Battery Runtime Calculator: How Long Until Blackout?
Battery Runtime Calculator: How Long Until Blackout?
You are camping, it is 8 PM, and your battery reads 12.6 Volts. You turn on the heater and the laptop. Do you have enough power to last until sunrise?
Or will the low-voltage alarm wake you up at 3 AM? Use this engineering-grade tool by ShockTrail to predict your exact runtime based on chemistry and load.
The Runtime Estimator
Enter your battery specs and the wattage of the device you want to run.
Battery Runtime Estimator
Discharge Logic by ShockTrail0.0 Hours
ℹ️ Includes ~15% inverter efficiency loss.
The “Peukert Effect”: The Hidden Thief
If you have a 100Ah battery, logic suggests you can pull 100 Amps for 1 hour. Wrong.
In 1897, German scientist Wilhelm Peukert discovered that lead-acid batteries lose capacity as the discharge rate increases. If you pull 100 Amps from a 100Ah lead-acid battery, it might only last 30 minutes, effectively becoming a “50Ah” battery under that stress.
Lithium (LiFePO4) Advantage: Lithium batteries have a Peukert Exponent very close to 1.0, meaning they are immune to this effect. A 100Ah Lithium battery will give you 100 Amps for roughly one hour. This is a massive efficiency gain explained in detail by Battery University.
Depth of Discharge (DoD): The 50% Limit
Just because your car has a gas tank doesn’t mean you drive it until it’s bone dry. Batteries are the same.
- Lead-Acid / AGM / Gel: You should never discharge below 50%. Going deeper causes sulfation and permanent damage. If you have a 100Ah AGM battery, you really only have 50Ah of usable energy.
- Lithium (LFP): Can be safely discharged to 80% or even 100% (depending on the BMS quality). This effectively doubles your usable power for the same physical size.
Inverter Efficiency Loss
If you are running AC appliances (110V/220V) like a TV or Microwave, you must account for the inverter. Converting 12V DC to 120V AC creates heat. A typical pure sine wave inverter is about 85% efficient.
Our calculator automatically deducts this 15% “tax” so you don’t get a nasty surprise. For ultra-efficient systems, consider using DC-native appliances as recommended by Northern Arizona Wind & Sun.
Real World Scenario: The Laptop Charger
Let’s do the math manually to double-check the tool:
1. Usable Capacity: 100Ah * 50% DoD = 50Ah.
2. Load in Amps: 60W / 12V = 5 Amps.
3. Inverter Loss: 5 Amps / 0.85 = 5.88 Amps actual draw.
4. Runtime: 50Ah / 5.88A = ~8.5 Hours.
If you switched to a Lithium battery in the same scenario, you would get nearly 17 hours of runtime.
Frequently Asked Questions (FAQ)
Does cold weather affect runtime?
Yes, drastically. At 32°F (0°C), a lead-acid battery loses about 20% of its capacity. Lithium batteries perform better but cannot be charged below freezing without internal heaters.
What is C-Rate?
C-Rate describes how fast a battery is charged or discharged relative to its capacity. 1C means fully discharging in 1 hour. Most Lead-Acid batteries prefer a slow C/20 rate (20 hours).
My voltage reads 12.0V, is it empty?
For Lead-Acid, 12.0V is roughly 50% charged (effectively empty for longevity purposes). For Lithium, voltage curves are flat, so 12.0V means it is almost dead (approx 10% left).
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