Which size battery to use on the tool

 Battery powered tools offer, obviously,  a variety of batteries ranging in power availability and output. Both of which relate to size as the output goes up.  These are measured, almost universally, in Amp Hours.

Amp hours doesn't actually or necessarily relate to how long the battery will last.  I mean, it does, but how the tool gets used will cause that to change.  Generally speaking, a 2AH battery will put out 2 Amps over an hour.  That's all good and we'll if the tool is consistently and constantly putting out specifically that amount of power.

Tools however, especially those that offer variable speeds and torque or power settings, will change how many amps are actually being output at any given time depending on use.  You can have a 5AH battery only last 15 minutes because the tool is actually putting out 3 or 4 times the power output.

So, be aware that the battery isn't necessarily a "bad" battery if it only lasts a short time.  You have to take into consideration the type of tool and the output it requires to run it given a particular task.  That 5AH battery will power a flashlight a LOT longer than 5 hours as compared to running a circular saw for about 15 minutes at max output.

Also, keep in mind that some tools are more meant for sporadic and intermittent use than persistent and consistent output.

For example,  a circular saw might get run on a jobsite for 4 hours persistently. An oscillating multi-tool might only be used intermittently on a job site for a 10 minute task then not used again for 3 or 4 hours.

We will want to put say, at least a 4AH on that circular saw to have the power and longevity to get through a task or series of tasks before running out of power.  We can put a 2AH battery that that multi-tool and have it available for use all day long depending on typical use.

Keeping a 2AH battery on the multi-tool is a good idea.  It keeps the tool lighter and not causing unnecessary muscle fatigue or strain.  If you anticipate a big, time consuming project, you might put a 4AH battery for that project, then back to a 2H battery in general. 

Conversely,  you almost never want to put a 2AH battery on the circular saw.  One, the power draw usually requires more than the small battery can provide at any one use.  Whereas having at least a 4AH battery on the saw will give the saw all the power it needs for the task, for a much longer time.

In fact, many tools, like saws and drills/drivers with high output needs will have a noticeable difference in how fast and/or how powerful they run just in going from a 2AH battery to a 4AH or higher battery as the bigger battery has more amps to draw on.

The power used in a tool is actually measured in Watts output.   So V×A=W (18V×5A=V), so... 18×5=90.  This basically tells us that an 18V, 5AH battery can provide up to 90 Watts of power at any one time.    A 2AH 18V battery can only provide up to 36 Watts at any one time.  If the power draw from the tool only requires say, 20 Watts of power sustained for a short time, both batteries can get it done.

However, if the task requires 50 Watts of power draw, the 2AH battery will come up insufficient and underpowered compared to the 5AH battery will will go full power easily. 

Of course, it's not like any of us are going to mathematically figure out how many Watts are needed to drill a hole through steel vs how many are needed to drill the same size hole through Pine.  It's not hard to figure though that the more effort required to drill or cut through harder, thicker materials is going to need more power. 

It probably won't take the drill much power to go into Pine wall studs  But you may want to change to a 5AH battery to start drilling into concrete.

So, to sum it all up, go ahead and keep a 1.5 or 2AH battery on that flashlight or multi-tool  to keep it light and easy to use.  But don't skimp, put at least a 4AH battery or bigger on that saw or drill to make sure you can get the best results as you go.

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