Connecting battery banks

mountainman · October 2019

With 2 separate systems. 1 array on 2 gc batteries. And another array charging 2 of the Same type gc batteries. Could I connect all 4 batteries at night?. Or would there be consequences?

mcgivor · October 2019

Connecting the batteries in parallel that have been charged, possibly discharged seperately, may well be at different state of charge, this may cause massive amounts of cascade current into the bank of lower capacity, causing arcing at the connection points which could result in explosion of hydrogen if done on the terminals of flooded lead acid batteries, or damage to switch gear if done remotely.

The better solution would be to permanently connect the battery banks in parallel, and charge as a single bank with two separate controllers / arrays. The one problem with doing this retrospectively however, is that the batteries may be of different age and residual capacity which would cause imbalances between strings. This configuration is better if done when all batteries are new and identical make and capacity.

mvas · November 2019

@mountainman, It appears that you have 4 identical GC Batteries. Is that true? Costco sells 6V 208AH GC2 batteries. Is that what you have? Is there a reason why you have two smaller battery banks, and not one large battery bank? How old are these batteries? I don't understand why you decided to parallel connect the batteries at night, but separate the banks during the day.

With two charge controllers on one large bank, verify that you do not equalize twice as often.

mountainman · November 2019

@mvas I have 4 identical us battery 208 ah. Problem is on 1 system I have a 30 mppt that is adjustable. The other systems pwm controller that's not adjustable. In the Morning both controllers play well together.
But by 11-12 the pwm stops. And only the mppt is producing power. So by charging the bank separately only 2 batteries to each controller i get more whs per day. I'm not home during most days so all my power needs are at night. I think the fix is to get 2 of the same adjustable controllers. And no I manually eq only when necessary.

papab · November 2019

Why not connect the panels to 1 large enough controller? That would be simpler.

@mcgivor's concern about massive current flow would only be a concern if 1 battery was very close to dead.

What you have is actually not a bad way to deal with parallel systems, you can make sure each bank is getting charged up.

Estragon · November 2019

It probably doesn't make a lot of difference that the pwm stops around noon, just that the other controller is finishing absorb and maybe doing the float. The current required is lower anyway, so as long as the one controller is doing a full absorb, not really an issue.

Even with my two networked classics, one will produce more than the other to absorb/float the bank.

Unless there's some other reason to split the bank, I'd keep it as one. It should take whatever total whs it takes to charge the batteries, and be about the same with the bank split or as one. Assuming the pwm absorb setting isn't optimal for your batteries, it might be better for them to have the controller with more appropriate voltage do the absorb for them all, IMHO.

mountainman · November 2019

@Estragon would it be a good idea to aim the pwm system in a sse direction and leave the highest voltage mppt system in the south position or aim those in a sse position for virtual tracking?
@papab like many people. in the beginning I didn't have a clue what solar could do or how much my loads where.
I looked at grid tie panels and shipping cost more than the panels for a small system.
So free shipping 4 100 watt panels and a pwm cc cost me $500. Then I realized it was over 100 foot from the camper in the shade to where the sun was. So it was cheaper to buy a epever 30 than large pv cables.
Then I added an inverter. TV and box fan then stereo.
I found that 400 watts on 400 ah wasn't enough even for a part time summer system.
I located 2 190 gt used panels local pick-up.
4 36 cell 100 watt panels and 2 60 cell 190 panels where not a good match for 1 controller. And the cost of a 60 amp 150 volt.
Was very expensive.
Since I already had 2 cc I used what I had.
Plus with 2 if 1 broke. I wouldn't be in the dark with no access to power lines.
I located 2 100 foot runs of direct burial 8 awg used house wire for free for the pwm.
That gave me 16.9 volts to the controller.

Estragon · November 2019

Having the pwm sse might work well. That would get you current for bulk charging a bit earlier, with the mppt adding more later. Assuming you get to absorb before ~noon, the mppt in full sun could finish absorb then float.

I have shading to the east, and main arrays facing a bit west of south. They're tilted steep, and produce much less later in the day. I also have small pwm arrays tilted vertical and facing more west. The small arrays are mainly for winter if/when the main arrays get snow covered, but also help by providing power to keep banks floating later than would otherwise be the case. Sort of the opposite of your situation, but the same general idea.

mountainman · November 2019

Thanks think I'll try the sse on pwm and see how it works out.

mike_s · November 2019

mountainman said:
I have a 30 mppt that is adjustable. The other systems pwm controller that's not adjustable. In the Morning both controllers play well together.
But by 11-12 the pwm stops. And only the mppt is producing power.

That's not a problem. It just means the batteries are charged as far at the PWM is concerned. Which will be most of the way. Assuming the voltages on the MPPT are correctly set, it will simply continue to charge them all to full. And, it should have enough time to do so if it takes over at 11-12.

Better than having the PWM constantly undercharge 2 batteries, while the other 2 get good charges.

mountainman · November 2019

The pwm is locked at 2 hours absorb at 14.6. the mppt is limited 3 hours at 14.8. personally I haven't seen a great advantage with mppt vs pwm in output.
Yes much more adjustable better for longer distance pv arrays. And you can use various panels.
But so far I've only used solar for summer. Seems ive read mppt shine's in winter.

mountainman · November 2019

Just a random thought. If running different oriented arrays on mppt is a bad idea. With my mppt array facing south.
And my pwm Array all in parelell with 2 panels facing sse and 2 facing ssw work.

mike95490 · November 2019

PWM can be oriented in any way, and simply parallel to the limits of the controller

This is so clumsy to try to explain fully, and I'm too tired

mountainman · November 2019

Thanks Mike

Connecting battery banks

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