Wednesday, March 21, 2007

More Thoughts on Battery Equalization

Lee Hart, an EV guru on battery maintenance and equalization, has graciously e-mailed with me regarding the best way to equalize batteries. He's currently working on upgrading his battery balancer (see link at right) to handle a wider range of battery technologies and applications.

Lee convinced me that the flying capacitor approach that I originally thought would work is not practical in real world situations. I think Lee's battery balancer design would do a great job; however, I'd like to cost reduce it to something more suited to my own needs.

My plan is to create a relay board like Lee's that only handles 10 amps per relay instead of 30. If I costed out the parts and boards correctly, I can probably get that down to $100 per relay board vs. Lee's assembled cost of around $200. For the main controller board, I plan to use a similar design, but simplify the design to meet a narrower set of needs and replace the high-cost components with lower-cost, harder to use components.

For example, instead of a $80 complex STAMP controller, I'll use a $6 PIC that has more flexibility but will require more complex programming. Instead of the $150 Vicor BatMOD module, I'm using a free 100-watt 32-volt isolated power supply from an HP printer and making a current controlled buck converter to step down the voltage and charge the batteries (about $10). Instead of the $60 Radio-Shack meter, Lee is working on using an LM331 voltage to frequency converter to measure voltage and current of the charger which should cost around $10.

Many of these components will be harder to calibrate and program, but I'm willing to do that to lower the cost and reduce the size of the system. I don't know how this will end up, but I'll learn a lot and that's the point.


Mal said...


Can you elaborate on why the switched capacitor technique is not viable please?

The solution looked so elegant.

I am begining the process for an AC conversion allong the lines of ( I just bought a motor on Friday and pick up a controller in the new year)

TimK said...

Hi Mal,

I agree that the switched capacitor approach is very elegant; however, you quickly get to a point of diminishing returns as the batteries equalize. I've read in research papers that you really want to get the batteries within 20mv of each other.

In my lab experiments, I switched a large capacitor between two UPS (12 Ah) batteries. Once the voltage got under 500mV, there wasn't enough current transfer between the batteries to equalize them in less than a week. If you extrapolate this to large (185 Ah) batteries, the current involved would take over three months to equalize.

As the voltage gets closer together, the current transfer gets smaller and smaller, limiting the equalization.

I've found, with flooded-lead-acid batteries, that the equalization charge at the end of the charge cycle works just fine for me and also uses far less components.

Have with your conversion!

Cheers, Tim