Replacing li-po with li-ion battery on small device?

[flippityflop]

so i bought this cheap portable device and it worked ok for the first week. then i started noticing it would get discharged more and more quickly, until it doesn't even power up. i reckoned that it was a battery problem. so i wanna replace it. i don't have any more spare li-po battery, but what i do have are a couple of AA-sized lithium ion batteries (3.7V). i want to solder an AA-battery holder, so i can simply swap the lithium ion batteries whenever they get discharged.

my question is, if i where to leave the AA-batteries in and plug in the recharging cord, which is USB, rated 5V, would it also charge up the AA lithium ions, just like it charges the old li-po??

assuming it does, can i charge (in parallel) more than 1 AA lithium ions?? i'm not sure about the current rating of the power charger for the portable device, so i might have to watch out for that...

[audioguru]

All rechargeable batteries have a capacity rating in mAh (milliamp-hours). Lithium is a dangerous flammable metal (burns white hot like magnesium flares) and catches on fire if a battery is over-charged or is charged when discharged too low.

Some rechargeable Lithium batteries have a protection circuit inside. Even my little $17.00 toy helicopter has a 60mAh tiny Li-Po battery with a protection circuit on the end where its wires are attached.

If your original battery has a protection circuit but your AA Li-Ion cells do not then you might have an explosion.
If the AA cells do not explode or do not catch on fire then if their mAh rating is much higher than the rating of the Li-Po battery then it might take weeks to charge them.