BATTERY TO BATTERY CHARGER Charging Auxiliary Battery

Wish we would have known about these when we converted a cargo trailer back in 2011.

Somewhere before starting our Sprinter Conversion we started researching these “Battery To Battery” Chargers.  Common slang for them is B2B Chargers.  12VDC input to 12VDC @ 30 amps output (the model we used).

Note: There are various configurations of these B2B’s in both Voltage and Amperage.

Since we are installing a 30 amp Solar Controller, and a 120VAC to 12VDC Converter Charger, we decided to venture off into a B2B of similar capacity.  We selected a 30 amp charging unit.  There are larger amp units, but we felt 30 amps is adequate for our needs.  Plus is doesn’t require an upgrade to our existing alternator, or the need to install a secondary high capacity alternator.

(See Image below) In the future we hope to link a diagram/schematic of this multi-charger layout.  It contains the B2B, Solar Controller-Charger, and a 120VAC to 12VDC Converter-Charger.  As well as the Auxiliary Battery Cut-off switches, and multiple individual DC Breakers used on the three Chargers for protection and quick Isolation.

Not seen is the face of the 120VAC to 12VDC Converter-Charger that holds the AC Breakers and DC Fuses.  Also not shown are the Chassis Alternator power cable DC Breakers.

We have had a 120VAC to 12VDC Converter Chargers in several rigs that output a max charging output of 30 amps.  They have been more than adequate for one or two auxiliary batteries.  If you need more capacity you will have  to design differently.

If you get into multiple charging sources and multiple batteries.  Make sure any and all are compatible. If you don’t know what you are doing, have someone build it for you, or at least design it for you.  We are talking battery compatibility, compatible charging outputs, shunts, meters, grounding issues to solve, proper wire gauges throughout, and proper protection fusing or circuit breakers.

In our case all batteries (2 Auxiliary and 1 Chassis ) are AGM.   Our Solar Controller Charger is manually switchable and set to AGM.   Our 120VAC to 12VDC Converter Charger is  jumper selected to AGM output.   Our B2B is “auto sensing” and properly sets itself to AGM charging on initial power up.

These B2B’s are a beautiful piece of technology.  They take your alternator output and ramp it up.  In the case of the 30amp unit, it has the ability to charge at up to 30 amps.

You won’t be able to come close to this type of charging amperage, using a passive battery isolator or an ignition relay type of configuration charging auxiliary batteries in parallel off an alternator.

Also these B2B’s are automatic in nature.  Automatic, in that they automatically isolate the “house” loads from the vehicle chassis battery when the vehicle engine is off.  So it eliminates the possibility of something on the “house” side, running down your vehicle’s starting battery.

But of course this convenient device is not cheap.

Brand: Sterling Power
Model: Sterling Pro Ultra Battery to Battery 30A
Price: $320.00 + Shipping to my location another $29.33
Source: Sterling Power Website