You can use either another vehicle or a jump box to start the vehicle. If the battery has a issue then charging it won't do jack ****, you could check the battery voltage when the charger is disconnected.

 

It's a 2018, so an original battery would be nearing its 3 yr limit. Ive tried to find info on the stock Motorcraft BXT90T5-590 but can't find if it's maintenance free or needs water checked. If the latter, I have no idea how I'd DO that without taking the battery out.

 

Maintenance free batteries usually don't provide for any checking of the electrolyte inside other than voltage or load testing.

 

Was able to jump it successfully and get it running, so I drove it a bit without shutting it off. Put it on the charger when back in owner's garage and it DID start on its own once, but not the second time. UPDATE: jumped it again and took to a local garage for a new battery that seems to have solved the situation. I was afraid it was something more complicated than that. (Maybe it IS more complicated, but no symptoms so far.)

 

That’s my question too

 

The "Save Battery" thing is to prevent the battery from becoming totally dead.

Over time batteries will discharge and no longer be able to provide the power to start the engine.

There could be several reasons for a good battery to lose it's charge.

Without getting into the electronics of it all,MOST times battery failure is due to improper or no maintenance. Some chemical reaction does take place at the battery terminal connections and sometimes the corrosion can be seen if looked for and sometimes it is still there , but behind the connections themselves..

Routine maintenance will usually prevent problems like this.
There are other problems that appear even if the charging system is working properly, BUT the charge can't get to the battery due to dirty high resistance connections.

One does not need to be a mech/tech to look under the hood and just see if there is any corrosion beginning. This little check can prevent a lot of problems .
Sorry for the long winded conversation., but doesn't cost anything and can prevent some inconvenience.