I've been a big believer in buying products that use AA rather than proprietary batteries. One advantage of AA batteries is that if you use up all your rechargeable AAs when traveling, you can alway find alkaline batteries in a convenience store. Also, if all you devices use AAs, you only need one charger.On the other hand, a disadvantage of AAs is that because of NiMH technology, and the shape of the cells, their total energy density is less than Li-ion batteries.
When using NiMH cells, a problem with I've encountered occurs when one of the four cells in the pack is bad. With a fully charged pack in my camera, I've been out of power after only five pictures because of one bad cell. Even if all all cells are good, it's best to match cells with similar characteristics. This was the purpose of this project. Yes, one can buy a battery tester, but its so much more fun to do it yourself!
My goal was not just to test the batteries as a set of four, but to get discharge data from each cell. I've seen some projects in which the battery pack is tested by connecting a power resistor and a clock to the terminals. The idea is that the when the battery is discharged, the clock stops, and you can make a calculation of battery capacity from that. There are two problems with this. One is that the battery isn't discharging at constant current, so you can't accurately compare your results to the battery's capacity rating. The other problem is that fully discharging an NiMH battery below 1 volt per cell will damage it. If you test in this manner, each subsequent test will result in worse performance.
The first step in the project was to do a little research on NiMH Batteries. I found a good reference in Sanyo's Twicell battery manual. Here are some of the more useful facts about NiMH batteries.
If you discharge the battery below 1V/cell, the cell can undergo a polarity reversal (!). This would cause the battery (a battery is a group of cells) voltage to drop suddenly. To give the cells a margin of safety, I limited discharge to 1.1V/cell.
The typical test discharge rate (It) is defined as the current at which the battery would be drained in one hour. Thus, a 2800 mAh (milliAmp-hours) battery can be drained at 2.8 amps. That would dissipate if a lot of power! I decided limit battery drain to 1 amp - a nice even number.
Here's the circuit:
The schematic is a little hard to read in the blog. You can right click it and open it in a new tab to enbiggen it.Here are the circuit details.
Current is drained through an LM317 three-terminal voltage regulator. It's configured for constant current mode with eight 10 Ohm 1/2 Watt resistors. Since full discharge current passes through these resistors, they must be big enough to dissipate that power. Increasingly higher rates of discharge can be selected as each of the switches are closed.
S1 => 1.2 Volts / (10 Ohms / 2) = 0.24 Amps
S1 + S2 => 1.2 Volts / (10 Ohms / 4) = 0.48 Amps
S1 + S2 + S3 => 1.2 Volts / (10 Ohms / 8) = 0.96 Amps
I only used the maximum discharge rate because any less results in a long test time. At the lowest setting, a full discharge can take ten hours.
One curious thing you may notice about the circuit is 1 ohm, 10 watt resistor. That's there because of the possibility of the maximum battery voltage exceeding 5 volts. Rather than attempting to measure the voltage of all 4 cells directly, the voltage is dropped 1 volt when draining the cells at 1 amp. In this way, the Arduino's maximum analog input is not exceeded. It does make calculation of the cell four's voltage a little strange-looking, though.
VB1 = A0
VB2 = A1 - A0
VB3 = A2 - A1
VB4 = A3 - A2 + (It * 1)
To convert these values to voltages and perform the calculations I imported a floating point library. In this way I was able perform the calculations and format the output directly to so that could be used by a plotting program such as GnuPlot. To use the tester I charged the batteries using the same charger that I travel with. I transferred the batteries from the charger to the tester. The tester was plugged into a laptop running a Putty terminal which logged all the output to a file. When "g" was typed at the keyboard, D7 went high, closing the relay and discharging the cells. If any cell reached less than 4.1 Volts, D7 would go low, stopping the test. From the saved data, a discharge profile could be plotted:
Several problems were encountered with the first version of the battery tester. The first was heat. I didn't expect that I'd need a very large heat sink on the LM317 since it was only dissipating four watts. I was wrong. My rule of thumb is that if a component is so hot that you can't hold your thumb on it, the component needs more cooling. I first tried adding a small 5 volt fan to the heat sink, powering it off the laptop's USB port. This cooled the LM317 sufficiently but made an annoying sound I suspected it was causing electrical noise that was making the battery readings erratic. I clamped ferrites on USB cable, but that didn't improved the accuracy of the measurements, so I removed the small heatsink, and installed the big heatsink you see in the photo. That took care of the temperature, but the measurements were still unstable. It turns out the source of the problem was the instability of the 5 volts supplied from the laptop via USB. A check of the Arduino's Vref pin showed that the voltage was varying all over the place. The solution was to use an external power supply to power up the Arduino, so the Arduino's regulator could provide a stable Vref.I've not done anything with this project recently, so I wanted to document everything. When I'm ready to start again, I won't have to start from scratch!
