Battery Voltage Maintain During Diag.

What battery Maintainer is everyone using During Diag? The one i'm using now, is not working in some situations .
 It will maintain over 13.5 volts KOEO. When i start the car,(KOER) it see's 14 volts then turns too a Minimal charge. Going Back too KOEO it will not go back to 13.5 volts without restarting it.    I wont know it untill i see the scan tool voltage drop below 12 volts.
  Looking to get at least 2 for both Garages.  
  Thanks for everyone's time.

I've been using a Mac battery charger, which is really a rebranded Clore:

www.amazon.com/Clore-Automotive-PL2320-B...er-20/dp/B007ESQW08/

The power supply setting will vary current to try to keep the system voltage at 14.0V. Works well on most of the vehicles I see, as long as there aren't a bunch of electrical loads on.

I'm Using the MAC version of that now. We need something more reliable. Maybe an inverter.