Also make sure you have some good circuit protection at the output of the charger -- a short and a reverse discharge into your charger is very dangerous for a SLA battery charger. One of my former employer's competitor had 1800 reports of problems, 22 fires with no circuit protection. The 12V battery can dump about ~400 watts into the charger. Best to put the protection at the connector. Their charger was the most simple of all. Just two rectifying diodes and a transformer. The battery acts like a big filter cap so they just set the peak to about 13.7 volts - the worst method for the battery. Also there is a newish book on charging SLA batteries. *Valve-Regulated Lead-Acid Batteries (Hardcover) - **ISBN:* 0444507469 - tells you everything about the charging methods you would need to know and how the batteries work. It's too expensive $180, I picked up my copy from the campus library to read. I put over 100k SLA chargers in the field last two years and used a regulator circuit - couldn't afford a micro or custom chip. There are now switch mode chargers you can buy at about ~4.00 OEM in 100k quantities. The regulator circuit caused not one problem and the batteries last a long time. We had one regulator fail out of the first 100k, but everything failed safe. The down side is that the regulator circuit makes the transformer bigger and heavy due to heat. Companies like Yuasa, Panasonic and PowerSonic have good application notes on charging also which show the basic circuits. In tests we found Panasonic and Yuasa batteries to be the best - months of testing many batteries in hundreds of charge/discharge cycles, many vendors. Even had some batteries swell and nearly ready to burst from cheap vendors. One guy returned one where his dog chewed through the cord 4 times, and he re-connected it - he was lucky because he didn't pay attention the polarity when he was re-connecting it, but got it right. I know the processor/custom chip solution is best because you can measure temperature, and correctly charge the battery in multiple modes - constant current and constant voltage as required. The book explains the best algorithm in great detail. Undercharging or overcharging the battery can hurt it and shorten its life. For many consumer applications a regulator is just fine, long enough battery life, low cost, and reasonable safety with circuit protection (fuses and/or diodes). You should be able to construct a charger in China for under ~US $4.00. Battery connectors are a big problem, however, not much out there to help -- just two reasonable vendors with adequate pins, Tyco and Anderson Power Products. Steve Russell McMahon wrote: >The perennial "how to charge a sealed lead acid battery realllly >cheaply and in a vaguely OK manner" question. More of what I >discussed/asked a month or so ago. If I meander on about this it may >prove useful to others and may prompt some useful input. I'm not going >to spec this in fine detail as the general requirement is clear >enough. > >Short description: Aim is to float a 12v battery at 13.7 volt nominal >at about zero cost. Rate of charge when not at float voltage is not >too too critical and can be adjusted by simple things like eg series >resistor and (gasp) limiting heat sinking on any regulator used to >force it into limiting / shutdown above certain power levels. > >Also scale answers down by 2/3 for an 8V battery. > >Input voltage is a vaguely constantish voltage which may be set within >reason with a say +/- 0.5 volt margin. battery has capacity in the >3Ah - 7 AH range. C/3 max charge rate is tolerable but as low as >/10 - C/20 for 3/7 AH battery probably acceptable. Typical bottom end >Asian SLA's spec sheets allow up to C/3 charge. > >An 'obvious' solution is simply a 13V7 voltage regulator with suitably >low reverse leakage (doesn't drain battery), stable-ish output voltage >and temperature characteristics that vaguely match what the battery >wants. ST make such - their PB137, 13V7 regulator. (Interesting name - >Pb = lead, 137 -> 13V7), now you'll remember it :-) ). > >2 V ish headroom needed for a 16V+ supply. Googling doesn't suggest >wide availability. Findchips (20 distributors) lists only Mouser at >$0.50/1000. I've little doubt that others do similar at a similar >price (as opposed to the largish range of 'proper" SLA charger chips >at a too dear $USN. > >The LM340/LM7805 or LM317 with scaling resistors that one would >otherwise be tempted to use is cheaper BUT have nasty problems with >good accuracy without adjustment . EG the LM317 adjustment current is >50 uA typical and 100 uA max so even with a stiffish reference divider >the variation in set point voltage at 12v can vary by about half a >volt depending on initial adjustment current. This is far too much too >ignore for SLA float purposes. Also, back feed down reference chain >from battery is far too high too be tolerable so a series output diode >is needed, detracting from regulation accuracy. PB137 is rated at 10 >uA draw AFAIR. > >An LM358, precisionish cheapish reference, a high side PFET (or pnp) >and a small bipolar driver, add R's to taste, would do better and >probably cost about the same. > >I can feel a processor based solution coming on - might as well keep >the processor busy. Vref will be calibrated during setup of the main >system and can then be accurate enoughish for this purpose. Processor >is AVR ATmega but all the above is processor independent (and almost >unrelated). Processor solution has advantage of allowing boost charge >to 14V4 ish and then drop back to float. > >Thoughts? > > > Russell McMahon > > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist