Battery Switching Circuit

[schlaefers]

Hello,
I'm currently working on a the power management system for a solar power blimp. My problem is this. I want to have several battery packs (4S A123s) in parallel from which I can flip a switch and discharge (make it run a motor or whatever load I need) one pack and flip another switch and charge (through a solar charger) another pack. Ideally I would not be physically switching but have my Microcontroller do the switching based on which battery pack is dead and which is ready to go. I've tried for about a week now to get it to work with MOSFETS but I can't get even the load side of the circuit to be correct. Anyone have any good idea?

 

[MikeMl]

Are you trying to put the FET in the positive lead, or the negative lead?

What is the voltage of the battery packs?

How much load current?

Can you just combine the packs using isolation diodes, or do you really need to switch off some pack(s)?

What is the Vdd for the PIC?

Is the Vss pin tied to the negative lead(s) of the batteries?

 

[schlaefers]

Hi Mike,
So the configuration I tried was to have the batteries positive terminal go to the top side of the load and the bottom of the load go through the FET to ground. I could get this to work but the problem is it only words for one battery. When more are added any of the MOSFETS being on will give the battery a path to ground. The voltage of each 4 series A123 pack is 13.2V. The load current will eventually be upwards of 30A pulsed but we will be happy to get 2-3A pulled now. We need to be able to pick which battery pack we interact with for the purpose of our project. One of three states, being charged, being used to run the load, and the last state is idle. The Vdd of the PIC is from 3-5V but we are currently running it off a separate power supply. A voltage regulator and PIC will the added to the load eventually. As for the Vss pin, I'm Sorry I dont know what you mean. The negative lead of the batteries is connected to the drain of the MOSFET through ground, which all the batteries go to. Thanks for you help.

 

[Diver300]

I can't see the advantage of having several battery packs connected separately. It's normally best to parallel the all of them. The voltage of LiFePO4 cells gives you a good idea of the remaining charge.

If you want to charge one pack and use another you need a mosfet to control the charging and another to control the discharging for each pack. However, if you are using mosfets, the body diodes will mean that it you won't be able to charge the lowest voltage battery without getting current come from the highest voltage one. You would need to put two mosfets to connect the charger, and two to connect the load, for each battery. It gets complicated. Why don't you want to parallel all the packs?

 

[SABorn]

Without having a common ground to all packs i fail to see how you can sense the voltage levels or even control the switching of each pack.

It would make more sense to me to switch the V+ side of the battery and to use a full isolation switching method like a relay.

Then you also will encounter a problem when you supply the micro with power from the packs as how will you supply the micro during switching phases, unless you switch one pack on before switching the other off.

Pete.