Charging a battery to 80% usually takes less generator time than reaching 100%, but the difference depends strongly on battery chemistry and charge taper. Compare the amp-hours required for both targets, then assess the finishing stage separately instead of assuming the last 20% charges at the same rate as the earlier portion.
Open the Battery Charging With Generator Calculator.
Compare equal starting points
An 80%-versus-100% comparison is useful only when bank capacity, starting charge, charger current, and concurrent loads remain consistent. Comparing one session that begins at 30% with another that begins at 70% says little about the cost of the finishing stage.
Choose a starting state of charge that represents the actual operating plan. Record whether SOC comes from a configured battery monitor or a rough estimate. A monitor reading with an uncertain synchronization history should not be treated as precise merely because it displays a decimal.
Also define the objective. You might need enough energy for an overnight load, a complete charge before storage, or a partial generator session followed by another approved charging source. Those purposes can justify different target choices, but each needs the battery manufacturer’s charging requirements respected.
Use the total bank capacity for its actual wiring arrangement. Two 12 V, 150 Ah batteries in series make a nominal 24 V, 150 Ah bank, while two in parallel make a nominal 12 V, 300 Ah bank. Confusing the two changes both the Ah calculation and the energy available.
Calculate the missing amp-hours for each target
For an illustrative 300 Ah bank starting at 40%, reaching 80% replaces 300 × (0.80 − 0.40) = 120 Ah. Reaching 100% replaces 300 × (1.00 − 0.40) = 180 Ah before charging losses are considered.
With a 60 A net current held constant, idealized times would be two hours and three hours. The final 20 percentage points represent 60 Ah, so constant-current arithmetic assigns them one additional hour. Whether the real finishing period takes one hour is a different question.
| Target from 40% SOC | SOC increase | Ideal Ah replacement | Ideal time at 60 A |
|---|---|---|---|
| 80% | 40 percentage points | 120 Ah | 2.00 h |
| 90% | 50 percentage points | 150 Ah | 2.50 h |
| 100% | 60 percentage points | 180 Ah | 3.00 h |
The 100% target requires 50% more replacement charge than the 80% target in this particular example: 180 Ah versus 120 Ah. It is not simply 20% more work, because both sessions start at 40%, not at zero.
That distinction helps explain why apparently small target changes can matter. Moving the target from 80% to 90% adds 30 Ah to this bank, while moving from 90% to 100% adds another 30 Ah. Equal Ah increments can nevertheless take different times when current tapers.
Use the corrected calculator
The corrected calculator clamps the required rise in state of charge at zero, so a target below the current SOC does not produce a negative duration. Its charging-current field now means net current available to the battery after simultaneous DC loads. The result is still a constant-current estimate with a chemistry efficiency assumption; absorption, taper, temperature, charger limits, and battery acceptance can lengthen real charging time.
Why 80% can be a useful lead-acid comparison point
Lead-acid charging commonly involves a bulk period followed by absorption, during which current declines. Victron’s charging manual uses approximately 80% SOC as a lead-acid bulk-stage illustration. The transition for a real bank is controlled by its voltage response and charging settings, not by a universal 80.0% switch.
This is why increasing charger current may save considerable time early in a session but less time at the end. Once the bank is accepting fewer amps under the required voltage limit, unused charger capacity does not force a faster finish.
An 80% target can help compare operating scenarios, but it is not a universal recommended endpoint. Repeated partial charging must be considered against the battery’s care requirements. Trojan’s AGM maintenance guidance calls for full charging after use for its covered products; follow the actual bank’s documentation.
If a generator session ends early, identify how and when the required finish will occur. An assumed sunny afternoon is not the same as a charging plan when weather, solar capacity, or loads prevent completion. A backup plan should account for those limits explicitly.
Why lithium needs a different interpretation
LiFePO4 banks can have a different current-acceptance profile from lead-acid. Do not import an assumed long lead-acid finishing period into a lithium schedule, or assume that all lithium banks finish instantly once they reach 80%.
The battery manufacturer specifies the compatible charge profile, maximum current, temperature limits, and any balancing or full-charge requirements. The BMS can restrict or disconnect charging under certain conditions, so a calculator’s time estimate does not override its protection.
In a lithium comparison, an 80% target may chiefly represent fewer Ah to replace. At higher SOC, cell behavior and charger voltage regulation still influence current. Use the actual charger and bank documentation to decide how much additional time to allow near the chosen endpoint.
Do not select a lithium chemistry factor simply because it gives a shorter result for a lead-acid bank. The tool’s chemistry selection changes an assumption; it does not change the installed battery’s physical behavior or permitted settings.
Account for loads during the generator session
The relevant current is net current into the bank. If a 60 A charger also supplies 12 A of DC loads, approximately 48 A remains for charging before other restrictions. For the 120 Ah replacement to 80%, the ideal time becomes 2.5 hours rather than two.
If loads vary, calculate operating blocks. A pump or inverter load that runs for only part of the session can make the average net current different from both its highest and lowest readings. A suitable monitor’s accumulated Ah or current log can improve the comparison.
Also separate generator loads from battery loads. An AC appliance supplied directly by the generator reduces available generator capacity, while a DC appliance supplied from charger output reduces net battery charging current. In some installations both effects occur, so the system’s actual power path matters.
When generator input capacity is tight, the charger may need an approved input-current limit. That can lower DC charge current below the charger’s headline rating. Enter the sustained current actually available to the bank rather than the largest number on the charger label.
Compare generator time and fuel without assuming a constant rate
Generator fuel consumption depends on its operating load and model. The finishing stage can leave the generator operating at lower demand while the battery accepts fewer amps. Multiplying every hour by a full-load fuel rate may overstate consumption, while assuming fuel falls in direct proportion to charge current may understate it.
For an illustrative logged session, suppose an early stage lasts 2.4 hours at a measured 0.35 gallon per hour, followed by a finishing stage lasting 1.8 hours at 0.20 gallon per hour. Fuel totals 2.4 × 0.35 + 1.8 × 0.20 = 1.20 gallons. The finishing stage contributes 0.36 gallon in that example.
Those invented figures demonstrate the method; they are not a predicted fuel curve. Replace them with model-specific consumption data or careful normal-operation measurements. Keep startup, warmup, and shutdown periods consistent when comparing sessions.
Use endpoints that the equipment can actually identify
A timer is useful for planning, but the charger and battery determine whether a full charge is complete. Record stage indicators, net current, and the manufacturer’s termination criterion. Avoid redefining 100% as the moment a rough calculated duration expires.
For a partial target, check whether the charging system offers an appropriate supported way to stop at the desired SOC. Do not alter voltage settings arbitrarily to force a percentage target; voltage and SOC are not interchangeable during an active charge.
Is charging from 80% to 100% always inefficient?
It can take longer per added Ah, particularly for lead-acid, but completing the required charge serves a battery-care purpose. Evaluate generator scheduling alongside the manufacturer’s requirements rather than treating shorter runtime as the only performance measure.
Can solar finish after the generator stops?
Yes, if the installed charging system, available sunlight, and remaining daily loads allow the required finish. Calculate the energy and charging window realistically, and keep an alternative for days when the expected solar contribution does not arrive.
References
- Victron Energy: Charge algorithm and charge-time estimation
- Trojan Battery: AGM and AES AGM maintenance
- Battery Charging With Generator Calculator