Battery Life & Charging

Alternative energy sources
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The usual. As I have two large solar PV systems here, and my lab assistant just put one in, and others are interested in things like this, here's where that stuff goes. This is mostly for things that work now, not "gee someday a fusor will do this" -- we know that, but it's not someday yet.
The hope is to save anyone embarking on this sort of thing a lot of wasted time and money, as at least I have been off the grid since 1980 and have had a lot of practice (and made mistakes you won't have to).

Battery Life & Charging

Postby Joe Jarski » Wed Jul 04, 2012 2:33 am

This is kind of a hypothetical question that I came up with while sizing my battery bank and had to ask it since it goes against 'conventional wisdom'...

According to Rolls battery datasheets, operating a battery bank at 25% DOD gives a 5000 cycle life, a 50% DOD gives a 3300 cycle life and 75% DOD gives a 2300 cycle life. So, if your system is sized for heavy usage at 50% DOD, but you have many days of light usage - say 25% (to make things easy), does it make sense to run two days between charge cycles and operate at 50% DOD to extend the battery life to 6600 days? Or even three days between, weather dependent of course, for a life of 6900 days? That's an extra 5+ years of life from the battery bank. I know the Rolls batteries typically last much longer anyway with proper maintenance, but if they normally last 20 years, then it could probably be stretched to 30 years.
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Re: Battery Life & Charging

Postby Doug Coulter » Wed Jul 04, 2012 7:29 pm

Wow, you've picked up on something I hadn't here. I'd been told by so many outfits, so many times, it was almost pure linear and if anything, advantage (over linear) with low DOD, that I'd assumed that was gospel, and have been running on that assumption myself for all these years (and getting 12 or so years out of L-16's that are going only 8 for most people). Seems they are saying it doesn't degrade as fast as linear.

Linear would work out to (more or less) the total amp-hours in/out, or through the batteries, as a life expectancy measure, almost no matter how you got it, only with real life batteries, it gets bad real quick (worse than linear) with higher DOD. They seem to be saying almost that, but that it doesn't keep getting better with shallow cycles. I dunno is the most honest thing I can say at this point. I'm getting good results with "the old religion", I can say honestly.

I rarely hit 50% here - because once you're getting there, the issue is likely to be "when is it going to be 100%", so you start making moves to stop the rundown. Although today was a great energy day (car charged despite AC on all day and some machine work) - it's still so hot, and sun going down I'm running the AC still and a generator that cuts the net draw down to about 10a, vs the 60a or so it would be without that little guy, for just that reason. In this case, the cost of that is balanced against my own belief I don't wanna run those guys down too hard into a week of "partly cloudy and T storms" predicted.

So, what they appear to be saying is it's worse than linear if you don't cycle things or only stay near the top. I offer just one data point. The car starts charging in the AM before the panels can really do that and charge the batteries - there's net drain (considerable) the first time it cuts on in the morning, and my relay system lets it do that for a couple minutes (down to 24.5v) before it cuts out, then lets them get to 28v or so (under charge) before allowing that again - I get about 5 cycles of that every day I'm charging the car in the AM. If anything, that's really perked these guys up! They seem to like a little cycling around the top, which is also the basis of all those anti-sulfation devices they sell to people (I never fell for that but...given this observation...).

I dunno. I'd guess a good deep discharge once in awhile would kill off any "memory effect" and hard-crystallization or passivation of the lead oxide, but I'd not do it on purpose - it happens enough by accident.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Battery Life & Charging

Postby Joe Jarski » Wed Jul 04, 2012 11:08 pm

It wouldn't be practical to do on a regular basis, but it's an interesting thought. I got started on that while looking at two parallel strings for the battery bank. Instead of running the whole bank as double the amp-hour capacity, I was thinking that you could alternate each half of the battery bank from one day to the next. The advantage wasn't so great in doing that unless you could really hit a sweet spot with capacity and usage.

