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As a general rule, a 100Ah deep-cycle lead-acid battery discharged to 50% will run a 50 watt load for 12 hours, a 200 watt load for 3 hours and a 400 watt load for 1.5 hours. For AC loads the times will be reduced to 70% to 95%, depending on inverter rating/load ratio.
- How do you calculate how long a battery will last?
- How long does a 100ah battery last running a DC load?
- Table – Battery depth of discharge chart
- How long will a 100ah leisure battery last compared to a car battery?
- Is it ok to use a deep cycle battery in my car?
- How long will a 12v 100ah battery run a 1000 watt inverter?
- How many hours can a 100ah battery last running A Fridge?
- Battery Run Time Calculator – How long can a 100ah battery last running a TV?
- Table – How long will a 100ah deep cycle battery last running typical household appliances?
- How long will a battery last calculator
How do you calculate how long a battery will last?
This can be a tricky question, as it’s not always as simple as just diving amp-hours by the load current to find out how long the battery will last.
This article will go through a few different equations in order to compute the total watt-hours of energy that can be extracted from your battery over time.
The main point is, there is no simple answer to this question. It depends on many factors like the battery capacity, your device’s wattage, how many watts you’re running per hour and the temperature of your environment.
The first step in calculating the life of your battery is to find out how many amp hours (Ah) capacity it has left to deliver. Of course, it’s stated by the manufacturer, assuming full charge.
Video – What are battery amp-hours?
How long will 100ah battery last in cold weather?
All batteries generate power through a chemical reaction and all chemical reaction are affected by temperature – the colder it gets, the worse they perform.
In the case a of a car battery, it’s unfortunate that as the efficiency goes down as the temperature drops, the engine needs more power to crank it.
At 32°F (0°C) battery power may be down 50% and more than 65% at 0°F (-18°C). At these low temperatures you can also double the amount of cranking power needed to start your car.
How long do lithium batteries last in cold temperatures?
As you might expect, lithium batteries perform better than lead-acid, losing only 10% of their capacity at freezing point instead of 50% like lead-acid.
All kinds of lithium batteries shouldn’t be charged or stored at low-temperatures.
How long does a 100ah battery last running a DC load?
This is the simplest situation to calculate as the battery delivers DC power to a DC load, so conversion isn’t required. We can simply use the current taken by the load and the Ah capacity to work out the hours run time:
- Battery run-time = Ah capacity/load in amps
- Example: Run time = 100Ah battery capacity / 10 amps = 10 hours
Simple, right? Well, actually it isn’t as simple as that.
The above assumes full battery capacity, but the battery may be partly discharged.
How do you know?
By checking the terminal voltage, which is proportional to the state of battery discharge.
Table – Battery depth of discharge chart
Percentage State Of Charge (12 V Lead-Acid Battery)
Battery terminal volts
Battery efficiency is also determined by other factors, such as temperature and discharge rate. Don’t think that a 100Ah battery can deliver 50 amps for 2 hours – you will be sorely disappointed!
The battery label normally specifies a maximum discharge rate. If this is exceeded, then the capacity reduces accordingly.
How much can you discharge a battery?
This is a vital question because it directly determines how much power you have available for use.
An ordinary car battery shouldn’t be discharge regularly more than 10 to 15%. If more, it’s life will be seriously shortened.
Car batteries are designed to deliver a lot of current quickly for cranking the engine, and then recharged quickly by the car’s alternator.
This means effectively that for running any constant load, you only have up to 15% of the battery capacity available for use.
Lead-acid car battery available capacity for constant load = 100Ah x 15% = 15Ah
How long will a 100ah leisure battery last compared to a car battery?
A lead-acid deep-cycle battery, sometimes called ‘leisure‘ batteries, can be discharge up to 80%, but 50% is recommended for regular use if you want to maximize it’s working life.
Lead-acid deep-cycle available capacity = 100Ah x 50% = 50Ah
How long will a 100ah lithium battery last compared to lead-acid?
