Got 4Ah, not 5Ah, battery 18V (20V) - done right thing?

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.