Strange,...(and perhaps poses another question / Post?) CHARGE VOLTAGE SAME?...as on your stereo...as is on...Provided charger? I would find manual SPECS of your stereos charge output.
I know that an optimal charge rate (first of all tecky talking) one must charge a batterey at a "higher voltage" (pressure) than the battery is itself. -or- There will be: (There is no difference of potential) (Ex: you can not charge a 12 V batt at 12 V,...nothing will happen)...The optimal "trickle charge") (other is a fast charge, and that will create more heat / warp / dammage faster)...is: Formula: Batt voltage X 1.25 = (perfect trickle charge) (Or you can just add 25%,...same thing...anything Xs 1. is naturaly,..the same) EX: Batt voltage + 25%.----anyway----just saying.
Of course the other factor is that one doesn't over heat (exceed mA rating of) the transformer / charger. One might have to convert this (Batteries are not rated in mA "milliamps" -- "One Thousanth" of an amp" 1000 mA + 1 Amp)...they are rated in mAh..."Milli Amp Hours" --or--Amp hours (Ammount of amps it can power for one hour) EX: A 10 amp hour battery, can run 10 amps for an hour---or---1 amp, for 10 hours...etc (We are deal with small batteries so it's probably less that an amp here) DON'T OVER HEAT THE CHARGER / TRANSFORMER. --- DON'T OVERDUE THE APPLIED VOLTAGE...warp yout batt
Guess:....Pop a volt meter on the charger (or look it up / read the nametag / sticker)...That is probably the nice charge rate
Sorry,...guess I'm getting a little deep.....love to share my knowlege