Wife started the 2018 impala with remote start. It timed out before she got in the car. Hit the start button and got the dead battery clicking.
I measured battery voltage, doen to 5volts. Removed neg cable and battery voltage recovered slowly to 11.
Has anyone seen or heard of this condition?
Will remove various voltage feeds and reneasure battery voltage with ground reconnected.
Any other thoughts on how to troubleshoot this? Or has this happened to anyone else.

Sounds about normal for a battery going bad.

If you can jump-start it, see if it is charging before replacing the battery. Could be a bad alternator.

If the alternator is charging, then likely the battery is bad. From a very high level, not knowing anything about how the car is used nor where it lives, if the battery is four years old or more,

Thank you for your inputs.
I've thought about a failed battery. Odd that it would remote start and then die -- but it is possible.
The car sits outside with 4 inches of snow on it. I will get back to it as soon as I can but we do have two other cars o it is not an emergency.
I have measured the battery voltage with the starter circuit removed and it had recovered (11+ v), went back down when I reattached the lead (5 v)..
I have heard that the car has another battery in it, news to me. But I will check if there is another one which could be causing the issue. Never heard of two batteries in an impala!
I have considered the starter, alternator, or a module.
I will eventually remove the snow and access the engine compartment again. I can get a charger on the battery where it sits. Might do that first to make sure it is up.

I was hoping the fact that the failure occurred following the time out of a remove start cycle might be indicative of a known issue. Guess this is only a random circumstance.
Will update.

Sounds like you have a good plan.

It could also be a bad alternator, and the bad alternator could be contributing to the battery going bad. A start and idle on a weak battery with no charging from a bad alternator could have just been the last straw on a long-deteriorating situation. Good reason to get it started and put a meter on it.

At the same time, you have said nothing about a charge light being on during previous drives, so that'* why I pretty easily lean towards it being a bad battery.

Warm enough to work outside.
Disconnected battery leads and charged for half hour, voltage at 12.5. reconnected leads.
Remote start unresponsive, car started from button.
Alt output 14.5.
Turned car off, let set to see if battery lost voltage, still 12.5.
Will use car locally tomorrow.
Alt OK, battery unknown but likely going bad.
Why remote start won't work still looking at.

It might have a reason in its little mind based on how often and how well it has started normally in its recent past. I'd say drive it around a bunch of times and see if it starts working again. Let'* say a week or so.

I let the car soak in the cold for a few days. Battery charged.
Went out and it started with push button. Would not start with remote.
Put code reader on it. The dashboard had a 'check theft monitoring system' warning on it (icon lighted)
Had some other faults listed which I attributed being the result of having battery disconnected for an extended period of time -- day or so.
Code came up as B101E-48; ecu software not programmed. Wasn't able to read any PIN code.
theft system icon eventually went out. Still, no remote start.

Car starts and runs ok. No idea why it gave me the original problem. Battery voltage was down to 9 v at one time. Alternator puts out 14.5 V.
I hesitate to mess with the theft system, I have a decent scanner but I don't have the training to fully utilize it.
I did record a four digit alarm code from it a while back but don't believe that is relevant here.

Will the BCM eventually program itself? Is this a dealer only problem?
Fobs for this car have always been an issue, using an aftermarket fob for the past few years, original one died. Programming was nothing short of a major problem.

It shouldn't lose all the normal stuff from having the battery disconnected. Likely the biggest issue would be the car alarm going off when reconnecting.

This car was used daily by my wife to go to her office, etc.
The remote start problem went away, it now works.
The car now acts like it did krikr to its recent episode that started this post.
I am scratching my head, probably never know what kind of glitch occurred or where.
Thank you all for your input.

OK, the odd word should be 'before'.
No idea how it was changed