Power Supply Issues with RFID Access Control Backup System

[charlescowless]

Hello everyone,

I'm new to electronics and need help troubleshooting an issue with a backup power system for an RFID access control reader in my residential complex.

System Components:
- RFID reader (12V, 6W consumption)
- 12V/8A power supply unit (PSU) with 13.8V output
- 12V/7AH VRLA battery as backup
- MIC 6A8 diode installed on PSU +12V line

Setup:
The PSU, battery, and access control system are connected in parallel. The PSU provides 13.8V to both the battery and the access system.

The Issue:
When there's a power outage, the system works as intended - the battery takes over and powers the RFID reader. However, when power is restored, the PSU fails to take control and charge the battery. Instead:

1. The PSU output voltage fluctuates between 1V-10V
2. The battery continues powering the system and discharging
3. The system only works properly if I:
- Disconnect the battery (which turns off the RFID reader)
- Wait for the PSU to stabilize (returns to 13.8V)
- Reconnect the battery

After this manual intervention, everything works normally - the PSU maintains 13.8V, powers the system, and charges the battery.

I've already tried installing a diode to prevent reverse current, but the issue persists.

Any suggestions on how to fix this so the system recovers automatically after power is restored?

Thanks in advance for your help!

 

[BradtheRad]

I guess you wish your power supply should act like commercial standby power supplies which go back to normal operation a minute after the grid comes back on.

So during a blackout your battery drains. Regardless how much its voltage declines, it is its own voltage regulator.

Grid comes back on. Connect the power supply and instantly its output voltage drops to battery voltage. The battery is yanking most of the power from the power supply! At the same time it has yanked the system down to a few volts.

To allow your power supply to maintain 13.8 V, you must limit current going into the battery. Through, say, 100 Ω series resistor (rated 2W). Sort of a trickle charge. Experimentation will get you eventual success.