A lithium-iron phosphate battery is inherently deep-cycle and can be discharge to 95% at normal temperatures with no damage.
LiFePo4 available capacity = 100Ah x 95% = 95Ah
For the rest of this post I’m going to assume the use of a lead-acid deep-cycle 100Ah battery as this is still the dominant battery technology in use.
If you wanted to know battery run-time hours for lithium iron phosphate batteries, simply multiply any of the results by 1.189 (this is 95%/80% – the difference between deep-cycle lead-acid capacity and LiFePo4 capacity.)
Is it ok to use a deep cycle battery in my car?
A deep-cycle battery isn’t able to deliver the hundreds of amps needed to crank a cold car engine. Not only are many amps required but they are delivered in a short time and deep-cycle batteries are not designed to do it.
A deep-cycle battery will be damaged in a very short time, even if it manages to crank the engine.
The only possible exception is to use a marine battery. A marine battery is designed to crank a boat engine and also can be deeply-discharged like a leisure (deep-cycle) battery.
In theory a marine battery would work in a car, if that’s all you have, but it wouldn’t make much sense as they are much more expensive than a standard car or truck battery.
How long will a 12v 100ah battery run a 1000 watt inverter?
For many off-gridders and home-owners installing emergency backup power, it’s important to know how long AC appliances can run on battery power.
As batteries are DC, and inverter is needed to convert the voltage to AC. This brings in more complications that have the effect of reducing battery run-time.
Modern inverters can be very efficient, up to 97% is often quoted. However, maximum inverter efficient occurs when the device is fully loaded.
When several household appliances are used, the load on a 1000 watt inverter will go up and down. This means the efficiency will vary as well. Lightly loaded it may be only 40 to 50%, but what does that mean?
50% efficiency means that for every 100 watts load you need 200 watts input power. This is a huge discrepancy, particularly if using solar where every solar watt is hard to come by.
If we assume a full-load inverter efficiency of 95% i.e. a load of 1000 watts then we can calculate the run time as follows:
Convert Ah to watts:
- 12 volt deep-cycle lead-acid available capacity = 100Ah x 50% = 50Ah
- 50Ah in watt-hours = 50Ah x 12 volts = 600 watt-hours
100Ah battery run time = available capacity / load / inverter efficiency, so
A 100Ah battery will run a fully-loaded 100 watt inverter for = 600 / 1000/ 0.95 = 0.63 hours
How many hours can a 100ah battery last running A Fridge?
I previously assumed constant loads for DC and AC, but often it isn’t like that. The worst case is an AC appliance such as a fridge.
Fridges have compressors on board which cycle on and off in normal operation. Sometimes the motor runs and sometimes not.
To make matters worse, the compressor motor pulls up to 3 times more current when starting than when running, so how do you know what power a fridge is drawing at any point in time?
You don’t. It needs to be estimated over time. Luckily the manufacturer provides exactly the information we need on the appliance label.
Annual energy consumption can range anywhere from 200kWh per year for a mini-fridge to 1000kWh for bigger fridge-freezers.
The key point is the annual energy consumption can be used to work out battery run-time as follows:
- Battery run time = battery capacity available in watt-hours / hourly consumption.
- Hourly consumption = annual kWh/365 days/24 hours
Assuming an average fridge annual consumption of 500kWh:
Battery run time = 600 watt-hours / (500 000/365 /24) = 10.34 hours
Battery Run Time Calculator – How long can a 100ah battery last running a TV?
Even though a TV draws on AC power, it does represent a mostly constant load, so it’s easier to calculate run time.
The average power draw of an LED 55″ screen TV is about 100 watt.
100Ah battery run-time = 600 watt-hours/100 = 6 hours
Table – How long will a 100ah deep cycle battery last running typical household appliances?
100Ah deep-cycle lead-acid battery with AC loads with inverter - 50% recommended discharge
Run time (hours)
Portable Air Conditioner
70 (average per hour)
Water Filter and Cooler
How long will a battery last calculator
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