How Long Does It Take to Charge a Battery?

The simple math

A charger's amp rating is how fast it pushes charge back into the battery. A battery's amp-hour (Ah) rating is how big the tank is. So the starting point is:

Hours to charge ≈ amp-hours to put back ÷ charger amps

Then add two things:

• A little extra charge. Batteries don't give back every amp-hour you put in. Lifeline, a major AGM maker, says to return 102% to 110% of the amp-hours you took out to fully recharge an AGM. Lithium wastes much less. Battle Born lists 99% efficiency for its 100 Ah battery.
• Top-off time for lead-acid. AGM, gel and flooded batteries need a slow finishing stage. More on that below.

Here's an example. You ran a 100 Ah AGM down to 50% fishing all day, so it needs about 50 Ah back, or about 53 Ah with Lifeline's 5% extra. A 10 A charger does the fast part (50% to 80%) in about 3 hours, then about 4 more topping off. Plan on 7 to 8 hours, which is overnight in the garage.

Our Battery Charging Time Calculator works this out for your battery type, size, charge level and charger.

Why the last part takes longer

A smart charger works in stages. Victron's Blue Smart charger manual lays them out clearly:

1. Bulk: full charger current until the battery reaches the absorption voltage. Victron says a lead-acid battery is about 80% charged at the end of bulk, and lithium is above 95%.
2. Absorption: the charger holds a steady voltage (14.4 V in Victron's normal mode) and the current drops off as the battery fills. Victron's chargers run absorption anywhere from 30 minutes to 8 hours depending on how deep the battery was discharged.
3. Float and storage: a lower voltage that keeps the battery full without overcharging it. In lithium mode, Victron skips float and holds a 13.5 V storage voltage.

That's why a bigger charger only speeds up the first part on an AGM. Lifeline's manual gives these absorption times for its AGMs, and they don't get shorter with a bigger charger:

From the Lifeline AGM Technical Manual. Lifeline also says absorption is done when the current falls below 0.5% of the battery's rating, which is 0.5 A on a 100 Ah battery.

Lithium spends almost all its charge in bulk, so it finishes much closer to the simple math. That's a big reason lithium is so popular for anyone who has to recharge between trips.

Charge times for a 100 Ah battery

These come straight from our calculator's math. For AGM: bulk hours = amp-hours from 50% to 80% (30 Ah) ÷ charger amps, then absorption. Absorption is 2 hours (Trojan's figure for AGM) or longer on a small charger, since during the taper it averages about half its rated amps on the last 20 Ah. For lithium: amp-hours to replace ÷ charger amps, plus about 25 minutes of top-off (Battle Born's figure).

Our math, at room temperature, with nothing else running off the battery. LiTime lists about 10 hours on a 10 A charger, 5 hours on 20 A and about 2 hours to 97% on 50 A for its 100 Ah lithium, close to ours. Lifeline's chart above runs longer for deep discharges, so if you own Lifeline AGMs, set 3 or 4 hours of absorption in the calculator's Detailed view. Check your battery's maximum charge current before using a big charger.

Notice the AGM column. Once the charger is about 20 A or bigger, the 2-hour finish is most of the time, so a bigger charger barely helps. On lithium, doubling the charger roughly halves the time.

Running gear while you charge (a fridge, a bilge pump, lights) steals some of the charger's output, so it takes longer.

What size charger to buy

Match the charger to the battery's amp-hours and to how fast you need it back:

From the Lifeline AGM Technical Manual, the Renogy 100 Ah AGM spec page, the LiTime 100 Ah spec page and the Battle Born BB10012 data sheet.

A few practical picks:

• A 10 A charger is the sweet spot for one 100 Ah battery charged overnight. NOCO's GENIUS10, for example, is a 10 A charger rated for batteries up to 230 Ah, with settings for AGM, flooded, gel and lithium.
• A 20 A or bigger charger makes sense for a 100 Ah lithium you need back in a few hours, or for bigger AGM banks.
• A 1 to 2 A maintainer is for keeping a full battery full in storage, not for recharging after a trip.
• A multi-bank onboard charger is the easy answer for a boat with a 24 V or 36 V trolling motor, since each bank gets its own output.

Whatever you buy, set it to the right battery type. Lithium wants about 14.2 to 14.6 V. Lifeline's AGMs want 14.3 V absorption and 13.3 V float. Our lithium vs AGM guide covers the differences, and our chargers page has picks for single batteries and boat banks.

