Servo - July 2010

FIGURE 5. Block diagram of the test system.

accuracy is of academic value only.

Look at the accompanying plots (in Excel) of a number of tests that I ran on eight Ryobi 18 volt battery packs. You can see that quite a few of the packs had very low mAh capacity. However, if you compare by colors (same battery pack), you can see that some of the tests also had acceptable results. The reason is that a full charge/ discharge(test)/charge/discharge(test) can sometimes restore a NiCad battery’s capacity to near normal. If it doesn’t, then that pack is probably defective. You can also tell by the plots whether a pack has any failed cells by a step voltage drop early in the discharge cycle. Figure 3 shows the actual data used to create the plots in Excel. If you look at it closely, you can see that the Charge/Discharge (test) cycles resulted in some packs more than doubling in mAh capacity (see tests B5B and B5C). If you have any battery packs that are NiMH, you will see a big difference in the discharge curves. The NiMH will have a higher and more constant voltage during discharge, but have a big step drop at the end when it reaches its capacity end. The three dark colored discharge curves (Figure 4) are NiMH battery packs.

Hardware Design

FIGURE 6. Test system electronics’ housing and connections.

FIGURE 7. The electronics consisting of the A/D modules, control circuit, and power supply.

Check out the block diagram of the original test setup in Figure 5. To make the connection to the battery easier, I modified each battery charger by adding terminals that connected to the charger’s battery terminals. Make sure that you understand that with some battery chargers there can be lethal voltages at these points if plugged into 120 VAC with no battery pack installed!!! NEVER PLUG IN THE BATTERY CHARGER WHILE TESTING OR PREPARING TO TEST A BATTERY PACK!

I have used DGH and Advantech serial A/D modules but any A/D that you can easily interface (serial A/D, serial DMM, Ethernet, USB) to your PC will work. The software provided is for example only and you will have to modify it (within VC#) to work with whatever A/D system you are using. A/D modules with a PC interface are readily available for a reasonable cost. With some persistence and patience, you can find a useable A/D module at even less cost (auction, closeouts, or using a microcomputer chip with A/D).