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

A Battery to Battery charger usually contains instructions to wire it between the Starter battery and the Habitation area battery with claims on boosting the charging rate. They have gained popularity because some Motorhomes have such poor Alternator 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 really serious fault exists in the old solution, even a two fold faster charging scenario is unlikely.

Yet big improvements on some Motorhomes are available by relatively simple changes, for less than £20. 

Just uprating the cable sizing and the Split Charge relay can usually bring most motorhomes with poor alternator charging rates close to the optimum. All without any battery degrading side effects. 

Most B2B installation guides specify that, as part of the installation, fat cabling is used, often greater than 16mm. It is this cabling that provides 95% of a B2B's improvement in most motorhomes, not the B2B device itself.

If you follow our guide at the bottom of this web page :

(from the section after the 'giant' relay) it contains information that will enable you to upgrade your own Alternator charging system to the optimum for around £20 in parts bought from eBay and 12v Planet.

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.

Most 'intelligent' Brake recuperation style Alternators that vary the voltage dependent on load, can be 'defeated' by simply putting on the Headlights. This headlight load will force the Alternator to apply the correct, continuous charge to the habitation batteries, even on vehicles with Brake Energy recuperation.

But it isn't the ideal situation and a device that intelligently controls the voltage despite the Alternators behaviour is more appropriate for most.

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 from Sargent, Schaudt, CBE or whatever system your motorhome has.

So our suggestion is not an old fashioned 'Battery 2 Battery charger' but an "Intelligent Alternator Charging Solution". Taking power directly from the Alternator additionally eradicates several possible voltage drop connections.

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 new style '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, Starter battery bypass, etc. 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. 

Apart from the installation of the 'charger', 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.     

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

The units we are talking about are made by the worlds biggest motorhome charger manufacturer, Schaudt.

They are the Schaudt WA 121525 and the 45amp version the Schaudt WA121545, see instructions and documentation below.

For a supplier, try Brownhills or look on eBay.

Manuals for the Schaudt WA121xxx range of Intelligent Alternator Boost Chargers -

Schaudt WA121525.pdf Schaudt WA121525.pdf
Size : 476.93 Kb
Type : pdf
Schaudt WA121545.pdf Schaudt WA121545.pdf
Size : 743.071 Kb
Type : pdf

                        How to Supercharge your Alternator charge rate for £25

The above units are one way of addressing a poor Alternator charge and Euro6 style ECU controlled Alternators, but you don't need to go down that route if the problem is just slow charging when driving. 
For around £25 and some minor work you can transform your Alternator charge rate just be uprating the cabling and intelligent wiring.

Where a Motorhome has separate Split Charge, Fridge, etc Relays, can we suggest you check that the Split Charge relay is of adequate rating if you increase the battery bank size? 
While some are 'OK' as far as the 'current rating' goes in passing the extra power from the Alternator to the bigger Habitation battery bank, they can create significant voltage drop.
It is often a good idea to uprate the 'Split Charge' Relay to a higher amperage, this allows you to take advantage of the lower voltage drop that bigger Relay switch contacts should give, especially after a few months use where burning may degrade a smaller relays contacts. 
Something like this 100A Relay below found on eBay for less than £9. 200amp for about £12.

Like the above relay, the best extra heavy duty relays also often have 'bolt on' connectors overcoming the limitations of the Spade type when currents increase. We sometimes see burnt Spade connectors on 'separate relay' installations when batteries are used beyond their life or the bank is increased. 

One way of installing a heavy duty relay is to use the output from the existing/old Split charge relay to drive the uprated relay. This prevents the bigger relays coils, which usually draw more current, in the new relay from overloading the low current Alternator D+ circuit. 
Don't drive the big heavy Duty relay direct from the D+ circuit unless you are sure the relay coils won't draw more than 0.5 amps max. 

If you get issues with the Ignition Warning light illuminating after fitting a Big relay, then that is a solid indication the relay has overloaded the circuit.

Along with a chunky relay heavy duty cabling can make quite a difference to the charging rate when Alternator charging on these vehicles. Especially relevant when extra charge current is being passed to a s econd battery.
But before you do anything check the impact the Fridge has on the voltage at the habitation battery, if it is significant then 

Starting with the Fridge/Freezer power supply cabling and connections may be a better idea. 
Often, fixing a problem with the Fridge draw will also give major benefit to the charging rate as well as improving Fridge efficiency. See further down the web page for a section on the Fridge.

