DC to DC Charger Time Calculator
A DC to DC charger can replenish an auxiliary battery while a vehicle is running, but the time needed depends on more than the charger’s advertised output. The DC to DC Charger Time Calculator estimates charging time using your battery capacity, starting and target state of charge, average charger output current, and assumed charging efficiency.
Use the result to plan a drive, compare charging scenarios, or estimate how long it may take to restore energy to a leisure or auxiliary battery. The result is an estimate based on a constant average charging rate. Real charging conditions can produce a shorter or longer time.
What the DC to DC Charger Time Calculator does
The calculator estimates the hours required to raise a battery from one state of charge to another. It first calculates the amount of charge needed in amp-hours (Ah), then divides that amount by the assumed average charging current and charging efficiency.
For example, a 100 Ah battery moving from 50% to 100% state of charge needs an estimated increase of 50 Ah. The calculator uses the current and efficiency you enter to estimate how long that increase will take.
The result is labeled Estimated charging time and is displayed in hours to two decimal places. If the target state of charge is equal to or lower than the starting state of charge, the result is zero hours because no increase in charge is required by the calculation.
The default values are illustrative. Your battery’s capacity, charger output, charging behavior, and operating conditions may differ.
How to use the DC to DC Charger Time Calculator
Enter the following information to create an estimate for your setup:
- Battery capacity (Ah): Enter the battery capacity used as the basis for your state-of-charge percentages. For a battery bank, use the capacity that applies to the bank as configured.
- Starting state of charge (%): Enter the estimated percentage of charge in the battery before charging begins. For example, enter 50 for a battery estimated to be half charged.
- Target state of charge (%): Enter the percentage you want the battery to reach. This may be a partial target, such as 80%, or a full-charge target of 100%.
- Average charger output current (A): Enter the assumed average current delivered by the charger at the battery over the charging period. This is battery-side output current, not the current drawn from the vehicle’s electrical system.
- Assumed charging efficiency (%): Enter the assumed percentage of delivered amp-hours retained by the battery. For example, 85% means the calculation assumes the battery retains 85 Ah for every 100 Ah delivered.
Review the Estimated charging time in hours. You can change the target percentage or average current to compare scenarios. Keep the assumptions consistent when comparing results, and remember that a charger’s rated maximum current is not necessarily its average current throughout a charge.
How the DC to DC Charger Time Calculator formula works
The formula calculates the positive difference between the target and starting states of charge, converts that difference into amp-hours, and then accounts for current and assumed efficiency:
Estimated charging time (hours) = Battery capacity (Ah) × positive state-of-charge increase ÷ 100 ÷ average charger output current (A) ÷ (assumed charging efficiency ÷ 100)
If the target is below the starting percentage, the positive increase is treated as zero. The efficiency adjustment increases the estimated time because the calculation assumes that only a portion of the delivered charge is retained.
Worked example using the default values
Suppose you enter the following illustrative values:
- Battery capacity: 100 Ah
- Starting state of charge: 50%
- Target state of charge: 100%
- Assumed average charger output current: 20 A
- Assumed charging efficiency: 85%
The required state-of-charge increase is 100% − 50%, or 50 percentage points. Multiplying 100 Ah by 50% gives an estimated 50 Ah increase in battery charge.
At an assumed average output of 20 A and an assumed efficiency of 85%, the calculation is:
50 Ah ÷ 20 A ÷ 0.85 = 2.94 hours
The calculator therefore displays 2.94 hours, or approximately 2 hours and 56 minutes. This is a planning estimate, not a prediction that the battery will reach full charge after exactly that amount of driving. In particular, charging current may decrease as the battery approaches a full state of charge.
Use cases for the DC to DC Charger Time Calculator
This calculator can help you think through several common vehicle and auxiliary battery situations:
- Planning a trip: Estimate whether a planned period of driving could provide enough charging time under your chosen assumptions.
- Comparing partial and full targets: See how the estimated time changes when you charge to 80% instead of 100%.
- Assessing a battery bank: Compare the estimated time for different battery capacities while keeping the other inputs the same.
- Comparing charging currents: Explore how a different assumed average battery-side output current changes the result.
- Planning around auxiliary power use: Get a starting estimate for restoring charge after using lights, a refrigerator, or other accessories.
These comparisons are most useful when the input values reflect your actual setup. A larger charger output may reduce the calculated time, but only if the battery and charging system can accept and deliver the assumed average current throughout the period.
Other factors to consider when calculating Estimated charging time
Charging taper can make the final part of a charge take longer than a constant-current estimate suggests. As a battery approaches its target, the charger or battery may reduce current. Time spent in absorption or balancing is not calculated separately.
Battery chemistry and management limits also matter. Different batteries accept charge differently, and a battery management system may limit or interrupt charging under certain conditions. Follow the battery and charger manufacturers’ instructions for compatible settings and operating limits.
Temperature can affect both charging performance and battery acceptance. A current that is achievable in one set of conditions may not be maintained in another.
Loads running during charging can reduce the charge reaching the battery. The calculator does not separately subtract power used by appliances or accessories. If loads are present, take them into account when choosing an appropriate average battery-side current assumption, and recognize that the actual result may still differ.
State-of-charge estimates can be uncertain. If the starting percentage is inaccurate, the calculated amount of charge needed will also be inaccurate. Similarly, use a capacity figure that is appropriate for the battery and for the percentages you enter.
Finally, the efficiency input is a simplified assumption about retained amp-hours. It does not explicitly model each electrical loss or changing charging stage. Avoid treating the illustrative 85% default as a measured value for every battery.
DC to DC charger time calculator FAQ
How long does a DC to DC charger take to charge a 100 Ah battery?
It depends on the starting and target charge levels, average output current, efficiency, and operating conditions. In the illustrative default example, charging a 100 Ah battery from 50% to 100% at an assumed 20 A average output and 85% efficiency takes an estimated 2.94 hours.
Should I enter the charger’s rated current or its average current?
Enter the assumed average output current at the battery over the charging period. The charger’s rated maximum can be a starting point, but actual current may vary, especially near the end of charging. If the average is lower than the rated maximum, using the maximum can underestimate time.
Why does the calculator include charging efficiency?
The efficiency input represents the assumed fraction of delivered amp-hours retained by the battery. A lower assumed efficiency increases the estimated time because more charge must be delivered to achieve the same increase in battery charge.
What happens if the target charge is below the starting charge?
The calculator returns zero hours. Its formula only estimates the time needed for a positive increase in state of charge; it does not estimate battery discharge time.
Does the result include appliances running while the battery charges?
No. Concurrent loads are not calculated separately. Accessories that draw power during charging can affect the current available to charge the battery and may increase the actual time needed.