Just downloaded my forscan app and plugged it in to my 2007 Ranger.

came up with error b1318, battery voltage low. I suspect the battery is on its way out; did multimeter alternator test, that’s ok.

can forscan check battery health (ie are any cells gone), or is it a garage job? 

Hi,

Software can't test battery effectively.  It does need a proper manual load test ideally.

Batteries have multiple failure modes - a bit like people, no good testing blood pressure if you are diabetic?
Capacity is the main one and difficult to measure without an actual discharge test or sophisticated garage equipment (Ford/Halfords etc often use Midtronics as their definitive test so probably the "best")
Other testers are available and cheaper ones can give a guide to condition if monitored regularly.
Internal resistance is one of the others factors (but there are lots more ways to die)
The classic high discharge tester is a guide to starting ability but doesn't show capacity i.e. it may be able to do it once but doesn't store enough energy to start repeatedly.
All in all it can be a complex affair and modern cars put high stress on batteries which are usually guaranteed against (rare?) "manufacturing faults" NOT the ability to start your car for 5 years or whatever:
https://www.batteryuniversity.com/articles/

Just my experience and we aren't lucky with batteries as, a bit like humans, you're always on borrowed time!😉