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How long to charge lithium battery packs comes down to one division: the battery capacity in amp-hours divided by the charger output in amps gives a minimum charge time, and the real time is longer because of the slow absorption stage and charging losses. A larger battery takes longer on the same charger, and a stronger charger finishes sooner on the same battery. The steps below show how to work it out from the labels on your own battery and charger.
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The quick answer: how long to charge lithium battery
Divide the battery capacity in amp-hours by the charger output in amps. That is the shortest possible time. Then add time for the absorption stage, which tops the battery off slowly, and for charging losses. Never set the charge current above the maximum charge current the battery maker lists, even if the charger can deliver more. This page covers the time calculation only: for the charging procedure, see the how to recharge a lithium battery guide.
Step 1: Read the battery and charger labels
The battery label gives its capacity in amp-hours and its voltage. The charger label gives its output in amps. Match the charger to the battery type before anything else. The Victron Blue Smart IP22 lists 12V and 30A, and its listing states that it also charges Li-ion batteries with a bulk – absorption – float algorithm. See the battery charger types guide for the charger kinds behind these labels.
Step 2: Divide capacity by charger output
Keep the example consistent: one 12V battery and one 12V charger. The two cases below are separate, because a 12V pack and a small cell are not charged on the same kind of charger.
Case A: a 12V lithium pack on a 12V charger
The Victron lists 12V and 30A. Take a battery labeled 30Ah: 30Ah divided by 30A is 1 hour as the shortest possible time. A battery labeled with double the amp-hours would need double that time on the same charger. The real time is longer than this figure because of the absorption stage and charging losses, so treat the result as a floor, not a clock time. If the battery maker lists a maximum charge current below 30A, divide by that lower current instead.
| Item | Stated figure | Use in the formula |
|---|---|---|
| Victron Blue Smart IP22 output | 12V 30A | Charger amps (divisor) |
| Example battery label | 30Ah | Capacity (dividend) |
| Result | 30Ah / 30A = 1 hour | Minimum time, before absorption and losses |
Case B: small lithium cells in a slot charger
Small cells use the same idea, with milliamp-hours divided by milliamps. The EBL charger (on Amazon) lists 8 independent slots for 1.5V lithium and 1.2V Ni-MH batteries, but its listing does not state a charge current per slot, so no time can be worked out from it. Read the current from the charger label, then divide the cell capacity by it.
Step 3: Allow for the slow final stage
Lithium batteries charge in stages, and the last stage runs slow. With the Victron, bulk fills most of the charge, absorption tops it off slowly, and float holds it. That is why a battery that looks nearly full still needs time on the charger, and why the charger, not the clock, decides when the battery is done. The EBL charger stops charging when batteries are full, and its LED indicators show solid green for fully charged and solid red for charging in progress.
Ways to shorten the wait
- Use a charger with a higher amp output, as long as it stays within the battery maker’s maximum charge current.
- Charge a smaller battery, or split the load across several batteries on separate charger slots.
- Match the charger to the battery type listed on its label.
FAQ: power away from the grid
How long does a lithium battery take to charge?
Divide the battery amp-hours by the charger amps for the minimum time, then add time for the absorption stage and charging losses. For example, 30Ah divided by 30A is 1 hour before those extras.
Can I use any charger for a lithium battery?
No. Use a charger that lists lithium batteries: the Victron lists Li-ion charging with a bulk – absorption – float algorithm. The EBL lists 1.5V lithium batteries for its slots. See the lithium vs lead acid charger guide for the differences.
Why does the last part of charging take so long?
The absorption stage tops the battery off slowly after the bulk stage fills most of the charge, so the charger slows the current as the battery fills.
How do I know when a lithium battery is fully charged?
The charger tells you: the EBL shows solid green when fully charged and solid red while charging. The Victron lets you monitor voltage and current over its wireless connection.
Does a bigger charger always charge faster?
A charger with a larger amp output fills the same battery sooner, but only within the battery maker’s maximum charge current. Use the charger that lists your battery type.
Related solar guides
See all batteries guides. Also see the battery charger maintenance guide and the how long a portable charger takes to charge guide.
