I have bunch of old power adapters (those with transformers) and some laptop power supplys that are around 16-20v but i needed 12v for charging lead acid batteries. Since 12v lead acids charge at around 14.4v i had to lower voltage from 16v (we in case with laptop charger from 19v) down to 14.4v to charge lead acid battery. Easiest way to drop down voltage is to use cheap ebay step down converters. They are really cheap 1-3€ and work really good and max current is 3A which is enough for me, well at least for most SLA and motorcycle batteries, or to tricle charge car battery. Since voltage can be adjusted from 1.5v to the input voltage (actually -2v from input since converter drops 1.5v and diode 0.5v) you charge all sorts off batteries, 6v, 12v and even 24v batteries becouse converter supports all the way up to 40v input voltage but i wont recommend that since it gets hot with more voltage difference (input-output). It also has voltmeter so you can use this as voltmeter so you can put it in your car to check voltage of battery.
Step 1: Tools & Materials
I recommend reading whole instructable before doing this becouse i maybe i didnt put something in parts list.
Drill ( i used 5mm and 1mm drill bits and largen holes with drill bits)
Hot Glue Gun
Dremel, rotary tool or hacksaw
File and sandpaper
Project Enclosure (100x60x25mm is the one i used)
Diode (should work on 40V 5A, more amperage is better)
LM2596 Step Down Module
Mini LED Voltmeter (i used 2 wire meter, selfpowered)
Banana Binding Posts (4mm)
Heatsink (i got mine from broken step up converter)
PCB connectors (for input, not sure how are those called)
Step 2: Add Heatsink on LM2596
LM2596 Converter can work up to 1A without heatsink. Since im going to max it (or just to be safe with 1A) i've added heatsink to cool it down. I got mine from cheap step up converter (100-150w version on ebay) and since it was a bit too large i cut it down with rotary tool. Remainig 3 fins i've put on top of LM2596. Under heatsink i isolated pins from potentiometer and capacitors. I forgot to take picture, but you have to unsolder potentiometer and put wires on 1-3 legs of potentiometer (1-2-3 on circuit, 1 and 2 are bridged) and connect them to 1-2 leg of potentiometer (the one you put on case).
Step 3: Holes & Testing
I used 5mm drill bits to make front holes for binding posts and potentiometer. 5mm is a bit smaller than diameter of binding posts so i enlargened them with same drill bit by pushing it on side (my holes were not in middle but after enlarging they were). After that i added PCB connector on back and connected it to step down module. I didnt mark polarity but they are same as front posts (the left one is positive, same as binding post on front). This goes to IN+ and IN-. Then i added two wires for front binding posts, postitive or red post goes to +OUT, negative or black goes to -OUT. Diode goes on positive post and arrow is facing towards post. Voltmeter is connected directly on post, so i can set voltage after diode (0.5v drop across diode) and i can use it to measure voltage of battery without damaging converter. When you connected everything, LED display should show voltage and when you rotate potentiometer voltage should change. Depending on input voltage, potentiometer should have one part where you rotate it but voltage doesnt change. That is fine becouse it is set to change voltage from 1.5v-30v so if you have 35v on input you can use whole rotation. My LED display doesnt work under 4v so i can measure that part, so i would have to fix that next time.
Step 4: Add LED Voltmeter
LED voltmeter connect directly to binding posts, after diode. Depending on where you have space in case you will put voltmeter. You can put it on side, maybe even behind if you want but i just put it on top. I made holes with rotary tool and then smoothen them with file and sandpaper to make sure voltmeter would fit perfectly. To make sure it wont move i added some hot glue on behind to hold it in place.
Step 5: Testing, Things to Replace
If everything is connected properly you can now set voltage and test it on some load. I tried it on 20w lightbuld and it worked fine. After that i set voltage to 14.4V and tried to charge battery. I measured 2.8A which is close to 3A so its working fine. On input there was 19v 5.45A laptop power supply.
Things to change:
- Add voltmeter that works under 4v (maybe one that has external supply)
- Maybe add battery to it so i can use it to power other stuff or test circuits with it
- Make another case with 5*3,7v lithium batteries to make this portable and adjustable from 1.5v-16v
- Maybe change diode, im not sure if this one is rated more then 3A but its really big
- Bigger heasink or holes, with 24v i charged 12v (14.4v) and heatsink got a bit hot so i might have to add some holes to move air thru case, it works fine when i remove top cover