Kevin's comments are very good and to the point.
If you research battery management techniques the term "cell balancing" comes up not "battery balancing" and the technique seems to be charge transfer between cells by various methods. The other method of shunting currents does not seem workable. There are some good articles but they get pretty deep into it. A good way to understand things is to look at some of the devices available which are the building blocks of BMS as opposed to building with discrete components. Linear Technologies and Freescale both are marketing devices and reading the spec sheets is a good start to understanding.
Like many I am using "batteries" (ie. fixed groups of cells ) and my concern where I started with this is that the batteries experienced improper charge voltages on one or more of the batteries when charged in series. As well, even though the charger has temperature compensation with the sensor on a battery terminal, it only knows what one battery's temperature and has no way of changing the charge voltage on individual batteries.
I took a short pass looking at electronic methods for shunting currents and quickly decided it was too complicated. I like simple solutions which I feel are more reliable.
A simple solution is to have one charger per battery with a temperature sensor on it's own battery. But there are some issues with that. For some reason, chargers with battery temperature compensation are hard to find and when you find them they are pricy especially with an appropriate current output around 25% of capacity.
The solution I have come up with (cheap too!) is to use one charger per battery and a set of relays ( 8 for 4 batteries) and switch the charger between batteries. Using a form C contact it can be arranged to insure that only one battery is connected to the charger at a time. Like others I have a disconnect switch in the battery string which is open for charging so that no circulating currents are possible and the charge only goes to one battery. Of course I am only concerned here with recharge at the dock rather than underway.
Another feature of this arrangement is that I can control the charge times and rate to individual batteries. I am using Lifeline batteries and they have a very specific charge profile specification and with this arrangement I can apply that quite easily.
Being rather a miser I have taken a simple battery charger which can be switched between absorption voltage and float voltage manually, and am controlling it with a microcontroller, using temperature sensor per battery and actual charge values for the applied current and voltage. That way my preferred charge profile is easily accomplished. For one thing I don't like to float charged batteries but rather end the charging when they are full as measured by the current at that point.
I very much appreciate the comments on the list regarding this string which have been very helpful in understanding things.
John
Posted by: John Acord <jcacord@gmail.com>
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