At 06:21 PM 9/21/2010, David Duffy (AVD) wrote: > I've been thinking of SLA battery chargers again. The project I have >in mind would require multiple (up to 20 units) 12V 5Ah SLA batteries to >be charged every few days at some sort of charging station. > >It would seem that a switching type regulator would be much better when >it comes to heat issues. There are a few low cost linear devices >around, but I'd really like to avoid the potential heat issues involved. I've done several linear-type SLA chargers in the past - all have=20 worked well but getting rid of the heat produced was never a=20 problem. Most of these are based on a LM317 regulator (used as a=20 smart transistor), a dual op-amp (LM358) and a TL431. The TL431 is=20 the actual regulator element, one section of the op-amp does current=20 limiting, the other section is a latch that determines the charging=20 mode (equalize or float). I had started work on a switch-mode charger but never finished=20 it. My approach was to start off with a significantly higher raw DC=20 supply derived from a standard transformer / rectifier / reservoir=20 cap and feed that to a Nat Semi 'simple switcher' regulator. The=20 feedback voltage was to be controlled the same way my linear chargers=20 are: that's a current-limit section driving a latch that selects the=20 appropriate feedback resistor. The charger always starts off in=20 equalize mode and stays there for as long as the charger is in=20 current limit. When the charger leaves current limit, it drops back=20 to float mode. I've done some SLA chargers where I hold the charger in equalize mode=20 for some defined time period after the charger drops out of current=20 limit (what some people call 'absorption mode') but it didn't seem to=20 improve battery life at all. As a result of that testing, my current=20 SLA chargers don't bother with that mode. My customers routinely get 5 to 7 years out of their SLA batteries=20 (all using my charge circuits) - I consider that to be a reasonable lifetim= e. dwayne --=20 Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax www.trinity-electronics.com Custom Electronics Design and Manufacturing --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .