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12V auxiliary battery size calculator for mobile detailing

Calculate the usable amp-hours required by a measured mobile detailing DC workload, then screen an exact auxiliary battery, BMS and recharge plan.

Model the usable amp-hours your real work cycle requires, then screen one exact battery and charging plan. This deliberately does not output cable, fuse, charger or alternator sizes.

Measured DC workload

Include every load that can operate from the auxiliary bank.
Use exact-model curves or measured simultaneous current at the real duty point.
Confirm voltage and how the auxiliary bank is separated from starting and propulsion systems.
V
Use the designed operating voltage at the auxiliary load.
A
Average current while the planned load set is operating—not the fuse rating.
A
Record the highest start or time-limited peak, with its duration kept in your source evidence.
min
Count only the minutes this load set actually draws current.
jobs
Use the busiest normal day the bank must support.
Ah
Add measured controls, standby, lighting or other demand not captured above.
%
A visible planning allowance; it is not a chemistry or cold-weather correction.
days
Use one when the system must recover fully before every shift.

Exact battery and recharge candidate

Use the current battery and BMS specification for the exact model.
Use capacity available at the relevant rate, temperature and manufacturer limits—not nominal Ah by itself.
Record continuous and peak current plus the permitted peak duration.
Use a battery-approved charger and an evidenced daily recovery figure after live loads.
Confirm mounting, environment, isolation, protection, ventilation and emergency instructions.
V
Match the exact battery/BMS voltage to the complete DC design.
Ah
Enter usable capacity supported by the exact evidence—not the front-label nominal figure.
A
Use the exact sustained discharge or BMS limit.
A
Use the limit for a duration at least as long as the actual peak.
Ah/day
Enter what the approved charging path can actually replace after concurrent loads.

Auxiliary-battery direction

Measure the DC work first

measured or exact-model current evidence for every planned load, the designed DC voltage and separation from the vehicle starting system, the simultaneous average operating current, the highest simultaneous start or peak current, active load minutes per job, planned jobs per day

Battery capacity cannot be inferred from a pump label or a vehicle size. Record real simultaneous current, peak demand, duty time, other loads and the recharge interval.

Usable capacity floor—
Same basis in energy—
Load per job0 Ah
Daily load0 Ah · 0 Wh
Capacity marginAdd candidate
Estimated days at loadAdd candidate
Daily recharge marginAdd charge plan
Peak demand—
This capacity floor uses entered usable amp-hours. It does not model chemistry-specific rate effects, ageing, cold performance, voltage drop, charge taper or inverter loads.
  1. 01

    Build a simultaneous load schedule from exact-model or measured current—not fuse labels, marketing watts or separate loads added as though they never overlap.

  2. 02

    Use usable capacity for the actual battery chemistry, discharge rate and temperature. Nominal amp-hours are not automatically available amp-hours.

  3. 03

    Check continuous and time-limited peak current against the exact battery/BMS statement, then verify every cable, connection, isolator and protection device separately.

  4. 04

    Confirm the charger, alternator interface, shore-power or solar arrangement is approved for the battery and can replace a normal workday before the next shift.

  5. 05

    Keep the auxiliary bank physically and electrically distinct from the starting and propulsion systems unless a competent design explicitly integrates them.

  6. 06

    Stop using a battery that is damaged, swollen, leaking, abnormally hot, giving off odour or triggering protection; isolate only as the manufacturer and emergency plan permit.

Calculate the pump current demand Create a competent-installer brief Add the installed battery mass

Electrical boundary: this tool deliberately does not size cables, fuses, isolators, chargers, alternator interfaces or enclosures. Those depend on the exact equipment and competent installation design.

Affiliate disclosure: As an Amazon Associate I earn from qualifying purchases. Retail results are not battery, BMS, vehicle, charging or installation approval. Outside the UK, use an appropriate local retailer and local electrical, transport and workplace rules.

Capacity begins with time

Amp-hours describe work—not installation safety.

Current multiplied by active hours exposes the demand. It still does not prove that a battery can deliver the peak, recharge before the next shift, operate in the installed environment or connect safely to the vehicle.

01Measure simultaneous load

Use real operating current and peak demand for the loads that overlap.

02Build the work cycle

Count active minutes, jobs, other demand and days between recharge.

03Use usable capacity

Apply the exact battery limits for chemistry, rate and temperature.

04Close the energy loop

Verify current limits, charging recovery, protection, mounting and isolation.

Five independent gates

Capacity alone is not a battery system.

  • Nominal voltage must match the complete DC design.
  • Usable capacity must cover the work cycle and stated reserve.
  • Continuous and peak limits must clear the measured loads.
  • The compatible charger must recover normal daily demand.
  • Mounting, ventilation, isolation, protection and emergency access remain installation decisions.

Start with the pump duty and current tool, add battery mass in the payload checker, and carry unresolved work into the fit-out brief.

Current-source boundary

Use the exact battery, BMS and charger documents together.

These primary sources support the amp-hour basis and the need to separate usable capacity, current limits, charging and installation. They do not approve a particular system.

Victron EnergyOfficial amp-hour capacity and discharge-rate explanationVictron EnergyOfficial battery-system wiring and design referenceHSEUsing electric storage batteries safelyHSEElectrical isolation and competent-work boundary
Source review: 13 September 2026. Recheck the exact current manufacturer documents and local electrical, transport and workplace requirements before purchase or installation.

Questions before checkout

12V detailing-battery questions

How big a battery do I need for a mobile detailing water pump?

Multiply the measured or exact-model operating current by active hours, add other daily demand, then allow for days between recharge and a transparent reserve. Use usable—not merely nominal—capacity for the exact battery.

Is a 100Ah battery always 100Ah usable?

No. Usable capacity depends on the exact chemistry, manufacturer limits, discharge rate, temperature, age and BMS or protection settings. Enter an evidenced usable figure rather than assuming the whole label capacity is available.

Can this calculator size the battery cable and fuse?

No. Cable, over-current protection, isolation, switching and terminals depend on current, length, routing, conductor and insulation type, environment, fault protection and exact manufacturer instructions.

Can I connect a detailing battery straight to the vehicle alternator?

Do not assume so. The battery, alternator interface, charging profile, vehicle electronics and starting-system separation need an exact compatible design.

Why does the calculator ask for peak current?

A battery or BMS can have separate continuous and time-limited peak limits. Capacity can appear sufficient while the exact system still trips or is overloaded during starting demand.

Does daily solar output count as guaranteed recharge?

Only enter recoverable amp-hours supported by the actual charging design and operating conditions. Weather-dependent generation should not be treated as guaranteed without an appropriate conservative plan.