Battery Charge & Discharge Calculator — Free Tool
Free battery tool
Battery Charge & Discharge Calculator
Enter any battery — see charge rate, charge time, solar panel count, MPPT sizing, runtime under load, and consumption per hour. All formulas shown.
Your battery
Your load & solar
⚡ Charging — how to refill this battery
🔋 Discharge — how long will it last
☀️ Solar panel & MPPT sizing
⚡ Inverter & charger sizing
Estimates based on stated inputs and standard derating. Real-world performance varies with temperature, age, cable quality, and charge profile. Always verify with manufacturer datasheets.
Frequently Asked Questions
What is the maximum charging current for a 100Ah LiFePO4 battery?
The standard charge rate for LiFePO4 is 0.5C, which means 50 amps for a 100Ah battery. The maximum safe rate is 1C (100 amps). At 50A, a fully discharged 100Ah battery charges from 10 percent to 100 percent in about 1.8 hours. The calculator above shows both standard and maximum charge rates for any battery you enter, along with the charge power in watts.
How long does it take to charge a 12V 200Ah battery from solar?
With a 400W solar array and 5 peak sun hours, a 200Ah 12V LiFePO4 battery charges from 20 percent to 100 percent in approximately 5.8 hours of sun. With 800W of panels, it takes about 2.9 hours. The calculator above computes the exact charge time based on your panel wattage, sun hours, and current state of charge, including system derating for real-world losses.
What size MPPT controller do I need for 400 watts of solar panels?
For a 400W array on a 12V battery bank: controller amps = 400W ÷ 12V × 1.25 safety factor = 42A. You need a 50A MPPT controller with at least 100V PV input voltage. On a 24V bank, the same array needs only 21A — a 30A controller works. The calculator sizes the exact controller amperage and PV input voltage for any array and battery combination.
How many amps does a 500 watt load draw from a 12 volt battery?
A 500W load draws 500 ÷ 12 = 41.7 amps from a 12V battery through an inverter. Including inverter losses (95 percent efficiency), the actual draw is about 43.9 amps. At this rate, a 200Ah LiFePO4 battery (90% DoD = 180 usable Ah) lasts 180 ÷ 43.9 = approximately 4.1 hours. The calculator shows consumption per hour in both amps and kWh.
Does this calculator show the formula so I can understand the calculation?
Yes. Every section — charging, discharge, solar sizing, and inverter sizing — shows the complete formula in a shaded box with your actual numbers plugged in. You can see exactly how the result was calculated and verify it yourself. The formulas use standard electrical engineering equations that work for any battery at any voltage.
What charges faster — LiFePO4 or AGM?
LiFePO4 charges roughly 2.5 to 5 times faster than AGM. LiFePO4 accepts charge at 0.5C standard (50A per 100Ah), while AGM is limited to 0.2C (20A per 100Ah). LiFePO4 also has 98 percent charge efficiency versus 85 percent for AGM, meaning less energy is wasted as heat during charging. Switch between chemistries in the calculator above to see the charge time difference for your specific battery.
Can I use this calculator for a 48 volt battery system?
Yes. Select 48V from the battery voltage dropdown and all calculations — charge current, charge time, runtime, solar panel count, MPPT sizing, and inverter sizing — adjust automatically. The calculator supports 6V, 12V, 24V, and 48V systems with LiFePO4, AGM, flooded lead-acid, and gel chemistries.