Battery question: Enough Kw/h from Amp/h?

SurfpathSurfpath Solar Expert Posts: 393 ✭✭✭
Hi,
I think I am regressing in my solar power knowledge.

But perhaps discussing this question will help me get sorted out:

There seems to be a signif. price difference between 420-450 amp/hr and 370-400 amp/hr batteries. Based on the below values and estimates, I'd like to see if I could drop down into the latter range.....

Estimates
  • Est energy needs: 6-7 Kwh/day
  • Est. daily depth of battery discharge: 30%
  • Est. Days with no PV/hydro: 1
  • Est. 5 hrs of sunlight/day

System
  • 8 x 6v batteries (48v).
  • 1980w of PV panels
  • 180w of hydro (will run in the evening - I know my sig says differently, but, say a very conservative 1 kw/hr a day)

I have read that a dual energy system, say PV plus wind/Hydro, can help one to reduce battery bank size, but I'd like to confirm this before pulling the trigger.

:cool:SP
Outback Flexpower 1 (FM80, VFX3048E-230v, Mate, FlexNetDC) 2,730watts of "Grid-type" PV, 370 AmpHrs Trojan RE-B's, Honda 2000 watt genny, 100% off grid.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery question: Enough Kw/h from Amp/h?

    Well it's simple (no it isn't). :D

    7 kW hours per day (including running the inverter?) on 48 Volts is 146 Amp hours. Unless you are not including the inverter and remember the conversion efficiency difference. Then it goes more like 146 / 0.90 (efficiency) + 10 (power used by inverter) = 172 Amp hours.

    Now if you want that to be 30% DOD the battery bank needs to be (172 / 0.30) 574 Amp hours. If you want another full day of power without charging and same DOD you have to double it. (This is why I prefer using 25% DOD and knowing you can take it down to 50% on day two if needed).

    But that is based on all power coming from the batteries. Nicely conservative and definitely will work. But with load shifting you can either get more power than you expect or use a somewhat smaller battery bank. So the question is: how manageable are your loads?

    The difference can be looked at from the PV output:
    You've got 1980 Watts of panel. Normally that will operate at an average of 77% efficiency. Over 5 hours that should be 7.6 kW hours DC or 6.8 kW hours AC (consumption and conversion of inverter estimated). But in a typical battery system once the batteries are charged the panels produce nothing of significance unless there are loads operating at that time. Thus the over-all end-to-end efficiency drops to near 50%. That looks like this: 1980 Watts * 5 hours * 0.52 = 5 kW hours AC.

    This is why we use the conservative numbers; better to have to much and not use it than to run out.

    Now if power consumption were steady it would be an easy thing, but chances are you do not use a consistent (7000 Watts hours / 24 hours) 292 Watts. If you did you could leave out the 5 hours during charge time (assuming enough panel to provide both charging and loads) or 1460 Watt hours worth (about 30 Amp hours @ 48 Volts).

    Since you also have some hydro power available you can count on that. But it is not very much at 180 Watts. If it were on for 24 hours it would manage another 4kW hours, which would make a significant difference. Again the problem is the the difference between when the power is produced and when it is used.
  • BB.BB. Super Moderators, Administrators Posts: 29,509 admin
    Re: Battery question: Enough Kw/h from Amp/h?

    What are your maximum sustained and surge power requirements... Very roughly for flooded cell batteries:
    • C/8 sustained rate: 400 AH * 48 volts * 0.85 inverter efficiency * 1/8 rate = 2,040 watts recommended max continuous load
    • C/2.5 surge rate: 400 AH * 48 volts * 0.85 inverter eff * 1/2.5 rate = 6,528 max surge rating (starting well pump, etc.).

    Just rules of thumb--But a good starting point to ensure your load requirements match the typical battery bank/wiring for long term reliability.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery question: Enough Kw/h from Amp/h?

