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Background: Building a radio control wheel bot. Using a motor controller that recommends independent power supply for 1.) power to the controller and 2.) power to the motors. Also want a 3rd power supply for accessories (lights, horns, etc.).

I am using 8 identical lead acid batteries. 6 in parallel for the motors, 1 for the controller power, and 1 for the accessories. All 3 systems are 12v.

I want to hard wire a solar charger (with controller) to charge the systems, but I will put a switch on all the positive connections to the charger, so I can choose when each system is being charged. I do not want to put switches on the negatives though.

So now the question: Is it ok to connect all the grounds from all the batteries together? Then, if I want to charge all 3 systems at the same time, I will turn all the charging switches on, which will effectively put all 3 systems in parallel. Is this ok?

Is it ok to charge one system while all the grounds are connected? Most likely i will want to charge the controller power and accessory at all times, but only charge the motor batteries when I'm not driving it (to prevent voltage/current spikes from going to the controller power).

Any explanation is greatly appreciated. Thank you!

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  • \$\begingroup\$ I believe it recommends separate so you don't deplete the motor controller power lower than 12v by driving it too much. Below 12v is no good for the controller apparently. \$\endgroup\$ – David Jun 18 '15 at 15:22
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    \$\begingroup\$ It'd help if you can link to the datasheet or info on the controller. Browning out is certainly a bad thing to have happen, but there are other ways to solve this than separate batteries. \$\endgroup\$ – Nick Johnson Jun 18 '15 at 15:27
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It's not only okay to connect the grounds together, it's mandatory if you want them to communicate or interact with each other in any way. Without a common ground reference, what you have is several entirely separate systems, and any communications between them would have to be isolated using something such as an optoisolator or a transformer.

Charging one set of batteries when they all share a common ground is also no problem.

That said, you really don't need separate batteries for different parts of your system, whatever the motor controller manual says. At worst, you will need separate regulation for high-noise parts of the system, and input filtering (such as LC circuits) to reduce noise from the motors. Just look at something like the Tesla- you don't see that having several different battery banks for different parts of the cirtuit.

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  • \$\begingroup\$ Thank you for the response. Puts my mind at ease that I won't fry the controller! \$\endgroup\$ – David Jun 18 '15 at 15:24
  • \$\begingroup\$ What if I were to up the voltage of the motor system to 36v. Is it ok to connect the grounds of a 36v and 12v system? \$\endgroup\$ – David Jun 18 '15 at 16:28
  • \$\begingroup\$ @David Yes, absolutely. Again, not just okay, but mandatory - if two parts of the circuit don't have a shared ground, they're not the same circuit! \$\endgroup\$ – Nick Johnson Jun 18 '15 at 16:38
  • \$\begingroup\$ Common sense +1 \$\endgroup\$ – Andy aka Jun 18 '15 at 16:55
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Your charging circuit doesn't sound like a good idea to me, because you will interconnect the batteries with different charge level. Imagine your motors were used a lot and depleted their batteries, while the accessories were not used at all. The moment you will start charging, you will short a fully charged battery to 6 discharged batteries, causing overcurrent.

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I would likely stop charging the the controller power and accessory batteries, and only charge the motor batteries. But that does leave room for operator error, so I understand your point.

The controller I'm using is a Roboteq AX2550. https://www.roboteq.com/index.php/docman/ax-documents-files/ax-documents-1/ax-userman-1/ax25xx-user-manual/88-ax2550-user-manual/file

Page 32 is where they discuss powering the controller.

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