Howdy.
I have a motorhome with a bank of house batteries and a bank of engine batteries. All 12v.
The coach sits most of the time. We run the generator daily. The generator charges the house batteries but it doesn’t charge the engine batteries. Over time, the engine batteries get low and then the generator won’t start.
Can I use a Victron Orion XS dc to dc 50amp 12v charger with the house battery as the input and the engine battery as the output to keep the engine battery charged?
Is the generator providing mains voltage at 120/230/240v (depending on your geographic location)?
If so, why not use an appropriate IP22 charger to charge the engine battery - less expensive than an Orion XS, and easier to configure and install.
Blue Smart IP22 Charger | Victron Energy https://share.google/zQgbxjvESTfpTwP5b
You could use the Orikn XS, but there are smaller Orions such as the Orion 12-12/18 isolated unit.
I second the BlueSmart charger idea. The IP65 12/5 is around 100.00.
Well @Clark that’s just a good idea. Hadn’t thought of that - generator provides 240v with 2 poles so I have access to either 120v or 240v.
One problem is that I don’t know where a 120v source is within the battery compartment. Hopefully that is easily solvable.
Another problem is that there’s an uncovered use case - if the generator is off, the house batteries will be charged by an array of six 290watt solar panels but the engine batteries will not be charged.
Will the DC to DC charger sense that power is being supplied from the solar panels to the house batteries and then channel this to the engine batteries?
That should work, but you probably have to turn off the engine shutdown detection, and experiment with the voltage thresholds to ensure charging starts and stops correctly.
However, I think using a 50A charger is a bit overkill for your application, which is essentially just a battery maintainer.
As previously suggested, a small mains charger could work, or, as I have done in my own campervan, use a dedicated battery maintainer such as the Ablemail AMT12-2. The only non Victron device I have across three separate installations!
I also use the ablemail 12-2 battery maintained on my boat, but I am not sure if it is available outside the UK.
There are a couple options for places to buy Ablemail chargers in the US. Ebay and Nohma.com
@Clark thanks for helping me think through this. I’d like to propose that the topic of engine shutdown detection in my use case is moot. This is because in my case, the engine only charges the engine batteries. If I’m using the Victron Orion XS dc to dc 50amp 12v charger with the house batteries as the input and the engine batteries as the output, then engine state detection should be moot correct?
I agree that the Victron IP22 trickle charger is a lower cost option, but using it does leave the aforementioned use case unattended. While more expensive, the IP22 device seems shores up my core use cases.
Is my thinking correct (aside from being expensive)?
Engine shutdown detection detects when the source battery is being charged by its voltage being raised higher than the resting voltage so that the target battery is then charged. Therefore, when charging the starter from domestic you would still use engine shutdown detection if you only want the starter battery charging when the domestic is being charged.
Pretty much, but take note of PWFarnells comments re shutdown. Your will need some way of triggering the XS when the generator starts, and more importantly stop charging when the generator stops, otherwise you risk running the house batteries down. Does the generator have an alternator, or just provide regulated 12v?
Also keep in mind you will need at least 8AWG cables, suitably fused in each leg, to handle the 50A charge.
Pretty sure 6 awg is the smallest acceptable cable size with an Orion 50 amp. I usually use 4 AWG.
You’re absolutely right - the bigger the better. Mine is connected with 16mm2, which I believe is 6awg?
However, the manual does indicate minimum 10mm2/ 8awg, which I guess would be for very short runs.
For the OP, in my opinion, this adds to the complexity of using the Orion XS rather than a small charger or battery maintainer for this application
My bad. A case of RTFM properly! 
