Universal Lead-Acid Battery Charger from Laptop Power Supply - Page 1
Forum section: electronics / beginners

author: drdm
author_rank: Contributor
timestamp: March 26, 2016, 05:49:29 pm
content: Hello everybody,I have to make a charger for a 6V 12Ah battery, witch I initially planned to do the old-fashion way(a transformer, a rectifier and an lm317 adjusted to 7,5V), but I've discovered that for the same price as a 1A 12V transformer I can buy an old laptop power supply, rated at 19V 4A. I've decided to use one of these and make a regulator, that can charge 6 and 12 volt batteries (I recon 7,5V and 14V output voltage respectively). From what I've read about lead acid batteries they must be initially charged with 1/10 of their amphour rating (1.2 amps for a 12Ah battery) and after they reach a certain voltage it should be maintained till full charge.I'm planing to make an adjustable voltage regulator based on lm317 with adjustable current limiting.These two circuits are given in the datasheet, but i can't figure out how to combine them(it is also not entirely clear to me how the high current circuit works).Does my plan make any sense or is there something fundamental that I am missing?

author: Kilrah
author_rank: Unknown
timestamp: March 26, 2016, 06:22:37 pm
content: Twice as much energy lost as heat as what goes into the battery? We're in 2016

author: drdm
author_rank: Contributor
timestamp: March 26, 2016, 08:45:29 pm
content: That is not of importance to me, but if you have a better solution I will be glad to see it. My main concern is if a laptop power supply is suitable for the task.

author: Seekonk
author_rank: Super Contributor
timestamp: March 26, 2016, 08:54:14 pm
content: You have a laptop power supply, no need to invent a new one. These all have an opto isolator and zener to set voltage. Switch zener with LM431 and make it adjustable.

author: drdm
author_rank: Contributor
timestamp: March 26, 2016, 09:17:26 pm
content: I still haven't bought the supply and for a coupe of bucks more i can get an universal one that has voltage adjustment build in. The bigger problem is that i need a current limiter since I don't want to feed 4-5A into a 12Ah battery. The limiter could be adjustable so different batteries could be charged.

author: knks
author_rank: Regular Contributor
timestamp: May 08, 2016, 11:48:01 pm
content: I use Laptop Power Supply (19V or so) to charge my car battery. Schematics very simple - just one resistor in seriesOf course I have to manually monitor the battery voltage, but since I only use it ~ once-a-year I think it's good enough

author: jeroen79
author_rank: Frequent Contributor
timestamp: May 09, 2016, 09:03:54 am
content: Quote from: drdm on March 26, 2016, 08:45:29 pmNot by itself.It's output voltage is too high and it may not have a suitable constant current limiter.But it can be used to bring the mains AC down to a more managable DC level to power the charger.How about geting a cheap CC/CV buck regulator.Then all you'd have to do is modify the CV pot so to switch between 6 and 12 volt batteries.

author: tszaboo
author_rank: Super Contributor
timestamp: May 09, 2016, 09:46:45 am
content: It is better to get a nice IC doing everything, like an L200, instead of coming up with a discrete solution.

author: danadak
author_rank: Super Contributor
timestamp: May 09, 2016, 09:47:08 am
content: http://www.ti.com/lit/an/slua115/slua115.pdfhttp://www.ti.com/lit/an/slua055/slua055.pdfhttp://ww1.microchip.com/downloads/en/AppNotes/01015a.pdfRegards, Dana.

author: agha.akhlas7
author_rank: Newbie
timestamp: June 22, 2016, 02:11:23 pm
content: You need to charge 12 vdc batterry with 14.5 vdc by regulating the laptop power supplySent from my SM-J500F using Tapatalk