Too many batteries, not enough panels?

So I found myself ten, new, six-volt, 275ah, flooded lead acid batteries.

But the only charging components I have are a xantrex c-40 charge control and a 135 watt solar panel................

My intention is to build a system to power my "weekend" cabin. Real basic stuff, lights, laptop, stereo. I have a 12 v, 1000 watt cotec inverter.

My thinking was that it didd'nt matter how fast the batteries got charged. Just that I had ample power stored up for the weekends.

Can any of you help me trouble shoot my logic? I was reading in the "off grid" forum that you need sufficient amperage to fully charge this amount of batteries that the 135 watt panel alone may not have. Do I need additional panels? Or perhaps I could just make the battery bank smaller? Or maybe, fingers crossed, this isnt an issue?

I am very much a solar noob, so any info would be greatly appreciated!:D

Comments

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Too many batteries, not enough panels?

    it's true that you will need more than that pv to charge the batteries up. batteries have a self discharge rate without you even using them and that one pv may not even be enough to offset that. float charge rates i recommend between 1 and 2% of the battery ah rating. you may get away with as low as around 3 or 4% for prolonged charging like you have in mind, but more is always better if you can do it.
    your battery bank i'm assuming it to be series/parallel for 12v operation and that makes the ah rating of the bank 1375ah so this is by no means a small battery bank especially for a cabin. making the bank smaller leaves the problems of do i have enough to cover my weekend needs and what to do with the other batteries?
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Too many batteries, not enough panels?

    Flooded lead/acid batteries like a charge rate between about 5-15% (see the note below from niel) of ah capacity. As Neil has suggested, you can live with less, but not too much less.

    One word of experience about off grid cabins and "just the basics". Most people under estimate their loads (often substantially) and over estimate their daily solar charging capability. I suggest that before you go much further you get a pretty good handle on your loads to give you a good idea of how much Pv you should budget for. One final note, no matter what you do, loads WILL grow with time. From "just the basics" to let's just add a TV, or a Micro, or,,,,

    Good luck, and welcome to the forum.

    Tony

    edit from niel to add that trojans don't like over 13% and some other types may be inline with that chemistry.
  • mshen11mshen11 Solar Expert Posts: 185 ✭✭✭✭✭
    Re: Too many batteries, not enough panels?

    following the charge rate rule... what happens if you have too much panels? other than buying more batteries, what can you do to prevent waste?

    you can set the charge controller to limit the charge, if so where does the extra current go?
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Too many batteries, not enough panels?
    mshen11 wrote: »
    following the charge rate rule... what happens if you have too much panels? other than buying more batteries, what can you do to prevent waste?

    you can set the charge controller to limit the charge, if so where does the extra current go?

    Doesn't "go" anywhere. Electricity is a *potential*, if you don't use it, it doesn't exist.

    What you are describing is a situation where the potential is higher than the usage, in which case, all that can be used will be, and the unused potential just does nothing.
  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: Too many batteries, not enough panels?

    Try using only 4 of them for now. Hook up 2 in series, and two pairs in parallel to get 12V 550Ah bank. Make sure you turn off your inverter when you leave. If you only stay there for the weekend, then:

    135 W * 4.11 = 555 Wh (solar harvest per day)
    555 Wh * 5 days = 2774 Wh (harvested during the week)
    2774 Wh * 2 = 5550 Wh (battery capacity needed for 50% DoD)
    5550 Wh / 12 V = 462.4 Ah (battery Ah needed)

    Closest you got is 275 Ah * 2 = 550 Ah. Going any larger does not make sense, you don't have enough solar panel power. If you could get another 135W panel, that would give you more breathing room.
  • System2System2 Posts: 6,290 admin
    Re: Too many batteries, not enough panels?

    Thank you all for your help! I think AntronX has a good point. I'll start with four of the batteries and learn about my own power consumption and the rates at which it is replenished. I'll work on PV expandability later after I get a handle the basics.

    Just one more thing concerning the batteries I will NOT be using. Any tips on proper storage?
  • jeffkrusejeffkruse Solar Expert Posts: 205 ✭✭✭
    Re: Too many batteries, not enough panels?

