Rechargeable Battery Fixed Load Test

The Rechargeable Battery Fixed Load Test

Looking at the Rechargeable Battery Capacity Results

Interpreting and Using the Rechargeable Battery Capacity Results

General Experience at Rechargeable Battery Review

RBR Fixed Load Test vs. Current Test

The Rechargeable Battery Fixed Load test method allows for a simple method of calculating current and energy because there is one fixed value or variable in the equation at all times.

The basic calculation in use is Ohm’s law : I = V / R.

From the data collected ( see Rechargeable Battery Test Platform ) we can calculate the total current and power ( watts or wattage ) delivered over a period of time. We use a dead rechargeable battery (cut-off) voltage of 0.8v for all our measurements. Others may use 0.8v, 0.9v or 1v.

The choice of the cut-off is not of great significance as there is little energy delivered after 1v when compared to the life cycle of the battery. We chose this value as many devices will continue to operate when the rechargeable battery drops below 1v. What is important is consistency and that each rechargeable battery tested is treated equally.

The total amount of energy delivered by any rechargeable battery typically drops as the battery is forced to work harder at higher and higher rates. Figure #1 below shows that the Ni-MH rechargeable battery has a relatively flat voltage during its use.

• At 9.7 ohms, the current at 1.2v is = 1.2v / 9.7 ohms = 0.124 Amps
• At 4.7 ohms, the current at 1.2v is = 1.2v / 4.7 ohms = 0.255 Amps
• At 3.4 ohms, the current at 1.2v is = 1.2v / 3.4 ohms = 0.353 Amps

Results are provided in the AA battery section.

Figure #1: Sanyo NiMH AA 2700 Battery at Various Discharge Rates.

The Rechargeable Battery Fixed Load Test

We have decided to use the fixed load test for several reasons.

• Designing and manufacturing a mutli-channel test platform was an attainable, affordable and an easy to manage objective.
• A multi-channel platform allows for more tests to be performed simultaniously.
• Using Data Acquisition units coupled with PCs was a practical approach.
• Using several Data Acquisition units coupled with PCs allows for growth and expansion.

When a rechargeable battery is tested for its capacity the following procedure is followed:

• All rechargeable batteries are conditioned ( cycled, charged and discharged ) until no significant change in capacity is measured ( +/- 2% ).
• All rechargeable batteries are charged using the same charger.
• All rechargeable batteries are charged at the same rate.
• All rechargeable batteries are handled at room temperature.
• Room temperature is 25 degrees Celsius.
• Room temperature may vary plus or minus 2 degrees throughout the year.
• AA, AAA rechargeable batteries are tested using a fixed load of 10 Ohms.
• The LaCrosse BC900 ( 350 mA discharge ) and the Maha Powerex MH-C9000 Wizard1 ( 500 mA discharge ) are used to verify our results.
• If there is a significant difference between the RBR Fixed Load Test results and the results from the the LaCrosse BC900 and the Maha Powerex MH-C9000 Wizard1( +/- 5% ), we re-run our tests to re-verify our results.

See some examples below.

Looking at the Rechargeable Battery Capacity Results

When looking at the rechargeable battery test results, keep in mind that rechargeable battery capacity and behaviour varies as no two rechargeable batteries are identical and all manufacturers claim a range of operation for their product.

• No two rechargeable batteries are identical or behave exaclty the same in whatever the identical circumstances are.
  
• Rechargeable battery capacity varies from cell to cell. This is normal.
  
• Even with the best rechargeable batteries, as noted by the manufacturer, rechargeable battery capacity can vary more than 5% and up to 10%.
  
• Some rechargeable battery data sheets from reputable manufacturers list a minimum capacity which can be up to 10% lower than the labeled or stated capacity ( see Sanyo_HR-3U-2500.pdf).
• Not only can each rechargeable battery have a different initial capacity, but you can also expect there to be some differences in each rechargeable battery's behaviour, including behaviour during charge and discharge.
• Although room temperature may vary by a few degrees, we do not expect that this small variation is a measurable factor.
• The capacity reported represents the best result. We do not always have the same number of samples of each rechargeable battery to form a comparable average.

In ideal circumstances Rechargeable Battery Review would have:

• A great deal more samples to test.
• More equipment to produce and log results.
  
• More sophisticated equipment to produce and log results from different tests ( such as the constant current tests ).
• More human resources to manage the greater number of tests and equipment, tabulate the results and manage the information ( including publishing ).
• A great deal more financial resources to accomplish all of the above.

Interpreting and Using the Rechargeable Battery Capacity Results

The information published here at Rechargeable Battery Review should be considered as a reasonable reference and not as an absolute value for an entire product line. When reviewing the information presented, please consider:

• The information presented in the previous paragraphs.
• The variation in performance that exists in all rechargeable batteries provided by all manufacturers.
• The rechargeable battery manufacturers data sheet when it is available.
• All the samples are processed and treated equally.
• Trends that may appear from products from the same manufacturer or brand. Relative to their labeled capacity, some manufacturers or brands consistently exhibit excellent performance. Relative to their labeled capacity, some other manufacturers or brands consistently exhibit average or below average performance.
• Each rechargeable battery test platform ( LaCrosse BC900, Maha MH-C-9000 or the RBR Fixed Load Test ) introduces error.

General Experience at Rechargeable Battery Review

Over the course of our tests and reviews, we have observed that most rechargeable batteries that come in a package generally operate within the same range. Generally, this operating range is within a 5% variation, sometimes with a 5-10% variation and not so often greater than 10%.

Here are a few examples:

RBR Fixed Load Test vs. Current Test

Here are a few examples of the test results from Rechargeable Battery Review's Fixed Load Test and the Current Tests performed on the LaCrosse BC900 and the Maha MH-C9000 Wizard1.

The majority of the tests results from the LaCrosse BC900 and the Maha MH-C9000 Wizard1 fall within +/-5% of RBRs Fixed Load Test. It may happen that one of the LaCrosse BC900 or the Maha MH-C9000 Wizard1 test results might vary more than +/-5% of RBRs Fixed Load Test.

Here are a few examples:

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