12 VOLT system question

2Guido · July 2013

Part of my system is a triple bank of jumbo 12volt AGM's parallel fed with the Coleman Diversion Controller powering 2 5kw inverters. The solar panels inputs are paralleled also at approx 1kw at 38 volts. The dump controller only measures voltage and amperage with no charge control. The measured amperage into the batteries in bright sun is around 20amps at 13.5 volts. Would you recommend an MPPT controller to improve my amps? and if so, a recommendation of type and brand and current rating would be really helpful...
Thanks !!!

BB. · July 2013

Re: 12 VOLT system question

Basically, power into the battery bank with PWM controller:

1,000 watt panels * 1/38 volts Vmp = 26 amps maximum from array/PWM controller
26 amps * 13.5 volts = 351 watts best case

Power into the battery bank with MPPT controller:

1,000 watts * 0.77 panel+controller deratings = 770 watts average good day
770 watts * 1/13.5 volts charging = 57 Amps typical average peak amps on good day

Only you can decide if that is worth ~$500 of MPPT charge controller...

Midnite makes very nice MPPT Controllers. Outback, Morning Star (and some others if you were outside the US) are also very good for the job. All have controllers that are good for 60 amps or more.

Many times, GEL batteries have fairly severe maximum charging current limits (like 5% of battery AH capacity--200 ah * 5% = 10 amps maximum rate of charge). You need to check the battery specifications for that.

How far from solar array to charge controller/battery shed? Midnite has higher voltage versions (Schneider/Xatrex has a very high 600 VDC max input controller).

Why do you have a dump controller? Do you have wind/water turbines? In general, dump controllers are not real good at battery charging (100% off or 100% dump--no average current at the level needed by the batteries for their properly charging).

-Bill

2Guido · July 2013

Re: 12 VOLT system question

Array to controller approx 50ft using 2 gauge copper cable. I have two small wind generators also (which are pretty much useless) hence the reason for the dump controller. However, the dump has only kicked in once for only a few seconds so the batteries are always receiving current. I have a 3kw 24 volt system also powering a different bank of batteries and inverters also using a coleman 440 dump controller. Seems like I need a midnight MPPT !!!!

Cariboocoot · July 2013

Re: 12 VOLT system question

If you have 1kW of solar panels with a Vmp of 38 (probably Voc 38, Vmp 30) going directly to a 12 Volt battery bank what you really get is more like Imp * battery Voltage or roughly the equivalent of 584 Watts (based on 33 Amps @ 17.5 Volts) of array. That's nearly half wasted, so yes the charging power would be improved by an MPPT charger: you should be able to get 64-ish Amps from 1 kW on 12 Volts.

I'd be re-evaluating the turbines and dump controllers.

BB. · July 2013

Re: 12 VOLT system question

I would suggest that you "define" your present system (# of panels, Vmp/Imp, battery bank AH capacity/voltage, DC and AC loads--Average peak watts/max surge, Watt*Hours per day, etc.).

Next, define the "optimum system" that would support those loads/battery bank (using our rules of thumb).

And then pick hardware to support those needs--Cost out system and figure out if the prices are within your wallet's capability. Rinse, repeat.

Then go out and buy hardware/reconfigure existing system. Don't go out and just purchase a new Midnite MPPT controller. It may not "fix" your system's other issues.

-Bill

12 VOLT system question

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