How to measure battery capacity?

2twisty
2twisty Solar Expert Posts: 199 ✭✭✭
All batteries lose capacity as they age. My GC2s, when new are rated at 208Ah.

My Charge controller knows the Ah capacity of my bank because I told it that I had 208Ah. As the batteries age, that number won't be accurate, and the SOC% reading that my charge controller reports will be inaccurate.

Is there a procedure by which we can determine the current Ah capacity of a battery bank once we've got them charged to full SG? Some way that we can then update the Charge Controller on some regular basis so that the SOC% that it reports is at least reasonably accurate?

This procedure would also be useful in plotting the lifespan of a bank. You could plot the "death curve," if you will.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How to measure battery capacity?

    I am not sure there is a good way to do what you ask.

    The proper answer is to run the application +10% more discharge. If the battery survives the test, then you have a "good battery" for your application.

    Say you want to discharge to 50%, add 10% more so discharge to 45% (calculated load). For example. a 100 AH battery bank, discharge by 55 AH under normal loads. If it works, then the battery is OK.

    A simple method you might try--Under standard temperature and and state of charge for your bank--Put a light load on the battery bank and measure the voltage (Va and Ia). After a bit of time, increase the load for a few minutes and measure the new voltage (Vb and Ib)--Then do this calculation:

    Battery Resistance = Va-Vb/(Ib-Ia) = Battery apparent resistance (probably on the order of 0.001 to 0.010 Ohms or so)

    You can do this once a month or several times a year and graph the results. See if you can find a pattern to aging (flat then curve up, straight line increasing, curved line increasing, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    Re: How to measure battery capacity?

    It's a difficult one to determine without running your batteries to the point where the inverter shuts down due to low voltage, however if you choose to go this route in determining your battery capacity it will give you the most accurate result at the expense of some battery life. Just make sure to recharge them immediately at the C/10 rate after your test. Note that at the point where inverters usually shut down there is still "20% or so of the Ah remaining in the batteries"

    A less accurate but far less harmful test is the "rapid discharge test". Put the heaviest quantifiable load you can on the fully charged battery and let it run until it has removed about 25% of the rated Ah of the battery. After removing the load let the battery sit for a few hours and check the voltage. In the case of a 12v battery the voltage should still be around 12.5v if it's in good condition. Also during the test take note of the voltage sag.

    As far as plotting a "death curve" is concerned I think it may be a hard trend to spot as capacity readings can vary due to charging conditions (high current rapid charging on sunny days vs slow trickle charging on cloudy days). Battery temperature will also mess with capacity readings. The onset of battery failure can sometimes be very sudden. In a controlled laboratory environment plotting a death curve may be possible, but due to the inconsistent charging and discharging regimes of off-grid systems it may be far more difficult to spot. Most off grid users develop a "feel" for their batteries and one glance at the dashboard can usually pick up a concerning change in performance...
  • ILFE
    ILFE Solar Expert Posts: 364 ✭✭
    Re: How to measure battery capacity?

    2twisty, when you get a chance, post an update on your blog, please? I have enjoyed reading it in the past.
    Paul
  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    Re: How to measure battery capacity?

    Wow. A request for a blog update! Ha! There's been a LOT happen since the last one! I'll add it to my list.. I've had other priorities lately. I mostly post small updates on for my friends, since my blog readership wasn't very large.
  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    Re: How to measure battery capacity?

    Soo.. What about this idea:

    After a full EQ and a rest, measure SG. Confirm that batteries are at 100%

    Place a load on the system and measure AH consumed until 25% of bank Ah rating has been used.
    Measure SG again. Determine TRUE charge level.
    Calculate true AH consumed, determine use ratio
    Calculate AH rating of battery.

    I have not worked out the math for this yet, hence the ambiguity in my steps above. The problem I can see with this method is that hydrometers are not suuuuper precise. They get you really close, but I'm thinking that it would not be close enough for the purposes of this calculation. Miight even have to go deeper on the discharge, maybe to 35 or 40% so that you get a more accurate result.

    Thoughts?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How to measure battery capacity?

    Yep... Sounds OK.

    Otherwise, discharge to the depth you need for normal operation +10% longer. As long as the bank meets your needs, it is still "good".

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 2twisty
    2twisty Solar Expert Posts: 199 ✭✭✭
    Re: How to measure battery capacity?

    ok. I guess I'll get started working out the math to calculate that.