Their chart has a few squiggles in it, so not exactly linear and it just gets cut off at 5,000 cycles although the trend is towards more cycles at shallower discharge.
Cycle Life.jpg

Maybe it just gets flaky beyond that point depending on how it's used??

It's good to know that you rarely hit 50% - I've been looking at similar capacity as you, although at 48V (683AH) or maybe 820AH and probably 32 245W panels.
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Re: Battery Life & Charging

Postby Doug Coulter » Thu Jul 05, 2012 10:51 am

I wouldn't want to do the switching, myself - I can pull such serious peak amps you'd be talking adding a safety/reliability issue to the system. Sometimes I threaten the life of my amp shunt pretty seriously. I often get that one milli-ohm resistor quite hot (melt solder - and this is big and has fins). Switches with way under that R value, reliably, are kind of scarce. More than that, you get fire. Fire plus gassing batteries - not good.

Remember also - if you temporarily disconnect batteries in operation (break before make) there are troubles due to that. Volts on the inverters and charge controllers can go to zero or too high before things can react properly. Xantrex's plan is for you to separately disable/disconnect everything else individually first - a lot of switches (which I don't have). I did try such a lashup way back when - it never really worked that well - you want all your batteries being trickle charged when you can, to keep up with self-discharge anyway, so they might as well be "live" in the system rather than inventing a little battery trickle charger for the ones switched out at any time. I decided it was too much trouble for me at any rate, and that having them all live at once was a good plan and kind of got rid of the problem of needing that particular "hot spare" by increasing the capacity enough that I had plenty of time to react to any issue before needing it.

Xantrex wants you to first disconnect panels from solar controllers, then the controllers from the battery bus, then the inverter...then you can do what you want but everything resets to an extent when you do that, as it loses things like its attempt at knowing the current state of charge and similar issues. Not a real big deal, but I'd guess you could over-volt fry some things if you weren't careful. For example, the charge controllers have about a 1 second response time...and are putting out a current - so volts can rise way up for that second if the load disappears.

I am not sure what the inverter does re grid pass through without battery volts on it - it might glitch your whole house power and have consequences on other gear? I've already eliminated most things here that would care and don't mind the 12:00 flashing on the various stupid clocks they put into everything these days, but...something to consider if there's anything stateful in your load profile.

I'd have to analyze that chart better after figuring out if one of the axes isn't kind of 1/x, but it looks to me like it goes up faster than linear at low cycle depth - which is how I've been assuming it was, and I can't complain about battery life here. I appear to be doing about 50% better life-wise than the norm. Or better - those last L-16's I took out of the other system were 15 years old! Most people change them out at 8.
I replaced them with 8-9 year old batteries that had been in this main system (and then sat for 3-4 years) - and those are still happy and chipper.

But I also have to say - I baby the heck out of them - and that's why I rarely hit 50% - I start worrying and taking action earlier than that. I pretty much shut down the shop at sundown (other than minor tinkering), and if I have to do something really power consumptive later at night (as when I did a demo fusor run live on G+) - there's a generator out there running to absorb at least some of that. (fusor runs take a few kw, due to all the ancillary junk and metrology and fans and you-name-it - a few computers on to do the show plus the data aq). I probably didn't need to do that - it was only a few kwh out of 24 I have (theoretically), but it made me feel better since I didn't know what tomorrow's weather was going to bring - you become sensitive to that when it determines what you're able to use, and I tend to be over-conservative that way and keep tomorrows options as open as possible.

I was up late last night for example, on G+, and just left the AC on as it was so hot/wet here (90F/90RH) - but I let that little honda generator take the first 40 amps of draw (about 1kw) from that (60 total, or 70 considering this computer as well)...half a gallon of gas for a late night party - fine once in awhile. Acts like preventative maintenance since I use generators so rarely anymore and it's good for them to run a little.

