How long to charge your battery?
By Sithum Imarsha Last reviewed
Charge time = Ah to put back ÷ charger amps ÷ charge efficiency, plus a slow last stage. A 100 Ah LiFePO4 battery from 20% on a 25 A charger reaches 95% in about 3 hours and is full in about 3.5 hours.
Your result
Time to full
3.5 hours
95% in 3 h days, then 0.5 h to finish
- To 95%
- 3 hours
- To 80%
- Last stage
- 0.5 h
- Ah to put back
- 75Ah
- Into the battery
- 25A
- Solar a day
- 62.2Ah
- Charge efficiency
- 99%
- Sun hours
- 5.5 h a day, Sydney in October (BOM)
Air temperature is 25 °C unless you type yours (Estimate), so winter output is conservative.
Off-grid for days? Size the battery for your fridge.
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How we calculated this
Ah to put back in the fast stage
A charger gives full current (the bulk stage) until the battery is about 95% full for LiFePO4, or about 80% for lead-acid (Victron charger manual). Ah = capacity × (bulk end − charge now).
75 Ah to 95%
Bulk time
Hours = Ah ÷ (charger amps − amps your loads use) ÷ charge efficiency (99% LiFePO4, 95% lead-acid). From solar: the Ah a panel makes in a day (panel watts × BOM sun hours × losses ÷ volts), less 24 hours of your loads, so the answer is in days of sun.
3 hours days of sun
The slow last stage
Then the charger holds the voltage and the current tapers off (absorption). Victron says lithium takes typically less than 30 minutes to reach 100%. Lead-acid takes 30 minutes to 8 hours, longer after a deep discharge. That is why a lead-acid battery rarely gets full on a drive.
0.5 h: 3.5 hours in all
| Type | Bulk ends at | Last stage | Charge efficiency | Recommended charge |
|---|---|---|---|---|
| LiFePO4 | 95% (Official source) | 0.5 h (Official source) | 99% (Official source) | 0.5C (Official source) |
| Lead-acid | 80% (Official source) | 0.5 to 8 h (Official source) | 95% (Official source) | No source |
Victron's battery manual asks for 2 hours of absorption now and then, so LiFePO4 cells can balance. We use the charger manual's 30 minutes for the time to full. The manual's own example: 100 Ah lead-acid on a 10 A charger is 8 hours of bulk plus up to 8 hours of absorption.
“Official source” means read in full on the maker’s own datasheet, manual or spec page, or in an official dataset (BOM). “Dated third-party source” means read on a dated third-party page, such as a product test. “Not yet checked” means one non-official source. “Estimate” means our own pick or reasoning: type your own figure.
Sources
- Victron: Lithium Smart battery manual
- Victron: BMV-712 Smart and SmartShunt manual
- Victron: Blue Smart IP22 charger manual
- Victron: which solar charge controller, PWM or MPPT?
- Victron: SmartSolar MPPT 100/30 and 100/50 datasheet
- NREL: PVWatts version 5 manual
- Bureau of Meteorology: Climate Data Online, mean daily solar exposure
- Redarc: BCDC in-vehicle DC-DC chargers
Two stages, not one
A smart charger works in stages. Victron’s charger manual describes them:
- Bulk. Full current until the battery is well above 95% full for lithium, or about 80% for lead-acid.
- Absorption. The charger holds the voltage and the current tapers off. Lithium takes typically under 30 minutes. Lead-acid takes 30 minutes to 8 hours, longer after a deep discharge.
So a simple Ah ÷ amps sum is close for lithium and badly wrong for the last part of a lead-acid charge.
The bulk stage
Hours = Ah to put back ÷ (charger amps − amps your loads use) ÷ charge efficiency.
- Charge efficiency is 99% for lithium and 95% for lead-acid by default in Victron battery monitors.
- Loads that run while you charge, like a fridge, take their share of the charger current first.
Victron’s own example: a flat 100 Ah lead-acid battery on a 10 A charger is 8 hours of bulk, then up to 8 hours of absorption. A 100 Ah lithium battery on the same charger is 9.5 hours plus 30 minutes.
From solar
A panel does not give a steady current. It gives energy over the day, which depends on the sun where you are. We use BOM peak sun hours for the town and month, then take off heat, system losses and the controller. The answer is in days of sun.
A flat 200 W panel on MPPT in Sydney makes about 70 Ah a day in January and about 27 Ah in June at 12.8 V. Loads you run all day come off that first.
Charge current limits
A bigger charger is not always better. Victron recommends 0.5C or less for its LiFePO4 batteries: 50 A for a 100 Ah battery. Most LiFePO4 batteries will not charge below about 0 to 5 °C: Victron gives +5 °C, Enerdrive and Renogy 0 °C. Check your own battery’s limits.
Questions
How long does it take to charge a 100Ah lithium battery?
From 20% on a 25 A charger: 75 Ah ÷ (25 A × 99%) is about 3 hours to 95%, then about 30 minutes to full. With a fridge drawing 0.76 A at the same time, about 3.6 hours.
Why is charge time not just Ah ÷ amps?
Two reasons. Some charge is lost (1% for lithium, about 5% for lead-acid). And the last part is slow: the charger holds the voltage and the current tapers off. For lead-acid the last 20% can take up to 8 hours.
How long to charge a 100Ah battery with a 200 W solar panel?
It depends on the month and the town. A flat 200 W panel on MPPT in Sydney makes about 71 Ah a day in January and about 27 Ah in June, before your loads. So 75 Ah takes about 1.1 days of sun in summer and about 2.8 in winter.
Can I charge a lithium battery faster with a bigger charger?
Only up to the battery’s limit. Victron recommends 0.5C or less for its LiFePO4 batteries: 50 A for 100 Ah. Check your battery’s own charge limit.
Does running the fridge slow the charge?
Yes. The charger feeds the fridge first. A 25 A charger with a 0.76 A fridge puts 24.2 A into the battery.