Dell PSU dps-600fb adjusting 13.8V output

[AndreyS]

I want to modify Dell PSU DPS-600FB so that I could use it to charge car battery. From that what I've read the voltage has to be 13.8V in order to charge normally it.
I saw the 12V output voltage on some Dell PSU can be adjusted via grid pins.
The voltage resistors are adjusted by manufacturer very precisely, I wouldn't want to change it, I want to use PSU in PowerVault 220s, and sometimes to charge the battery.
What I did so far: I checked pins on PowerVault's main board to figure out how to run PSU. So bottom left pin was grounded on it. I attached image with known to me pins (marked).
Unfortunately, there isn't much on the web regarding this PSU.
The PSU is controlled by CMOS OTP-based 8-bit microcontroller packs Microchip PIC16C72A (main web page, datasheet).
One guy told to add a 2.1 k ohm resistor to +S to - S get 13.6 volts. But where +S and -S are?

[madires]

The voltage regulation is done by an analogue feedback via an opto coupler. Find the voltage divider at the output voltage and you'll know. The MCU does possibly some management and monitoring stuff, like monitoring voltages. Maybe it would also need a mod to be happy. Unless the 3.3V and 5V outputs aren't based on buck converters, those voltages might increase too by your 13.8V mod.

[lukaq]

In most cases you have pot to adjust output voltage. Some supply's need external pot or resistor. But in many cases, you will hit OVP before you get that 13.8V or higher. I'm not sure if that is the case on all DSP's, but I would say on most of them.

[NiHaoMike]

What about build a boost converter? With a tapped push pull design, the MOSFETs won't even have to handle much current.