Battery Balancing: Keeping cells equalized

After finshing up at University of Illinois, I spent some time at developing firmware for non-dissipative battery equalizers. These devices are basically clever capacitive charge pumps that charge a capacitor across a higher voltage cell and discharge across a lower voltage cell.

This ends up giving you essentially a constant voltage on your batteries, no matter how you charge or discharge them. In principle, this is a very good idea, and this works well for ultra caps and other energy storage devices which are prone to charge imbalance.

The downside is that the capacitive devices typically have no knowledge of how much current they are moving, which can cause overheating if they are used with batteries that are extremely out of balance. I implemented the switching algorithms for a family of equalizers on a PIC, as well as added voltage-sensitive frequency changing and some other neat features. Unfortunately, current sensing nor overcurrent fusing was added due to the substantial cost and losses associated with it, which ultimately lead to overheating issues.

Some things still start out on the white plugboard. This is a PIC12F device in the protoboard with a probe or connector attached to each pin of the device. The people at Microchip have a nice development kit in their ICD2 and ICD3 are really slick, and even work with these very small devices. The RJ-25 hanging off the protoboard is the ICD2 connector, and the pots are there to simulate battery voltage..

Here's a picture of the circuit itself plugged into one of my favorite o-scopes of all time. This is a Tektronix 2232, which is an analog scope with a digital sampler kind of hooked on the side. It could do measurements and such on the screen using cursors and things, but was entirely analog in the signal path. We also had a newer Tek all-digital scope, but I actually rather liked using this one.

The two meters at left are monitoring voltage, and teh scope shows the switching waveform output from the PIC. This PIC12 replaced a 555 timer circuit and several other discrete components, while also providing added features, including a status LED and voltage-dependent switching frequencies.

This is the prototype unit for four cells using the controller above. This photo is here as it illustrates an interesting struggle with cost reduction. Even though the principle of capacitive battery balancing is sound, the application specifics can be a bit more difficult. Of note here was the packaging and manufacturing costs, as a capacative equalizer needs lots of big caps for its size, which for a low enough loss were only available in through-hole packages. This made the boards quite expensive to manucture.

In order to save cost, several meetings were held with manufacturers who could do the job for cheaper. Some of these were rather elegant solutions, some were not. One company came with a design the specified that they should be potted using beach sand as a cost saving measure over conventional potting compounds. This has kept me amused ever sense, as the devices sounded like a baby rattle when they were moved around, which is not in my mind an entirely virtuous feature for a device to extend the life of a set of batteries.