Slow charging is actually better for battery longevity than fast charging. For example, my phone will limit it's charging speed at night when I have an alarm set, because it assumes I won't need it until my alarm goes off, and the slower it charges the less damage is done to a battery.

In general, lithium batteries are damaged by heat. Rapid charging creates extra heat, so it's worse for the battery. Manufacturers work hard to minimize the damage of quick charging, but it's still somewhat better to slow charge when you have the time.

So what other posters may be missing here is that, depending on the device and how it's powered, "slow charging" may not actually be fast enough of a charge to keep it charged while powered on.

So while its true, slowly charging your battery is better for it, in this case, you may be receiving the warning because the device may eventually power down while in use.

If that is indeed the case, in the meantime if you charge it while it is off and use it until the battery gets down to about 30% and then turn it off to charge again and you should be fine. Honestly pretty safe just mildly frustrating, if you ask me.

Source: a friend's girlfriend's old laptop which we discovered needed its fancy charging cable to both charge and stay on and a regular usb cable actually failed to give it the proper power delivery. She was having all kinds of issues with it and it took me digging through the whole manual for the device to figure it out.

Lemme throw a wrench into everything. Sometimes faster charging can be better than slower charging.

Part of the USB power delivery spec is PPS or programmable power supply. PPS lets your device specify the exact voltage down to 0.02V or 20 mV. Heat is generated by inefficiency, and PPS' goal is to get the voltage just right so there's as little power loss as possible. My iphone charging at 5v 1amp "slow" charging gets hotter than the full 20 watts fast charging.

Technically any charging of a Lithium battery is "bad" for its health. Batteries are usually rated for a certain number if charge cycles from the manufacturer and after that number of cycles will start to lose capacity. The best advise I've seen regarding battery health is to keep it from charging to %100 everytime. As others have stated, the charging speed can also have an impact as it creates heat and heat will also wear out the battery.

As for your specific device it seems like it's consuming more power than the charger is giving it, so even when plugged in and turned on it may be discharging the battery still as it's not getting enough power. Either you buy a new charger that can put out more power or you turn it off while charging it. Either case shouldn't be bad for the battery health.