Capacity testing

CALLD
CALLD Solar Expert Posts: 230 ✭✭
in Solar Beginners Corner #1
I'm keen of giving my 24v 400AH battery bank a capacity test, but want to avoid running them down to the inverter's 20.8v LVD unnecessarily. Is this possible? They aren't proper deep cycle batteries so I don't want to punish them anymore than being in a RE application already is. Is it possible to use data gathered from a half-hour run at a C/3 load? It's the most brutal test I can think of. It would involve me hooking up a 2000w electric heater and a few other appliances to bring the total load to 2500w, which is the maximum rated power of my inverter. If I take voltage readings every 5mins and plot a graph could I tell anything from the discharge curve? Maybe like trying to project where the final cut-off point will occur? Obviously would have to take Peukert's law into account.

Has anyone done this?
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #2
    Re: Capacity testing
    CALLD wrote: »
    I'm keen of giving my 24v 400AH battery bank a capacity test, but want to avoid running them down to the inverter's 20.8v LVD unnecessarily. Is this possible? They aren't proper deep cycle batteries so I don't want to punish them anymore than being in a RE application already is. Is it possible to use data gathered from a half-hour run at a C/3 load? It's the most brutal test I can think of. It would involve me hooking up a 2000w electric heater and a few other appliances to bring the total load to 2500w, which is the maximum rated power of my inverter. If I take voltage readings every 5mins and plot a graph could I tell anything from the discharge curve? Maybe like trying to project where the final cut-off point will occur? Obviously would have to take Peukert's law into account.

    Has anyone done this?
    Well, 20.8 V is death to your Batteries, that is meant to protect the Inverter more than the batteries. A 24V bank should be about 23.8V - 24V lightly loaded @ 50% dod. If you don't have a battery monitor to count the amp hrs you can use a known load over a period of time to measure the amps removed to the 50% level. Many times a real loading is more meaningful. How long do you expect them to last and what are your loads. If you expect them to last 20 hrs and they only last 10 hrs they you have to examine where you are at.

    If they are NOT deep cycle batteries, you definitely need to be careful.

    This is for Surrettes Deep cycle.

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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #3
    Re: Capacity testing

    Thank you @Blackcherry04. I am concerned that these hybrid type batteries may have lost some of their capacity because the voltage under load seems to drop much faster than I expect. I'm getting down to 24.0v after removing just 50AH from them after they've been charged to the point where they will no longer accept any meaningful current. That's a 400AH battery with a +/- 15Amp load on it for 3 hours and 20mins. I cannot verify their SOC with a hydrometer because they are sealed, but they do have those little green magic eyes which are showing "fully charged" so that's the closest thing I have to a SG reading. The furthest I've pushed them is by discharging 3.6kw/h as shown on a kill-a-watt meter from them over 16 hours with no incoming charge. The voltage under very light load was 23.2v. I then applied a heavy load of 2300watts on the inverter which dragged the voltage down to 22.2volts after 5mins. The SOC at the point should have been around 50%. Seems a bit low when looking at the discharge curves for deep cycle batteries. Unfortunately as is the case with many cheap and nasty products, information from the manufacture is non-existent. I can't even get charging instructions for these batteries nevermind discharge curves...
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #4
    Re: Capacity testing
    CALLD wrote: »
    I cannot verify their SOC with a hydrometer because they are sealed, but they do have those little green magic eyes which are showing "fully charged" so that's the closest thing I have to a SG reading.
    If they have a magic eye SG indicator, then there must be loose fluid electrolyte.
    Chances are that you do not have truly sealed batteries (not freely vented, able to be mounted in any orientation). You probably have "maintenance free" batteries which are basically normal FLA batteries that have extra headroom above the plates for more electrolyte and cell vent caps that are very hard to open.
    You can probably pry them open carefully to check SG, but it is not clear whether that would really tell you anything.
    They may have high internal resistance as well as some loss of capacity.
    SMA SB 3000, old BP panels.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #5
    Re: Capacity testing
    CALLD wrote: »
    I then applied a heavy load of 2300watts on the inverter which dragged the voltage down to 22.2volts after 5mins. The SOC at the point should have been around 50%.
    Thats a huge load, The voltage sag could be from the batteries being unable to sustain their power or the cabling not being capable of that much loading. Parallel strings tend sag if all the connections are not up to snuff.

    Post a picture of your Battery setup and let us see it. What size cables do you have ??
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  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    #6
    Re: Capacity testing

    Thank you Blackcherry04 and inetdog, your info is definitely helping me to form an opinion on what may be the problem. I've long suspected that the internal resistance may be very high in these batteries, but it may also be down to how I've set things up since the very beginning. The battery bank has sort of evolved along with my learning curve. It has also grown. I started out with half the amount of capacity in the beginning but quickly added more as I realized that DOD was such an important issue especially with these batteries. I've pretty much taken nearly every rule with regards to mixing new and old batteries, wiring in parallel strings and charging parameters and "bent them". These batteries have really been my training wheels. Just trying to determine how long they can still keep on going before some sort of cascading failure occurs.

    If it's of any use these are the batteries in question:

    http://www.battery.co.za/wp-content/themes/battery/dl/Excis%20April%202012.pdf

    This is pretty much all the manufacture gives...
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