Vehicles, trucks & marine

Truck & Heavy Equipment Battery Bank Calculator

Size the starting bank of a truck, bus, tractor or machine (12 V parallel or 24 V series-parallel) from cranking requirement and optional overnight hotel loads.

Electrical system

Required total CCA (OEM / engine maker) (optional)

Engine displacement (if CCA unknown) (optional)

CCA per battery

Capacity per battery

Overnight/accessory consumption (optional) (optional)

Max discharge for accessory loads

Formula

CCA(req) = OEM value, else ≈ 2 × displacement(ci)  (diesel rule of thumb)
P(cranking) = ⌈CCA(req) ÷ CCA(battery)⌉
P(hotel) = ⌈Ah(hotel) ÷ (Ah(battery) × DoD)⌉
P = max(P(cranking), P(hotel));  N = P × (V(sys) ÷ 12)

Worked example

12 V truck, OEM requires 1,875 CCA, batteries of 925 CCA / 100 Ah, 150 Ah overnight at 50% → P = max(3, 3) = 3 batteries in parallel.

Limitations and warnings

In 24 V systems two 12 V batteries are wired in series per string; series does not add CCA, so the bank CCA equals the sum across parallel strings.
Battery box space, cable sizing and OEM approvals limit the number of batteries. Treat the displacement rule as orientative only.

Four values that are often confused

Nominal capacity (Ah): Charge a battery delivers under rated conditions, usually over 20 hours (C20) at 25 °C.
Usable capacity: The part of the nominal capacity you can use without damaging the battery — about 50% for lead-acid, 80–90% for LiFePO4.
Cranking current (CCA): Current deliverable for a short time in the cold to start an engine. It measures power, not stored charge, and cannot be converted to Ah.
Stored energy (Wh): Ah × voltage. The right unit to compare batteries of different voltages.