How to make a smart charger behave like a dumb charger

[e100]

I have a 12v 40 Ah sealed lead acid battery bank which I use as a continuously connected UPS for my home network. Everything is permanently connected to the 12v. There is no 12v to mains converter involved. The continuous current draw is about 2 amps.

To keep the battery charged I bought a CTEK 12v 3.8amp charger as that seems to be popular with people who know about car stuff.
https://www.ctek.com/uk/products/car/mxs-3-8-uk
Instruction manual
https://www.ctek.com/storage/9E4D59B2526B4C3C54C61873C881CAACFC4C93F306DE1C8A2788E26EA6E506DA/f9b26d5eaeeb4787a51e5fc933ae1eae/pdf/media/687dd00827b44e44afee8695c5d0efab/MXS_3.8-manual-low-UK-EN.pdf
This is one of those fancy 7 stage chargers with a microcontroller. The 7 stages are shown on page 5.

It seems that this was designed to charge a disconnected battery, not one being continuously discharged via a load.
The net result is that it never raises the battery voltage above 13.8v. The indicator lights on the front say that it has reached stage 7, but I don't think it has.

The voltage waveform I see is pictured below. The time axis along the bottom has 5 minutes per division.



Every few minutes it goes into a charging phase then pauses at 13.8 to monitor the voltage. The 2 amp load causes the voltage to drop rapidly until it hits 12.5v and then it triggers a charge cycle again. So the average voltage of the battery is only about 13.1v. From what I have read I should be aiming for something between 13.8 and 14.4v.

So how can I trick this smart charger into charging to a higher voltage?

[Gyro]

13.6 - 13.8V is a typical float charge voltage for a 12V Lead Acid battery. Higher voltages are used for a cyclic charge, where the battery is experiencing heavy discharges (eg. automotive use), but just sitting at low load, you don't want to exceed the float charge voltage.

The charger appears to be monitoring the battery properly - 12.5V is about right for a 100% charged battery.

[NiHaoMike]

Get a separate 12V power supply module (something like 4-5A would be ideal) and a Schottky diode. The power supply should be set to about 14V (might be better to use a 15V module and turn it down) and the diode prevents reverse current flow back to the charger/battery. If you like, you could use a relay (powered by a separate wall wart) to bypass the diode to eliminate the losses, which also lets you use a regular diode since that would only operate for the very brief time it takes for the relay to switch.