I really studied this at one time, and what's interesting is that if a battery bank is too large, (regular discharges less than 20%) you're not really getting your money's worth, as the # of cycles x amount of discharge is less. Likewise, a battery that is discharged regularly deeper than 70% starts eating life too. It's surprisingly even however, from 20% DOD to around 70%; more cycles less discharge, or less cycles, more discharge, you get about the same total KWH total out of the pack over its life either way. (just take a cycle life chart for most batteries, and start multiplying cycles by DOD at different points) The bottom line is, you might save some money upfront with a smaller battery bank, but it won't last as long. Therefore, the replacement cost ( labor to move out old bats, move in new) is higher with a smaller bank, so the total operating cost over the system life is higher, too. Also, the charge rates that battery companies list are really for big, dumb on-grid chargers, not the 3 stage PWM units with temp comp that we use. I've found systems with really low charge rates to actually last a very long time. We're trading possible under charging and sulfation to avoid chronic over charging (and over heating) which is the real killer IMHO. Somewhere between is optimum.
I still do 5 days storage to 100% DOD, (or 4 days to 80%, same thing) for our avg. off grid systems, and tweak it down if we need to save money. Big systems with a good generator can work fine at less than 3 days. Of course local weather is a factor in this too. We get long spells of cold, cloudy weather followed by a couple of weeks of nice sun. For GT w/ backup, I size for more like 8 hours. As was said before, they'll probably die from the float service, not the cycling. Even a really small battery will still work if the grid is out for a longer time, they'll just have to do more when the sun is out, and go to bed early. R. Walters r...@solarray.com Solar Engineer On Dec 1, 2009, at 3:15 PM, Travis Creswell wrote: > 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: re-wrenches-boun...@lists.re-wrenches.org > [mailto:re-wrenches-boun...@lists.re-wrenches.org] 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, wind...@wind-sun.com 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...". > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org >
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