Originally Posted by
woodway
As it turns out. I do design electronics for a living, specifically I’m running a team that designs battery chargers 😀
I won’t call Shimano, but I did take a look at the manual for the di2 internal battery charger (the one with the usb connector), and it says:
”• The charging time of an AC adapter with a USB port is about 1.5 hours, and that of computer USB port type about 3 hours. (Note that the actual time will vary depending on the amount of charge remaining in the battery. Depending on the specifications of the AC adapter, recharging via the AC adapter may require as much time (about 3 hours) as recharging via PC.)”
The Shimano charger looks to have a boost converter in it (the battery is 7.5v and USB is 5v). It may be that in the past they had trouble with their converter if the port could not supply enough current. But it appears that if they did, they have fixed the problem. Cheers!
That's the same note they had going back to at least 6800 Ultegra. Shimano warrantied batteries for my customers in 2021/22, so I wouldn't be so confident that anything changed.
"Use an AC adapter with a USB port with a voltage of 5.0 V DC and with a current equal to or higher than 1.0 A DC. If the one with a current lower than 1.0 A is used, the AC adapter may heat up, potentially causing a fire, smoke, overheating, destruction, electric shock, or burns."
"
Do not connect the charger to PC when it is on standby. This may cause a PC failure depending on its specifications."