Formulas and estimation rules
Total power is measured in watts. Remember these formulas:
- Watts = Volts × Amps
- Amps = Watts ÷ Volts
- Volts = Watts ÷ Amps
Amp-hours (Ah): It measures the amount of electrical charge (amps) that a battery can provide over time. One amp-hour means that a battery can deliver a current of 1 amp for one hour.
Watt-hours (Wh): A watt-hour is a measurement of energy used by an electrical device over time. One watt-hour equals to the energy used by a 1-watt device for one hour.
Battery depth of discharge DoD): Most batteries like lead acid, AGM, and gel types should not be discharged beyond 50% of their capacity, while lithium batteries can be discharged up to 100%. For example, if you have a 70ah lead acid battery, you can only use 35 amps before it needs to be recharged.
Battery discharge efficiency: Unfortunately, batteries are not 100% efficient when discharging. The efficiency rate will depend on the chemistry of the battery.
Based on directscience.com data:
- Lead-acid batteries discharge efficiency ≈ 80 − 85%
- Lithium-ion batteries discharge efficiency ≈ 90 − 95%
Here's a table on 70ah battery usable amp-hours after considering discharge efficiency (Lead acid: 85%, and Lithium: 95%).
| Battery type | 70ah battery usable capacity (in ah) |
|---|---|
| Lead acid | 59ah |
| Lithium (LiFePO4) | 66ah |
Must watch this video to understand the basics of batteries (capacity, charge, and discharge mechanism)
What does 70ah mean on battery?
70ah or 70 amp-hours battery means it can provide 1 amp for 70 hours or 10 amps for 7 hours "theoretically".
We usually calculate battery capacity in watt-hours to find out how much actual power it can hold.
Formula: Battery capacity Wh = Ah x Volts
12v 70ah battery is equal to 840 watt-hours and 24v 70ah battery is equal to 1680 watt-hours. This means the same amp-hour 24v battery stores double the power of a 12v amp-hour battery.
Now let's dive into the calculation part and at the end, I'll share why is it impossible to get a 100% accurate runtime.
Related post: Battery Amp Hours To Watts (Ah To Watts) Calculator
5 Steps to calculate 70ah battery runtime
Steps
70ah battery in watts = 70×12 = 840 watt-hours
Battery capacity in wh after DoD limit = 840 × 50% = 420wh3. Multiplying the battery capacity after DoD by 0.85 for lead acid and 0.95 for lithium-ion to take into account battery discharge efficiency.
Battery usable watt-hours: 420 × 0.85 = 357whBattery AC watts = 357 × 90% = 321 AC watt-hours 70ah battery runtime on LED light = 321÷20 = 16 hours
Turns out, a 12v 70ah lead acid battery will run LED light for about 16 hours.
How Reliable Are These Estimates?
Estimating 100% accurate battery runtime is nearly impossible to achieve. There are many other factors that will affect the battery lifetime on load. Let's discuss some of them.
1. Peukert's law
Peukert's law says that the faster a battery is used, the less energy it can provide compared to what it says on the label. This is because when the battery is discharged quickly, more energy turns into heat instead of powering devices.
This rule only applies to lead acid batteries. On the other hand, lithium (LiFePO4) batteries can be safely discharged at 50% of their capacity (continuously) without causing any power losses.
Here's a chart on 70ah lead acid battery usable capacity in amp-hours with respect to how fast it is being discharged.
| Usable 70ah lead acid battery capacity | Hours of discharge |
|---|---|
| 70ah | 20 hours |
| 63ah | 10 hours |
| 61ah | 8 hours |
| 58ah | 6 hours |
| 56ah | 5 hours |
| 49ah | 3 hours |
| 42ah | 2 hours |
| 35ah | 1 hour |
Must read: Battery Charge And Discharge Rate Calculator: C-Rating To Amps
2. battery life cycles count
Batteries have a limited number of times they can be used and charged. This is called the life cycle.
Lead-acid batteries last a few hundred cycles if maintained well. Lithium batteries can last thousands. But the more a battery is used and charged, the less it can hold its maximum capacity.
After around 500 cycles, a lead acid battery can lose about 20% of its charge-holding capacity, while a lithium battery may retain about 80% of its original capacity even after 2000 cycles.
3. effect of Temperature on batteries
The temperature affects how well your battery will work. Batteries work best when they are at a temperature between 68°F and 77°F (20°C to 25°C). If it gets too hot or too cold, the deep cycle battery won't work as well. Ask your manufacturer for a more accurate number.
High temperature: When batteries are exposed to high temperatures, their internal resistance increases, which results in a reduction in their overall capacity.
This means that the battery will be able to store less energy and will not last as long. In addition, high temperatures can cause the battery to degrade more quickly, reducing its lifespan.
Low temperature: On the other hand, low temperatures can also negatively affect the performance of your battery. When the temperature drops, the chemical reactions that take place within the battery slow down, reducing its capacity.
This means that the battery will not be able to store as much energy, which can be a problem during periods of low sunlight, such as during the winter months.
Tip: To make sure your battery works the best, it's really important to keep it at the right temperature. You can do this by using a special cooling system or putting the battery in a spot where it won't get too hot or too cold.
