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From: "Jim Wilkins" <muratlanne@gmail.com>
Newsgroups: rec.crafts.metalworking
Subject: Re: Got 4Ah, not 5Ah, battery 18V (20V) - done right thing?
Date: Thu, 16 May 2024 13:10:23 -0400
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"Richard Smith"  wrote in message news:m1le49hl6y.fsf@void.com...

Hi there

Ran SDS out of battery in one session at the mine - first time.

Is 18V - believe it's badged in the USA as 20V, but the physics of the
Li-ion cells surely says in reality it's the same batteries and
voltage (?).

Went by tool store - no 5Ah batteries I asked for.
Said well it's 12 UKPounds per Amp-hour, and we do have 4Ah
batteries.

Thinking - well take two identical 4Ah batteries, swap occasionally,
running-down charge in both?

4Ah battery is better in every other tool which takes that form of
battery - screwdrivers, twist-drill drills, etc.

But the draw of the "big" SDS drill - the biggest which takes the
"small" batteries - that power draw would go across more cells if a
5Ah battery.  Would have been better?

What do you say?

Regards,
Rich Smith


PS - SDS drill drilling the sockets in granite to insert the
feathers&wedge to split granite boulders - simply a regular workhorse
now.
Also use for "misc" other tasks - drill holes into which hammer in wood
for screws holding light-fittings, etc, etc.

------------------------------

My understanding is that Lithiums can be formulated for various tradeoffs 
between high discharge current and high Ah rating. Unless you find a spec 
sheet for the cells with the discharge current rating and know how much the 
drill draws you have to trust the manufacturer's competence and honesty.

In this spec sheet example the maximum discharge current is 6.5A, about 
twice the Ah rating.
https://shop.baltrade.eu/339,Battery_18650_Li_ion_3400_mAh_Panasonic_NCR_18650B_Lithium_ion_cell
Notice that they give a typical and a minimum mAh capacity. Like steel, 
batteries vary.

The C rating is the current the battery would provide for 1 hour before 
falling to some minimum safe or useful voltage, though it's usually measured 
at a lower current and longer time to get a more favorable higher number.

The Ah you can use in practice may be less than the rating because the 
battery's internal resistance rises as it discharges and will lower the 
output voltage under heavy load due to internal IR drop, so the battery 
terminal voltage hits the minimum before it's fully discharged. The 
indication is a higher battery voltage after removing the load, or with a 
lighter load that causes less internal voltage drop, if you have one.

My job was once building the production line test stations that determine if 
electrical products including op amps and computer memory meet their spec 
sheet rating. Much of what I bought from Radio Shack and on Amazon didn't 
quite pass my testing but was still usable, with care.