How this is calculated
The bank must take back its capacity multiplied by the depth discharged. That part is arithmetic. The hard part is that the current going in is the smaller of what the source can make and what the battery will accept, and acceptance collapses near the top.
So the clock splits. The constant-current stage runs until the bank’s own acceptance has fallen to the current on offer. Above that point the charger holds the voltage steady and the current falls away by itself, down to the termination current the datasheet names.
Worked example: a 400 Ah 48 V lithium bank, half discharged, on a 1,600 W array. Take off 14 percent for system losses and 2 percent for controller conversion, divide by a 57.6 V charge voltage, and about 23 A reaches the battery. Bulk carries it from 50 percent almost to full in 8.5 hours; absorption adds minutes, because at 0.06C it never meets its own ceiling. Over 4.2 peak sun hours, two solar days.
Make that same bank flooded lead-acid and it becomes 7.3 hours of bulk plus 3.2 hours of absorption: 10.5 hours in all, two and a half solar days at the same 4.2 peak sun hours. The off-grid battery bank calculator sizes the bank; this page says whether your source can fill it.
The model uses rate ceilings, tapers and termination currents published by Rolls and Victron, and lands within four percent, on total time to full, of both worked examples in the Rolls Battery User Manual V7.4.
What the three stages actually do
Bulk is the charger working flat out. Every amp the source can make goes in and the terminal voltage climbs on its own. Rolls states that this stage typically brings a lead-acid bank to about 80 percent, which is why bulk misleads people.
Absorption starts the moment the charge voltage is reached. The charger holds that voltage and the current does whatever the battery permits, which is less and less. For a 48 V bank at 25 °C, Rolls sets 60.00 V in its regular-cycling flooded table (the off-grid one) and 58.80 V for AGM, ending at a tail current of 2 percent of the 20-hour rating for flooded and 3 percent for AGM.
Float is the holding voltage afterwards, 54.00 V flooded and 55.20 V AGM. It replaces self-discharge and nothing else. A system at float by mid-afternoon has spare array to spend on loads.
The ceilings the datasheets set
Every chemistry has a rate ceiling, and the two families write it differently. Lithium uses a C-rate, a multiple of capacity; lead-acid a percentage of the 20-hour capacity.
| Chemistry | Recommended rate | Range allowed | Ends at | Charge efficiency |
|---|---|---|---|---|
| Flooded lead-acid | 10% of C/20 | 5–20% | 2% tail current | ~80% |
| AGM | 20% of C/20 | 10–30% | 3% tail current | ~80% |
| Gel (OPzV) | 20% of C/20 | 10–30% | 3–4% tail current | ~85% |
| LiFePO4, 10–35 °C | 0.5C | up to the label maximum | 0.05C tail current | 92% round trip |
| LiFePO4, 0–10 °C | 0.2C | temperature limited | 0.05C tail current | 92% round trip |
| LiFePO4, −20 to 0 °C | 0.1C | parallel strings recommended | 0.05C tail current | 92% round trip |
Read the lead-acid rows as a floor as well. Charging a flooded bank under 5 percent of its 20-hour rating prolongs the charge and, in the manual’s words, increases the potential for sulfation buildup. Cold bites at the other end: charging LiFePO4 below freezing.
Why the top fifth takes the longest
The plate area left to react shrinks as it converts, so the current a lead-acid battery will swallow falls in step with how full it is. Above about 80 percent the charger is no longer in charge of anything; it waits.
Take a 600 Ah flooded bank at 48 V, half discharged, on a 60 A charger, exactly the recommended rate. Bulk covers 50 to 80 percent in three hours. The last fifth takes four.
Lithium iron phosphate holds its acceptance almost to the top. The Rolls LFP manual ends the constant-voltage stage at 0.05C and suggests 20 to 30 minutes of absorption for a single string, mostly to give the cell balancer something to work with.
The generator trap. A generator sized to fill a lead-acid bank fast burns full fuel for the three easy hours, then idles into a four-hour taper it cannot speed up. Stop it at 80 percent and let the array finish.
Cycling in a partial state of charge
Off-grid banks rarely reach 100 percent in winter, and what that costs depends on chemistry.
For lead-acid it is cumulative. Rolls describes deficit cycling as sulfation that gradually lowers available capacity, warns it can take months to become noticeable, and calls for a corrective equalization every 60 to 180 days on such banks.
Lithium iron phosphate carries no such penalty. The same maker states its LFP batteries may cycle or be stored in a partial state of charge, and that equalization is neither required nor wanted. Victron adds that service life slightly improves with partial charging.
One caveat survives: cell balancing happens near the top, so a bank that never finishes can drift. Rolls advises charging each battery individually once a year for one or two parallel strings, twice for three or four.
