How Long Will 70Ah Battery Last? (In Real World Conditions)

Curious to know how long your 70ah battery will run different appliances? It is nearly impossible to give you the exact estimated runtime of your battery because many factors play a role in it.

Most of the information out there will give you ‘theoretical‘ numbers.

But I’ll share the most important factors which affect battery performance. And 5 easy steps to calculate your 70ah battery runtime on any load.

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 type70ah battery usable capacity (in ah)
Lead acid59ah
Lithium (LiFePO4)66ah
Table 1: 70ah battery usable capacity after its discharge efficiency rate

Must watch this video to understand the basics of batteries (capacity, charge, and discharge mechanism)

Batteries 101: Understand the battery basics – Ah, mAh, Wh, Volts, c-rating.

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

Follow these steps to calculate your 70ah battery backup time on load.

Steps

1. Converter battery Ah into watt-hours (Wh). Let’s say you have a 12v 70ah battery.

70ah battery in watts = 70 × 12 = 840 watt-hours 

2. To take into account the battery depth of discharge limit, multiply your battery capacity in watt-hours by 0.5 if you have a lead-acid battery. For example, if you have a 70ah lead-acid battery.

Battery capacity in wh after DoD limit = 840 × 50% = 420wh

3. 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 = 357wh

4. Multiply the battery usable capacity with the inverter efficiency rate to take into account for inverter efficiency rate. Most of the inverters are about 90% efficient.

Battery AC watts = 357 × 90% = 321 AC watt-hours 

5. Divide the battery AC watt-hours by the total load connected. Let’s say you’re running a 20-watt LED light with a 70ah battery.

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 capacityHours of discharge
70ah20 hours
63ah10 hours
61ah8 hours
58ah6 hours
56ah5 hours
49ah3 hours
42ah2 hours
35ah1 hour
Table 2: 70ah battery usable capacity with respect to discharge time (hrs)

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.

How long will a 70ah battery last? – chart

Here’s a chart on how long a 12v 70ah battery will last while running different home appliances.

ApplianceWattage consumptionHow long will a 70ah lead-acid battery last
Fridge300 watts1 hours
TV60 watts5 hours
Trolling motor720 watts20 minutes
Ceiling Fan120 watts2.5 hours
Computer300 watts1 hours
LED Bulb20 watts16 hours
Dishwasher1800 watts10 minutes
Toasters1200 watts16 minutes
Table 3: how long will 70ah battery last?

Summary

  • 70ah battery will last anywhere between 15 hours to 15 minutes running different appliances.
  • 70ah lead acid battery will run a TV for about 6 hours.

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Chris Tsitouris is a renewable energy professional with 10+ years of experience as Director of Engineering at Solar Spectrum, previously working as Project Manager at SunPower and Energy Analyst at the National Renewable Energy Laboratory. As a thought leader, Chris has authored numerous articles and research papers.

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