Ah, or amp-hour or ampere-hour is an abbreviation that shows a battery’s charge capacity or the current that can be delivered at a regular rate as the battery gets depleted in one hour. For instance, if you have a cordless tool that uses a 1.0 Ah current, this means in the right conditions, it will drain the 1.0 Ah battery pack in one hour.
What this also means is that a 2.0 Ah battery will power the tool for long as compared to the 1.0 Ah tool with all factors constant and the current flowing at the same rate. However, you will need to note that a 4.0 Ah battery pack will not always last twice as long as the 2.0 battery pack. At times, it will even exceed the double runtime. However, all this depends on the design of the battery pack.
An overview of Series batteries vs. parallel batteries
When you connect two batteries in parallel, these are usually coupled conductively from one end to the other. For instance, when you connect two AA batteries with 1.5 V each and attach them in a way that the positive terminal is connected to the negative one, the voltage of the two batteries will show as 3.0 V.
However, when two batteries are connected side by side, they are said to be connected in parallel. If a piece of copper is crossed across the 2 AA batteries with 1.5 V each and another copper piece on both terminals, they will still produce 1.5 Volts. The only difference is that they will last twice as long when they power devices.
The battery packs that are used in power tools have tremendously advanced such that two 2.0 Ah batteries will not necessarily produce 4.0 Ah. Even though the 4.0 Ah battery will have twice the battery cells of the 2.0 Ah, their controls and circuitry differ.
This is why you find that the Milwaukee Red Lithium higher capacity battery performs better than the compact batteries. On the other hand, the heavy duty Makita LXT tools need to be powered by a high capacity battery pack rather than compact ones.
This is more than just the runtime. Additional channels help the higher capacity batteries to produce more power. When current is overdrawn from Li-ion batteries, this can damage them. This is why you find that most of the modern batteries feature a safety function. When a tool tries to draw more current than necessary, the tool will be shut off by the control circuitry until it has cooled down or there is a reduction in the load. If you have double the cells that you intend to draw batteries from, you will have a great headroom.
For instance, a 2.0 battery pack is limited to a point that the maximum current flow isn’t more than 1.0 Ah so that the cells are protected. You can design your 4.0 Ah batteries in a way that each cell delivers a maximum current of 1.0 Ah. When you join the two groups in parallel, the battery can support the maximum current of 2.0 Ah.
However, there are engineering limitations…
in the real world and you will rarely find battery packs that deliver more power than the smaller packs with the same voltage. When we ignore the engineering limitations of the real world such as the buildup of heat, power tools can’t handle the power that is more by 100%.
Practically, when a compact battery pack delivers a peak continuous current of 1.0, there may be a need to design a higher capacity battery that can support a maximum current of about 1.2. Incase a tool fails to use the excess power, in the extended capacity, this will cause an increase in the total runtime. Since the total load is shared by battery cells, this could reduce the maximum load losses such as the one that occurs as a result of heating. Therefore, double capacity batteries have greater efficiency that enables them to have a more than double runtime.
Ah refers to the total charge time of the battery pack. A higher value can translate into extra power and more power. High capacity, as well as compact battery packs, have a current draw ceiling, and therefore, you might not enjoy double the power.