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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.
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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.
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| 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 EtchellsBecause 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. 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
Imaging ResourceAqualab: Which Battery Will Do