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Battery Fixed Load Versus Constant Current Test PDF Print E-mail

Battery Fixed Load Versus Constant Current Test


The rechargeable battery fixed load versus constant current test is often a topic of discussion when testing and comparing rechargeable batteries.

One test uses a fixed load and the current provided by the rechargeable battery drops as the voltage drops. The constant current test forces the battery to deliver the same amount of current during the entire test. The rechargeable battery fixed load versus constant current test comparison is usually more important in batteries where the voltage drop over time is less constant. This is relevant with the alkaline battery in which the voltage curve can vary greatly with changes in loads. The alkaline battery is not well suited to high drain devices.


Rechargeable Battery Fixed Load Test


There is nothing complicated about the Rechargeable Battery Fixed Load test and there are few variables to control. The only area of concern is the actual wattage or capacity of the fixed load. If the fixed load is driven to close to its capacity, losses will result due to heat.

Imagine trying to fill a barrel with water. You pour water into a container connected to a hose which dumps into the barrel. The hose can transfer only 1 gallon per minute.

So long as you don't pour more than 1 gallon per minute into the container, the container will always be empty. You haven't exceeded the capacity of the hose.

If you pour more than 1 gallon per minute into the container, the container will fill up ( more resistance ). You are exceeding the capacity of the hose.

In electrical terms, excess current results in heat. It also results in a higher equivalent resistance. You can only push so many electrons across a specific surface area of a given conductor or material. Long term excess current results in load failure ( the resistor in this case ).



Rechargeable Battery Constant Current Test


The rechargeable battery constant current test draws current from the battery at the same rate during the entire test. The graph below ( from the Duracell NiMH Technical Bulletin ) shows how the voltage drops over time as constant current is drawn from the battery.

duracell_constant_I_21degrees.gif
                                                        Constant Current Graph


Today's electronic devices use very small battery sources to power all those small portable electronics. Constant current or current driven sources can include devices using DC-DC voltage converters, certain amplifiers and motorized devices ( CD walkman for example ).

Real World Devices

The use of each test is really a reference for comparing data. You can't compare any two objects unless they were tested under the same conditions.

In real life, there are few if no constant current or fixed load devices. A simple example of a fixed load device would be light bulb. An old fashion flashlight has a bulb with a wire element that has fixed load characteristics. Heat plays a part in this devices energy consumption, so it is not a perfect example.

A CD Walkman uses a motor which normally draws a fixed amount of current. But in reality, none of our devices use the same amount of energy or current from the battery at any given moment. Music, voice, volume, light, picture etc ... all vary over time or use, and therefore so does energy use. This is true for all our devices from laptops to cell phones, MP3 players or cameras. No two moments of energy consumption are the same.

Battery Fixed Load Versus Constant Current Test, the Real Meaning


If you take a look at the Rechargeable Battery Fixed Load test page and the graph, you will see that the voltage is fairly constant over the greater part of the battery discharge cycle. This is typical for a good NiMH Rechargeable Battery. In general, the NiMH Rechargeable Battery will provide a relatively constant current through a fixed load for the majority ( > 90% ) of its discharge cycle ( which is approximately 1.2v / R ).

The constant current test requires a circuit that can drain the rechargeable battery at the same current regardless of voltage changes in the battery. This is not necessarily trivial and can require a feedback circuit that allows for monitoring and control to keep the current drain fixed at a constant rate. This is usually a microprocessor based system with A/D and possibly D/A capability.

A feedback circuit basically monitors a value ( in this case current ) and then gives the information back to another circuit that can make adjustments to that value as needed. In the constant current test, adjustments would be made up or down depending on whether the current is too low or high.

The constant current test can contain calculation errors because there is a time delay between what value is detected, when the current is adjusted and what value is captured.

If you look at the LaCrosse BC900 or the Maha MH-C9000 Wizard1 tests, you will see they use a constant current discharge. You will also not that they vary and that they are never exactly the specified value (ie. The manual may specify 500mA, but the display says 493mA ).

The fixed load test is an easy, reliable and predictable test with no unknown losses when an appropriate fixed load wattage is used.

Using the rechargeable battery fixed load versus constant current test is a matter of choice. These are "close" or "representative" tests to real devices and are used in a controlled setup which allows us to compare rechargeable batteries.

The environment is well controlled so we know that each product is treated equally and this is what is important.

GB Dec, 2006 

 
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