    Another way of looking at it: you're talking about a difference in capacity of 50 Amp hours. At 48 Volts that's about 2.4 kW hours. That's a lot of power. In fact it's about as much as I use in total at the cabin.
  • SurfpathSurfpath Solar Expert Posts: 393 ✭✭✭
    Re: Battery question: Enough Kw/h from Amp/h?
    "the problem is the difference between when the power is produced and when it is used"

    Coot,
    A small lightbulb just came on. You are right that I can't assume efficient power production during the entire day from the PV because "once the batteries are charged the panels produce nothing of significance unless there are loads operating at that time." That hadn't really sunk in 100% before. The low 50% estimated end-efficiency you state now makes total sense...

    Likely, prime power-use for me would be 6pm-11pm during the weekdays. A chunk of my estimated 6-7kw/h will be used during this time. The hydro will help to defray some of this, however that little lightbulb tells me that I should really try to shift more of this load during the day. To make my night-time draw on this proposed battery bank less.

    What will run during the 8am-6pm day ought to be fairly consistent . . . the 1.2 kw/d fridge and a 1.2 kw/d chest freezer. I could turn the temp down on the freezer in the morning and then back up at night when I get home. But this seems like a bit of a pain to remember. I could also start a load of laundry in the morning at 7:30am, but this is not really in prime sunshine time yet.

    Perhaps I need to 1) revisit my loads again to better match my desired battery bank. 2) revisit the concept of only running the hydro in the evenings, or 3) throw down some more greenbacks and keep the bank around the 500 mark.
    -SP
    Outback Flexpower 1 (FM80, VFX3048E-230v, Mate, FlexNetDC) 2,730watts of "Grid-type" PV, 370 AmpHrs Trojan RE-B's, Honda 2000 watt genny, 100% off grid.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Battery question: Enough Kw/h from Amp/h?
    Surfpath wrote: »
    I could also start a load of laundry in the morning at 7:30am, but this is not really in prime sunshine time yet.
    -SP

    Some washing machines and most dishwashers can be set up for a delayed start of up to 8 hours. That, or an external timer, would let your washer start after the sun is high even though you are not there.
    A dryer is another matter entirely, since they tend to have safety switches that you have to press to start. (I suppose you could wire that to a relay too, but it would not be a good idea!)
    SMA SB 3000, old BP panels.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Battery question: Enough Kw/h from Amp/h?
    Surfpath wrote: »
    There seems to be a signif. price difference between 420-450 amp/hr and 370-400 amp/hr batteries.

    I have noticed that also. It's because mass produced L16 (370-400ah) are cheap. Some folks prefer to undersize their batteries to take advantage of mass production prices. Sure, they might not last as long, but they also cost less.

    For example, suppose that you buy 8 L16 (375 ah) for $1800 and they only last 5 years. Compare that to buying 24 crown 2 volt (500ah) batteries for $14,500 which last for 15 years. Or buy 8 Rolls s600 (450 ah) for $3350 and get 10 years. Of course, the lifetimes I used in the example are wild guesses... any of those batteries could be made to last less than a year! That's the big problem... if we knew how long our batteries were going to live, we could make more rational purchasing decisions.

    Since you are shopping for your first set of batteries, I would weight the cheapness factor a bit higher.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SurfpathSurfpath Solar Expert Posts: 393 ✭✭✭
    Re: Battery question: Enough Kw/h from Amp/h?
    vtmaps wrote: »
    Since you are shopping for your first set of batteries, I would weight the cheapness factor a bit higher.

    VTmaps - that is a good point (1st time batteries so keep it $$ reasonable)

    I see good prices out there for Deka and US battery L16's. Perhaps I should be considering these?

    I hear that these latter L16 brands are so called "3-5 year batteries," vs. Trojan L16's which are apparantly "5 year" batteries, vs. Surrete L16's which are supposedly "7 year batteries."

