DC-DC Charger Calculator: How much does the DC-DC charger charge while driving?
A 30 A DC-DC charger puts about 58 Ah into the house battery in two hours of driving – a 200 Ah LiFePO4 from 30% is then at just under 60%. This calculator shows what your drive delivers in charge, how long you have to drive to reach the target state of charge and how much current the charger draws from the alternator for it.
Result
State of charge after 2 h of driving
59%
58 Ah recharged (838 Wh)
- Driving time to target state of charge
- 4 h 49 min
- Energy charged
- 838 Wh
- Input current (alternator)
- 34 A
- Charge per driving hour
- 29.1 Ah/h
- Size the wire gauge and fusing of the charger's wiring per the manufacturer's specification (wire gauge calculator for the voltage drop).
Show calculation
- 1To recharge: 200 Ah × (100% − 30%) = 140 Ah
- 2Effective charging current: 30 A × 97% = 29.1 A
- 3In 2 h of driving: 29.1 A × 2 h = 58 Ah
- 4Driving time to target: 140 Ah ÷ 29.1 A = 4.8 h
- 5Input current from the alternator: 30 A × 14.4 V ÷ (92% × 13.8 V) = 34 A
How it's calculated
charged [Ah] = charging current [A] × charge efficiency × driving time [h]
driving time to target [h] = capacity × (target − start) ÷ (charging current × charge efficiency)The DC-DC charger is a DC/DC converter: it takes in energy at starter battery voltage (about 13.8 V with the engine running) and puts it out at charge voltage (about 14.4 V). Because of its efficiency it draws more current on the input side than it delivers:
Input current [A] = charging current × 14.4 V ÷ (efficiency × 13.8 V)Worked example
A 200 Ah LiFePO4 is at 40% in the morning. How full is it after three hours of driving with a 50 A DC-DC charger?
Inputs
- Charging current of the DC-DC charger: 50 A
- Driving time: 3 h
- House battery: 200 Ah
- Battery type: LiFePO4 (lithium)
- State of charge before the drive: 40 %
- Target state of charge: 100 %
- Efficiency of the charger: 92 %
- Charge efficiency of the battery: 97 %
Result
100%
State of charge after 3 h of driving
- Driving time to target state of charge
- 2 h 28 min
- Energy charged
- 1,728 Wh
- Input current (alternator)
- 57 A
- Charge per driving hour
- 48.5 Ah/h
Calculation
- To recharge: 200 Ah × (100% − 40%) = 120 Ah
- Effective charging current: 50 A × 97% = 48.5 A
- In 3 h of driving: 48.5 A × 3 h = 146 Ah (requirement 120 Ah reached)
- Driving time to target: 120 Ah ÷ 48.5 A = 2.5 h
- Input current from the alternator: 50 A × 14.4 V ÷ (92% × 13.8 V) = 57 A
The variables explained
- Charging current of the DC-DC charger (A)
- Rated current per the nameplate. Some chargers derate in heat or at low input voltage.
- Driving time (h)
- Pure driving time with the engine running. Stop-and-go and start-stop phases reduce the charge.
- Efficiency of the charger (%)
- Typically 90–95%; determines the input current from the alternator.
- Charge efficiency of the battery (%)
- How much of the supplied charge is stored: LiFePO4 95–99%, lead-acid 80–90%.
Common mistakes
- A charger too big for the alternator: a 60 A charger plus the vehicle's own demand overwhelms small alternators – check the manufacturer's approval and rating.
- Feed wire too thin: long runs from the engine bay to the rear need large gauges, otherwise the charger derates because of low voltage.
- Vehicles with smart alternators and no ignition signal: the charger must reliably detect the running alternator (voltage threshold or ignition signal), otherwise the starter battery gets drained.
- Calculating with the driving time from the navigation app: breaks and traffic jams don't count.
Assumptions and limits
- Charge voltage 14.4 V, input voltage 13.8 V.
- Constant charging current (no absorption phase, no thermal derating).
- The alternator can deliver the input current in addition to the vehicle's demand – that depends on the vehicle and must be checked.