Multiple batteries / multiple boards configurations

General discussion of using Roboclaw motor controllers
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Vinc
Posts: 2
Joined: Sun Jan 10, 2016 2:50 am
Multiple batteries / multiple boards configurations

Post by Vinc »

Hi,

I'm investing the possibility to use Roboclaw 2x7A or 2x15A (motor sizing still in progress) for a project, but not received them yet, and I was wondering how the boards can manage:
1) the use of several LiPo (or Li-Ion) batteries, either in parallel or in series, connected to a board (I'm thinking of the regenerative behaviour and thus the configuration of the numbers of cells in that case)
2) the use of one battery pack to power several Roboclaw boards at once: similar issue: how the regenerative behaviour is dealing with that and how to configure the number of cells

It's maybe a bit out of topic since it doesn't necessarily apply only to Roboclaw board but more general batteries topic but it's a fundamental question to know if I will be able to deal with the Roboclaws at the end.

Thanks for any help !

Vincent
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Basicmicro Support
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Joined: Thu Feb 26, 2015 9:45 pm
Re: Multiple batteries / multiple boards configurations

Post by Basicmicro Support »

1. Regeneration will pushe back current(power really) into the batteries. If the batteries are in parallel then each battery will take a different amount of that power depending onhow low that particular battery is. Lipos have a pretty flat disipation curve so the charging curve is also pretty flat so in general the power will distribute evenly between all the cells.

2. In general the use of Multiple Roboclaws on one battery isnt any different than using one since a singal Roboclaw actually has(can have) two motors on it you end up with multiple sources and sinks of power. The battery just becomes one of those sources/sinks. With multiple motors at any given instant some motors will probably be sinking current while others are sourcing it. It is more likely in a multiple Roboclaw system on a singal power rail that very little power will ever go back to the battery(the battery looks like a higher resistance than the individual motors so they will tend to use the regen energy instead of pulling it form the battery.

If you are worried about a worse case scenario the main one is if you were in a situatio where the majority of the motors are in regen caused by an external source(for example if the unit was a wheeled vehicle and was being towed or the unit was rolling down a very long hill). Then you could conceivably over charge the Lipo batteries. In actual practice you have a couple options.

1. You can have the roboclaw disipate the power as heat if the rail voltage gets too high. It does this by braking the motors. This caused the energy to be disipated in the motors and the mosfets of the Roboclaw but none of that energy goes back to the batteries. The drawback to this is there is no way the Roboclaw can protect itself from excessive heating if this condition lasts for a long time.

The point at which braking happens can be set by the user. The default is to trigger this at the maximum voltage allowed by the Roboclaw(eg 34v on most boards). You would want to set this to the maximum voltage your combined battery cells should be allowed to reach when charging(eg 4.2v times number of cells in series).

2. You can added an exteranl votlage clamp. This is the same as above but instead of the Roboclaw disipating the regen power, a very large resistor will do it instead.

3. You can add a power contactor to disconnect the batteries if an over voltage happens. This is more complex because you would also need to properly handle the very large voltage spike that would happen when disconnection occurs but would completely disconnect the batteries from the motors in an excessive regen situation.
Vinc
Posts: 2
Joined: Sun Jan 10, 2016 2:50 am
Re: Multiple batteries / multiple boards configurations

Post by Vinc »

Thank you for the quick and detailed reply.
Seems that everything can work as expected then, especially because our application will be serial mechanisms (not wheeled vehicle) the worst case scenario is really not likely to happen and especially for a long period of time so setting the maximum voltage value will do.
And thank you for explanation of the regeneration "shared" across motors, I didn't think about it and that's good to know.

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