Sometimes the best way of uprating the cable is to run a second parallel cable, rather than ripping out the old and replacing with a single larger cross section cable. Using a single big fat cable can give issues when it comes to connectors, as many readily available connectors won't take very fat cable.
We suggest that DIY solutions are better off utilising two thinner cables, bear in mind that just adding a second cable the same size as the original cable will double the current carrying capability and reduce voltage drop significantly. 
However we would suggest a slightly thicker, 10mm, second cable would work even better.
Uprated 50amp/6mm cable is available here for just £1.69 a metre :
So using this cable and a £6 relay from eBay you can supercharge your Swift motorhome Alternator charging for less than £20.

But even better would be this 10mm/70A cable at £2.57/m can be found here :

It might be worth thinking about running a separate chunky Negative/Earth cable directly from the Starter battery negative Post to the Habitation Negative battery post on a British built Motorhome, even if you don't uprate the Positive cable. 
Even better would be to run the Negative/Earth lead from the Alternator body to the habitation battery.
Don't trust the Base vehicles Earth to be perfect, especially on a Fiat, Peugeot or Transit Base, as the Engine 'Earths' are known to be a weak point. 

If you suffer Alternator failure, quite common on Motorhomes with enlarged battery banks or Lithium upgrades, then take the opportunity of the Garage working in that area to attach a decent sized (minimum 16mm/110amp?) Earth cable from the Alternator body/Lug to the Starter battery negative.
A second Alternator Positive feed would also never hurt, and obviously it would become an essential item if the Alternator has been uprated.
Also think about asking for new Engine/gearbox earth straps.

Uprating the relay and twin cables can be as good as installing a B2B if you increase the Habitation battery bank size.       
The aim should be to get the habitation battery voltage to within 0.1v of the voltage at the Starter battery when the Alternator is spinning in order to ensure the enlarged battery bank gets the best possible Alternator charge. 

Clearly, if the voltage at the Habitation battery is already the exact same 14.4v of the Starter battery when under Alternator charging and Fridge 14.4v load, then you already have a perfect set-up and don't need to uprate anything.

Poor cabling, connectors or relays between the Alternator and the habitation battery bank will not only reduce the real world charge to the Habitation batteries, extending charge time, but also place extra strain on the Alternator. 

Remember the Earth Points focused on here are that of the Alternator, not just Engine or Chassis. 
You might have a fantastic Gearbox earth point but if the mounting between the Alternator body and the Engine Block  isn't good, you will have problems. 
When a modern Motorhome Alternator can pass up to 220amps, it should have it's own dedicated Earth strap to the vehicle body and then onto the Starter Battery.
If you have a Battery To Battery charger or a big battery bank, the Alternator Earth may be critical to reliability and usability. While the B2B instructions might say to take connection points from the Starter battery, it is obviously more sensible to go direct to the Alternator for both Positive and Negative, where appropriate, as this is where the mega amps will come from. Same applies to the feed for a big Split charge relay.


Sometimes it is the extra load imposed by the up to 19 amps of a Freezer/Fridge that will take an already poor charging voltage further into the Doldrums. So fixing a poor Fridge wiring set-up first can also have major dividends with Alternator charge rates.
If you suffer significant voltage drop to the Fridge, the Fridge will obviously be less effective as well as impacting charging. 

Some vehicles take the Fridge/Freezer feed from the habitation battery terminals (via Fusing and usual D+ switching) when the best cable run will be from the Alternator. 
Some converters wire the Fridge from the Starter battery (via Fusing and D+ switching) which is a much better option than using the habitation battery, but still not as good as an Alternator option.   

While the advantages of these different 'start' points for the wiring may not be an immediately obvious cause for voltage drop at the habitation battery, we have seen a better than 1.0 volt improvement in habitation battery charging just by moving the Fridge feed from the habitation battery terminals to the Starter battery terminal.

We were surprised to see just how much the charge current also went up with the improved voltage from rewiring the Fridge. One owner reported that the charge times were 'slashed by half', but that wasn't ratified by scientific testing.

If the original Fridge feed comes from the habitation battery, find the Fridge D+ operation circuit in the Power Controller/Fuse unit and use it to operate a new big relay that takes it's feed directly from the Alternator B+ post and a big Fat Earth from the Alternator body mounting.  
Not only will the Fridge be more efficient but the Habitation battery charge voltage will rise significantly.

I think you will be surprised by the results that just basic, easy cable changes can make. We have seen almost 18amps per 100Ah battery which matches the best of the B2B's.