    I realize that longevity is actually relative to how well you manage your charging and baby your batts. I witnessed friends using up a nice bank of "5-7 year" AGM's in 24 months.

    So far I am thinking about a Trojan "signature line" L16, or a bank of cheaper Deka/US L16 Batts. Again, your advice has helped me to narrow it down.
    -SP
    Outback Flexpower 1 (FM80, VFX3048E-230v, Mate, FlexNetDC) 2,730watts of "Grid-type" PV, 370 AmpHrs Trojan RE-B's, Honda 2000 watt genny, 100% off grid.
  • Eric LEric L Solar Expert Posts: 262 ✭✭
    Re: Battery question: Enough Kw/h from Amp/h?

    I faced a similar decision early this year and went 'cheap and cheerful' with 8 of the smaller L-16s, because the price difference locally was quite significant. After less than one year I can't say much about reliability except to say that battery sg and voltage are doing fine (like new, actually). If they last at least 18 more months I'll have been better off financially going this route. So far, so good.
    Some washing machines and most dishwashers can be set up for a delayed start of up to 8 hours. That, or an external timer, would let your washer start after the sun is high even though you are not there.

    I would add to this comment that something I'm also finding is that the latest generation of charge controllers (I have Midnite Classics) have a lot of flexibility in how you can set up opportunity loads using a solid-state or mechanical relay plus a little signal cable (like cheap Cat 5 cable). E.g., you can program them to activate a mechanical relay to turn on a load (washer, heater, air-conditioner, etc) when the batteries have been in absorb voltage (say) for x minutes or hours (where "x" is some value you choose), and to turn off loads when they fall below a set voltage (like float voltage). Or, you can program the Midnite Classic to signal a solid-state relay to vary the voltage to a resistive load like a water heating element. This can let the element use whatever surplus power is available from the solar panels without interrupting the normal battery charging cycle or other loads. In other words, with the right charge controller you'll probably find that there's some pretty cool stuff you can do with opportunity loading.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Battery question: Enough Kw/h from Amp/h?

    i wouldn't advise being away from a washing machine that is running. several times i have seen failures occur while i was out with the result of water all over the place. the simplest is the water hoses breaking, but more serious things do happen like transmissions going. the water lines i got tired of them breaking so i opted for the metallic mesh reinforced hoses. i haven't had a hose failure in 15yrs now and still counting.;)
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Battery question: Enough Kw/h from Amp/h?
    niel wrote: »
    i wouldn't advise being away from a washing machine that is running. several times i have seen failures occur while i was out with the result of water all over the place. the simplest is the water hoses breaking, but more serious things do happen like transmissions going. the water lines i got tired of them breaking so i opted for the metallic mesh reinforced hoses. i haven't had a hose failure in 15yrs now and still counting.;)

    I also have never had a failure with the mesh reinforced inlet hoses, but I have had the drain hose fall off the pump outlet at the bottom of the machine. (No way to shut that off, so the entire contents of the washer ended up on the new hardwood floor. And we were home at the time!)
    SMA SB 3000, old BP panels.
  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Battery question: Enough Kw/h from Amp/h?

    yes, when they go they sometimes go in royal style don't they? when i was out and the hose burst it was continuous water running until i got back. i do have a basement with a floor drain, but it sprays and travels a bit before reaching the drain, plus all of the wasted water upped my bill and i get billed twice for my water as it is outgoing on a sewage bill based on water consumption. yes, if i had a source other than the local water company i wouldn't pay a sewage bill.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Battery question: Enough Kw/h from Amp/h?

    Am I the only one besides Cariboocoot who manually turns on the water pump to fill a small storage or pressure tank. My shurflo pump is on a spring-wound timer. If my tank is empty I just wind it up to 15 minutes.

    If I have a leak, it can't be larger than the 30 gallons my tank holds. In order to keep this thread on the topic of batteries, I mention that a water leak cannot turn on my water pump and run down my battery.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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