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Glen Carter avatar image
Glen Carter asked

Help with battery switchs for motorhome?

I am in the process of designing the system for my motorhome looking for help to configure the system disconnects. The main components:

  • 4- 440W solar panels > 4 DC breakers/disconnects > 4 MPPT 100|30 > Lynx Power In
  • 8- 100Ah Lion UT-1300 > 2 - Lynx Power In
  • MultiPlus II 12 | 3000 | 120 2x120
  • Orion-Tr Smart DC-DC Charger Non-Isolated 12 | 12 | 30

I plan on a Battery switch on the starter battery negative terminal.

The coach system, I am planning to use a Shallco Bi-Stable Latching Relay BR-300-12-1-34 on the main negative terminal of the Lynx Power In before the Smart Shunt.

This should provide 100% battery disconnect from system. At this point the MPPT should turn off.

My question is: with the motorhome being all electric (no propane) is there any need for additional battery switches on the 12V input to MultiPlus II and/or the Solar output from the Lynx Power In?


This is my preliminary diagram eleganza.jpg


Multiplus-IIMPPT ControllersLithium Batterysystem designlynx distributor
eleganza.jpg (174.6 KiB)
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4 Answers
Mike Dorsett avatar image
Mike Dorsett answered ·

As an engineer, I don't like putting switches or fuses in battery negative lines. This can cause unexpected issues with other components in the system. Switching the positives gives far more predictable results. Also, you need to disconnect the PV input to the MPPT BEFORE disconnecting the battery - as this can also cause either damage to the unit, or voltagew overshoot on the output.

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Glen Carter avatar image Glen Carter commented ·
Thanks Mike, good to know. I was planning to review the MPPT warnings again. I'm dealing with just needing to have a way to disconnect everything for storage. Will definitely create a shutdown checklist. Otherwise, with the solar, having the power on and just turning off the Inverter is acceptable.
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Kevin Windrem avatar image
Kevin Windrem answered ·

Take care with switching DC power to the Multi. There is a HUGE inrush current when applying power that can damage switches/relays and cause batteries to go into protect mode. You should relay use a precharge circuit.

The Lynx BMS has a built-in recharge and DC power relay but is very expensive especially if you don't need a Victron BMS.

REC makes one that also includes a relay for DC power switching.

In reality, when the Multi is turned off it's DC current draw is tiny. I doubt you'd notice if you left it on. The same is true for the smart shunt.

Note that if you power down the shunt you will need to set it to NOT reset to 100% SOC when the power returns. You will also need to run the batteries through a charging cycle before it will report SOC.

I also think switching the positive side of the batteries is better/safer.

You should have a circuit breaker/fuse ahead of the Multi. That way you can disconnect the grid if something isn't right.

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Glen Carter avatar image Glen Carter commented ·
Thanks Kevin. I don't have a need for a Lynx BMS. The DC battery switch will hardly be used, however in case of emergency, I plan to have the battery kill switch by the door, thus the reason for the Bi-stable latching relay (maybe a switch protective cover will be a good idea).


I agree on little parasitic load with everything off and with 880W of solar panels exposed (panels on slides, 2 on top exposed at all times)

As for the grid/shore power 120V/240V, I could just disconnect the cord, however I plan to install a 50A hardwired surge protector/power management device.

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Glen Carter avatar image
Glen Carter answered ·

As I further learn about the system components, I'm thinking a Smart Battery Protect 12/24-Volt 65 amp on my coach 12V load with a remote switch at the door will protect battery from low voltage and allow me to turn off all 12V loads.

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kkesp avatar image
kkesp answered ·

Use a disconnect between Lynx and inverter, keep it simple every connection adds resistance to the system. I'm curious why you plan to have 4 MPPT charge controllers? With 4 panels in parallel, or 2 series strings run in parallel you could use 1 150/100 MPPT. If you had 4 panels in series you could use 1 250/100 MPPT. Seems like extra complexity, and cost. Use a combiner box on the roof then you only have to drop 2 wires to 1 appropriately sized disconnect breaker.

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kevgermany avatar image kevgermany ♦♦ commented ·
Some of the panels are on slide out rails.
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