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                Battery to Battery chargers or B2B's versus "Inteligent Alternator Charging Solutions"

A Battery to Battery charger is device which connects between the Starter battery and the habitation battery with claims on boosting the charging rate. They have gained popularity because some Motorhomes have such poor charging installations with just a basic Split charge relay and undersized cabling that the room for improvement is obvious.

However, some of the marketing claims for these devices of up to 3 times faster charging are just not realistic, unless some serious fault exists in the old solution.

In the past most Battery to Battery, B2B, chargers have claimed to faster charge the habitation battery bank when driving by increasing the charging voltage to a higher level than that recommended by the Battery manufacturers. 

The higher voltages then cause the battery to deteriorate, losing capacity and battery efficiency, by accelerating corrosion of the battery Plates. This higher rate of fluid loss is acknowledged by the B2B manufacturers as an unwelcome side effect. 

Therefore within a year some batteries treated in this way, have less capacity and take longer to charge than if the batteries had been left with standard Alternator charging. 

So in some ways using a higher than recommended charging voltage is self defeating once the batteries have been used for a while.

Yet improvements on some Motorhomes are available by relatively simple changes, like just uprating the cable sizing and the Split Charge relay, without any battery degrading side effects.

The optimum long term charging is achieved by ensuring that the batteries receive the voltage and currents as specified by the battery manufacturer, no more to ensure damage doesn't occur, but no less or slower charging will result

We think the optimum and most cost effective approach is to ensure any voltage drop is the minimum possible, paired with the most efficient batteries.


Therefore the 'B2B' we suggest as the most appropriate takes power directly from the Alternator, not the Starter Battery and supplies it to the Power Distribution unit/Controller. So our proposal is not a 'Battery 2 Battery charger' but an "Intelligent Alternator Charging Solution". Taking power directly from the Alternator eradicates several potential voltage drop connections.

While this may not outperform a Sterling B2B's 'forced charge' system during it's first few months of use, in the longer term it will outperform any old style Battery 2 Battery charger.

The loss of efficiency and capacity of a Battery that is 'force charged' is not generally understood by many who run B2B's, who often also do so in association with Solar Charging.
If you compromise a batteries efficiency, it will take more Solar amps to get fully charged and because the compromised battery is shrinking in real terms you also get back less Solar amp hours.

Ask anyone in the know out on the web, they will all tell you fast charging will accelerate Internal battery corrosion and that corrosion not only restricts a batteries ability to accept charge, but also give it up.

Have a look at the picture below from Bosch, it shows a battery that has corrosion on the Grid and one that hasn't. 

This corrosion is a layer that grows between the Grid and the Paste through which electricity has to flow.
You can see why the 'reduced corrosion' Bosch and Varta Powerframe batteries in the right hand photo have a claim of 70% better electrical flow than conventional corroded batteries.

The B2B's out there do what they say 'on the Tin', but the marketing forgets to mention the beneficial effect can be only short term and shortens the batteries life significantly.

Most aftermarket Battery to Battery chargers are installed in exactly that way, battery to battery, bypassing the Motorhome Electronic control systems and creating issues in modern Motorhomes. Until recently that was the only option.

But that has now all changed.

There is now a solution available that offers faster charging from the Alternator to the habitation battery that works in harmony with the existing Motorhome electronics, not against it.

These units stick rigidly to the voltages and currents recommended by the battery manufacturers, so a standard Wet Acid battery will get the recommended 14.4v. 

The faster charging is achieved by optimising the power from the Alternator by ensuring that whatever the input voltage into our B2B, the optimum is applied to the batteries. 

Even if a Euro 6 or Brake Energy Recovery system is installed, where the Alternator 'suspends charging' for a time, the B2B will go on providing a continuous voltage that is matched exactly to the battery.

There are units that have more features, like different settings for Gel, AGM or Wet/flooded batteries. They ensure that each battery type gets exactly what the battery manufacturer intended, e.g.14.3v for Gel or 14.7v for AGM, not necessarlly the Alternator's fixed 14.4v output

It does this regardless of the Alternator voltage whether it is 11v or 15v.

Even better, when the battery is full, our 'B2B' does not go on forcing 14.4v into the battery. 

Just like a mains charger, it drops down into 'Float'/'Maintenance' mode of 13.4v saving fuel and prolonging battery life. 

Some of these B2B units turn a 2016 Hymer into a fully optimised AGM battery charging machine. 

Apart from the installation of the 'B2B', the wiring is unaltered in functionality. Although there are minor physical changes to insert the 'B2B', the functionality remains the same as before. Everything works EXACTLY as the Motorhome designer intended, just that now the Elektroblock/Power control unit has a rock steady, optimised Alternator feed that will ensure fast charging to the battery manufacturers optimised limit.   


These units can make 2015 - 2018 Hymers Alternator charging fully optimised for AGM batteries.   

On a Euro 6 Motorhome (or Caravan Tow car) the new charger ensures the Habitation area batteries will get a continuous charge, not one that drops when the Motorhome ECU 'deactivates' the Alternator to save fuel. 

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