Question on Cerbo GX Relay wiring for batter heaters

DIY
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1

Hey folks,

My goal is to properly wire up 4 Ultraheat battery heaters to be controlled by my Cerbo GX to keep the batteries above 41F so that they can always accept a charge - I live in northern USA and will be taking my van to Canada for skiing frequently. The heaters consume 1.8A at 13.5V each, for a total of 7.2A (100w).

The amperage of the heaters is higher than the specs on the Gerbo GX relays (max 6A at up to 30V). As such, I was going to put the heaters on a relay. I have read some concerns about using standard automotive relays as when the coil is disabled (relay turned off), it can induce a reverse voltage spike and possibly damage the Cerbo GX. I was thinking that instead of using an automotive relay I can simply use a solid-state relay.

I do have a question of wiring. I was going to power the load side of the relay from my DC panel with a 10A fuse (a fairly low value to ensure that a failure in the heater will not result in a short that can melt the battery). On the control side of the relay, do I need yet another circuit from my panel with a 5A fuse which will go from panel to COM on Cerbo GX, and then another wire from NO pin to my SS relay, then the relay to ground? I feel this is the best option, but then I will have two fuses in my panel for the heaters (one for Cerbo Relay and one for SS relay to power heaters). I could also bridge the Cerbo power pin (as the Cerbo is also powered by 12v), and put an inline fuse for 5A, but then I have this random inline fuse that might not be easy to diagnose if I don’t remember it being there…

One other small detail - my fuse panel is on a Battery Protect device so if the system is unsafe to discharge due to very low temps, it won’t be powered either. This does have an implication that the system can only keep itself warm if it is at least warm enough to discharge the battery as my heaters will use a circuit on this panel. My goal is to always keep the batteries warm enough to discharge and charge, then either be driving my van (DC2DC charging) or plugging into shore power. I don’t expect much solar charging at the ski hill due to winter and clouds.

How would you wire up battery heaters?

2

Have a look at the FAQ linked below, it has a diagram in showing you need power for both the control circuit and the load circuit, it does not show fuses.

Regarding your question about temperatures so low you have a load disconnect and loose heater power rather misses the fundamental point that you have also lost Cerbo power so you have no control of the power relay. If this is a concern, the battery heating power should be supplied from a charging source upstream of any battery disconnect, by a charger that is not shutdown by the Cerbo and has its own separate thermostatic controller, again powered by the charger. Cold black start.

3

Which faq link?

4

My cold black start plan is an extension cord to shore power through my rear door seal to power a 1500w heater placed near my electrical box.

5

You could use a Smart Battery Protect 65A, they have Bluetooth, so you can set it to what you need. Use the relay on the Cerbo to control the remote switch on the battery protect. This will allow you to protect the battery from the lowest voltage you are prepared to go and also allow remote on-off from your phone, VRM cerbo automation etc.

6

oops

7

Yes there is generally a voltage spike when DC voltage is removed from the relay coil. This is easily managed by placing a diode across the coil since the spike is opposite polarity to the energizing voltage. If you shop around, you may find a suitable relay with an internal diode. If not, a 1N4002 should do the trick.

You can use the same source as will feed your heaters. You can switch the negative side of the coil with the Cerbo’s relay output. This way there is no need for a fuse in the line coil’s wiring (the coil will limit current in that line).

8

Or you can search for “RC Snubber” on a site like Amazon. I don’t know if their capacity is enough; they all appear to be the same.