    How are you going to equalize the batteries? That takes a lot of power.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Too many batteries, not enough panels?
    mshen11 wrote: »
    following the charge rate rule... what happens if you have too much panels? other than buying more batteries, what can you do to prevent waste?

    you can set the charge controller to limit the charge, if so where does the extra current go?

    too many amps from too many pvs will stress a battery with, at minimum, higher maintenance. damages could occur with warping of plates and possibly pushing out on the casing. the controller will not limit bulk current to a battery unless it is over its own limitations as a controller. it won't know that one may have a 10ah battery that can't take 60a into it for example.
    as to waste it is rare that people have so many pvs as to have waste and one usually tries to size things up for cost reasons mostly, but to align components to work most efficiently also. i suppose if one has waste that more loads could be placed onto it. if not then the alternative may be to feed the grid with it if it's available.
    now some controllers can act as a diversion type and redirect the current after bulk charging, but you still need loads for it so sizing a system is important and knowing the capabilities and functions of components is very important. it would still pass up to the controller's limit the full pv current available until bulk charging is done and that could be too much for a mismatched system that has too small of a battery bank capacity.
    also note that when controllers go into other charge stages that they do limit the current some.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Too many batteries, not enough panels?

    I would seriously consider NOT using just four. The reason being is that other batteries should not be added to an existing string. Additionally, how are you going to keep the others charged?

    Perhaps use some, sell others and buy some more PV so that they come closer to matching. As for tips for proper storage,, the short answer is don't store them. If you must, store them cool, keep them on float charge.

    I suggest you do a search on this site for the battery FAQs and read and learn all you can about batteries. (I would provide the link, but my satellite connection is deathly slow right now.

    Tony
  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: Too many batteries, not enough panels?

    I doubt his batteries will get unbalanced if he tries that method out for few weeks or few months. Few years? - yea. I gave him that advice to get started with already existing equipment and not spend any more money. Try it out and get real world experience, then see if you need more components.

    To goodhustle, make sure you have a volt meter to monitor battery voltage. Print out this: http://otherpower.com/otherpower_battery_metering.html and take it with you to the cabin. Note that these voltages are true only when your battery is unloaded, or disconnected from loads and while not charging. You don't want to take your battery below 50%. Your unused batteries need to be fully charged before stored and then top off their charge every month or hook them all up for 12V and put them on 1A trickle charger. I assume you are not going to store unused batteries at your cabin.
  • System2System2 Posts: 6,290 admin
    Re: Too many batteries, not enough panels?

    Thanks for the info!

    I had stars in my eyes when I bought all these batteries. But I really want to subsribe to the "never use more than 20% of your bank's capacity" advice. I was hoping these would be a 10 year investment with much expandability possible.

    I'm thinking about 2 x 200 watt Evergreen panels of the 12 volt flavor. Please send word if you think this is a bad move. (200w seems alot for a 12v panel)

    But for now I'll just use 4 of the batts and keep the system small as its obvious I have some things to learn before sinking any more money!

    If I dont purchase any more PV's in the next couple months I figure on selling/trading the unused Batts. The info on Batteries/metering/storage is very helpful. Thank you all again!:cool:
  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: Too many batteries, not enough panels?

    Yea, 400W is much better. 400W * 4.11 * 5 days = 8.22 KWh (per Week) * 2 = 16.44 KWh (battery @ 50% DoD) / 12V = 1370 Ah.

    12V @ 275 Ah * 5 = 1375 Ah. That's 10 of your batteries.
    400W * 0.735 = 294W * 0.97 = 285W / 12.5 = 23A of solar charging current. / 5 = 4.6A per battery. Don't know if that's enough to euqualize your batteries. It's enough for me, but I don't use flooded batteries.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Too many batteries, not enough panels?

    you still will want to keep the other batteries in good shape with a float charge on them or the batteries will self-discharge and also sulfate.
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Too many batteries, not enough panels?

    Check these links out!

    http://www.batteryfaq.org/

    http://www.windsun.com/Batteries/Battery_FAQ.htm#Lifespan%20of%20Batteries

    (My connection is better today!)

    Tony
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