Hello Travis,
Just for my $.02.. I gave up a while ago on the textbook 3 days of autonomy, but for a slightly different reason (including yours). We would give a battery bank 3 days of autonomy, and then place all kinds of alarms and load shedding at 50% SOC, but nothing to tell folks when they have used up one day of their energy. (This is particularly relevant with small systems with LVD charge controllers.) Consequently, everyone (we are talking about remote installations in the jungle with no alternative) keep using the power until the alarm sounds. The panels can charge back up usually not much more than one day. The battery lives its (shortened) life out going between 50% SOC and 70% SOC, unless of course the ever important generator is part of the system. There is no magic to 3 cloudy days versus 6, or 8 or 12, so .. Might as well deal with it sooner rather than later.. Walt SunEnergy Power International From: [email protected] [mailto:[email protected]] On Behalf Of Travis Creswell Sent: Tuesday, December 01, 2009 2:16 PM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Other's thoughts on Autonomy? was concord batteries, EQUALIZE Them! IMHO, one of the worst design boo-boo's is going past more then 2 days of autonomy. Personally, I no longer size much over one day because it's my anecdotal observation that most batteries die of old age and being ignored long before cycles get them. Speaking mostly about quality deep cycle flooded. Lots of good things result; -50%-75% smaller battery bank means a $20,000 battery bank just turned into $5,000 bank which frees up a ton of money for more modules and now-a-days you can buy a lot more PV with that money. More array mean far less reliance on autonomy. I'll take the trade all year long. In the summer we have 3 to 4 weeks of sun and one day of clouds and in the winter we get 3 to 4 week stretches with 1 sunny day. Autonomy doesn't really matter in either case from what I've seen. The larger the bank means more self discharge losses, which on large battery banks gets significant as they age. 5-15 years later you'll still have all that array but no matter what you're looking at new battery bank. -If you study the quality deep cycle manufacturers literature you'll see that you'll see that anything over 1 day of autonomy is too much to allow the array to actually charge the battery bank anywhere near the recommended amps and just like rust, sulfation never sleeps. -Less cells to water -Less space required -Given that a surprisingly high percentage of off gridders totally screw up on their first bank, no matter how much we all try we might as keep the stupid tax of replacing a 2.5 yr old battery bank to a minimum. -All of this discussion about cross paralleling, buss bars, TLC with a gazillion connections and multiple strings goes away. -And the best part is we don't have to carry all of the lead into the basement and even better back out of the basement! Just my .02. Feel free to strongly disagree but let's be polite about it. Travis Creswell Ozark Energy Services _____ From: [email protected] [mailto:[email protected]] On Behalf Of R Ray Walters Sent: Tuesday, December 01, 2009 12:44 PM To: RE-wrenches Subject: Re: [RE-wrenches] concord batteries, EQUALIZE Them! I used to think that one string was optimal; until I had a single cell failure take out an entire system for weeks. (try operating a 24 v system at 22v! ) I now think that 2 parallel strings is optimum, 3 is OK, and 4 is max. At 4 parallel strings, we start spending more time looking to make sure all connectors are the same exact length etc. to insure equal operation. But of course how do you account for varying internal resistance of the batteries......?? I've done 4 parallel strings at 144 DC of sealed batteries on an electric vehicle, but we were very careful with our resistances, I even switched to smaller wire, on closer strings, and calculated out the exact resistance, so all strings were theoretically equal. This set actually just died, but achieved its manufacturer's predicted cycle life. (B&B battery, 350 cycles to 80% DOD) So if you're careful, 4 strings can work well. Worst I've seen was 20 golf carts paralleled in a 12 v system, (10 strings) and they didn't pull the main connections from across the set, just connected to one end. The results were very predictable, with the furthest batteries being chronically under charged, and the closest ones being over cycled to a premature death. Ray Walters On Dec 1, 2009, at 11:28 AM, [email protected] wrote: You gotta wonder about why the customer bought such a battery layout, or why the installer sold that kind of configuration (which ever it was) with so many small batteries. We would never recommend going over 2 parallel banks, but sometimes the "customer knows best...".
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