Lots of PWM???

Hi. I am working on a project that is going to need 10 PWM output signals to charge 10 li-poly or li-ion batteries. Yeah thats a lot of PWM!!! :)
The problem is that I need a way to produce the signals with an accuracy of 10 bit, and:
As i am going to charge 3.7v li ion with a 12v supply, using only pwm to drop the voltage to the battery (battery + to power supply+, minus to NPN transistor, and emitter to ground, pwm to transistor's base), how high does the frequency have to be so as not to kill the battery with the 12v spikes??? Perhaps using some caps connected in parallel to the battery??? Anyone knows how to calculate the value??
Is there any I2C, serial, etc, external hardware PWM???

Thanks a lot!!!!!! Sorry for my english :)
Manuel

i don't have a clue about battery charger, but to me a DC voltage instead of PWM would be safer... but i could be wrong.

Assuming i'm right, the easy way is to use 10 bits DAC. You can find some I2C one. Now you just need to hook few of them on 1-2 i2C bus and you're in business.

10 DAC would also require 10 ADC on the PIC for some 'feedback' control. But that's easy.

Fortunately, i'm not the only one here and some may have better tips and or idea.