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Battery Charging Formula Reference

This reference consolidates all formulas needed for battery charging calculations — from basic CC phase time estimation to total energy consumption accounting for charger efficiency. Use these formulas to size chargers, estimate charge times, and plan energy budgets for any battery chemistry.

Fundamental Formulas

Charge Time (CC Phase)

CC Time (hours) = Ah to Charge / Charge Current (A)

This is the primary formula for estimating bulk charge time. The Ah to Charge is the difference between starting and ending state of charge multiplied by rated capacity.

Ah to Charge = Capacity (Ah) × (End SOC% - Start SOC%)

Example: 100Ah battery, 20% to 100% → Ah to Charge = 100 × 0.80 = 80 Ah. At 50A charge current: 80/50 = 1.6 hours.

Charge Current from C-Rate

Charge Current (A) = Battery Capacity (Ah) × C-Rate

The C-rate defines charge speed relative to capacity. A 0.5C rate on a 100Ah battery = 50A. A 0.2C rate = 20A. Higher C-rates charge faster but increase heat and may reduce cycle life.

C-Rate = Charge Current (A) / Battery Capacity (Ah)

To find the C-rate of your charger: divide charger current by battery capacity. A 30A charger on a 100Ah battery = 0.3C.

CV Phase Time Estimation

CV Time (hours) ≈ 0.3–0.5 h (LFP) CV Time (hours) ≈ 2–4 h (Lead-Acid)

The CV phase time depends on battery design and termination current. For LFP, the CV phase is relatively short because the voltage curve is flat and current tapers quickly. For lead-acid, the absorption phase is much longer as the charger must push current into increasingly resistant plates.

Total Charge Time = CC Time + CV Time

For practical purposes, charging from 20% to 80% skips most of the CV phase, making it significantly faster than a full 0–100% charge.

Energy Consumption

Energy to Battery (Wh) = Ah to Charge × Nominal Voltage (V)

LFP nominal voltage: 3.2V/cell (12.8V for 4S, 25.6V for 8S, 51.2V for 16S). NMC nominal voltage: 3.6V/cell.

Energy from Source (Wh) = Energy to Battery / Charger Efficiency

Charger efficiency is typically 90–98% for modern switching chargers. A 95% efficient charger means 5% of input energy is lost as heat.

Charger Power Draw (W) = Charge Voltage × Charge Current / Efficiency

Example: 50A at 14.6V with 95% efficiency → 50 × 14.6 / 0.95 = 768W from the power source.

Chemistry-Specific Parameters

Parameter	LFP	NMC	Lead-Acid
Cell nominal voltage	3.2V	3.6V	2.0V
Charge voltage (12V pack)	14.4–14.6V	16.8V	14.4–14.8V
Recommended C-rate	0.5C	0.5C	0.2C
Charge efficiency	95–98%	95–98%	80–85%
CV phase duration	0.3–0.5 h	0.3–0.5 h	2–4 h

Worked Example: Full Charge Calculation

Given: 200Ah 48V LFP battery, 100A charger (0.5C), 20% to 100% SOC, 96% charger efficiency

Step 1: Ah to charge:

200 Ah × 0.80 = 160 Ah

Step 2: CC phase time:

160 Ah / 100A = 1.6 hours

Step 3: Total time with CV taper:

1.6 h + 0.4 h = 2.0 hours

Step 4: Energy to battery:

160 Ah × 51.2V = 8,192 Wh

Step 5: Energy from source:

8,192 Wh / 0.96 = 8,533 Wh

Step 6: Average charger power draw:

8,533 Wh / 2.0 h = 4,267W (~4.3 kW)

This is a substantial charger — typical for large RV, marine, or home battery systems. Ensure your AC input or generator can sustain this power draw for the full charge duration.

Try It

Use the Charging Time Calculator to compute exact charge times using these formulas with your specific battery and charger.

Open Charging Time Calculator

Next Step

Calculate the C-rate for any charger and battery combination with the C-Rate Calculator.

Open C-Rate Calculator

How Long to Charge a Battery

Practical guide to battery charging times across common capacities and charger sizes.

Charger Sizing Guide

How to select the right charger size for your battery based on chemistry, capacity, and use case.

Frequently Asked Questions

What is the formula for battery charge time?

Basic charge time = Ah to Charge / Charge Current (A). For LFP, total time includes a CV taper phase of approximately 0.3–0.5 hours. For lead-acid, the absorption phase can add 2–4 hours. Total energy consumed = Ah × Voltage ÷ Efficiency (typically 0.95 for lithium, 0.80 for lead-acid).

What is the CC phase in battery charging?

The Constant Current (CC) phase is the bulk charging stage where the charger delivers a fixed maximum current while battery voltage rises. For LFP, this phase delivers approximately 70–80% of total capacity. The CC phase ends when the battery reaches its maximum voltage, at which point the charger switches to CV mode.

What is the CV phase in battery charging?

The Constant Voltage (CV) phase follows the CC phase. The charger holds voltage at the battery's maximum while current tapers. This phase tops off the remaining 20–30% of capacity. For LFP, the CV phase adds 20–60 minutes. For lead-acid, the absorption (CV) phase can last 2–4 hours.

How does charger efficiency affect charge time?

Charger efficiency determines how much energy is lost as heat during conversion. A 95% efficient charger means 5% of input energy is lost. For a 1,024 Wh charge, the charger draws 1,078 Wh from the source. Lower efficiency chargers draw more power and take slightly longer to deliver the same charge.