I understood that the split charger system was not suitable for use in vans fitted with intelligent alternators. Mine is a 2017 Kombi Euro 6. See below for info from excellent site
https://www.travelvolts.net/
Smart alternators are installed on modern European Vehicles (Euro 5, Euro 6 + and newer engine models). The object of this new system is to utilise a vehicles wasted kinetic energy during braking / deceleration cycles experienced in every day motoring and rapidly convert as much of that energy (which is usually wasted as braking heat) into useful electricity and store this energy in the starter battery. Then, during acceleration and cruising release this energy back into the vehicles running system as “free electrical energy“ thus reducing the time where an alternator loads the main engine. This increases MPG/KPLand lowers CO2 emissions. However, in order for this system to be effective, the starter battery must have ‘free space’ to boost the energy into the battery, this requires the battery to be about 20% depleted (low enough to allow more power to be boosted into it but not too low as to prevent the engine from restarting when switched off). To replenish this ‘free space’, during deceleration or braking events, the voltage on the alternator shoots up to 15V or more. This higher voltage fast charges the starter battery to replenish its capacity. As you are using the inertia of the vehicle to charge the battery, rather than fuel, it is seen as ‘free energy’. Then the voltage drops to about 12.4V to allow the free energy to be consumed by the vehicle allowing the battery to deplete itself by about 20% ready for the next speed reduction and so on and so forth. Albeit an improvement in terms of emissions, there are knock on effects regarding the auxiliary charging systems on board commercial vehicles, read on.
Problems with Smart Alternator / Regenerative Braking
The system requires a 20% empty starter battery for the system to work. It needs the space to “dump” the fast energy build up during braking. This is in direct conflict with the auxiliary charging system requirements, why? 1) No charge going into the batteries during the 12.2-12.4V phase (which is totally by primary system design). Therefore, if a simple relay charging system was used to charge the auxiliary system it would not be charged during this time frame. This will be a problem if you require a charged auxiliary battery during travel or at location to location. 2) Very high battery charge rate during vehicle deceleration / braking due to alternator high voltage. This is relatively problem free for the starter battery as its relatively full. However, a large empty auxiliary bank could experience high currents at high voltages (much higher than their recommended level) which would be detrimental to the battery leading to premature destruction. Problem with using voltage sensitive/controlled relays? 1) Most VSR / VCRs have time delays before activating. 2) Even when the relay engages then at low voltages the batteries do not charge but at high voltage the aux batteries will get damaged due to massive current in rushes. Also remember that the inherent software control system prevents the battery from being over 80% charged, so even when the battery is charging it will prematurely stop charging due to the software limitations which must leave that 20% space for the unit to be able to dump the braking power, so you can never fully charge a battery using a relay or fet controller you must use an active power product like the CTEK Dual, Ripca EL1225 or Sterling BB1230. (other products are available and prices vary hugely)