> 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. On 12 February 2014 15:44, Richard R. Pope wrote: > Yes, this will be big trouble. Most LA batteries should not be > charged at over 14.4 volts. Once you get above this voltage the > electrolyte will start to boil. The plates will also overheat and > buckle. This will quickly destroy the battery. > If you need to use PWM and MUST keep voltage to the average and not the peak value then addition of a series inductor at the battery and a reverse biased diode from the Vin side of the inductor to ground produces a "buck converter. If PWM is fast enough this can be a typical powdered iron cored toroid or similar, but for very slow PWM or very high currents an old large transformer core can be used with suitable airgap and windings to produce a high current high inductance inductor. Something which becomes so large if done with a commercial powdered iron product can probably be achieved at a cost mainly of time and effort. Things like microwave oven transformers or 2:1 stepdown/up mains transformers (often free, especially if burned out) may have enough copper and lamination steel without buying anything. Russell --=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 .