Braun series 790 battery - Braun Pulsonic Rechargeable Cordless Men's Razor - Silver

Question edited for clarity.

Check the battery for voltage with a digital voltmeter. Then check the charging circuit.

There is nothing to calibrate. There is misinformation on the internet stating that this is necessary. It is incorrect

I don't know about this specific model of Braun shaver (I have a Braun Model 7526).

I have looked at the instructions for a couple of models of Braun Shaver, and the instructions are similar so it may help you to replace the batteries in yours.

Here is one of the instructions:
Braun 7526 battery problem
http://www.fixya.com/support/t1171024-braun_7526_battery_problem

Open the shaver:
1) Use a small flat screw driver pry off the bottom (where the cord plugs in)
2) Remove the silver sides
3) Using a small torq driver remove the 4 screws
4) Open shaver
5) Remove circuit board with batteries
6) Remove and replace circuit board with batteries (or solder in the new batteries)

And here is a link to instructions for a few different ones:
http://www.fixya.com/support/t1094906-needs_new_batters_open_razor_7526

http://www.fixya.com/support/t6710169-change_battery_cost

http://www.fixya.com/support/t771258-braun_battery_replacement

http://www.service.braun.com/line/SH/S5772/S5772_4_CHIN.pdf

Maintenance Braun

Try removing the foil and cutter head. Wait a few minutes and then refit them. It may be on some sort of electronic timer that after so many hours of shaving, the unit automatically comes up with a "change foil" icon and then wont let you use it until its replaced.

It is a light to tell you to replace your cutter cassette and foil. It will reset after 7 shaves. It usually comes on after 18 months or the cutter is worn. Page 11 in your manual.

It appears that the battery needs to be replaced. shaveroutlet.com has the battery you need and installation video. They can also replace it for you
Braun Lithium Ion Battery for Pulsonic and Series 7 Model Shavers
Braun Series 7 Shaver Disassembly Tutorial Professional Battery Replacement for Most Models

pit back in the charge station for at least 4 hours and switch the power on for the charger
many small battery chargers will flatten the battery if the power is switched off
if that fails , contact amazon for a return letter to get you warranty refund

These cleaning/charging units are prone to going bad. There are no parts for them, so, you will need to replace it.
You will find at shaveroutlet.com
Braun Series 7 and Pulsonic Clean Charge Base