How/why is it that two batteries with the same voltage may preform at different levels of power based on their amp hours? For instance, a 2Ah battery powering a weed eater at full charge, and the weed eater not working as powerfully as it would with a fully charged 4Ah battery?

Forgive me, I’m new to hobbies involving power tools but I’m excited to learn!

In: 4

The amp-hours rating is a measure of capacity. 40 Ah would mean that the battery is able to deliver 40 amps of current for one hour (or 20 amps for 2 hours, 10 for 4, etc.)

Now *power* measures the rate of energy use and is equal to volts times amps. The smaller capacity battery will obviously not last as long, but also generally may not be able to deliver as much *instantaneous* current as the larger battery either. Say the 40 Ah battery might be able to deliver a peak current of 80 amps (if this were sustained it would drain the battery in 1/2 hours), whereas the 20 Ah battery might max out at 60 amps, say. If the tool needs more than 60 amps instantaneous power, the smaller battery will not be able to deliver it. Its voltage will (temporarily) drop and the tool will not operate at full power.

Edit- I just realized I multiplied your example batteries’ capacities by 10, but it doesn’t matter -the principle is the same

Ideally both batteries should perform the same, difference being that one lasts twice as long before being empty. Ah literally stands for amp hours so if you drew 4 amps from them consistently the 2Ah battery would last for 30 minutes and the 4Ah battery for an hour

The 4Ah battery would likely have more cells in parallel, which in theory would allow it to push out more amps because it would split the load between the cells, but the weed eater shouldn’t push the batteries all the way to their limit and both should output as many amps as the weed eater needs.

Also as the battery gets discharged it’s voltage slowly goes down but with lithium batteries it isn’t that noticable, the voltage of a single cell is about 4.2V when fully charged and at around 3.6V it would be considered “empty” and discharging it beyond that would cause damage so any tool would cut out at that point to protect the battery

So tldr if the smaller battery is noticably weaker at full charge I would be slightly concerned

They are not directly related.

Amp-hours is a measure of energy content of the batteries.

Power is a measure of how quickly you can use that energy.

To use a water analogy (NOTE: We will assume that the water is pressurized and maintains the same pressure in all examples)…

Amp-hours is related to the volume of your water tank. A larger tank would have a greater “amp-hour”.

Power is related to diameter of the hose you attach to the water tank. A larger hose will allow you to exert more “power”, but will deplete your tank quicker.

The 2Ah rating means the battery should be able to provide 2 amps of current for 1 hour.

An extra rating of a battery is the C rating.

The C rating multiplied by the Ah rating tells us the maximum current the battery can constantly provide.

Eg:

* 2Ah battery with C rating of 10 could provide 20amps

* 4Ah battery with the same C rating of 10 could provide 40amps

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