I would like to design a battery capacity system for my AGM lead acid battery (12v, 40Ah). My idea is that I have values of the battery voltage and its corresponding capacity, which I will use to plot a curve which I can then interpolate with the current voltage reading. I will be measuring the voltage of the battery with a INA219 chip.

I contacted the company that manufactures the battery and was told that designing such a battery capacity monitoring system is not useful as an aging battery would eventually have reduced capacity despite an unchanged full charged voltage reading. They gave an example where the battery might start of with say 13.2V but may have reduced capacity (by 10Ah) in a few years while still retaining its maximum voltage reading (13.2V). Hence, plotting such a curve may not be useful. Also, I would not want to fully discharge the battery with the intention to regularly update changes to this voltage-capacity curve, since doing so would significantly impair the lifespan of my battery.

Does anyone have any thoughts on alternative methods that I can monitor the capacity of the battery while taking into account capacity changes caused by an aging battery? Otherwise how can I continue using the voltage-capacity plot method while taking into account the changes caused by aging?

  • \$\begingroup\$ So you want to convert observed voltage into remaining capacity? There are a lot of variables. At a minimum you absolutely have to account for the load. A load current will cause the battery voltage to drop due to effective internal resistance of the cell. Same goes for charging, if you plan to operate your estimator during charging. \$\endgroup\$
    – mkeith
    Jan 22 '18 at 4:42
  • \$\begingroup\$ What was wrong with the advice given to you by the battery manufacturer? \$\endgroup\$
    – Solar Mike
    Jan 22 '18 at 6:55
  • \$\begingroup\$ @solar mike: Nothing wrong about their advice but I wanted to see if anyone here has any other suggestions on how I can overcome this limitation? Perhaps there might be alternative solution to measuring capacity for my battery type, and if so, I thought this might be a good place to get some advice. \$\endgroup\$
    – Craver2000
    Jan 22 '18 at 7:14
  • \$\begingroup\$ in flooded cells (which unfrotunately you don't have) you can measure the specific density of the electrolyte. By measureng voltage you're really building a thermometer. \$\endgroup\$
    – Jasen
    Jan 22 '18 at 9:05
  • \$\begingroup\$ @Jasen: Thanks for your comment. Do you mean that measured voltage corresponds more to changes in temperature than battery capacity? \$\endgroup\$
    – Craver2000
    Jan 22 '18 at 9:35

Here are some typical discharge curves for AGM lead-acid, in this a case a 12V 60AH battery (40Ah should be similar but with all currents multiplied by 40/60):-

enter image description here

Note that the time scale on this graph is logarithmic so the curves are a bit deceptive. However you can see that as current increases voltage reduces, and capacity is reduced at high current (eg. at 60A it lasts 30 minutes which is only 30Ah). So you can use voltage to estimate remaining capacity, but to get reasonable accuracy the effect of current draw must be factored into your calculations.

how can I continue using the voltage-capacity plot method while taking into account the changes caused by aging?

Capacity may be reduced due to shedding of active material from the plates, electrolyte loss, sulphation, impurities or contamination etc. Some of these things also increase internal resistance, causing greater voltage drop at higher current.

Capacity loss may occur suddenly and cannot be reliably predicted, so to get the actual capacity you need to measure it. The obvious method is to fully charge the battery and then discharge it until empty. Alternatively you could just discharge until you reach the minimum working voltage in your system at normal current draw, as this is the effective capacity in your application. If only done occasionally this calibration cycle should not significantly affect lifespan.

You can measure internal resistance by applying or waiting for a step change in current, then calculating resistance from the voltage difference. This figure can then be used in the calculations you do to estimate remaining capacity.

  • \$\begingroup\$ Thanks Bruce, that clarifies my understanding a lot. Seems that measuring capacity is much more complicated than I thought. And as for the other point, I note from your explanation that actual capacity measurement still requires occasional 'full' discharge of the battery. You mentioned about the need to factor in current draw in my calculations? Are there any algorithms available for this? \$\endgroup\$
    – Craver2000
    Jan 23 '18 at 6:35
  • \$\begingroup\$ The reason for me asking is that if the current draw from my load varies from time to time, then subscribing to one of the logarithmic curves (with fixed current value) shown above may not be accurate anymore. \$\endgroup\$
    – Craver2000
    Jan 23 '18 at 6:35
  • \$\begingroup\$ The curves are not actually logarithmic, they just look that way because only one axis is logarithmic. Voltage drop during discharge is relatively linear until the battery is almost flat, so a linear extrapolation may be good enough. The effect of current draw can be included by modeling internal resistance and adjusting capacity to match the known battery characteristics (if you don't have data for your battery then use the graph above). How complicated it gets depends on what accuracy you want - many would be happy with 5 'bars' showing approximate percentage of charge remaining. \$\endgroup\$ Jan 23 '18 at 7:13
  • \$\begingroup\$ You can't tell what the actual capacity is until you discharge the battery. At full charge the voltage will always be maximum, no matter what the capacity. As you discharge the battery the current and voltage drop can provide an estimate, which gets more accurate the closer you get to full discharge. After measuring capacity you may assume that it probably won't vary significantly short term, but long term it will (thus the need for occasional capacity checks). Or you could just show the capacity left as a proportion of full capacity (whatever that is). \$\endgroup\$ Jan 23 '18 at 7:24

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