Solar Power Battery Bank
Monday, October 13th, 2008
Mr. Yago:
I recently setup a small off-grid solar power system in my residence. I am using three 110w, ~6amp Mitsubishi 12v panels wired in parallel, charging two 12v MK/ DEKA Gel-Cell batteries with a 184 aHr capacity (at a 10 hour rate) wired in parallel. The controller in use is a Xantrex C40. The wires in use are all oversized (#2/0 AWG as between batteries and inverter, #10 AWG as between panels and controller and controller and batteries).
I have been running the system for about a month now. Is it too late to expand my battery bank capacity? I have received advice to the effect that once you initiate a battery bank’s cycles you can’t then expand the bank later on.
My problem is that I am discharging the bank too quickly. I have only a chest type deep freezer drawing power, and when the compressor is running it draws 10 amps. The compressor runs 10 minutes out of every hour. My bank and panels produce/ store enough electricity to run the freezer for a 24 hour period, but after a nighttime of no sunlight my battery state of charge is about 12.4 volts.
It is difficult for me to imagine that load that requires 10 amps an hour will defeat a battery bank that can produce 30 (183aHr x 2 = 363/ 12 hours = about 30 amps) amps an hour in a single night, but it has been.
In sum, may I add two more 12v batteries to double my bank’s size at this point? Also, does my energy consumption and battery depletion look accurate to you or is something amiss within the system?
Thank you for your time, and amazing articles.
Respectfully,
Matt Houghton
Matt:
You have several possible problems to deal with. First, you could have the worlds largest battery bank, but if your freezer removes each day more stored energy than you are putting back from the solar, it will only take a few days for this system to run down and never catch up. I would first determine if this load is more than you thought and it may pay you in the long run to buy a more efficient refrigerator.
For example, If you review my recent article about building a solar trailer, you will see eight (8) 350 amp-hour deep cycle L-16 batteries being charged by a 600 watt array. The array can actually be extended to 1000 watts when we set it up as a display, but when sitting next to my home we only use 600 watts of array. This system will keep the battery charged for months at a time and the only load is the refrigerator-freezer. However, it is $3,000 SunFrost refrigerator-freezer which I think still holds the world’s record for being the most energy efficient. Also, it is being powered by 600 watts of solar, not the 300 watts you are trying to use.
If you really want to do this and not need to increase your battery and solar array size, I would consider switching to a SunDanzer Model #DCF225 12/24 volt DC top load freezer and then you will not need the inverter. It will operate straight from the battery and I think the 300 watt solar array you have should easily power one of these unless you live in the extreme North.
Next, since your batteries are still new enough, I do not think you would get that much of a “mis-match” if you did add more batteries, but the problem is you are operating everything at only 12 volts and this is requiring you to wire everything in parallel which is not good. We sometimes wire batteries with two parallel strings, but when you increase the number of strings you can get all kinds of imbalance and even the risk of one battery string discharging into the other if it has a weak cell. If you do decide to increase the battery bank size and you stay with 12 volts, I would switch to 6-volt deep cycle golf cart batteries which would more than double your amp-hour capacity without having to make four parallel strings.
Good Luck!
Jeff Yago
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