How to start 15" MacBook with lower wattage supply/fake cable and no battery
1. Device will start in SMC Bypass mode (Hold power button and plug n magsafe cable. release after 5 seconds), device will start with no magsafe light and full fan
2. pull up SMC_BATLOW_L to 3v3_S5 Lineline. Device should immidiately start. On 820-2915 board that would be R5040 pin 1 to pin 3 of Q5040.
Why its important to have a good power supply
Even slight overvoltage(~25V) might cause mainboard damage. If you take in the device with cheap Knock/off charger - ALWAYS tell customer to buy a good one. It would also be reasonable to warranty if customer ignores your recommendation. In most burned ISL6259 cases there is a knock-off charger somewhere in customer's backpack.
Magsafe 1&2: get a dead OEM 85W charger and use its cable to have a reference cable: such cable will work with every Macbook. This is really important if you troubleshoot SMC/ISL6259/Battery. You also need to check it with original working charger after the repair: if you have bad MagSafe Sleep mode, your device wont run from OEM charger but will work on bench!
Important note: Refer to MacBook Battery compatibility page for the list of boards which wont boot / properlz start with no battery wven on proper charger.
USB-C Models: you need a 100W power supply to autostart 16" macbook, so thats also must have for your bench.