I have built a 12v rover – designed to be fully autonomous with a 12v deep cell marine battery as its only power source.
It uses the following components:
1. [Raspberry Pi3 through a power converter][1]
2. [Sainsmart 16 Channel Relay Board connected to the PI to control motors.][1]
3. [LED Monitor (Draws 2A at 12V)][1]
4. Four 60 Watt Hobby Motors
It works great. I can write the code and drive it around the yard.
However – when the motors are engaged, the LED Monitor shuts off and comes back on again – as if it lost power. The PI seems to be unaffected and everything works fine – except for the momentary rebooting of the monitor.
A voltage meter shows 12.5 at the battery – drops to 11.9 when the motors are engaged, and stays at 11.9 until the motors stop and then goes back to 12.5.
My guess is that when the motors are engaged, it draws a large amount of current, more than the battery can provide, and causes the monitor to reboot. My guess is that this is not good for the components long term.
My question is – can I somehow isolate the motors from the circuit, even though they use the same power supply – so that the momentary start-up draw does not affect the rest of the components? What is the proper way to wire such a circuit?
Best Answer
No way.
Marine batteries are ginormous and are not going to be "laid low" by four 60w (5A) motors starting up. That's 20A nameplate, and even if it surged at 10x, or 200A, that's still a fraction of their cranking amps. A marine battery is only going to dip a volt or so.
The problem is your wires. I bet you have a pair of wires coming from the battery which then come to a junction where they split to Raspberry Pi, LED board and relay board (which I gather powers the motors). That pair of wires is not near thick enough for the jobs (and that may be saving you).
Try doing a separate "home run" for each, back to the battery terminals proper.
So the battery terminals will now have 3 wires each, one for the Pi, one for the LED and one for the relay board. Given the amperage of the Pi and LED, #18-20 should suffice.
If testing proves this out, I would combine the Pi and LED board into a single homerun, so you now have 2 homeruns.
Also as a sanity check here, make sure you are never gating on both relays on a side at the same time, as this will effectively short the battery through the relay board. And remember relays have a throw time: don't turn off forward and instantly turn on reverse relay, if the reverse relays make faster than the forward relays break, that's a dead short.
If that were to happen, it would cause a lot of voltage drop on the (I presume) small supply wires from the battery to the relay board. That would explain the Pi and LED being browned out.
But before you upsize the relay board feed wires, consider - the small wire may be saving your relay board and motors, by creating a lot of voltage drop during a short condition, and that current limiting would be saving parts.
I have a 3' interconnect inside a panel going to a generator rated for 35A, but has a system flaw where it can overload massively. The last guy tried to assure the #12 wires don't fail again by upsizing to #6. (60A in buildings, like 100A in chassis). I found melted wire insulation oozed into the strands, so it clearly overloaded 200A+ before blowing the generator. All he did was move the failure point from the $1 wire to the $8000 generator. Don't be that guy.