And I found out the reason is this.
As you know, all of the PSPs' batteries are Lithium-Ion batteries. The more you use them, the shorter its life time be. That is normal. Those kind of batteries can stand for 1000 recharges before they completely die. So the cause of this problem is that your battery is becoming weaker and weaker day by day in a very small ratio. Don't be afraid, this is normal.
If the CPU is overlocked on 222 MHz and the PSP is charging VIA wifi, that means that your battery is in its early ages. So it is strong and can handle its full power (charge while wifi led is on).
If the CPU is overlocked on 222 MHz and the PSP is not charging, that means that your battery is weaker than before by a small ratio. This ratio determines it.
Wifi drains so much juice from the PSP. Anyways, overlock the CPU on 333 MHz to check if there isn't any problem. All PSP users would eventually notice this. This is just the normal aging of the battery.