Sterling Power BB1240 12V 40A Bidirectional DC-DC Battery Charger

The Sterling Power BB1240 is the Saturn series charger built for smaller overland rigs, camper vans, and purpose-built day trucks where space is tight and alternator headroom is modest. At 40A input and 38A output, it delivers fully regulated, 4-stage alternator charging without asking more of your electrical system than it can comfortably give. We typically recommend the BB1240 as a strong starting point for builds in the 100–200Ah range — though the right charger always depends on your alternator capacity, your loads, and how you actually use the rig.

Like every unit in the Saturn series, the BB1240 applies the correct charge algorithm for your battery chemistry rather than sitting at a fixed output voltage. Six preset profiles cover Lead I and II, AGM I and II, and Lithium I and II — set it once and it handles bulk, absorption, and float autonomously. A low-temperature cutoff protects lithium cells in freezing conditions, and six self-recovering protection circuits manage over-voltage, under-voltage, over-current, and over-temperature faults without manual intervention.

Bidirectional reverse charging is standard. When your house bank climbs above 13.2V (13.5V on lithium profiles), the BB1240 automatically feeds up to 13A back to the starter battery to maintain it at a healthy 13.4V — so if solar is looking after your house bank while the rig sits, your starter is looked after too. If the starter ever gets too low to turn the engine over, Jump Start Assist mode forces the reverse charge to run for 20 minutes to get you back in action.

The BB1240 pairs cleanly with other Saturn series units for expanded capability. Running a BB1270 already and want more output? Add a BB1240 for a combined 110A with granular current control across six power level combinations via the optional BBR remote.

Key Specs:

• Input voltage range: 9–16V
• Rated input current: 42A at 13.68V
• Rated output: 38A at 14.41V (~550W continuous)
• Reverse charge current: 20A
• Efficiency: 95–98%
• Dimensions: 163mm × 130mm × 56mm
• Weight: 0.8 kg
• Recommended input cable: 10mm² / 8AWG
• Recommended input fuse: 50A
• Recommended output cable: 10mm² / 8AWG
• Recommended output fuse: 50A
• Certifications: CE, UKCA, E-Mark
• Cooling: thermostatically controlled variable-speed fan
• Compatible: lead acid (AGM, gel, flooded), lithium LiFePO4

Compatible with smart/variable alternators (Euro 6/7+). Ignition feed or vibration sense activation modes. Optional BBR remote available for display monitoring, custom charge voltage adjustment, and output current limiting.