Old APC Smart-UPS 1250 not charging

[Bart_I]

Model: Smart-UPS 1250  Serial # S93121584xxx 

Batteries have recently been replaced.  Unit will run on AC, and will run on battery power when batteries are fully charged.  However batteries will drain down over several days when running on AC power. Batteries will fully charge (27.7V) with external charger applied, but will decay to <20V when UPS is running on AC power (measurements taken with DVM on battery with AC connected and unit running on AC) and Battery LEDs will reduce from all bars to 1 bar (after several days of running on AC power).

I disconnected the battery with AC plugged in and running, and the voltage on the battery leads was ~17.7V.

It seems that the battery charger function is not working properly.  This unit is way past warranty, and sending it to the factory for service is probably not possible, or would cost way more than replacing the unit. 

I would like to repair it myself if possible.  I am an Electronic Engineering Technologist by trade and training, so I understand the risks and hazards associated with these repairs, and know how to perform repairs safely.  I acquired this unit second hand, so I don't have any manuals or other documentation.  The Schneider support website lists this model as discontinued, with no documentation available for download..

Is there anywhere that I could find a Service Manual or Schematic for this unit?  Is there a known procedure that needs to be performed when replacing batteries that might be preventing this unit from charging a known good battery? 

Any assistance is greatly appreciated.

[Brother]

Hi Bart,

I realize it's been a while and since no one replied i will 😉

When you search online for "smart-ups 1250 service manual" there are a few results. I got mine from https://elektrotanya.com/apc_smart-ups_900_1250_2000_os-0702_a_e701a_rev10_sch.pdf/download.html this will give you a base to work with. As u already know, there must be an issue in the charging circuit area. I usually start with eyeing for burn marks, bulby elco's, blackened resistors, thyristors, diodes etc. It can be in the bias charge circuit too. Perhaps just a diode in the rectifier. Measure everything with your dmm that you can and don't forget wiring cables and flaky connections. IC's you have to look for the expected signals on the pinouts in the datasheet of the resp. chip. Did you test the big fuse in between the batteries yet? Usually the default charging voltage for the batteries is too high which will wear them out in 2-3 years. If you search well you'll probably find the procedure how to decrease this voltage a bit via the serial port once you've got the charging circuit working again. This will make your batt's last longer at the cost of slightly lower power which is peanuts imho.

3Cheers!

Brother

[Bart_I]

Thanks Brother!  Much appreciated.