Battery Life and Cable Drop Calculator

What this generator does

Converts capacity into watt-hours, applies the fraction you will use and the conversion loss, and divides by the load — then, separately, works the resistance of a cable run and the voltage it loses.

How to use this tool

1. Type the capacity and the voltage it was measured at.
2. Say what the load draws in watts, and how much of the battery you are willing to use.
3. Read the hours, and the honest milliamp-hour figure at five volts.
4. Underneath, put in the cable run to see what arrives at the far end.

Understanding the controls

Capacity in mAh

As printed. On its own it means nothing until you know the voltage it was measured at, which is why the next field matters.

Cell voltage

3.7 volts for a lithium cell, 12 for a leisure battery. Multiply by the capacity and you have watt-hours, which do compare across voltages.

Load in watts

What the thing being powered draws. Watts divided into watt-hours gives hours directly, with no unit juggling.

Conversion loss

What the converter takes on the way. Around fifteen per cent for a USB power bank; less for a good inverter, more for a cheap one.

Fraction you are willing to use

Running a lead-acid battery flat shortens its life sharply, so half is a common limit. Lithium tolerates deeper discharge.

Current in amps

For the cable calculation, what the load draws. Amps, not watts, decide the drop.

Run in metres, one way

The distance to the load. The current has to come back, so the resistance counts twice this — the mistake that halves most hand calculations.

Conductor area in mm²

The cross-section of one conductor. Doubling it halves the resistance, and halves the loss.

System voltage

What you are sending. The same loss in volts is a far bigger fraction at 12 volts than at 240, which is why low-voltage runs need thick cable.

Most you will lose, per cent

Three per cent is the usual target. The page finds the smallest standard conductor size that stays inside it.

Common use cases

• Working out how many phone charges a power bank really gives
• Sizing a leisure battery for a fridge or a light in a van
• Comparing two batteries whose capacities are quoted at different voltages
• Choosing cable for a twelve volt run where the drop matters
• Seeing how much power a long thin cable is wasting as heat

How this generator works

Everything is done in watt-hours, because watt-hours survive a change of voltage and milliamp-hours do not — the check converts them back to the milliamp-hours on the label to prove the conversion. The delivered energy is the stored energy times the fraction you will use times what survives the converter, and dividing by the load gives hours. The cable half uses copper's resistivity over twice the run, which is checked explicitly, then Ohm's law inverted: the drop divided by the current must give back the resistance, and what arrives plus what is lost must equal what was sent. The suggested conductor size is verified by recomputing the drop at that size rather than trusting the search.

Randomness and fairness

Nothing is random. The same battery and the same cable always give the same answer.

For how randomness is produced across the whole site, see how Generate Random works.

Limitations and good to know

• Battery capacity falls with age, with cold, and with a heavy discharge — the label is a best case.
• Conversion losses vary with load; a converter is least efficient at very low draw.
• Cable figures are for copper at room temperature; hot cable has more resistance, and aluminium considerably more.
• This is arithmetic, not an electrical design: current ratings, fusing and installation rules are a separate matter.

Privacy and your data

The figures are worked out in your browser. Nothing about your batteries or wiring is transmitted or stored.