My generator (8.5kw residential) exercises at the same time every week EXCEPT after we have a power failure (very short ones). After that it does not exercise at the regular time and I have to reset the exercise button. My serviceman insists this SHOULDN'T happen but is not finding out why it is happening. Any help would be appreciated. Thanks.

Problem with this, guessing that normally the power generated is somehow dumped when there is no real load applied.

Digital voltage regulator with ±1.5% no-load to full-load RMS regulation.

https://www.google.com/search?q=20+RESA%2C+testing+with+no+load

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With the brand name of a generator, this does not matter in most cases, but it may happen that the circuit breaker is defective or is only ejected, nevertheless, always use a multimeter for operating reliability, if that does not help the second thing Is a large capacitor intern where the plug blocks are located mostly at the rear of the generator.

The capacitor may be deformed or even short-circuited in rare cases.
fault finding repair tips for brushless capacitor compensated generator

I would suspect the control board for the generator is defective.

Is it possible that some other timer is kicking in on Friday? Something not related to the ATC-300?

Did it always do this? Is there a factory 'exercise' (non-load) built into the transfer assembly?

What does the fuse power?

Press and hold the exercise button at the day and time you want it to exercise. It will remember this time and will then auto exercise. The ex led blinks and for example 7 times, that means 7 days until ex, then 6 and so on. I hope this helps, emerson power have manuals, or I can send one let me know

If the unit is using a wet cell acid battery, it's possible that dirt mixed with hydrogen build up on top of the battery caused the second failure. The battery top should be kept clean and should be checked at least 4 times per year. Any stray ignition source can cause the hydrogen to explode, including lightning.

It's also possible that the battery float charger needs to be adjusted or possibly has a problem. Some float chargers are adjustable. The higher the charging rate the more off-gassing the battery will experience and increase the chance of a failure.

If the battery has exploded, the entire generator and housing should be cleaned (washed) thoroughly to remove any remaining battery electrolyte (acid).

Be careful - if you aren't comfortable working on this type of equipment, it's best to call a local dealer and find a service vendor.

http://www.homegeneratorsystems.com/service_support/manuals/index.cfm?rd=1

This link will take you to their site where you can download their manual.

Setting Exercise Timer
The Home Standby Generator is equipped with an exercise
timer that will start and exercise the system once every
seven days. During this exercise period, the unit runs for
approximately 20 minutes and then shuts down. Electrical
load transfer DOES NOT occur during the exercise cycle
(unless an utility power outage occurs).
A switch on the control panel is labeled “Set Exercise”
(depicted on page 10).The specific day and the specific time
of day this switch is pressed is programmed into the
control board memory.This date and time is then used to
automatically initiate the system exercise cycle.
To perform the Set Exercise procedure:
1. Choose the day and time you want your Home
Standby Generator to exercise.
2. On that day and time, set the AUTO/OFF/MANUAL
switch to OFF.
3. Press and hold down the “Set Exercise” switch for two
seconds.
4. Set the AUTO/OFF/MANUAL switch to AUTO.“Set
Exercise” is complete.
For example, if you press the “Set Exercise” switch on
Sunday morning at 10:00 AM, the unit will run an exercise
cycle the following Sunday at 10:00 AM (+/- 1/2 hour).
NOTE: “Set Exercise” will only work if the unit is in the
Automatic mode and this exact procedure is followed.The
exerciser does not need to be re-set if the 15 Amp fuse is
removed or changed.The exerciser will need to be re-set if
the 12 Volt DC battery is disconnected.
If you want to change the day and time the unit exercises,
simply perform the “Set Exercise” procedure at the exact
weekday and time you want it to take place.
• To prevent possible injury that might be caused by such sudden
starts, always set the AUTO/OFF/MANUAL switch to OFF.
• Remove the 15 Amp fuse before working on or around the
generator or transfer switch.

On your Automatic Transfer Switch ( the little gray box you see on the right in the picture above) you have two blue buttons. One is a Manual test switch. The other has a clock on it and this is your exercise set button. Hold it down for a FULL 3 seconds if you want it to excercise without a load (not powering your home) Hold it down for a FULL 6 seconds if you want it to exercise with a load every week. Your generator will start and run for 20 minutes, and then shut down. Now every week your generator will exercise at the time and day that you pushed the set button.