by Doug Coulter » Thu Jul 05, 2012 9:51 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.