Two battery banks with one charge controller?

Re: Two battery banks with one charge controller?

As long as the charging set points are the same there isn't much problem with switching between banks like this.

However, from someone who has tried this, you will soon discover it is a pain in the anatomy to do so and all-too-easy to forget to flip the switch. Then you will start dreaming of automation; using the charge controller's AUX function to tie in the second bank during Float to keep it up or to activate the AC charger connected to the inverter during that stage to achieve the same end.

And then you go get different batteries of the correct capacity when you tire of the Rube Goldberg management method and instead look for opportunity loads to utilize the PV power with.

Re: Two battery banks with one charge controller?

TucsonAZ wrote: »

I guess that leaves me with two questions:

1) Where is the damage normally caused with a battery imbalance, the charging, discharging, being connected to each other? Is one of those less damaging?

The reason I ask is because I could use one of those 1 or 2 or 1+2 style battery switches and charge them together but come nighttime I could swap over to the main bank and still have the reserve bank, or if it was a cloudy day I could select one of the two banks based on the weather.

2) So the charge controller should never be hooked up to the panels without the battery bank connected, is that correct? I thought I read you should always disconnect the PVs first then the battery bank, if that's the case would the same apply with the switch as it will momentarily disconnect from the system as I'm switching it. So should I disconnect the array before doing any switching?

Damage is a somewhat inaccurate term.
With small differences in capacity there is a slightly shortened lifespan for the one capacity battery over the other. With large differences the decrease in lifespan becomes worse. If the charging regime is adjusted to suit the larger batteries it has a more adverse effect on the smaller ones (chronic over charging). if the charging regime is adjusted to suit the smaller batteries it has a more adverse effect on the larger ones (chronic under charging).

Here you are looking at a 25% difference in capacity. So if you study the peak current range using the standard percentage values you have:
125 Amp hour batteries; 6.25 Amps minimum, 16.25 Amps maximum.
100 Amp hour batteries; 5 Amps minimum, 13 Amps maximum.
With the peak charge rate around 10 Amps (X2 for the parallel banks) it will be within the range for either size and you probably will not notice any detrimental effects from the mismatch at all.

Note that if you add other mismatching factors (such as different brands/types, case sizes, chemistry) in any problems will get worse.

Some charge controllers will get messed up if you take the battery off before the PV, others won't. If you follow the simple rule of disconnecting PV first (and reconnecting it last) it will work with any charge controller and possibly save you some serious grief (and money).

But frankly I would not be concerned about having to isolate the batteries with such a small capacity difference. If they were 100 Amp hours vs. 200 Amp hours it would be a no-go.