Battery Ah question

[henry d]

Just wanted to check to see if I understand correctly. I have an battery operated chainsaw and it is great but didn't last as long as I hoped when we cut down a neighbours tree. Batteries (2) are 18v 3Ah but I can get another set of 4Ah. These should last 1/3 longer than the 3Ah batteries. Is it generally that simple yes? 

[Dave-G]

1/4 longer as a rough guess. Think of them as larger pots of the same voltage. And they will take longer to charge/refill.

Edited January 26, 2023 by Dave-G

[billytheghillie]

As above post, i got a Dewalt drill with 2  18v  5.0ah batteries they last a very long time, and quick to charge.

[sportsbob]

1 hour ago, henry d said:

Just wanted to check to see if I understand correctly. I have an battery operated chainsaw and it is great but didn't last as long as I hoped when we cut down a neighbours tree. Batteries (2) are 18v 3Ah but I can get another set of 4Ah. These should last 1/3 longer than the 3Ah batteries. Is it generally that simple yes? 

Yes you are correct.

Here is some science in simple terms, with the first example you can draw 3 amps out of it at the 10 hour rate so 300ma for 10 hours = 3A, the second example has 33% more capacity so all the figures increase by 33% assuming 100% efficiency. Now things start to change when you discharge faster than 1/10 C ( C= capacity ), assuming you discharge at the 1hr rate  the 3Ah battery will probably be 10 to 15 percent down on its stated capacity and so will the 4Ah ( the observed rule seems to be it will be at about 87% efficiency ). One thing that may also change is the batteries capability to discharge at higher currents this will cause a larger voltage drop across the terminals which will then lead to more current being drawn and the battery will as a result of this discharge even faster than the mathematical equations due to the variables highlighted.

[Gordon R]

I have no idea if this is scientifically correct, but friends and my sons' friends insist that a cordless 18v impact wrench exhibits more power with a higher amp battery. They are adamant that a 2Ah battery will produce less power than a 5Ah, despite both being 18v. 

[henry d]

Many thanks everyone 

[JohnfromUK]

50 minutes ago, Gordon R said:

I have no idea if this is scientifically correct, but friends and my sons' friends insist that a cordless 18v impact wrench exhibits more power with a higher amp battery. They are adamant that a 2Ah battery will produce less power than a 5Ah, despite both being 18v. 

Not so in theory - it should produce the same power - but for only 2/5 of the time.  However, in practice, the smaller battery may well have a bit of a limitation on peak current draw - that may in fact make it less powerful.  Therefore it may well be true, but the key is that the battery is higher Amp hour which is not the same as higher peak current (Amp) rating.  IF the peak current rating is the same -the peak power should also be the same.

Edited January 26, 2023 by JohnfromUK

[sportsbob]

1 hour ago, Gordon R said:

I have no idea if this is scientifically correct, but friends and my sons' friends insist that a cordless 18v impact wrench exhibits more power with a higher amp battery. They are adamant that a 2Ah battery will produce less power than a 5Ah, despite both being 18v. 

They could be correct because the motor will draw an amount of Watts, Watts are volts time amps and as the higher capacity battery can typically deliver the same amount of amps with a lower drop in terminal voltage you will get more overall power to the motor.

[12gauge82]

10 hours ago, Gordon R said:

I have no idea if this is scientifically correct, but friends and my sons' friends insist that a cordless 18v impact wrench exhibits more power with a higher amp battery. They are adamant that a 2Ah battery will produce less power than a 5Ah, despite both being 18v. 

They are correct, I have noticed this many times myself. 

[bigroomboy]

14 hours ago, Gordon R said:

I have no idea if this is scientifically correct, but friends and my sons' friends insist that a cordless 18v impact wrench exhibits more power with a higher amp battery. They are adamant that a 2Ah battery will produce less power than a 5Ah, despite both being 18v. 

As others have said in theory that isn't true, but in reality is comes down to how the battery was designed to have a higher capacity. It could be the type of cells chosen can output more power or the simple way to think of it is if an extra set of cells were out in parallel to achieve the extra capacity then they can provide additional current or provide the same current with less voltage drop.

[udderlyoffroad]

To be honest, size of battery for a given power tool mostly comes down to balance.  I find a drill can be heavy and awkward with a honking great 5 Ah battery, especially if you're holding it over your head driving in plasterboard screws.   For the OP, I'd suggest that not only will the run time be better with his chainsaw, but he'll barely notice any weight penalty.

However...if it happens to be a Lidl 'Parkside' brand chainsaw, be careful.  For the money it costs, it's excellent value.  But I have noticed the 4 Ah battery got rather toasty after using it hard this summer.  It then needed to cool down quite a bit before it would accept a charge.

[Windswept]

Speaking of heat, with my battery chainsaw I'm sure it performs better if the battery is at room temp rather than outdoors if it's freezing.

This may be obvious but ensuring the chain is very sharp and the depth guides are not too low are also get important with a battery saw.

[henry d]

8 hours ago, udderlyoffroad said:

To be honest, size of battery for a given power tool mostly comes down to balance.  I find a drill can be heavy and awkward with a honking great 5 Ah battery, especially if you're holding it over your head driving in plasterboard screws.   For the OP, I'd suggest that not only will the run time be better with his chainsaw, but he'll barely notice any weight penalty.

However...if it happens to be a Lidl 'Parkside' brand chainsaw, be careful.  For the money it costs, it's excellent value.  But I have noticed the 4 Ah battery got rather toasty after using it hard this summer.  It then needed to cool down quite a bit before it would accept a charge.

I went for Einhell (sp) from tool station as it was on offer with batteries and charger and I had a voucher for money off.

I've got the 4Ah and they are same size and weight (by feel) so I am covered now if I have to cut for more than an hour. 

[Jonty]

On 26/01/2023 at 14:38, Dave-G said:

1/4 longer as a rough guess. Think of them as larger pots of the same voltage. And they will take longer to charge/refill.

For my simple brain, that’s a great analogy/explanation Dave - thank you 👍