Charging from solar

A solar panel's watt rating is what it makes in perfect lab conditions. In the real world you get less. Our calculators plan on getting about 75% of the panel's rating into the battery over a day. So:

Panel watts ≈ watt-hours to put back ÷ (sun hours × 0.75)

Watt-hours are amp-hours times volts. Two examples with 5 sun hours, which is a decent summer day in much of the US:

• 100 Ah lithium from 20%: 80 Ah × 12.8 V = 1,024 Wh. 1,024 ÷ (5 × 0.75) = 273 W. Round up to a 300 W panel.
• 100 Ah AGM from 50%: 50 Ah × 12 V = 600 Wh, plus 10% for charging losses = 660 Wh. 660 ÷ (5 × 0.75) = 176 W. Round up to 200 W.

LiTime's numbers agree. It says 400 W of panel with about 4.5 effective sun hours fully charges its 100 Ah lithium in a day. That's 400 × 4.5 × 0.75 = 1,350 Wh, a bit more than the battery's 1,280 Wh.

Those panel sizes put the energy back in about a day. The charge itself runs a little longer, because lithium needs a short top-off and an AGM's slow finish doesn't speed up for solar. Our calculator puts the 300 W lithium example at about 1.1 days of 5-hour sun, the 200 W AGM example at about 1.4 days, and LiTime's 400 W example at about 1.2 days.

You also need a charge controller between the panel and the battery. Its output rating in amps should be at least the panel watts divided by the battery voltage. A 300 W panel on a 12 V battery is about 25 A, so a 30 A controller. Our Power Station & Solar Sizer works out panel size for a whole day of camping gear, and our solar guide covers aiming and shade. Panels are on our solar panels page.

Charging from your truck or boat motor

An alternator can charge a second battery while you drive, but don't count on it to fill a deep-cycle battery quickly through a simple wire. Vehicle alternators aim at keeping the starting battery topped up, and a long thin wire to the back of the truck loses voltage on the way.

For lithium, a DC-to-DC charger is the right tool. Victron's testing found that charging lithium straight from an alternator can overheat the alternator, and that a battery's protection circuit cutting off the charge can spike the voltage and kill the alternator's diodes. Victron points to its DC-to-DC chargers as the fix. A DC-to-DC charger limits the current and gives the battery the right voltage, and its amp rating works just like a wall charger in the table above: a 20 A DC-to-DC charger puts back up to about 20 Ah for every hour you drive.

An outboard's charging output varies a lot from motor to motor, so check your manual before counting on it to refill a trolling motor bank. Most anglers charge those at home or at the dock.

Cold weather charging

• Lithium: LiTime rates its 100 Ah battery for charging only from 32 °F to 122 °F and warns that charging in freezing temperatures damages the cells. Some batteries, like Battle Born's heated model, have a heater or a low-temperature cutoff that stops charging in the cold. Know which kind you have.
• AGM: Renogy rates its 100 Ah AGM for charging from 5 °F to 104 °F. Good chargers adjust the voltage for temperature, which matters for lead-acid.
• Everything charges slower in the cold. Plan for extra time in late fall and on the ice, or bring the battery somewhere warm first.

Questions people ask

How long does it take to charge a 100Ah battery?

For lithium, about the amp-hours used divided by your charger's amps: roughly 5.5 hours from empty on a 20 A charger, 10.5 hours on a 10 A. For AGM from half full, about 7.5 hours on a 10 A charger and under 4 on a 20 A, because the last 20% always takes at least a couple of hours.

Can I charge a lithium battery with a regular charger?

Use a charger with a lithium setting. Lithium wants about 14.2 to 14.6 V and no float stage. Avoid lead-acid repair modes like "recondition" or "equalize" on lithium. Victron's recondition mode, for example, pushes a 12 V battery to 16.2 V and is for lead-acid only.

Can I leave my battery on the charger all the time?

With a smart charger or maintainer that drops to float or storage, yes. That's what those stages are for. Victron's chargers drop to a storage voltage after float and run a short top-off once a week. Don't leave an old-style charger with no automatic shutoff on a battery.

Will a bigger charger damage my battery?

It can if it's over the battery's maximum charge current. Renogy's 100 Ah AGM tops out at 30 A, and Battle Born's 100 Ah lithium at 50 A. Check your battery's spec sheet.

How long does a trickle charger take to charge a dead battery?

A long time. A 2 A trickle charger needs about 25 hours just to put 50 Ah back, and more for the top-off. Trickle chargers and maintainers are for keeping a full battery full, not for recharging after a day on the water.

Why is my charger taking so long to finish?

On AGM and flooded batteries, the absorption stage tapers off slowly, and Lifeline says it typically takes 2 to 4 hours. It also takes longer if anything is running off the battery, if it's cold, or if the battery is old and losing capacity.