Hello, I make batteries.
Let me answer your question.
1. You said your battery is 3.7V-100MAH. At first glance, you can see that it is very small.
2. You plan to charge with a 5V-300MAH charger.
Correct "this should be 5V-300MA".
3. Certainly not possible.
Why?
Lithium battery chargers need to pay attention to at least two points.
1) Charging Current: Look at your current capacity of 100mAH and your charging current is 300mA. It's three times bigger. It's deadly for the battery.
2) Let's talk about voltage: For your 3.7V battery, the optimal charging voltage should be 4.2V-4.3V.
Yours is now 5V, which is a bit high.
Last topic:
Maybe you will ask why, why can you use 5V to charge your cell phone if it is also 3.7V .
That's becausethat there are circuits in the mobile phone.
Thank you for your help!