[RE-wrenches] Other's thoughts on Autonomy? was concord batteries, EQUALIZE Them!

[R Ray Walters]

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
ray at 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-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at 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, Windsun at 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...".
>  
>  
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