Ecobatteries for NiMH rechargeable batteries

Extreme Battery Tests

To help customers on which brand of NiMH rechargeable batteries to use
I have done some simple tests.

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Manager Ian Jenkins

Constant Current 1

Most electronic devices like digital cameras, GPS or portable audio run at a constant current. If the current drops when the battery is exhausted the device compensates by slowing down.

To achieve consistent results a Maha MH-C9000 charger analyzer was used. Using this it is possible to charge and discharge batteries at specified currents for up to 15 charge/discharge cycles.
All AA batteries were charged at 1600mA (AAA 600mA) and discharged at 1000mA (AAA 500mA) down to 1v.

Make Manufactures Capacity mAh Discharge Current (Amps) Experiment Capacity (mAh)
Maha AA 2700 1 2451
Vapex AA 2700 1 2372
Panasonic AA 2600 1 2318
Yuasa AA EnTime 2500 Instant 1 2255
Energizer AA 2450 1 2109
Vapex AA 2100 Instant 1 2001
Maha AAA 800 Instant 0.5 781

Constant Current 2

A Computerized Battery Tester made by West Mountain Radio was used. Using it all AA size batteries tested were discharged at a constant 2 Amps (AAA 0.8Amps) down to 0.9v. Although digital cameras usually run on a max of 1.2 Amps, the higher current was chosen to obtain results quicker.

Make Manufactures Capacity mAh Discharge Current (Amps) Experiment Capacity (mAh) Watt hrs
Maha AA 2700 2 2622 3.12
Sanyo AA 2700 2 2560 2.98
Maha AA 2500 2 2411 2.83
Sanyo AA 2500 2 2405 2.77
Eveready AA 2500 2 2394 2.80
Maha AA 2300 2 2309 2.70
Panasonic AA 2600 2 2278 2.66
Sanyo AAA 900 0.8 910 1.06
Maha AAA 800 0.8 785 0.95

Watt-hours vs mAh and Simple Run Times

Rechargeable batteries are ususlly rated in milliampere-hours (mAh), a measure of how much current they can provide over time. A rating of 1600 mAh means that the battery should theoretically be able to supply 1600 milliamps (mA) for one hour, or 160 milliamps for ten hours, etc. The best NiMH AA cells today carry ratings of 1800 to 2700 mAh.

mAh is really only part of the story. What is more important is how much total energy a battery can deliver.

Energy is measured in Watt-hours, the product of voltage and current over time, or volts times amperes, measured over hours. (A milliamp is 1/1000 of an ampere.) To measure total energy, we need to measure the voltage and current moment by moment throughout the battery's discharge, multiply the two values together, and total up all the individual readings.

Further Fallacies of mAh

" The standard way of measuring mAh gives wildly optimistic values when compared to what the batteries actually deliver in typical digicam usage. The problem is that digicams gobble power in big gulps, while battery-testing standards measure power delivered in small sips. Batteries are much less efficient when driving heavy loads than light ones. Thus, even if a manufacturer tests and reports their batteries' capacities truthfully according to the accepted standard, the resulting numbers may have little to do with how well the batteries perform in real-world digicam usage. " The Great Battery Shootout! by Dave Etchells

Because of this load-dependent behavior, I set up my battery test system to run the batteries under loads closer to those seen in typical digicams. As a result, the mAh capacities I measured are generally quite a bit lower than the manufacturer's claims, but do give a much better idea of how the batteries will do when plugged into an average digicam

Constant Resistance

Several resistors were soldered together in parallel to give a circuit with 2 batteries in series.

Voltage readings of the batteries were taken every 20 seconds until the final reading was 1.85v That is 0.9 v per battery plus 0.05v to allow for 1 battery discharging below its reccomended low voltage value.

To automate the recording process a battery profiler circuit was made up using a picaxe microcontroller.
This turned on a relay connecting the batteries to the load, took a voltage reading every 20 seconds then sent it to the computer's serial port .

The results were displayed using Hyperterminal then placed into an Excel spreadsheet so the batteries capacity in mAh and Watt-hrs could be calculated

The tests were repeated untill the results were within 2% of each other for at least 3 runs giving a standard deviation of less than 2%

Notes

Large wattage resisters were used so any heat produced in the resisters would be dissipated and the resestance would not vary as the temperature was constant.
When the tests were finished at 0.9v per battery it was found that the battery voltage rose after the load was disconnected. This shows that these batteries still have reserve capacity which was not measured.
Although digital cameras run on a current of around 800mA, the higher value of 1800 was chosen so as run times would be shorter giving the batteries an extreme test.

RESULTS

Make	Manufactures Capacity	Discharge time to 1.85v (min) 2 cells in series	Experiment Capacity (mAh)	Watt hrs
Uniross AA	2000	61	1923	2.20
Sanyo AA	2100	61	1944	2.24
Powerex AA	2000	63	1966	2.24
Powerex AA	2200	66	2073	2.38
NexCell AA	2200	68	2144	2.48
NexCell AAA	800	23	690	0.76
Sanyo AAA	800	23	687	0.74

NOTES

As my aim was to get the results to 2% or less certain precautions had to be done. The terminals in the battery holder had to be made from copper circuit board as the terminals in the plastic holders were found to give a small but significant resistance.
Battery terminals had to be cleaned before each days testing.
The wire between the battery and the battery profiller circuit had to be heavy copper and soldered to the terminals in the battery holder.
Charging had to be uniform. Batteries were charged using a Maha MH C401FS charger then left on trickle charge for at least 2 hours.

Links to Battery Testing Sites

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Aqualab: Which Battery Will Do

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