Maximum battery discharge rate?

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vtmaps
vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
in Solar Beginners Corner #1
My energy conservation efforts have gone so well that I can afford to run power tools (table saw, circular saw, etc) from my batteries. I am concerned, however, about maximum discharge rates. I have read repeatedly in this forum that the max discharge rate for flooded batteries is C/8, and the max surge is C/2.5. My trimetric meter shows a discharge rate of C/4 for my table saw, and this may be sustained for 4 to 5 minutes at a time.

Would someone please explain why C/8 is the max recommended discharge rate? What are the consequences of exceeding this rate? I do understand that the batteries are less efficient (Peukert) at high discharge rates. I also understand that battery temperature is increased at high discharge rates. Are there any other issues?

I just reread the Wind-Sun Battery FAQ to see what it says about max discharge rate: Not much, but there is this quote:
Battery FAQ wrote:
It will not hurt a deep cycle battery to be used as a starting battery, but for the same size battery they cannot supply as much cranking amps as a regular starting battery.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #2
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    Re: Maximum battery discharge rate?

    As far as I understand it, they're not "hurt" at all by a high discharge rate- they'll just have a reduced capacity, as you've pointed out. The maximum rate will depend on the battery type, forklift batts for example have their capacity rated at C5, so that's a pretty normal discharge rate for them.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #3
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    Re: Maximum battery discharge rate?
    stephendv wrote: »
    As far as I understand it, they're not "hurt" at all by a high discharge rate
    You told me what I wanted to hear :-) Thanks! --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
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    Re: Maximum battery discharge rate?

    it is also possible to overheat batteries and even warp the plates. i don't think it will occur if it is a case of discharging just around the c/8 rate for most fla types. (12.5% actual, but we often go with 13%) this is more apt to happen much higher in the discharge % area and can often parallel a battery's charging ability and as we've stated before, this can vary from one battery to another. i can say most would handle up to about 20%. maintenance on such a battery may be higher and i normally would not recommend doing this every day as it could take some lifespan away from the battery with high stresses like that.
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  • marsofold
    marsofold Solar Expert Posts: 45 ✭✭
    #5
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    Re: Maximum battery discharge rate?

    My Johnson Control golf cart batteries are officially rated at 75 amps for 100 minutes. Most of the electric car enthusiasts routinely cruise at about 90 amps and briefly accelerate at 150+ amps. Since golf cart batteries hold 220 amp-hours, that means that it is perfectly safe to discharge them at C/2.5 and to surge them at C/1.5. I personally have run my well pump for ten minute intervals at C/4 without any problem. Be aware of two things, however: the electric car people often find it necessary to replace their batteries at two year intervals. And when pulling heavy currents the voltage output will drop considerably. When I ran my well pump at C/4, I saw my output voltage quickly drop, then maintain at 11.6 volts. Too much lower and the inverter would have kicked off. Of course an hour later (at night without any charging) the voltage recovered back up to 12.5 volts. Ratings notwithstanding, I had probably discharged 1/4 of my total battery storage capacity in that 10 minutes. So while it can be done, the capacity loss at high current levels is significant.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
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    Re: Maximum battery discharge rate?

    marsofold,
    if you notice the 20% i said was relative and equal to about a c/5 rate so it is a sizable draw. now some batteries can dump more high currents for short periods and they tend to lean towards starting batteries and not deep cycle. gc batteries are a cheap tweener and the fact that you state they are only lasting a few years may be reflective as well as reinforcing of what i said. just because you can go c/2 or c/3 does not mean you should do it as that is still tough on a battery.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #7
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    Re: Maximum battery discharge rate?
    niel wrote: »
    just because you can go c/2 or c/3 does not mean you should do it as that is still tough on a battery.
    But why is it tough on the battery? If heat is the only issue I am not worried... I only plan to draw C/4, and only for a few minutes, and only in the winter when my batteries are at 55° F or lower. I am trying to avoid using my generator for just a few saw cuts in the shop, but if its going to take years off of my batteries, I won't do it.
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
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    Re: Maximum battery discharge rate?

    heat is always an issue as batteries do not last long when made hot. capacity is also a bit less due to the high discharge rate too so it is taking away much of the cycle life of the battery. would you do this to your car and take it up to 120mph every time you use it even though it can and it is just for a few minutes at a time? do this often enough and come back to tell me how it's doing after just a few years of this compared to the same car that is not abused by taking it to that extreme. batteries aren't magic boxes.
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  • marsofold
    marsofold Solar Expert Posts: 45 ✭✭
    #9
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    Re: Maximum battery discharge rate?

    Here is info which shows the surge and draw of a typical 3/4HP pump (like the one I have). I know that it is pretty much a guess, but would running one for 10 minutes abuse golf cart batteries (I have six total with three paralleled 12v 220AH strings -total 660AH)? Note that it's only 1/5 of a second on the surge...
    Attachment not found.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #10
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    Re: Maximum battery discharge rate?
    marsofold wrote: »
    Here is info which shows the surge and draw of a typical 3/4HP pump (like the one I have). I know that it is pretty much a guess, but would running one for 10 minutes abuse golf cart batteries (I have six total with three paralleled 12v 220AH strings -total 660AH)? Note that it's only 1/5 of a second on the surge...
    If your inverter can handle the initial surge, I think you will be drawing your batteries down at about 120 amps before inverter losses (6amps AC x 240 volts / 12 volts DC = 120 amps). As battery voltage drops below 12 volts the draw will be higher still. Therefor you will be discharging by at least C/5.
    Have you actually tried this to see if the inverter can handle the surge?
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #11
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    Re: Maximum battery discharge rate?

    a fraction of a second surge should be fine for the batteries and as vtmaps said, it is the inverter that may be picky about it if it picks up on a voltage sag that it deems to be too low. odds are the surge and voltage drop happened for too short of a timeperiod for the inverter to care.
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  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
    #12
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    Re: Maximum battery discharge rate?
    marsofold wrote: »
    Attachment not found.

    Just another thought;

    This graph shows the "Well Pump Starting Current", not necessarily the battery current when starting said pump.

    Correct me if I'm wrong but the inverters with high surge current ratings are able to deliver better surges than other systems because of larger internal capicator banks. This should help decrease the 0.2 second peak current draw from the batteries and cables. No?

    Alex
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