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Faq

FAQ Batteries: charging voltage, cable thickness, series/parallel (24/36/48 V), off-grid & self-built motorhome

Welcome to the comprehensive FAQ on batteries. Here you can read all about types of batteries, the correct charging voltage, how to calculate capacity, how to connect safely and how to connect batteries in series and parallel. You will also find practical tips for off-grid applications and the self-built camper.

What types of batteries are there and which one do I choose?

  • Lead acid (wet batteries)
    Inexpensive, widely used as a starter battery. Must be upright and able to gas, so ventilation is important.

  • AGM batteries
    Maintenance-free, resistant to vibration and suitable for RVs, boats and UPS systems. Can be discharged deeper than wet batteries.

  • Gel batteries
    Low maintenance, low gas and cyclically strong. Suitable for applications requiring deeper discharges, such as recreation and medical equipment.

  • LiFePO4 batteries.
    Lightweight, high usable capacity (up to 90-95%), fast charging and long life. They are ideal for off-grid applications, solar power and self-built RVs.

What is the best charging voltage for 12 V batteries?

The charging voltage depends on the type of battery:

  • Lead/AGM: absorption approximately 14.25-14.4 V, float 13.5-13.8 V

  • Gel: absorption approximately 14.1-14.4 V, float 13.5-13.8 V

  • LiFePO4: absorption approximately 14.2 V (±0.2), float usually 13.5 V or no float

Preferably use a 3-stage battery charger with bulk, absorption and float mode. For LiFePO4, a suitable charging curve or BMS-controlled system is necessary.

How do I calculate battery capacity (Ah)?

  1. Calculate daily consumption in watt-hours (Wh): power in watts × usage time in hours.

  2. Divide this by the system voltage (e.g. 12 V, 24 V or 48 V) → this gives the required amp-hours (Ah).

  3. Correct for depth of discharge (DoD): lead about 50%, AGM about 70%, Gel about 80%, LiFePO4 up to 90-95%.

  4. Add a 20-30% margin for weather, aging and peak usage.

Example: daily consumption 800 Wh with a 12 V system → 800 ÷ 12 ≈ 67 Ah.
With lead-acid batteries you choose about 135 Ah (because only 50% usable).
With LiFePO4, about 70 Ah (95% usable) will suffice.

HowTo: calculate cable diameter

The right cable cross-section prevents voltage loss and overheating.

Roadmap:

  1. Determine the current: amps = watts ÷ volts.
    Example: 1000 W at 12 V = 83 A.

  2. Measure the return length (out + back).
    Example: 2 m out = 4 m return.

  3. Choose a maximum voltage drop of 2-3% of the system voltage.
    At 12 V this is 0.24-0.36 V, at 24 V 0.48-0.72 V, at 48 V 0.96-1.44 V.

  4. Use a cable table or calculation aid to choose the required cross-sectional area (mm²).

  5. Place a fuse as close as possible to the plus pole.

Practical example:
2000 W inverter on 24 V → 2000 ÷ 24 ≈ 83 A.
At 2 m out (4 m return), 25-35 mm² cable is recommended.

HowTo: Connecting batteries in series and parallel (6 V, 8 V and 12 V)

With serial and parallel connection you can easily create higher voltages or larger capacities.

Serial (higher voltage, Ah stays the same):

  • 2 × 12 V = 24 V

  • 3 × 12 V = 36 V

  • 4 × 12 V = 48 V

  • 4 × 6 V = 24 V

  • 6 × 6 V = 36 V

  • 8 × 6 V = 48 V

  • 3 × 8 V = 24 V

  • 6 × 8 V = 48 V

Parallel (same voltage, more Ah):

  • 2 × 12 V parallel = 12 V, double Ah

  • 3 × 12 V parallel = 12 V, triple Ah

Series-parallel (both advantages):

  • Example: two strings of 2 × 12 V in series (each 24 V/100 Ah), then in parallel → 24 V/200 Ah.

  • Example: two strings of 8 × 6 V in series (each 48 V/200 Ah), then in parallel → 48 V/400 Ah.

Safety rules:

  • Use identical batteries (same brand, capacity, age).

  • Place fuses as close to the positive terminal as possible, preferably per battery or per string.

  • Use the correct cable thickness.

  • Always check polarity with a multimeter before connecting a charger or inverter.

  • For LiFePO4: check if the BMS allows series and parallel use.

Practical calculation examples battery banks

Example 1: 24 V / 200 Ah with 12 V batteries (100 Ah)
Make two strings of 2 × 12 V in series (each 24 V/100 Ah). Put those in parallel → 24 V/200 Ah. Total of 4 batteries.

Example 2: 36 V / 200 Ah with 12 V batteries (100 Ah)
Make one string of 3 × 12 V in series (36 V/100 Ah). Make two and put in parallel → 36 V/200 Ah. Total of 6 batteries.

Example 3: 48 V / 280 Ah with LiFePO4 12.8 V/280 Ah
Make one string of 4 batteries in series (48 V/280 Ah). If you want 560 Ah, make two strings and put them in parallel. Total 8 batteries.

Connect safely - checklist

  • Provide ventilation for lead-acid batteries.

  • Always use appropriate fuses and cable thickness.

  • Keep the correct order: when disconnecting, minus first, then plus; when connecting, vice versa.

  • Secure batteries securely and provide strain relief for cables.

  • Set charging curves and temperature compensation correctly.

Off-grid and self-built camper tips

  • Preferably choose LiFePO4: lightweight, deeply dischargeable, long life.

  • Build the charging path logically: solar panels → MPPT controller → battery bank → DC consumers or inverter to 230 V.

  • Size the battery based on daily consumption + 20-30% margin.

  • Provide sufficient peak power in the inverter.

  • Use monitoring via a shunt or BMS.

  • For storage: LiFePO4 rather store partially charged, lead rather on trickle charger.