How Long to Charge a 100Ah Battery With a 10 A Charger

Charging time from capacity, power and a starting level

A 5,000 mAh phone battery at 3.7 V holds 18.5 Wh. On a 20 W adapter at 85% efficiency it fills from empty in 1 h 5 min; on an old USB 2.0 port the same charge takes 8 h 42 min. The calculator works from the battery rating in mAh, Ah or Wh and either the charging current or the charger's wattage, charges between any two levels, and shows the charge rate in C next to the time, so a figure that no battery would accept stands out.

Seven entries, in the order the form asks for them

Battery capacity and its unit - the rating on the label: 5,000 mAh on a phone, 100 Ah on a 12 V battery, 56 Wh on a laptop pack.
Battery voltage - nominal: 3.7 V for most lithium-ion cells, 1.2 V for NiMH, 12 V lead-acid, 12.8 V LiFePO4.
Charging current or charger power - amps or milliamps from a charger spec or a USB meter, or watts from the adapter label.
Charging efficiency - optional. Leave it empty and the result gives a range for 80% to 90% plus the lossless minimum.
Starting level - optional, the percentage on the screen now. Empty means a flat battery.
Target level - optional. Empty means 100%; type 80 if you stop there.
Read the result - the time, the charge rate in C, the time to 80%, and a table of the same charge on other sources.

Current or watts: which number to enter

Both routes give the same time when the numbers describe the same charger. They differ in where the number comes from and what it already includes.

Question	Charging current (mA or A)	Charger power (W)
Where you find it	cell charger spec, USB meter, battery charger display	adapter label, phone or laptop spec sheet
How it becomes power	current × battery voltage, so 1 A at 3.6 V is 3.6 W	used as entered
Main trap	a spec that lists the maximum, not the current actually flowing	the adapter can supply more than the device takes
Pick it when	loose cells, NiMH packs, 12 V and 48 V batteries	phones, tablets, power banks and laptops on USB

The arithmetic behind the time, on three batteries

Energy to add is the capacity in watt-hours times the share you are filling. Time is that energy divided by the power the battery actually stores: t = E × (target - start) ÷ (P × efficiency). The charge rate is the stored current divided by the amp-hour rating; 1C would fill the battery in one hour.

1

Phone on a 20 W adapter.
5 Ah × 3.7 V = 18.5 Wh. At 85%, 17 W reaches the battery, so 18.5 ÷ 17 = 1 h 5 min, with 80% reached after 52 min. The adapter delivers 21.76 Wh for the job. Charge rate: 0.92C.

2

18650 cell on a 1 A charger, efficiency left empty.
3,000 mAh at 3.6 V is 10.8 Wh, and 1 A is 3.6 W. With no losses the floor is exactly 3 h; at 90% and 80% the result gives 3 h 20 min to 3 h 45 min. The rate is a gentle 0.33C.

3

12 V 100 Ah battery on a 10 A charger.
1,200 Wh to add, 120 W from the charger, 102 W stored at 85%: 11 h 46 min, and the charger supplies about 1,412 Wh. Because this battery is over 100 Wh, the table under the result lists charge rates from 0.05C to 1C instead of USB ports.

A 5,000 mAh phone on eight USB power sources

The rows below come from the calculator with the phone from example 1, charged from empty to full at 85% efficiency. The USB 2.0, USB 3.0 and Type-C figures are the port limits of 2.5 W, 4.5 W and 5 V at 1.5 A or 3 A; USB Power Delivery goes up to 100 W in its standard range. The last three rows are what the adapter could supply, which is more than this battery would take.

Source	Watts	Time, 0 to 100%	Charge rate
USB 2.0 port	2.5 W	8 h 42 min	0.11C
USB 3.0 port	4.5 W	4 h 50 min	0.21C
USB-C at 1.5 A	7.5 W	2 h 54 min	0.34C
USB-C at 3 A	15 W	1 h 27 min	0.69C
20 W adapter	20 W	1 h 5 min	0.92C
45 W adapter	45 W	29 min	2.07C
65 W adapter	65 W	20 min	2.99C
100 W USB PD	100 W	13 min	4.59C

A few more setups, each typed into the calculator as written:

Phone from 20% to 80% on the same 20 W adapter at 85%: 39 min.
20,000 mAh power bank from 20%, 20 W at 85%: 59.2 Wh to add, 3 h 29 min.
56 Wh laptop at 11.55 V on 65 W at 90%: 57 min if the laptop sends all 65 W to the battery, which it rarely does while switched on.
NiMH AA, 2,000 mAh at 200 mA (0.1C) and 66%: 15 h 9 min.
LiFePO4 100 Ah, 12.8 V, from 10% to 90% at 20 A and 95%: 4 h 13 min.

Four ways a charging estimate goes wrong

Typing the adapter's maximum for a device that takes less. A 140 W adapter on a 5,000 mAh phone gives 10 min at 6.05C, and the calculator flags it. The phone's charge controller decides how much it accepts, so use the phone's own maximum charging wattage.

Treating the last 20% like the first 80%. Lithium-ion chargers hold a constant current, then a constant voltage (4.2 V per cell for typical lithium cobalt oxide cells) while the current falls, and stop near 3% of the starting current. The straight line is honest up to the switch and optimistic after it, which is why the result shows the time to 80% separately.

Charging from zero when the screen says 35%. A phone plugged in at 35% needs only 65% of its rating. Leaving the starting level empty overstates the wait by more than half (100 ÷ 65 = 1.54).

Using 5 V for a phone battery. USB runs at 5 V, but the cell inside is about 3.7 V, and mAh on the label refer to the cell. Entering 5 V inflates the energy by 35%.

Charging questions with numbers in the answers

How long does it take to charge a 100Ah battery with a 10 amp charger?

At least 10 h from empty, since 100 Ah ÷ 10 A = 10. With 85% efficiency on a 12 V battery the calculator gives 11 h 46 min. From 50%, the usual floor for lead-acid, half of that.

What does a charge rate of 1C mean?

A current equal to the amp-hour rating: 5 A for a 5,000 mAh battery, 100 A for a 100 Ah one. At 1C the battery would fill in one hour if the current never dropped; 0.5C takes two hours and 0.1C ten.

Is a 65 W charger safe for a phone that supports 25 W?

USB Power Delivery has the charger and the device agree on a voltage and current before the power rises, so the phone draws what it supports. Enter 25 W, not 65, to get its charging time.

How do I turn Wh into mAh?

Multiply by 1,000 and divide by the voltage. A 56 Wh laptop pack at 11.55 V is about 4,848 mAh. The calculator accepts Wh directly, so the conversion is only needed for comparing labels.

What efficiency should I type in?

Wikipedia lists a charge and discharge efficiency of 80-90% for lithium-ion and 66-92% for NiMH. Those are round-trip figures, so the charging step on its own is at least that efficient. Leave the field empty to see the 80-90% range next to the lossless minimum.

Why does my phone charge slower in the cold?

Between 0 and 5 °C (32 to 41 °F) lithium-ion cells can be charged only at a reduced current, and chargers cut back to protect them. The calculator assumes room temperature.

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How long to charge a 12 V 100Ah battery with a 10 amp charger?

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