Saturday, May 03, 2008

My Latest Off-Grid PV System Upgrade

Because of a recent system upgrade I now have better control over the amount of power I use, and the amount of power I keep in reserve. While I want to benefit from as much power as the system is capable of providing on a day-to-day basis, I find myself cutting back, knowing that I’ll need stored energy in the event of a grid power failure. I need to strike a balance between the amount of power I use, and the amount of power I keep in reserve, but I find myself wanting more of both.

Since my solar panels have the ability to fully charge the existing battery bank by noon on a sunny day, it’s obvious that I have the capacity to charge a larger battery bank. It seems that a battery bank upgrade will allow me to get more from the system on a daily basis while at the same time increasing the amount of stored energy. For that reason, I’ve just replaced my old battery bank, increasing the amp-hour capacity from 420 to 675.

I’ve known that I could greatly improve system performance with a bigger battery bank for some time, but I’ve been holding off because my old batteries are still in good shape. Since batteries of different sizes, types, and ages shouldn’t be included in the same battery bank, I was confronted with the problem of what to do with the old batteries. I decided to isolate the two battery banks from each other, and to use a switch to connect or disconnect the old battery bank as needed. The diagram below shows how I did that:


The new ones are 6-volt batteries, most commonly used in golf carts. I series wired three pairs of these to create the equivalent of three 12-volt batteries at 225ah each, and paralleled the three pairs for 675ah at 12-volts. I’ve installed a heavy-duty switch, allowing me to switch in bank 1, bank 2, or both banks at the same time. I’ll switch in the new battery bank for day-to-day operation, only switching to the old bank once in awhile to keep it fully charged. In the event of a power failure, when I’ll need the extra capacity, I’ll switch in both battery banks. With both battery banks connected, I’ll have one large battery bank with an awesome 1095 amp-hour capacity.

Understanding that batteries should not be allowed to fall to more than 50% of their capacity, I now have a useable capacity of 547 amp-hours, a significant upgrade from the previous 210 amp-hour useable capacity I previously had. After monitoring the new battery bank for a few days I observed that:

· It does indeed take more time to fully charge the bank.
· Loads are powered for a longer period of time each evening, after the sun goes down.

In the event of an extended power failure I’ll have light, refrigeration, communications, the ability to prepare food. I’ll be able to use my bio-fueled stove for heat, and fans to circulate fresh air. I’ll be able to watch TV or listen to the radio. I’ve had these capabilities previously, but not to the extent that I have them now. This will serve me well in the event of an extended power failure. I’ll need to continue upgrading the system if I expect to use it to charge the electric vehicle I hope to purchase within the next two years, but I’m getting there. I’ll be doing some capacity tests soon, and I’ll record the results on this blog. Check back later.

For information concerning my recent system automation upgrade, check my February 25th post: http://solarjohn.blogspot.com/2008/02/ive-automated-my-off-grid-pv-system.html

John

3 comments:

Damon said...

Hi SJ,

Since I'm fairly limited on space, and I want to test out technologies that may not be quite mainstream enough (or cheap enough) for everyone to use, and because my stuff is non-critical too, I'm looking at Lithium-chemistry batteries for my next expansion. Basically I'm hoping to get to an additional usable ~1.5kWh capacity in not *much* more than the size and weight that I already have for my 40Ah lead-acid with its usable ~0.25kWh.

That 1.5kWh would allow me to run my servers or fridge/freezer for 2 days, for example, although actually finding non-roof space to gather enough sunlight midwinter is pretty tricky!

I'm trying to get more data (and pricing) on the Valence and PowerStream batteries.

Have you thought of trying non-lead-acid storage?

Rgds

Damon

Solar John said...

I have thought about other types of storage, but I wanted the most bang for the buck. Space and weight are not a problem for my PV system, so the golf cart batteries I bought seem to be the best value at the present time.

Having said that, I am excited about recent advances in lithium batteries. I think the price will come down once they're mass produced for cars like the Chevy Volt and the Th!nk. sj

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