The Lee Hart/TMSI Battery Balancer, Rev A Overview

Here are some comments about the Rev A Balancer extracted from email correspondence with Lee:

I designed it to be easy to build and program, because I don't have the TIME to do it if it was hard to build and program! Conceptually, my Battery Balancer more like a general-purpose multimeter than a dedicated-purpose E-meter. Bigger and more expensive, but more versatile and easier to use.

It's not a production item. It's just something that I built for my own use. For reliability's sake, I laid out PC boards. I ordered 10 sets of boards because of economics (it was something like $600 tooling plus $60 per board). I've been selling the boards and parts kits to interested parties who are fully capable of building, testing, programming, and using the product all by themselves without a lot of hand-holding. The total parts cost runs around $500.

Fundamentally, it consists of:

You can use the on-board Vicor, or your own external DC/DC converter (Todd PC30 etc.). The Vicor has the advantage of small size, high efficiency, and implements a proper 3-stage charging algorithm. You can use as many Vicors as you want; the terminals are there for paralleling them (each one delivers up to 16.5v at up to 15 amps).

You also need an off-board digital multimeter with RS-232 interface. I used this as the A/D converter. It was an easy way to get high quality, noise-resistant, UL listed, guaranteed isolated readings. The meter is unmodified in any way, so it is available for any other use if needed.

The programming is unique to every application. You have to write it yourself, but the BASIC stamps are pretty friendly and I can provide examples to start from.

In my own case, I have it check each battery voltage, and find the lowest one. It charges that battery for a time proportional to the difference between its voltage and the average pack voltage, and then checks all the voltages again.

It also checks the temperature probes (one in each battery box), and turns on the fan if too hot or the heater if too cold (via two of the general purpose relays).

It also watches charging voltage, and turns off the charger (via the 3rd general purpose relay) if something goes wrong and the charger fails to turn off.

There are lots more sophisticated things I have experimented with. It can test batteries, by using a general purpose relay to connect a load resistor. It can run an LCD dash display and keypad (but for now I use a laptop computer on the seat).

If AC power is present, it keeps balancing until done. If there is no AC (isolated input is off) it stops balancing after a couple hours since balancing power is coming from the pack itself.

Currently [05 Sep 2001], I have my BatMod set to a) bulk charge at 15a constant current, b) finish at 14.4v, c) equalize at 1a max to 15.5v max. For daily charging, the Balancer normally shuts off the BatMod without equalizing. I tell it to equalize 1-2 times a month.

It serves as my 12v DC/DC converter, too. My accessory battery is just a spare propulsion battery. Whichever one is the weakest is used as the accessory battery. This means there is so much accessory battery power that I don't notice whether the Balancer is actively charging the accessory battery or not. 90% of the time the lowest battery is not the accessory battery.

You don't have to do this, of course. It is just what I chose to do.

[5 Jul 2002] I only ordered 10 sets of PC boards, and they are all gone. I basically bought parts to build 10 sets, but some parts you have to buy larger quantities (resistors 200 pcs minimum), so I have a few misc. parts left.

We could make another run of them. The tooling is already paid on the PC boards, so the next batch would be cheaper (as long as the number of changes are small). But [...], I could use some help in all the detail work! It takes forever to order dozens of parts, count 'em out, and ship 'em to people!

It's an "open source" design; I don't care if anyone wants to duplicate it. I think it's "good for the cause" of EVs.

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Revision History:

06 Sep 2009: Relocated from main Balancer page.
22 Sep 2002: Added links to bare PCB images.
05 Aug 2002: Initial revision.