My own system definitely wouldn't mind another 50% of batteries now. But that's a relative-sizing issue. Till I got the additional panels, the issue would have been, gawd, once you run 'em down, you'll never get them charged up again - too many batteries has that and the self-discharge issue, whereas too many panels only makes me want to find more ways to use the extra when I have it, which has been quite often this season. That's certainly the more-fun challenge to be faced with!

Batteries really do not like sitting at partial charge, they seem to be much happier with at least some minor cycling - but minor, more is not better from what I can tell.

I've rejuvenated some things that weren't terribly practical before - been getting conventional electrically heated showers frequently (for me), running a water distillation rig, A/C in summer (and space heaters in the winter) to use this extra power rather than letting it fall on the floor via the charge controllers protecting the batteries from it. It's been happening a lot that I already have the car fully charged, since I don't drive too much (not even every day, and my normal errand loop only runs the car halfway down - heck, now I can drive harder and not sweat hypermiling!). Since I added panels that could do that nearly everyday (twice on a good day in summer) - that's some serious kwh to find uses for when I can't really store any more. It's a fun challenge compared to how it used to be!

I'm pondering whether to get more batteries and loosen my operating plan to use a lot more at night, or keep my "good habits" and find other uses for the discretionary power as it arrives at the moment. There's a round-trip loss with batteries, after all, and anything you can usefully use up on the fly skips that one - and often provides luxuries I'd learned to live without, kind of nice to have them back. As you're probably not used to not having some of these things, and have an intrinsically higher demand than I do - I'd suggest you have more capacity on just about everything than I do - maybe double. I've made myself real easy to make happy, in effect, compared to how most live. Or at least, start about where I am and learn how that is going to work out. If you're still going to have grid, at least your main power draw from that can be timed to get good time-of-use rates and that can be a big win - a lot of Volt owners are proving that one out.

I'm still running this pretty darn conservatively. I try not to run things down to the point where one good day won't take them all back to the top again, even in the presence of my big loads. I'm not sure at all that I need to, but it's what I'm actually doing, and it does work out nicely for lifetime issues.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Battery Life & Charging

Postby Joe Jarski » Fri Jul 06, 2012 10:25 pm

The last few months I've been averaging 13kWh per day. That's after my initial crack at reducing my consumption by unplugging things that I don't need anyway, which cut it in half. And it doesn't effect the way I operate at all.

My plan as of now is to put in 32 245W panels and a single string of batteries at 820AH and 48V. That should give me 36kWh of power and make my average battery discharge about 36%. I can make plenty more improvements to conserve power when I need to - it'll be a learning experience, but I have to start somewhere.
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Re: Battery Life & Charging

Postby Doug Coulter » Sat Jul 07, 2012 11:21 pm

Do you have time-of-use data? That would be key to sizing things the best. But yes, that large a system will definitely get you going nicely. One issue I've had with the otherwise excellent Xantrex stuff is that there is a limit to how many things will go on their little proprietary xanbus hookup and still work. Evidently it's the inverter that's too talkative, but it's also what normally provides the power to the system control panel etc.
So if you wind up with over about 4 solar charge controllers (and you will in that system), then you can't have a single unified system with it all on the same buss, and to have a system control panel for the charge controllers it will need a +15v separate supply for it. Not a big deal, but it's a PITA not to have it all together, as you lose some of the logging of the *net* power, instead getting "this much went in" and "this much went out" as separate things on separate control panels.

Don't for get that if you're where there's a lot of snow, you have to angle the panels enough to shed it - and leave plenty of room beneath them or it piles up there and there's no place for more to fall off. My new setup does that for the steep ones on the front, and I'm very much looking forward to cooler times and not longer having to scrape them off when it snows. Should work out about right anyway where you are.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Battery Life & Charging

Postby William A Washburn » Sat Aug 04, 2012 8:39 am

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Re: Battery Life & Charging

Postby Starfire » Sat Aug 04, 2012 12:14 pm

http://www.ebay.co.uk/itm/180905261367? ... 1423.l2649

2 off 4.2 AH 4.2v Lithium rechargeable
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