The lithium battery for your Auto-Sleeper motorhome.
Auto-Sleeper, built in Willersey, Worcestershire, covers everything from compact Fiat Ducato campervans to the flagship Mercedes-Benz Sprinter Burford, and every model leaves the factory with an AGM you can only half use. A TITAN drops into the same locker, delivers the full rated capacity, and charges faster from alternator, solar or mains - whether you are on a Fiat or a Sprinter base.
Two platforms, one British builder
Auto-Sleeper, based in Willersey near Broadway in Worcestershire, is one of Britain's longest-established motorhome manufacturers. The current range splits cleanly across two base vehicles. The Fiat Ducato carries the full Fiat coachbuilt range - the Broadway (EB, EK TB LP, EL and FB layouts) and the Nuevo EK Plus - together with the Fiat Premium campervans (Symbol, Symbol Plus, Warwick Duo, Fairford, Kingham and Warwick XL) and the Fiat Active campervans (FL 635, FG 635 and KB 635). The Mercedes-Benz Sprinter carries the premium range - the Bourton, Burford and Burford Duo coachbuilts, and the M-Star campervan.
If you are buying used, older Auto-Sleeper coachbuilts from before approximately 2020 may be on a Peugeot Boxer. The Boxer shares a platform with the Ducato and accepts the same DIN-format battery, but the alternator management differs slightly - the DC-DC charger recommendation applies to it regardless.
Every Auto-Sleeper leaves the factory with a Sargent electrical system - typically the EC700 or EC900 series - and a standard AGM leisure battery. The factory fit is usually 90Ah to 100Ah on the Fiat range and 100Ah upwards on the Sprinter models. Because you can only safely take an AGM to around 50% depth of discharge before you start damaging it, the usable reserve is considerably less than the label suggests. A TITAN lithium in the same or a similar footprint gives you close to 100% usable capacity, charges far faster, and handles the deep cycling that touring motorhome life demands without degrading over time.
For the broader picture of switching to lithium in a coachbuilt, the motorhome battery guide covers the fundamentals. If you are comparing Auto-Sleeper against other brands, the motorhome brands hub has the full range.
Sizing an Auto-Sleeper by model
Starting points by model group. The Sprinter-based premium models have more payload headroom and larger habitation spaces, so can typically carry a bigger bank without compromise.
| Model | How it gets used | Good fit |
|---|---|---|
| Fiat Active campervans (FL 635, FG 635, KB 635)Panel van, 2-4 berth, Fiat Ducato Euro 6E | Weekend touring, lights, fridge, phones, occasional hook-up | 100Ah to 150Ah |
| Fiat Premium campervans (Symbol, Warwick Duo, Fairford, Kingham, Warwick XL)High-spec panel van, 2-4 berth, Fiat Ducato | Extended touring, fridge, heating, solar top-up, more devices | 100Ah to 150Ah |
| Fiat coachbuilts (Broadway EB, EK TB LP, EL, FB; Nuevo EK Plus)Coachbuilt, 2-4 berth, Fiat Ducato Euro 6E | Multi-night stops, off-grid, heating, fridge, multiple devices | 150Ah to 180Ah |
| Mercedes coachbuilts (Bourton, Burford, Burford Duo) and M-StarPremium Mercedes-Benz Sprinter, larger habitation space | Extended off-grid, inverter use, larger appliances, full-tour set-up | 180Ah to 230Ah |
These are starting points, not fixed rules. The honest figure comes from your own loads, not the model name, so total them in the battery size calculator. Battery lockers and tray dimensions vary between Auto-Sleeper model years and specification levels - measure your existing space including terminal height before ordering.
Where the leisure battery lives in an Auto-Sleeper
Auto-Sleeper locates the leisure battery in one of several positions depending on the model and base vehicle:
- Under the driver or passenger cab seat. The most common location on Fiat Ducato-based Broadway, Nuevo and Fiat campervan models. The AGM typically sits in a tray or wooden box beneath the seat. Access usually requires removing the seat bolts - the seat is heavy and carries seatbelt anchor points, so refer to the handbook. The flat DIN profile of the TITAN fits the same tray and keeps the centre of gravity low.
- Under the rear bench or dinette seating. Some Nuevo and earlier Broadway layouts position the battery under the offside rear bench. This space often allows a slightly larger battery or a side-by-side parallel pair where extra capacity is needed.
- Habitation locker or dedicated compartment. Mercedes Sprinter-based models - the Bourton, Burford and Burford Duo - commonly mount the leisure battery and ancillary electronics in a dedicated locker or external compartment, giving more room to install a DC-DC charger, MPPT controller and fusing alongside the battery.
Because the TITAN case is sealed to IP67, it handles the damp and vibration of an underfloor or locker mount without issue. The RJ45 comms ports drop to IP32 while a cable is connected, so keep any plugged port clear of standing water. Locker and tray dimensions vary between Auto-Sleeper model years and trim levels - measure your space and include terminal clearance before ordering rather than relying on a brochure figure.
Charging an Auto-Sleeper from alternator, solar and mains
An Auto-Sleeper charges its leisure battery from three sources: the engine alternator while driving, roof solar through an MPPT controller, and the mains charger on hook-up. Lithium takes all three faster than the AGM it replaces, so a decent motorway run or a sunny afternoon puts real capacity back rather than a slow overnight trickle.
The alternator is where both current Auto-Sleeper platforms need extra thought. The Fiat Ducato Euro 6E runs a variable-voltage smart alternator as part of its energy recovery system - the output voltage rises and falls with driving conditions rather than holding the steady high voltage lithium needs to charge reliably. The Mercedes-Benz Sprinter VS30 generation, fitted to all current Auto-Sleeper Mercedes models from around 2019, uses an even more aggressive smart alternator strategy that drops to very low voltages at cruising speed. On both platforms the fix is a DC-DC charger - also called a battery-to-battery charger - wired between the starter battery and the leisure bank. We would fit a Victron Orion every time: it takes the variable alternator output and converts it to a clean, controlled charge for the leisure battery, while protecting the starter battery from being drawn down. A split-charge relay is a simpler fit, but it is less reliable for lithium on a smart-alternator vehicle and some relay types allow a backfeed that quietly clips the top of your usable capacity. Feeding straight off the alternator with no relay and no DC-DC is not something we recommend.
Plan the roof with the solar sizing guide, check your Sargent charger or any external unit against the compatibility list, and size the battery around the gap your charging sources cannot cover overnight. Every TITAN carries a custom BMS with a built-in low-temperature heater so it charges safely down to -30C, and comes with a lifetime, fully transferable warranty.
