Want to save some cash on a B2B?


Full Member
This might be of interest for those who fancy some experimentation?

Following on from my testing of different Split-Charge options, I thought it might be useful to share the "Inverter+Mains Charger" option which turned out to work pretty well, together with a more complete setup rather than my 'quick and dirty' test setup.

This is an alternative approach for a B2B setup where there is an existing Mains Smart Charger installed. More Complex than a basic VSR but provides an automated proper multi-stage charging regime to the Leisure Battery at a lower cost than a dedicated B2B Charger if budget does not run to an off-the-shelf B2B.

Cheap B2B.png

(Fuses and -ve cables not shown)

Using a pair of SSRs each side - one for Live & one for Neutral to ensure Mains and Inverter 240V lines are kept fully isolated from each other.
Using single 5 Pin relay with NO and NC outputs ensure only one pair can be active at any one time
An alternative to using SSRs is to use a Contactor (Auto-sensing 240V Relay) but typically more expensive.
The size of Inverter must be matched to the maximum output of the Mains Charger.e.g. if you have a 30A charger, a 500W Inverter (which can deliver 40A) would be the minimum. Using an undersized Inverter will not work out correctly.

If any one does give this a go, the info is provided without warranty and guarantee and risks are down to the installer.

I am fitting an alternate version of this design in the next couple of days.
Instead of using SSRs, I am using a CU DIN Changeover Switch - https://amzn.to/2EN6U79
So using a Manual selection rather than automatic, and a 1000W inverter to feed into my 120A Charger (but current-limited to an output suitable for the Inverter (upto 80A for a 1000W device)).
Seperate to the B2B bit, I am incorporating a 3-way heavy duty DC switch which will let me bypass the Inverter B2B if I wish, or to use the Inverter off the Leisure Battery without any rewiring (giving me a backup/extra Inverter if needed).

Any questions, just ask :)


Full Member
I have a Cyrix-CT Battery Combiner working as a Split Charge and the plan was to do basically what is in first post, but feeding the inverter via the Cyrix (as oppossed to a Switched Relay as shown in the diagram) so connection is made automatically between Inverter and Starter Battery when the Battery Combiner (aka VSR) is enabled.
But it turns out the Victron Cyrix is more intelligent than I am :p

The Cyrix-CT devices actually check the voltages on both sides of the device and if either is under 2V the unit will not turn on. As there is no battery on the output side, just the Inverter, the Cyrix will never turn on and so the Inverter will never get powered.
I'll need to replace the Cyrix CT with a basic VSR that just switches on when either side hits a set value (13.3V usually) and does no clever voltage checking.
Last edited:

Users who viewed this discussion (Total:0)