Tacklebox ’n Tools

Battery Charging Time Calculator

Find out how long your boat, RV, camper or trolling motor battery takes to charge from a plug-in charger, an onboard charger, your alternator or solar, and whether your charger is the right size.

How to use it

  1. Pick your battery and how many you charge. Pick a group size, or type in the amp-hours from the label.
  2. Tell it how empty it is. Where the battery is now and how full you want it. A battery monitor shows this best.
  3. Choose how you charge it. A plug-in charger, an onboard charger with one bank per battery, the alternator through a DC-to-DC charger, or solar.
  4. Read the results. Hours to charge, whether it's ready in the time you have, whether your charger is too big or too small, and what to buy.
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Your setup

An example is loaded to start. Change any field and the results update. Simple shows the essentials. Detailed adds more settings, a comparison table and the math.

View

Your battery

Battery type
Ah
batteries
Amp-hours (Ah) are printed on the label. For a power station, divide its watt-hours by 12.8, or use our power station and solar calculator.

How empty it is

After a day of trolling or a night of camping, 50% is common. A battery monitor shows it best.

How you charge it

Charging from
hours
The slow last stage of a lead-acid charge. Trojan lists up to 4 hours for flooded and 2 hours for AGM. Use your charger's setting if you know it.

Time you have

hours
Overnight is about 10 hours.

How the numbers are worked out

Amp-hours to put back

The amp-hours you need back are battery Ah × (charge you want − charge it has now). A 100 Ah battery at 50% needs 50 Ah to reach 100%.

Some charge is lost as heat, so a little more has to go in. We use the charge efficiency Victron's battery monitor manual gives: 95% for lead-acid (its default) and 99% for lithium (its recommendation).

Lithium

Lithium (LiFePO4) takes close to the charger's full current almost the whole way up, so the time is Ah to put back ÷ (charger amps × 0.99). To reach 100%, we add the short top-off stage Battle Born lists: 20 to 30 minutes per 100 Ah (we use 25).

Check: LiTime says its 100 Ah battery is full in about 5 hours at 20 A. This calculator gives 5.5 hours from empty.

Lead-acid: bulk, then absorption

Flooded and AGM batteries charge in stages. In the bulk stage they take the charger's full current. Trojan's charging charts show bulk ending at roughly 80 to 90% full, so we use 80%. Bulk time is Ah up to 80% ÷ (charger amps × 0.95).

Then the absorption stage holds the voltage while the current tapers off. Trojan lists up to 4 hours for flooded and 2 hours for AGM. We count that time for the full 80% to 100% stretch, and a share of it if you start above 80% or stop short of 100%, since Victron notes a shallow discharge needs little or no absorption. If the charger is so small that the last 20% would take longer at half its amps, we use that longer time instead.

Power-Sonic's rule of thumb for a fully flat sealed battery is Ah ÷ charger amps × 1.75, which comes out longer than this model. An older or basic charger may land closer to that.

Alternator and solar

A DC-to-DC charger is treated like a plug-in charger at its rated amps, counted only while the engine runs.

Solar amps are panel watts × 0.75 ÷ 14.4 V. The 0.75 covers standard system losses (NREL's PVWatts model uses 14%) plus heat and imperfect aim on portable panels. It is our planning assumption, the same one our power station calculator uses, not a measured figure. 14.4 V is a typical charging voltage for both lead-acid and lithium. Days are hours of full sun needed ÷ sun hours per day. Sun-hour choices are rounded from NREL-based sun maps.

Charger too big or too small

  • Flooded: Trojan recommends 10 to 13% of the battery's Ah. Victron advises C/5 (20% of Ah) or less once the battery starts gassing. We flag over 20% as too big and under 10% as slow.
  • AGM: Trojan uses 20% of Ah in the bulk stage. Power-Sonic says the starting current must not go over 30% of Ah. We flag over 30% as too big and under 10% as slow.
  • Lithium: Battle Born allows 50 A per 100 Ah, and Victron recommends 50 A or less per 100 Ah. LiTime recommends 20 A for its 100 Ah. We flag over 50% of Ah as too big. Check your own battery's spec sheet, since some allow more.

On a plug-in charger, a DC-to-DC charger or solar with more than one battery, we assume the batteries are wired together and share the current. An onboard charger gives each battery its own bank, so each one gets the full amps per bank.

What it leaves out

  • Battery age. Rolls lists the charge efficiency of its AGM batteries at about 80%, dropping about 1% a year after the third year, so a tired battery takes longer.
  • Things running while you charge, like a fridge or fish finder. They eat into the charger's amps.
  • Cold. LiTime says not to charge its lithium batteries below 32 °F unless they have low-temperature protection.
  • Solar that changes through the day. Sun hours roll a whole day of weaker and stronger light into full-sun hours.

Planning estimates. Always set your charger to your battery's type, and follow the battery maker's charging limits. Charge lead-acid in a place with fresh air, since it can give off hydrogen gas while charging. For wiring a DC-to-DC charger, size the cable and fuse with our wire size calculator, and see our lithium vs AGM guide if you're thinking about switching.

Sources