Common banks against common charge sources
All rows are 48 V nominal, half discharged, at the datasheet charge voltage. Array figures take 14 percent system losses and 98 percent controller conversion. Click a heading to sort.
| Bank | Chemistry | Charge source | Current | Bulk h | Absorption h | Total h | Source kWh | Solar days |
|---|---|---|---|---|---|---|---|---|
| 200 Ah | LiFePO4 | 1,200 W array | 18 A | 5.6 | 0.1 | 5.7 | 5.2 | 1.4 |
| 200 Ah | LiFePO4 | 50 A charger | 50 A | 1.9 | 0.2 | 2.1 | 5.2 | — |
| 400 Ah | LiFePO4 | 2,400 W array | 35 A | 5.6 | 0.1 | 5.7 | 10.4 | 1.4 |
| 400 Ah | LiFePO4 | 60 A charger | 60 A | 3.3 | 0.1 | 3.4 | 10.4 | — |
| 400 Ah | LiFePO4 | 100 A charger | 100 A | 1.9 | 0.2 | 2.1 | 10.4 | — |
| 400 Ah | AGM | 2,400 W array | 34 A | 5.0 | 1.5 | 6.6 | 12.0 | 1.6 |
| 400 Ah | AGM | 80 A charger | 80 A | 1.5 | 2.0 | 3.5 | 12.0 | — |
| 400 Ah | AGM | 120 A charger | 120 A | 1.0 | 1.3 | 2.3 | 12.0 | — |
| 600 Ah | Flooded | 3,600 W array | 51 A | 4.0 | 3.8 | 7.8 | 18.0 | 1.9 |
| 600 Ah | Flooded | 60 A charger | 60 A | 3.0 | 4.0 | 7.0 | 18.0 | — |
| 600 Ah | Flooded | 120 A charger | 120 A | 1.5 | 2.0 | 3.5 | 18.0 | — |
| 800 Ah | Flooded | 4,800 W array | 67 A | 4.0 | 3.8 | 7.8 | 24.0 | 1.9 |
The two 120 A rows sit at the maximum the manual allows rather than the recommended rate, where a large generator-driven charger lands. It halves the clock and is the upper bound before the manual warns about overheating.
A mains charger on a 120 V outlet runs out of circuit before the bank runs out of appetite. A 15 A branch circuit is 1,800 W on paper and about 1,440 W for a load that runs for hours; at roughly 90 percent conversion that is about 22 A of charge at a 57.6 V lithium charge voltage.
A 230 V socket has far more headroom. A 16 A circuit is about 3,680 W on paper and near 2,900 W for a load that runs for hours; at roughly 90 percent conversion that is about 45 A of charge at 57.6 V, so the bank’s own acceptance limit usually stops you first.
Quick answers
How long does it take to charge a battery bank?
Longer than capacity divided by charge current. A half-discharged 400 Ah 48 V lithium bank refills in two hours on a 100 A charger; a 600 Ah flooded bank on 60 A takes seven, four of them tapering.
What is the difference between bulk, absorption and float?
Bulk is full current until the charge voltage is reached. Absorption holds that voltage while current falls away. Float is the lower holding voltage that keeps a full bank full.
How fast can I charge a lead-acid battery?
Rolls sets flooded charging at 10 percent of the 20-hour capacity, allowed between 5 and 20 percent; AGM at 20 percent, allowed between 10 and 30.
Why is lithium so much quicker to fill?
It accepts 0.5C at room temperature and keeps accepting almost to the top, so the constant-voltage tail lasts minutes rather than hours.
What does partial-state-of-charge cycling do?
It sulfates lead-acid and slowly removes capacity, which is why the manual calls for equalization every 60 to 180 days. Lithium iron phosphate is rated to cycle and sit part-charged.
Sources
Sources: Rolls Battery Engineering, Rolls Battery User Manual V7.4, August 2024 — charge currents for flooded, AGM and OPzV gel, absorption time formulas and worked examples, tail currents, charge voltages at 25 °C, charge efficiency, deficit cycling. Rolls Battery Engineering, VB-Series LFP Drop-In Battery Operating Manual, revision 1.2, 17 March 2026 — charge current by temperature, absorption voltages, 0.05C termination, partial state of charge. Victron Energy, Lithium Battery Smart 12,8 V and 25,6 V datasheet, retrieved September 2026 — charge current and 92 percent round trip against 80 percent for average lead-acid. NREL, PVWatts Version 5 Manual, NREL/TP-6A20-62641, September 2014, Table 6 — the 14 percent default system loss.
Go deeper
Size the bank with the off-grid battery bank calculator, then find the array that refills it in a day with the panel-to-battery calculator. Still choosing chemistry? Lithium against lead-acid compares them, and the LiFePO4 voltage chart turns a resting voltage into a state of charge.