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Battery Capacity Measure Unit
Last Updated: 06/07/2016

Battery Capacity Measure Unit

You may confused by many different terms with battery capacity, such as Wh, mAh, Ah...Here are some tips may help you understand:

1. Wh means Watt-hour.

100 Watt-hour battery can deliver 100 watt power for 1 hour, 20 watt power for 5 hours.

This is a unit BiXPower likes to use since it is much more accurate and can compare between different batteries. A 200 Wh battery has more capacity than 100Wh battery for sure.

2. mAh or Ah, means Milliamps Hour (mAh) or Amps Hour (Ah).

This is a very misleading unit. When a battery is rated with mAh or Ah, it should always associated with a voltage. A 10Ah @ 12V battery capacity is bigger than a 10Ah @ 5V battery.

However, there are a lot lot of lithium batteries on market that do not state voltage when claim their mAh ( or Ah) capacity. In these cases, most likely they measure with lithium battery cell voltage, which is 3.6V or 3.7V. By time 3.6 (or 3.7), then divided by 1000, it will be its accurate Watt-hour capacity. For example, a 10,000mAh battery actual capacity is 10000 x 3.6 /1000 = 36 Watt-hour.

You can compare two battery capacities by Amp-hour only if they have same voltages. If two batteries have different voltages, using Watt-hour will be more accurate.

1 Amp-hour battery can deliver 1 Amp current power for 1 hour.
1 Watt-hour = 1 Amp-hour x 1 Volt.

We recommend to compare battery capacities by Watt-hour, not Amp-hour. Watt-hour is a more accurate unit to show the power capacity than Amp-hour when compare batteries with different output voltages.

3. Battery capacity and maximum output power are two different concepts. A 300 Watt-hour battery may only output maximum 100 Watt power.

Think of a car gas tank. Capacity is how many gallons gas the tank can hold. Output power the speed the car can drive. A battery with big Wh ( or mAh) capacity is just like a car with big tank. A car with big gas tank may drive very long distance, but it does not mean the car can drive at very high speed, A battery with large Amps-hour capacity may last long, but it does not mean the battery can output large Amps current.

4. Rated capacity and actual output capacity:

A battery Watt-hour capacity usually is calculated based on battery cells nominal capacity. For example, if a battery cell capacity nominal capacity is 3.7V x 2350mah=8.7 Wh and a battery pack is consist of 18 of cells, then the battery capacity is rated as 8.7 x 18= 156.6Wh.

A battery cell capacity is rated under standardized test conditions and makes it possible to compare different battery cells. However, the actual power output of a battery cell in real conditions is usually less than the nominal power output. Conditions are never as perfect as in a laboratory.

When use many batteries cells to make a battery pack, it may need to add many extra control circuits to provide safety protections and some charging/discharge function. Those extra circuits need to use some power from battery cells also. This will make the battery pack actual output power less than the battery cells power output added together.

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