Help with high current battery switching without putting the load through the switch

[Zardichar]

Hey guys, I am new to the forum and came to here to ask for help on a project that I am trying to construct.

I have 3 lithium polymer batteries, one 7.4v, 11.1v, and 14.8v. The current draw will be around 150a for an extremely short period of time (a couple milliseconds) and then will settle to around 30a continuous.

I want to use a 1 pole, 4 position (1 for off) rotary switch, but none of them can sustain very high currents. So that is where I need your help.

I made the following circuit for it: (sorry for low quality)

but I quickly realized that when the switch is turned to any of the positions, then the higher voltage battery would just dump current into the other two, so that would be very, very bad.

Please correct anything that is wrong in this post and please help me with this project.

All comments are welcome and I thank you guys in advance!

 

[davenn]

Hi there
welcome to the forums

I have uploaded your image to the forum as it took forever to load/view on photobucket

what is that 3 legged device on the left ?

I doubt you will find any normal rotary switch that can handle a few amps let alone 30+++ amps
you are probably going to have to go to SSR's (Solid State Relays) and control their switching using a normal rotary switch and low current
Be prepared to spend some serious money for the SSR's

cheers
Dave

 

[Zardichar]

Hi there
welcome to the forums

I have uploaded your image to the forum as it took forever to load/view on photobucket

what is that 3 legged device on the left ?

I doubt you will find any normal rotary switch that can handle a few amps let alone 30+++ amps
you are probably going to have to go to SSR's (Solid State Relays) and control their switching using a normal rotary switch and low current
Be prepared to spend some serious money for the SSR's

cheers
Dave

The 3 legged device is an IRLB3034 mosfet. Gate a source are connected via a 220 ohm resistor and the gate connects to the negative input with a 100 ohm resistor. The drain is connected to the negative output.

So you don't think there is any cheap way to do this?

 

[CDRIVE]

You may want to Google Cole Hersey (sp?) switches. They've been making heavy duty marine switches for decades. They are large rotary switches designed to switch large Lead Acid storage batteries. They ain't cheap either though.

Chris

 

[KrisBlueNZ]

You want to select one of the three batteries at a time and feed it to the output of the circuit?

I think you should be able to do this with three of those big MOSFETs, IRLB3034, one for each battery.

Connect the positive terminals of the three batteries together; this becomes the positive output of the circuit.

The negative terminals of the three batteries go to the source terminals of individual IRLB3034 MOSFETs, and their drains are tied together. The commoned drains are the negative output of the circuit. This is convenient because you can use a non-isolated common heatsink on those MOSFETs.

Each MOSFET should have your 220 ohm gate-source resistor, and a way to supply a bias voltage to its gate. This bias source needs to be able to provide a large surge of current to bring the gate voltage up very quickly, because the turn-on time needs to be very short, otherwise the energy dissipated in the MOSFET (as it passes through its linear region in between fully OFF and fully ON) could easily blow it up.

As it stands, you could easily violate the SOA (safe operating area) of the MOSFET during switch-on. When the SOA is violated, the MOSFET is practially guaranteed to lose its magic smoke instantaneously, and, usually, become an immediate short circuit, which will be a problem if a different MOSFET turns on - a heavy current will flow in the second MOSFET and probably blow it up too.

I'm assuming that this 150A surge will occur every time a MOSFET is turned on, correct? So the heavy load will be present during the time that the MOSFET is switching from OFF to ON, right? This is likely to be a problem.

Using more than one MOSFET for each battery might be an option. Also, it might be possible to control the switch-on current surge by inserting an inductor in the current path, to reduce the dI/dT in the MOSFET at turn-on. But these are just possibilities; heavy current switching is outside my experience.

There is also a small issue with generating the gate voltage. Each MOSFET's source is at a different voltage, you need to switch the negative side of the voltage generator to the MOSFET you are dealing with. The gate voltage supply needs to have the ability to deliver a lot of current for a short time.

Chris and Dave have suggested other options - solid state relays, and very heavy duty rotary switches. I think you should look into those possibilities as well.

It would also be very helpful if you would tell us more about your project. For a start, can you describe the load, and the aim of your project?

This high-current switching stuff is outside my experience. I will PM another user, GonzoEngineer, and ask him to have a look at this thread.