### Author Topic: Batteries and mixing voltages  (Read 1053 times)

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#### stridera

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##### Batteries and mixing voltages
« on: August 13, 2013, 04:17:42 PM »
I have a large 48V motor that I'm planning to use for a primary drive wheel.  I also wanted to mix 2 24V motors for flywheel control.

My thought was to get 2 24V batteries and connect the 24V batteries in parallel to power the smaller motors, and run one line in series to power the 48v motor.

My 48V motor controller has a 5V power rail that I can use to power the sensors and microprocessors to control the controllers.

Is there a problem mixing voltages like that as long as I keep them on distinctly separate circuits?  Should I add optocouplers between the 48v->5v powered microcontroller and the 24v motor controller?

Am I explaining this well enough?

Thanks guys!

#### jwatte

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##### Re: Batteries and mixing voltages
« Reply #1 on: August 13, 2013, 08:31:01 PM »
Yes, the main problem will the the "floating" ground reference of the second motor. It will be at +24V compared to the ground of the other motor. This is assuming what you really mean is that the batteries are coupled in series, and each of the two motors is across one of those two batteries.
Might be safer to just put the flywheel motors both across the "bottom" battery, and live with the asymmetric discharge.

#### stridera

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##### Re: Batteries and mixing voltages
« Reply #2 on: August 14, 2013, 12:07:09 AM »
Why would using one battery for the 24v circuit be better than using them both in parallel?  Other than the connection with the batteries directly, the two circuits wouldn't be touching.  (I could even use optos to separate the control circuits.)  The 24V motors would be ground to the 24V batteries, so it wouldn't be floating.  Am I wrong in this?

So:

48v Motor <-> 48v Motor Controller <-> (Series = 48V) 2x24v Batteries (Parallel = 24V) <-> 24V Controller <-> 24V Motors
^- 5v Power Out to power Sensors and send control signals to both controllers.

(Or can even do a voltage divide myself using the 24V Line.  Can use optos to electrically separate the control lines between the two circuits.)

If there is anything I can read to explain this better, please link it to me.  Thanks again.

#### jkerns

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##### Re: Batteries and mixing voltages
« Reply #3 on: August 14, 2013, 08:30:43 AM »
What do you mean by "parallel"?

If you have them wired in series to get the 48 volts:

ground -> negative terminal A, -> positive terminal A -> negative terminal B -> positive terminal B - >48 V

You can not also wire them in parallel where the two negative and two positive terminals are connected  because you will just end up shorting across the batteries:

| negative terminal A --> positive terminal A |
ground-> |                                                                     | -> 24 volts
| negative terminal B --> positive terminal B |
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#### stridera

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##### Re: Batteries and mixing voltages
« Reply #4 on: August 14, 2013, 01:28:00 PM »
Yeah, you're right.  I don't know what I was thinking.  :/

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