I have not followed the thread from the beginning so maybe I missed something but I would say that your PWM scheme is fine as long as there is enough series resistance to limit the peak current and the PWM frequency is high enough (even a few hertz would probably be fine). As a real example, let's say that you are charging the battery from 15.5V with 1 ohm of series resistance and the battery is around 50% state of charge with an open-circuit voltage of 12V and that this voltage will change very little up to 10 Amps of charge current. Then, during your PWM "on" time, 3.5 Amps will flow. Note that your PWM is really controlling the average current and not the average voltage, since even a 1% duty cycle will result in net charge current greater than zero (not an average voltage of 0.155 V). However, if you have a really "stiff" 15.5V voltage source and the charge current is limited only by the internal behavior of the battery, then during your PWM "on" time there could be 100s of Amps which would put lots of stress on several of your components, especially the battery and the PWM switching element. Sean On Tue, Feb 11, 2014 at 11:50 AM, Larry Bradley wrote= : > Question for the lead-acid battery experts: > > As you recall from where this all started, I want to charge the starting > battery from the alternator, when the alternator voltage could be 15.5 > volts or so. The plan was to use PWM, and limit the "average" voltage to > 13.6 or so. > > However, the PEAK voltage will still be 15.5 volts. > > Will this be a problem? We are talking about a normal car starting > battery, not a marine deep cycle. > -- > Larry Bradley > Orleans (Ottawa) Canada > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .