eBay SMPS as a Lead Acid Battery Charger - Page 1
Forum section: electronics / projects

author: tribat
author_rank: Contributor
timestamp: December 04, 2010, 11:41:25 am
content: I'm not sure how aware people are of this but SMPS modules on eBay are dirt cheap and very available these days. Chinese sellers are selling them for very low prices, and from what I hear their pretty decent quality too. At least the ones from seller called 'stoneined2009'.Here's an example:http://cgi.ebay.com/DC-12V-12A-145W-Power-Supply-Regulated-Switching-New-/170567024030?pt=LH_DefaultDomain_0&hash=item27b6963d9eCool thing about them is that they seem to have a pretty decent voltage adjust range, and from what I hear the range is usually wider than the specs say, for example that model goes up to 14.6V.The voltage range needed for lead acid batteries is about 13-15V so one of those 13.5V models with 12-15V voltage range should be perfect (http://cgi.ebay.com/350W-13-5V-DC-Regulated-Switching-Power-Supply-K004-/180455569172?pt=LH_DefaultDomain_0&hash=item2a03fd7714)I was wondering what kind of modification would those things needs to work as a lead acid battery charger? Could you just set the voltage to 13.5V and hook it to a battery and forget about it? Or would there have to be some kind of current limiting or something?I read about charging lead acid batteries and the way I understood it is that 13.5V is the magic voltage where a battery can be kept fully charged indefinitely without damaging it. It won't charge the battery very fast but will eventually get the job done. Still I wonder if the voltage adjust trimmer could be replaced with some circuitry that adjusts the voltage of the SMPS automatically.

author: Bored@Work
author_rank: Super Contributor
timestamp: December 04, 2010, 12:20:06 pm
content: Quote from: tribat on December 04, 2010, 11:41:25 amPointless. The idiot claims "worldwide" shipping, but then excludes whole continents.QuoteIf a seller starts with a lie like "Shipping to: worldwide", I immediately exclude him as a supplier I trust.

author: Balaur
author_rank: Unknown
timestamp: December 04, 2010, 01:02:09 pm
content: The problem with Chinese electronics is that the quality ranges for perfectly designed & made, high-quality, not expensive devices to absolutely POS stuff.As a general, personal observation, the power supply units tend to be on the POS side, especially those on eBay. Cheaply made, unreliable capacitors (of course), idiotic design, especially on the SMPS units, stupidly high ripple, but cheap as air, though.You can buy one and be very happy with that or you can have a horror story like this gentleman:http://www.ludens.cl/Electron/chinverter/chinverter.htmlCheers,Dan

author: tecman
author_rank: Frequent Contributor
timestamp: December 04, 2010, 02:13:54 pm
content: Quote from: tribat on December 04, 2010, 11:41:25 amWhat you are looking for is called "float charge". It will maintain, but not overcharge a battery. It is nominally 2.15 volts per cell, but that is at somewhere around 70 degF. The specific voltage varies with temperature. This drops about 0.1 volts for every 10 deg increase. You can read more at:http://www.hss.doe.gov/nuclearsafety/ns/techstds/standard/hdbk1084/hdbk1084.pdfpaul

author: tribat
author_rank: Contributor
timestamp: December 04, 2010, 10:02:30 pm
content: Quote from: tecman on December 04, 2010, 02:13:54 pmBut will just setting the power supply to that voltage be enough? Do you have to also a throw a current limiting circuitry there?

author: TechGuy
author_rank: Regular Contributor
timestamp: December 06, 2010, 04:01:10 pm
content: QuoteUnfortunately not, if you want to make sure your batteries last. Over charging your batteries will cause excessive electrolysis causing hydrogen to be release dehydrating the cells and causing sulphur deposits (sulfation) on the lead plates. Sulphur deposits act as like insulators blocking plate surface area need to run the chemical reaction to produce electricity. A good SLA charger will have three modes (if I recall correctly):1. High current charging mode, which will provide lots of current when the battery voltage is low. Current is constant and charging voltage is maintained within a few volts difference, but not excessively high to cause dehydration and sulfation. (Athough, my understanding is that battery temperature is an important factor in charging but I suspect few charges include temperature monitoring).2. A low current charging mode, when the battery voltage is near full charge. This takes place when the battery voltage reaches need its full charge, and the voltage difference between the charging voltage and the battery voltage drops below ~1V ( I think around 1.2V)3. A Floating charing mode that constantly monitors the battery voltage and tops off to address self-discharge.Also consider that in 24/36/48 voltage system where several batteries are connected in series. Some batteries will recharge faster than others. Good chargers will have charging taps to monitor and change all batteries in series equally so that all get properly charged. Otherwise some batteries will get overcharged and others undercharged.Consider that Batteries are expensive and there replacement cost is high. Better off buying a quality charger than a cheap charger that becomes expensive when you have to prematurely replace your batteries.

author: tribat
author_rank: Contributor
timestamp: December 06, 2010, 10:02:10 pm
content: Quote from: TechGuy on December 06, 2010, 04:01:10 pmI thought with a floating charge it's impossible to overcharge the battery, also a full battery with a floating charge uses very little current, is it necessary to still limit it?QuoteHow exactly does a battery charger monitor the batterys voltage? If it just measures the voltage across the the battery terminals it will just be the floating voltage what the charger is using currently. Even if it shuts down the the charging while taking voltage measurements it would take hours of rest for the battery to display a voltage that could be used to determine the state-of-chargeQuoteI only need to charge one battery or multiple ones in parallel