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View Full Version : [Question(s)] Battery bank monitoring etc.



SteamBoat
10-28-2008, 03:51 PM
Howdy

Iíve got a question about how to go about monitoring and voltage control of banks of batteries.

There are systems that equalize the charges on banks of batteries. After several charge and discharge cycles batteries can get out of balance which substantially shortens the life of the battery pack. One that Iíve seen is simply a device that you connect across a pair of batteries with three light gauge wires. (Not cables) The way I think it works is that as current goes into and out of the batteries it compares the voltage and shunts some of the current around the battery with the lower charge during discharge and shunts some around the higher charged battery during charge. This should result in balanced voltage across the pair. To work on a bank of batteries you get n-1 units (where n is the number of batteries) and hook them up to your system.

Besides the above mentioned benefit, I think itís a good idea to monitor your power source at a greater granularity if you can. Throw in temperature and current monitoring and you can really keep track of whatís going on.

What I want to do (I think) is not just replace the above mentioned device but also be able to monitor and control the system via a PC. What I am thinking is that the control of the system would go in the PC instead of being hardwired on the little electronic device. One set of monitors that pick up the voltage of each of the batteries and then a set of, I donít know, transistors that handle the current shunting based on what was picked up by the sensors.

Could you/I get away with one voltage sensor and electronically switch between the various batteries fast enough? Would it work best to have the current shunting units local to the batteries or to the control system? How do you control the current shunt? Do you use a toggle or do you send some signal to a smarter unit that can very the current shunt depending on voltage difference? Could I talk Trossen into creating a system that would do this for me? (There is a market isnít there?)

Why might this be worth doing? Well, on a little robot that has a small battery pack it probably isnít worth the time or effort. Especially for a one off. Batteries are too cheap. However, when dealing with an electric vehicle, such as a car or boat, it can make a big difference if you can get another year or two out of your $1500+ stack of batteries. It could also be exceptionally useful in cases where itís hard to change the batteries like when itís on another planet. J

I guess what Iím asking for is some input on how you might go about this. What kinds of sensors? What kind of shunts? Exactly what are the design considerations for a system that would not only work for me but be applicable to other people in the robotics or EV community.

Thanks much.

MArk B.

ScuD
10-28-2008, 04:30 PM
To be honest, I wouldn't use the computer as the "control". Cells, batteries, especially the newest technologies, do not take abuse lightly.

I'd lean more towards a real-time control with a PC uplink with the possibility to change process parameters through the GUI on the pc.

This way you could put a sensor on each cell with a shunt on each cell for an optimal solution.

Either way, battery control is not the simplest of control solutions, and may cause a lot of headaches :)

jes1510
10-29-2008, 08:08 AM
I just came across this page this morning:
http://www.societyofrobots.com/member_tutorials/node/101

It has details on a circuit to measure voltage, temperature, and current drain of a battery. Hopefully it's helpful.