Two battery banks with one charge controller?
TucsonAZ
Solar Expert Posts: 139 ✭✭
I asked something along these lines before but have a better picture of my system now so I'm revisiting it.
My battery bank consists of four 12v 125ah AGMs that will be set up as a 24v bank, feeding this bank is 1,380 watts of solar panels via a MidNite Classic 150 charge controller. I will be traveling with this as it's mounted on an RV roof but I'm in zone 2 so I get a TON of sun, more than my battery bank can handle I'm sure.
I also have two of the same batteries only in the 100ah version which I'm unsure of what to do with at this point.
Is there any reason on earth I can't just hook up a marine style battery selector switch and swap over to the smaller bank to top it off every once in awhile? It would take me all of 20 seconds to flip the breakers on the PV array, flip the selector switch and turn the PVs back on.
I could also use a charger off of my inverter and just turn it on when the main bank is topped off as was suggested in my other post but that seems more complicated honestly.
My battery bank consists of four 12v 125ah AGMs that will be set up as a 24v bank, feeding this bank is 1,380 watts of solar panels via a MidNite Classic 150 charge controller. I will be traveling with this as it's mounted on an RV roof but I'm in zone 2 so I get a TON of sun, more than my battery bank can handle I'm sure.
I also have two of the same batteries only in the 100ah version which I'm unsure of what to do with at this point.
Is there any reason on earth I can't just hook up a marine style battery selector switch and swap over to the smaller bank to top it off every once in awhile? It would take me all of 20 seconds to flip the breakers on the PV array, flip the selector switch and turn the PVs back on.
I could also use a charger off of my inverter and just turn it on when the main bank is topped off as was suggested in my other post but that seems more complicated honestly.
Comments
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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?
Just add them to your bank. Sure, you will get some uneven, sub-optimal wear, but so what.I am available for custom hardware/firmware development
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Re: Two battery banks with one charge controller?
I too would suggest just paralleling the batteries into one bank (assuming they are all AGM batteries from the same manufacturer with the same charging voltage specifications). Just monitor them with a volt meter/DC Current Clamp Meter 1-4x per month to make sure they are all sharing current/voltages equally (any divergence would need to be investigated).
Also, as you put parallel strings together, each battery string should have its own fuse/circuit breaker to limit current in case there is a short circuit somewhere in the battery bank (cable failure, wrench drop, shorted cell, etc.).
Such failures are not likely to happen--But if they ever do--You will appreciate not watching your heavy copper wire burning the insulation off and glowing red.
Lead Acid (and other high current battery chemistries) are really quite scary as the cells and banks become larger and larger. Throw in hydrogen gas, sulfuric acid, and no "off switch"--The fun never ends.
-BillNear San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset -
Re: Two battery banks with one charge controller?
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? -
Re: Two battery banks with one charge controller?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. -
Re: Two battery banks with one charge controller?The reason I ask is because I could use one of those 1 or 2 or 1+2 style battery switches
<snip>
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.
Battery selector switches come on two flavors:
make-before-break or break-before-make.
If you break one battery connection before you make the next, then yes, you need to switch off the PV while making the switchover. The make-before-break switch has both batteries connected in parallel for a moment while making the switchover.
read this for another perspective on battery switches, and to understand why they are not allowed in a residential system:
http://www.amplepower.com/primer/burnt_sw/index.html
--vtMaps4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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