Thanks, Sean. The 15 volt source is indeed "stiff" - it is regulated output of a 70 amp alternator. The battery being charged is fairly well charged. Even with the 15 volts across it directly, the charge current is low. The main thing I need to avoid is gassing. --=20 Larry Bradley =20 Orleans (Ottawa) Canada=20 On Tue, 2014-02-11 at 13:00 -0500, Sean Breheny wrote: > 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 i= s > 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 seri= es > 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 Amp= s > will flow. Note that your PWM is really controlling the average current a= nd > 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). >=20 > 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 lot= s > of stress on several of your components, especially the battery and the P= WM > switching element. >=20 > Sean >=20 >=20 >=20 > On Tue, Feb 11, 2014 at 11:50 AM, Larry Bradley wro= te: >=20 > > Question for the lead-acid battery experts: > > > > As you recall from where this all started, I want to charge the startin= g > > 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 t= o > > 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 .