How Long Will a 100ah Battery Last?
While it would be nice to think that a 100Ah battery powered a 1amp motor for 100 hours, or powered a 2 amp motor for 50 hours, it just isn’t that simple.
Part of the reason is that a battery only delivers as much power as is necessary at the moment: More work requires more power, while light work requires less power. The amount of power also depends on the efficiency of the engine, which determines how much energy is lost or brought to the work. Battery life also depends on impedance and heat, which varies based on the battery cell size. So there is no single simple formula that will help you calculate exactly how long a battery will last. However, we do have some tools to help us estimate.
Difference Between Amps and Amp Hours
Amps and amp hours are actually completely different things. Amps are a measure of electrical current. Amp hours are a measure of storage capacity. The number of amp hours on a battery is like the size of a gas tank on a car; obviously bigger is better, because then the car can store more fuel. But if you ask how long you can drive on a bigger gas tank, it quickly becomes clear that the question can’t be answered as is. How long you can drive depends on how full the gas tank is, how heavy the car is, whether you are going uphill, how fast you are going, whether you are towing anything … the size of the gas tank is only one factor, and to determine how many hours a car will go on a tank of gas, other information is needed.
How to Calculate Work Hours on a Battery
For most batteries, the Ah rating is calculated over 20 hours. So a battery is listed as 100Ah if it can deliver 5 amps per hour for 20 hours.
In order to determine how much power a device draws from a battery, you need to know not only the amp hours of the battery, but the voltage being drawn from it.
The basic formula is watts = volts x amps. In the case of a battery rated in amp hours, you could also rewrite it as watts/volts = amps, and then determine time.
For example, a 1.2 watt LED light operating on a 12 volt battery uses 0.1 amps (1.2/12 = 0.1). In this case, a 12 volt 100Ah battery would last for approximately 1000 hours.
However, we never get full power capacity from a battery, so many professionals simply subtract 30% from the total as an estimation tool, which would give us 700 hours of light in this example.
Why You Never Get all the Power Capacity From a Battery
Of course, just because a battery should provide a certain amount of power for a certain length of time, doesn’t mean it does. In the real world, other factors apply.
Batteries don’t discharge evenly
Batteries give power on a curve that decreases over time. As everyone with a battery-powered power tool knows, the battery stops delivering full power when it gets below 20-30% capacity. The voltage reduces, so the battery may continue to operate for the full amount of amp hours, but no longer deliver the same amount of watts. Most battery manufacturers prefer that you recharge a battery when it reaches 50% capacity.
Batteries are rated for 5 amps over 20 hours of time
Amp hours are all estimated based on 20 hours of time. If you use a battery for longer than 20 hours, even if you are within the total power capacity of the battery, there may be a significant drop in performance.
Chargers may not fully charge
Some chargers don’t charge batteries to 100% of capacity, either due to low quality or by design. Even if all the indicators are full, when measured, a “full” battery may only be at 80 or 90%.
Battery size matters – Comparison
When a battery is under light load, as with an LED light, it is not being called upon to deliver very much power. When a battery is under load, as with a power tool, it needs to deliver power, and that generates heat. As with many electronic devices, heat impairs function. Small battery cells dissipate heat more efficiently, while large battery cells heat up more slowly. Either way, when a battery gets overheated, it stops working regardless of capacity. So working a battery harder not only drains the battery more quickly, but may cause the battery to stop due to overheating, even when there is charge remaining.
Cycle Life of a Battery Count
When you are concerned about how long a battery will last, it’s important to consider not just how many hours it might run in continuous operation, but also the entire lifespan of a battery, called it’s “cycle life.”
Lithium-ion batteries are prized for their durability, and batteries from the major tool manufacturers are expected to last for 3 years or 1,000 recharge cycles.
Nickel-cadmium batteries are also expected to last for 3 years or 1,000 charges, but both batteries behave very differently during their life span.
Ni-Cad batteries need “deep discharging” every 2-3 months, in which the battery is completely emptied and then completely charged. Li-Ion batteries last longer if they are never deeply discharged, and never used below about 40% of their capacity.
When a Li-ion battery is discharged below about 2.5 volts per cell, the battery “dies” and cannot be recharged any more (some chargers have a “boost” function that can recover these batteries, but only if they have not been in a deep discharge state for very long). Ni-Cad batteries can be more easily refreshed from the deep discharge state.
In either case, long periods of storage, either on the charger or on a shelf, will reduce the life of the battery. For full, consistent power, batteries should be used and charged often.
In the world of batteries, 100 amp hours isn’t 100 hours at all. There are online tools to help you calculate battery life, which is a great way to get specific answers for your situation. This is a good site to help you calculate the battery life.