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Old 10-12-11, 10:47 AM   #1
bennelson
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Default 2 Charge controllers on same battery bank!?

Hey Guys,

I have a question for you.

I have a bank of batteries running at 48V. I am charging them with a 48V UPS (uninterruptable power supply.)

I have some PV. Very basic, just several amorphous panels - 12V/15w from the 45 watt solar panel set available at Harbor Freight.

I have a Xantrex solar charge controller that will run at 12/24/48V.

What I would like to do is be able to charge the batteries from either the UPS (which can charge at up to 10 amps) OR from the solar (which would be a much lower amp rate, but FROM THE SUN!)

My question is, how do I do this? Would I need a manual switch? Could I simply just have BOTH hooked up at the same time?

I have noticed that the UPS needs to be "pre-charged" before connecting a battery pack to it, similar to what you do in an electric car with the motor controller. If I had to manually switch back and forth between solar charging and UPS charging, that may play into it.

I have two sets of batteries. One is AGMs on a rack near the UPS. The other is AGMs in my electric motorcycle (also running at 48V). I want to be able to charge the motorcycle with this system, and after that's charged, charge the stationary set of batteries. I would have one or the other set of batteries connected to the charging system at all times.

I want to be able to USE the batteries by either driving the electric motorcycle OR feeding the power back through the UPS to power my garage or house in event of a blackout.

I already have it all figured out and working for the UPS blackout protection, and have 50 amp Anderson disconnects to be able to easily connect either the cycle or the stationary battery pack to the UPS. I just want to figure how to integrate a little bit of solar power.

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Old 10-12-11, 02:08 PM   #2
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Interesting setup idea Ben.

What if you just ran the AGMs on the rack in parallel with your bike's pack? Just plug in and have the batteries balance each other? Possibly too much current?

Anyway, it sounds like the bigger problem is switching from the PV to the UPS and back. Honestly this would be a great place to use an arduino unless you really have a good way of monitoring things and manually switching them. I don't think you'd want them both hooked up at the same time. One or the other will always be backfeeding power into the other because one will have a higher voltage than the other. It would be just like you can't use two chargers on one battery.
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Old 10-12-11, 03:26 PM   #3
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The stationary batteries are about 100 ah and the ones on the cycle are 55 ah, so to put them in parallel with each other, I don't think makes sense.

I did see a setup a while back that a guy did with a some relays and a cheap-o grid-tie inverter. It charged batteries, and once the batteries were charged, it kicked the solar power over to the grid tie to reduce his household energy usage by a tad.
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Old 10-12-11, 03:51 PM   #4
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I believe you can just leave the solar panels connected to the charge controller connected to the pack.

You shouldn't have to disconnect the charge controller to charge the pack from the grid/UPS charger.

The solar charge controllers will simply see voltage greater than X, and won't do anything if that's the case.
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Old 10-12-11, 04:01 PM   #5
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That's sort of what I was thinking.

What if I just left the solar charge controller connected and simply UNPLUGGED the UPS from wall power when I wanted to exclusively use solar?

Then the UPS isn't charging the batteries, the voltage is lower, and the solar sends power to the batts.

If I want, at night, I could just plug the UPS back in.

Granted, it's manual, but there are plenty of times that I might want several days of solar, and then just use the UPS to top the batteries off.
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Old 10-12-11, 04:19 PM   #6
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Your UPS will likely top off your AGMs and hold them at a high (float)voltage that will keep
your solar PV controller from ever turning on. But the Xantrex has an adjustable float voltage..
The problem with cranking it up, to use the PV, might be excessively high float voltage.

You might want to put the UPS AC power input on an SSR, that is controlled by the amount of sun available.
That would give the solar a chance to work.

When you wanted to use the chargers to charge up a discharged bank or motorcycle, both sources should feed power in,
while the pack voltage was low..

My backup system is doing nothing these days, but staying in float all day long..
500w of tracking PV array going to waste..
Wish it was running a GTI.. But that would be illegal..
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Old 10-12-11, 04:22 PM   #7
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Sounds good to me. I'd do that.
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Old 10-12-11, 04:57 PM   #8
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So when pack voltage is low, you don't see any problem having BOTH sources of charging hooked up to the same batteries?

Another possibility would even be sometime like just having the UPS on a TIMER, so that it only turns on to charge at night-time hours.
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Old 10-12-11, 08:30 PM   #9
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Yeah, it might not work. Your charger does PWM and that might mess with the UPS charger..
But, maybe it's like my TriStar TS-45, which has a dip switch setting to reduce RF noise.
It turns off the PWM and goes into an OFF-ON type of charging.
(Not as effective as PWM, but does the job)..

Hey, I might have to try that out on my back-up rig..
I'll be able to listen to AM radio again during sunny days!!

Anyways, your PV charge controller should take itself out of charge mode,
when the bank hits it's target voltage (pre-set set-point).
So, if the bank voltage is being held high by the UPS charger.. Your PV goes unused.

But, if the bank is at 11.5 volts, both units will (should) try to feed it current..



I wonder about just using solar during sunny days and turning on the UPS charger at night..?.
If you had some real PV power, that would be the way to go..

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