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I am trying to characterise a 9 V battery using a pulsed load measurement. The battery is a 9 V zinc-carbon eveready super heavy duty. The load is a controllable constant current source. When I discharge it for extended periods I observe curious behaviour.

Equipment

The circuit is as follows:

schematic

simulate this circuit – Schematic created using CircuitLab

Omitted for clarity are a 100k resistor pulling down the non-inverting input to the op-amp for when there is no control voltage, and some decoupling.

The MCP6241 is a rail to rail op-amp so when the control voltage is 0 V I get sufficiently low current draw through the load resistance. It is powered by a single 5 V rail. The control voltage defines the current through the load as V=IR. I am pulsing a 100 mA current so the voltage gets to 500 mV. This is tested and functioning pretty well (it's really about 496 mV).

The FET is logic level and seems to work fine being turned on by the op-amp. It does heat up but there is a small heatsink, and by my calculations it shouldn't reach the limit of the operating temperature range.

The battery voltage is measured using a buffered DAC with high input impedance, while the control voltage is generated by a buffered ADC. Both are a part of an NI myDAQ, sampling at 2 kHz then downsampled to 500 Hz.

Results

The test is a high-performance pulse characterisation (HPPC): the battery is pulsed with a load current and then allowed to 'recharge' back up to its open circuit voltage when the load is removed. The measurement was designed for rechargeable batteries (lead-acid, li-ion) and is detailed here. NB it is not designed for the zinc-carbon battery I am using. Here are 6 sequential pulses overlayed to demonstrate the difference in behaviour:

Battery discharge characteristics

The plot is zoomed to the relevant time window that shows the problem. The lower the curve, the later in the measurement they are, i.e. the top line is the first discharge. I expect the pseudo-linear discharge characteristic from the later curves, but at the start the discharge seems to drop the voltage very low and unstable.

Question

Is this expected behaviour of a battery when it is pushed too hard? Is this a temperature issue or something to do with the battery chemistry?

Or is it more likely to do with the measurement equipment? I don't think it is because the equipment functions fine for the later curves.

Has anyone seen similar behaviour?

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    \$\begingroup\$ Look for a datasheet of the battery. Then search for maximum current and puls loads. \$\endgroup\$ – Uwe Jun 26 '18 at 17:07
  • \$\begingroup\$ @Uwe The data sheet unfortunately doesn’t contain such detailed information as I don’t think these batteries are intended for any intensive products. The smallest load they suggest is 180R for a max 9/180 = 50mA current draw for only 0.5 h a day. I may need to adapt my measurement strategy to match the battery composition... \$\endgroup\$ – loudnoises Jun 26 '18 at 18:37
  • \$\begingroup\$ Very interesting. I like how it seems to "repair" itself after the first two pulses. I suspect that a 5Ω load is more (too little) than this type of battery was designed to handle. \$\endgroup\$ – Misunderstood Jun 27 '18 at 3:43
  • \$\begingroup\$ @Misunderstood yes I think you’re right, which is unfortunate because I wanted to drain it in a reasonable amount of time instead of over the course of a week. Oh well! \$\endgroup\$ – loudnoises Jun 27 '18 at 4:24
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    \$\begingroup\$ The datasheet you found contains the information that the useable capacity decreases with increasing load current. You get about 330 mAh at 5 mA current, but only about 60 mAh at 25 mA. If you want higher current, you have to use larger cells or another cell chemistry with much lower internal resistance. You may use 6 D-cells connected in series to get 9 V and more current. But if you discharge carbon-zinc cells in less than about 60 hours, you could not use the full capacity. \$\endgroup\$ – Uwe Jun 27 '18 at 11:32
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Unfortunately not a definitive answer, however I think this explains the behaviour...

I completed an additional 9 pulses to keep testing the battery. The minimum battery voltage should be around 4.8 V as per the data sheet. When the open circuit voltage dropped beneath 8 V the load would drop the voltage beneath 1 V.

Overlayed

Overlayed measurements

Concatenated

Concatenated measurements

Conclusion

Given these results I think that the test is simply not well designed for carbon-zinc batteries, which are in turn not designed for such needy loads. I do not believe that this is a result of the measurement equipment, and I do not have the relevant chemistry degrees to be able to comment on the battery composition. Thanks to commenters Uwe and Misunderstood for their suggestions and ideas.

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