12v 12ah sla Charging circuit - Page 1
Forum section: electronics / projects

author: creexpert
author_rank: Contributor
timestamp: February 27, 2014, 02:31:04 am
content: Hello everyone,I have a prototype high powered lantern running off of a 12v 12ah sla battery. At first, our intentions of charging the battery was using a simple wall-wort with the 2.5mm barrel.. but we have found issues with this method(clients with similar units which we had repaired recently, did not use the same wall-wort supplied with the unit, so input voltage was all over the place)We want to prevent this issue and use IEC AC inputs(like found on computer power supplies) and put the charging circuit inside the unit.. any suggestions.. we would like to have a led light when charging as well as when finished.. we already have low battery circuits which a lower voltage cut off to the LED circuitThe lantern will not be plugged in when in use.. only when chargingI have spent several weeks researching this.. DC is new to me, but AC is foreign so I am unsure which circuits I find online to be safe/efficientThere is roughly $15usd budget for the charger from wall to battery(circuit, power entry connectors, and cord) and would even consider a commercial product that could be used internally.. internal space should not be an issue.. have roughly 6"x6"x6"

author: macboy
author_rank: Super Contributor
timestamp: February 27, 2014, 03:21:08 pm
content: The first problem with your idea that I see is that ditching a UL listed AC/DC adapter in favor of an internal power supply, also makes your equipment automatically NOT UL listed. As I understand it, if you bring the AC line into your equipment, then you will need to then go through the expensive process of testing it for electrical safety. When you use a listed external power supply, you don't need to do that.After that, the most basic, and perfectly functional charger for a SLA battery is a constant voltage, current limited supply. The current should be limited to around 1/10 of the capacity, so about 1.2 A in your case (1 A is fine too). The voltage at the battery terminals should be maintained to 13.8 V at room temperature (a little higher if colder, lower if warmer). As the battery charges, at first voltage will be below 13.8 V and the current limiter will be in effect. At some point (around 70% full or so) the voltage will reach 13.8 V and the current will slowly taper off until it reaches just a few or few 10's of mA, equal to the self-discharge/leakage current of the cells. The battery is now simply being maintained at full charge. It is safe to leave the 13.8 V charging voltage on the terminalsindefinitely; there is no need to detect full charge and turn it off. You could bring a suitable DC (or AC) voltage into the box and do the voltage and current regulation in the box.

author: creexpert
author_rank: Contributor
timestamp: February 27, 2014, 04:46:09 pm
content: I see your point.. so lets say we go with a wall-wort proving the needed dc voltageDo you have a circuit or commercial suggestion for the charger itself?I have been considering this one, but the cost of the uc3906 has me searching for a better solutionhttp://www.electroniccircuitsdesign.com/battery-charger-circuits/uc3906-battery-charger-controller.htmlJason

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 27, 2014, 05:53:42 pm
content: What about an IEC input wall wart mounted inside the device? A 15V regulated supply and two diodes will get you right on the recommended float voltage.

author: creexpert
author_rank: Contributor
timestamp: February 27, 2014, 06:47:41 pm
content: Thanks for the great Idea.. now to find an 18v version Or a new circuit