Learning to identify end of life for Lead acid batteries.

ShenanigenZ
ShenanigenZ Registered Users, Users Awaiting Email Confirmation Posts: 1

I am kind of new to how to test batteries and am coming along issues with pre-existing system. What we have is six 4v batteries for a 24v system. These batteries have a history of heavy use and have been discharged fully which I know is a problem. So when I measure across each battery I am getting 4v and seeing 24v when they are all connected. The specific gravity is good with each cell being around 1.25. I feel like my meter is rounding the actual voltage because when I turn it on the inverter does register it around 25v. The problem is once you place the bank under load it does well but then dies quickly and certain cells will register 0v. The inverter never registers full charge just constant float charging.

My question is are these batteries toast? I get confused because with the voltage being good and the specific gravity being good I just thought the battery should be good. We have tried an equalization charge but that hasn't helped. I am kind of new to batteries so and information would be awesome.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin

    Welcome to the forum SZ.

    First answer is your batteries are probably toast. Especially if they are over a few years old. (all in my humble opinion).

    You really need to be measuring/logging voltages in the XX.YY accuracy. 24 volts is not accurate enough to understand what may be happening to your batteries.

    More or less:

    • 21.0 volts is dead (on a 24 volt FLA battery bank)
    • 23.0 volts is roughly 50% discharged when measured under moderate loads (roughly 5% to 20% rate of discharge)
    • 24.0 volts is very rough 50% discharged (after setting for 3+ hours at ~75F/25C and measuring the resting voltage
    • 25.4 volts resting is ~100% state of charge for a Flooded Cell Lead Acid battery bank).
    • 27.0 volts is the "float voltage" for FLA bank (long term maintenance charging current)
    • 29.5 volts held for ~2-6 hours is "Absorb" charging (bringing battery from ~80% SoC to 100% SoC (state of charge)
    • 30-32 volts is the float voltage (aiming for 5% to 2.5% rate of charge once a month to equalize).

    Note that rates of charge are based on 20 Hour Battery Capacity specification (i.e., 100 AH battery * 2.5% rate of charge = 2.5 amps charging current).

    Monitoring the specific gravity of each cell is the "gold standard" for determining state of charge for FLA batteries (AGM, Sealed batteries cannot get into cells to measure SG in normal operation).

    For example, take the above numbers and divide by 6 to get your 4 volt battery numbers. For example, your full charge 4 volt battery should be 25.4 volts / 6 = 4.233 volts per battery (or divide by 12 if you can get the per cell voltage of 2.12 volts per cell).

    With your batteries, you have 6x 4 volt batteries (or 12x 2 volt cells, if you can get to their intercell bus bars) and measure their voltages. And because you have 6 (or 12) otherwise identical devices. How they behave during different phases of operation can tell your a lot.

    For example during discharge. The first 4 volt battery that drops below ~3.83 volts (23.00 volts battery bus under load), that is the weak cell. Ideally, if all cells matched (same age, same capacity), all cells should drop evenly.

    During charging, if you have a "low voltage cell", it could be a 1) shorted cell or 2) simply a less state of charge cell that needs "equalizing" to be brought up to the same SoC as the rest of the cells.

    Or, if you have cell with much higher voltage, it could be an 1) open cell (high resistance), or 2) a cell that is much higher SoC than the rest of the cells ("good cell/battery", in the midst of a bunch of weak/under charged cells.

    There are lots of ways to "murder" an FLA battery. And there are ways to help get a longer life (proper charging, not letting a battery with less than ~75% SoC sit for days/weeks between charging), over charging (too high voltage charging, too long of charging at high voltage), over charging and overheating, letting electrolyte get too low and expose plates to air/oxygen, simply lots of cycling over the years, or a battery bank that is aging out.

    I guess you are in BC Canada... Cold temperatures reduce battery activity (storage capacity), but for every 10C below 25C, the batteries will age 1/2 as fast.

    Typically, you can try recovery of your battery bank (review charging current/voltage/time under charge; equalization). And look at your hardware and usage (try to get >~90% SoC once to three times every week (don't go for 100% SoC every day--That is hard on FLA batteries).

    And their is reviewing your energy needs and system. For a FLA battery bank, ideally with solar you are looking at 10% to 13%+ rate of charge. 5% rate of charge can work for a summer/weekend cabin. 10%+ is better for full time off grid.

    For solar and full time off grid, the (optimal) FLA battery bank design usually works out for 2 days of storage (no sun) and 50% maximum discharge (for longer battery life under typical operation). Or your Battery bank AH * Voltage is ~4x you daily load.

    Sizing the solar array to maintain your loads and have your XX% state of charge (most FLA deep cycle storage batteries recommend a 10% minimum rate of charge) and for your energy usage per day Vs hours of sun per day (by season). All pretty easy to do (if you "know" your loads).

    For our friends up North, it can be difficult to get enough sun for days/weeks at a time during winter--So you are left with running the genset (and burning $$$).

    And lastly, which is really firstly, you should look at energy conservation. It is almost always cheaper to conserve (turn stuff off, find the most efficient load for the purpose, insulation, etc.) than to generate the energy.

    Let us know how you want to proceed... Lots of things we can talk about, but want you to lead us where you want to go...

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset