Amp-Hours Explained for Boaters

Amp-hours are the most important number for any battery that runs a trolling motor, electronics or a house bank. They tell you how much energy the battery stores, and with a little math, how long it will run your gear. This guide explains what amp-hours mean, how they’re measured, and how to use them to choose and size a battery.

What is an amp-hour?

An amp-hour (Ah) is a unit of charge: one amp of current flowing for one hour. A battery rated at 100Ah can, in theory, deliver:

• 1A for 100 hours,
• 5A for 20 hours,
• 10A for 10 hours, or
• 50A for 2 hours.

In practice, it’s not quite that simple, especially for lead-acid batteries, but the basic idea is right: more amp-hours means more runtime.

How amp-hour ratings are measured

Most deep-cycle batteries are rated at the 20-hour rate. That’s the capacity the battery delivers when discharged evenly over 20 hours. A 100Ah battery at the 20-hour rate delivers 5A for 20 hours before it reaches its cutoff voltage. Trojan, for example, lists its batteries’ capacity as “20-hr rate.”

This matters because lead-acid batteries deliver less total capacity at higher currents, an effect known as Peukert’s law. A lead-acid battery rated 100Ah at the 20-hour rate might deliver noticeably less if you pull 50A from it, as a trolling motor might. Lithium batteries are much less affected, delivering close to their rated capacity across a wide range of loads.

Reserve capacity vs amp-hours

Many marine batteries list reserve capacity (RC): the minutes a fully charged battery can deliver 25A before voltage falls to 10.5V. It’s useful for comparing batteries for running loads with the engine off. You can roughly estimate the amp-hours available at 25A by multiplying RC by 25 and dividing by 60. A battery with 180 minutes RC delivers about 75Ah at 25A.

Usable amp-hours

Not all of a battery’s rated capacity should be used every time.

• Flooded and AGM lead-acid: for good cycle life, stop at about 50% state of charge. A 100Ah lead-acid battery gives about 50Ah usable.
• Lithium (LiFePO4): can routinely use 80–100% of its rating. A 100Ah lithium battery gives about 80–100Ah usable.

That’s why a 100Ah lithium battery can run a trolling motor nearly twice as long as a 100Ah lead-acid battery. See lithium vs lead-acid amp-hours and depth of discharge for more.

Estimating runtime

Runtime (hours) ≈ usable amp-hours ÷ average current (amps)

Battery	Usable Ah	Load	Estimated runtime
100Ah lead-acid	~50Ah	10A trolling	~5 hours
100Ah lithium	~90Ah	10A trolling	~9 hours
100Ah lead-acid	~50Ah	2A electronics	~25 hours
60Ah lithium	~54Ah	4A livewell plus graph	~13 hours

For lead-acid under heavy load, expect somewhat less than this estimate because of the Peukert effect.

Finding your average current

Trolling motors rarely run at full power for long. A motor that draws 40A at full speed might average 10–20A over a day of mixed use. To estimate:

• Check the motor maker’s current draw at different speeds.
• Use a battery monitor or the battery’s Bluetooth app to see real draw.
• Note how much charge you use on a typical day.

For electronics, check the manual’s current draw, often listed in amps or watts. Divide watts by 12 to get amps on a 12V system.

Amp-hours in series and parallel

• Series (for 24V or 36V): voltage adds, amp-hours stay the same. Three 100Ah 12V batteries in series make a 36V 100Ah bank.
• Parallel (for more capacity at 12V): amp-hours add, voltage stays the same. Two 100Ah 12V batteries in parallel make a 12V 200Ah bank.

Because voltage changes in series, comparing banks in watt-hours is clearer. A 36V 100Ah bank stores three times the energy of a 12V 100Ah battery. See amp-hours to watt-hours and kWh.

Sizing a battery with amp-hours

1. List each load and its average current.
2. Multiply by hours used per trip.
3. Add them up.
4. Add 20–30% margin.
5. Double it for lead-acid, or divide by about 0.9 for lithium.

Example: a fish finder at 1.5A for 8 hours (12Ah), a livewell at 3A averaging 50% duty for 6 hours (9Ah), and lights at 1A for 2 hours (2Ah) total 23Ah. Add 25% for 29Ah. You’d want about 58Ah of lead-acid or 32Ah of lithium.

Amp-hours and charging time

Amp-hours also tell you how long charging will take. Charging time (hours) ≈ amp-hours to replace ÷ charger output (amps), plus extra time for the final absorption stage on lead-acid. Replacing 50Ah with a 10A charger takes roughly 5–6 hours for lead-acid, and about 5 hours for lithium, which charges efficiently right to the top.

Tracking amp-hours on the water

A shunt-based battery monitor or a lithium battery’s Bluetooth app counts amp-hours in and out. After a few trips, you’ll know exactly what a typical day uses, which makes sizing your next battery much easier.

Common amp-hour mistakes

• Comparing lead-acid and lithium labels directly. Compare usable amp-hours instead.
• Forgetting the Peukert effect. Heavy trolling loads drain lead-acid faster than simple math suggests.
• Ignoring age and temperature. Older batteries and cold weather reduce available capacity.
• Comparing banks of different voltages in amp-hours. Use watt-hours.

More reading

The marine battery buying guide puts amp-hours in context with other specs. For lithium-specific advice, read the lithium marine battery buyer’s guide. The BCI group size guide shows typical capacity by case size, and buying a marine battery online covers what to check before ordering.

Products mentioned

• EPOCH 12V 100Ah Eco
• Battle Born 100Ah
• Trojan SCS150 (100Ah, 20-hr rate)
• Browse trolling motor batteries.

Frequently asked questions

What does 100Ah mean on a battery?

It can deliver about 100 amp-hours, such as 5A for 20 hours, at its rated discharge rate.

How long will a 100Ah battery run a trolling motor?

At a 10A average, about 5 hours for lead-acid or 9 hours for lithium.

Is reserve capacity the same as amp-hours?

No. Reserve capacity is minutes at 25A. Multiply by 25 and divide by 60 to estimate amp-hours at that load.

Do batteries in series add amp-hours?

No. Series adds voltage. Parallel adds amp-hours.

Why does my lead-acid battery run out sooner than expected?

Heavy loads, age, cold weather and the Peukert effect all reduce available capacity.