Interesting battery capacity statement.

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gww1
gww1 Solar Expert Posts: 963 ✭✭
in Off Grid Solar & Battery Systems #1
I have an 804 hour fork truck battery from giant battery. I was surfing their web site and though this statement was interesting.
80% Depth of Discharge (DOD): A Fully Charged battery, discharged to 1.75 volts per cell.

· Example: A 600AH battery (rated @ 6hrs) will provide a full 600AH over 6hrs and still have 20% reserve capacity that is never used.

· You are not limited to 80% of the 600AH (480AH) as one might assume.

· Discharging a battery below 80% will drive the voltage below 1.70 volts per cell, which will overheat and damage the battery and any equipment it is powering.

· Sealed batteries usually have a 50% depth of discharge, and will be damaged by an 80% discharge.

· All GB batteries have an 80% depth of discharge, but can be used in 50% (or less) applications.

1.75 volts is 42 volts on a 48 volt battery bank. I was under the assumtion that 630 amps taken was about 80% discharge but this says 804 amps taken at about 40 amps per hour is 80% discharge. That about 38kwhs of capasity.

What keeps confusing me is that if I take 230 amps out at an overall rate that I am sure is an average of 20 amps per hour with maby a few high draw hits, my trimetric will say over 70% soc and the sg will be at 1.220.

If I take 330 amps out the battery meter will read 60 soc but the sg will read at or below 1.150 which is my lowest reading on the hydro meter. This is right at 100% discharge.

The manufactuer say use the voltage while on the fly but the paper sent with the battery says take out of service at 1.170 sg.

My voltages are holding well and reflect the trimetric soc. I have my battery low voltage settings at 44.8 but most times see the low voltage on the cc at 45.7 or above when the battery actually goes off line. The battery always rebounds above 48 volt and mostly closer to 49 volts

I always wondered about that.

I have the grid and am not really having issues with too low of capasity but would eventually like to understand what is really going on. I have taken 500 amps out one time and had issues with the inverter not liking below 44 volts. Mostly I take between 200 and 300 amps out daily.

Just found the statement interesting but still don't really know where I am and what it means compared to my real life experiances.
gww
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    Re: Interesting battery capacity statement.

    A vendor can rate a battery to the job that it is expected to do--I.e., rate for 600 AH over 6 Hours and still have a 20% reserve.

    Certainly reading all of the information you can get on your battery bank (brand, model number, etc.) is very important.

    We here try to design to a "generic" set of assumptions. Gets us close enough to make accurate estimates on the hardware/system design and capabilities. When picking the final equipment--Need to read specifications and adjust where needed.

    One interesting thing about Lead Acid batteries--They are about 100% efficient when working with Amps and Time... I.e., take out 100 AH, you need to replace it with ~100 AH.

    However, energy wise, it is only 80% (or a bit better). We draw 100 AH at 12.0 volts (average discharge) and recharge 100 AH at ~14.5 volts:
    • Energy Eff = 100 AH * 12.0 volts / (100 AH * 14.5 volts) = 0.83 = ~83% cycle efficiency (average worst case for flooded cell lead acid battery)
    Charging from 20% to 80% State of charge is usually very efficient. Charging from 90% to 100% is very inefficient. And equalization is near 0% efficient (gassing water to hydrogen and oxygen and charging "weak" cells).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
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    Re: Interesting battery capacity statement.

    Bill
    Thanks for the response. I read all the battery manufacture stuff trying to learn if things are working as they should. They many times give conflicting advice. I just can't decide what to really rely on and still use to the max without me being the one to cause damage.

    My voltages hold well but the sg is all over the place. Once a month I want to go to 50% dod and once a month I want to EQ for guaranteed 100% soc. I hear this is best for "my" battery health. The battery maker believes every thing is going well and my plates are still not fully formed. I just can't decide if I am getting a 50% discharge as the sg says 100% dod and the trimetric says 60% dod and the amp hours used says 60% dod and the voltage says 60% dod. The charging is easier as the sg goes up pretty strait although some times the time and voltage to get it to go up is different. At the very top of the charge cycle I get a bit erratic readings but can usually tell the top as everything quits raising for a while.

    The statement from the manufacture about capacities make my batteries seem to be in worse shape then I thought if I go by sg but I am smart enough to know I just might be too dumb to realize what is really going on.

    I do know that regardless of sg I have had the battery to 500 amps out and did not seem to have heating and "damage?" but at 300 amps out it shows sg at 100 % discharge.

    The battery is still doing fine for my uses but I hate not knowing for sure what I am doing IE; good or bad?

    Hey, thanks for the response.
    gww
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