 12 and the other set in parallel the combine both sets in series to equal 24v
Being able to expand is what I was wondering about?

Also, if I decided to sell a 12v or 24v unit would it make much of a difference in resale if they had a PWM instead of MPPT. Reason being with a small sytem I have learned here that a MPPT will not really do much more than a PWM?

Thanks again guys

The batteries would be identical. I also have learned here a PWM is really as good as a MPPT on small systems of a couple of batteries or panels or a system slightly larger. If I sold the unit to someone it would still be a small system for them and I just wondered if it having a PWM instead of a MPPT would make it harder to sell. They could be like I was " all I knew was you have to have a MPPT until I learned it is not necessary and PWM is fine on small systems.
I will say even though I knew a couple of the answers to the batteries it is always nice for someone to reintegrate those answers in my head!
Thanks !

 

[Donald Siegel]

I bought a 2-position marine battery switch instead of trying to parallel new batteries to my lifepo4 24 v battery. When one battery runs low flip the switch to the full one, and avoid battery mismatch complications.

 

[Chefwhowa]

Question is good to put a fuse on my solar panel wire going to the all in one unit? Or I don't need to worry about putting one on.

 

[gnubie]

Think of it this way. Your all-in-one (AIO) is doing it's best to pull as much power from the array as it can. As part of this MPPT controllers run the panel voltage up and down to find the optimal point by drawing increasing amounts of current, often approaching the short circuit current of the array in the process.

In a system where the short circuit current of the array does not exceed the ratings of the AIO there is no fuse that you can use that will blow and also allow your system to produce the maximum amount of power from the panels. The panels, the wiring back to the AIO should be rated to handle the short circuit current of the system as a matter of course. The AIO itself should be able to limit the current flowing into it as part of normal operations.

So no need for a fuse. You should have a properly rated isolation switch to let you safely disconnect the panels from the AIO. Pulling the plug is a last resort as it will likely result in arcing which will damage the connectors for the brief time the arc would persist as the plug and socket separate.

I know there are people reading this that are suspicious of the possibility of low voltage arcing, so here's a youtube clip for background.