I recently put together a smart charger to replace the dumb charger that I built using an LM317 Voltage Regulator. I wanted something that could fast charge a battery at C/2 or higher. The batteries that this charger is configured for are 12V 4.5AH SLA batteries. However, the modifications necessary to charge any SLA battery are easy to make by just changing a few of the resistor values and potentially adding wider tracks on the high current bus. It bulk charges at a current limit of 2.5 Amps until the voltage across the terminals gets to about 14.7 volts. Then it continues charging by regulating the charge voltage at 14.7 volts until the current drops to .25 amps. From there it switches to a float charge of 13.5 volts to offset the batteries self discharge rate. Because this was the first circuit that actually required me to design and etch my own pcb as well as calculate out some of the part values and select suitable parts, I started out with a schematic that was 99% of what I needed. The app note that I used is here:

http://focus.ti.com/lit/ug/sluu023b/sluu023b.pdf

The attached files include pdf's of the schematic and pcb layouts as well as the ExpressPCB and ExpressSCH files. I have also included a BOM with all of the appropriate Mouser part numbers.

Please note: I am not an Engineer! I am just an inexperienced hobbiest and tinkerer. I am not responsible for any adverse consequences that may arise if you try to build this circuit. As you probably know, improper charging of a battery can cause horrific outcomes, especially if your goal is to fast charge them.

 

A bit more information on the PCB's. The top layer is a ground plane. I soldered the ground wire that goes to my 24V power supply directly to the copper plane. The positive wire from the 24V supply gets soldered to the large copper tab at the top left of the PCB.

Since my boards were not professionally made they did not have any through hole plating. This made it difficult for the components that had one pin connected to ground because I couldn't just solder it to the pad on the bottom layer because that pad was not electrically common with the top layer. What I ended up doing is just bending the grounded pins horizontal and soldering them to the surface of the top layer rather than inserting them into the hole in the board.

Also if you look at the high current line on the bottom layer towards the bottom right of the board you will see a break in the copper trace with two large pads on it. I put that in there so that I could solder a 0 - 3 Amp Analog current meter in series with the circuit. I also soldered a 0 - 15V Analog volt meter across the positive and negative terminals of the battery plug.

The volt meter actually causes a problem which I was never able to fix. When the BQ2031 is first turned on it goes through a battery qualification test to determine if a chargeable battery is connected to the output. One of the tests is to try to pass a current through the output and if no current will flow it will know that a battery is absent. The analog volt meter that I am using I guess passes just enough current through it to trick the charger into thinking that a battery is present. It doesn't begin charging but it also doesn't show a full fault condition so the BQ2031 never makes it out of the pre-qualification test. It will eventually time out I believe but I have never left it that long. I get around this by just making sure that I have a battery attached to the charger before turning it on.

The battery plugs into a 2.1mm DC connector. I used that because I soldered 2.1mm plugs on my 12V batteries to make it easy to plug them into my projects. Certainly you could come up with your own method for attaching your batteries to the charger.

 

To bump the charge rate up to 4 amps, would I have to change anything besides R20? Also, do you remember what voltages you used for Vblk and Vflt? I'm trying to do the same thing you are, but with a much larger battery (17 aH).

 

To bump the charge rate up to 4 amps, would I have to change anything besides R20? Also, do you remember what voltages you used for Vblk and Vflt? I'm trying to do the same thing you are, but with a much larger battery (17 aH).

Sorry for taking so long to respond to you on this but just in case you check back........R20 would need to be changed to .0625 Ohm resistor which should present .25 volts to the current sense pin at your target current or 4 amps. You would of course want to check the rest of the components in the high current path to make sure that they are specd to handle it. I think I have Vblk at 14.7V and Vflt at about 13.5V. I will hook up one of my batteries tonight and measure it with a voltmeter. I would definitely suggest you read through this application note as it will answer a lot of questions for you.

http://www.ti.com/lit/an/slua017/slua017.pdf

 

Awesome! Thank you!

 

Nice project.. I seen your comments about the top layer ground plane.. how hard would it be to change it back (since I am going to have these boards professionally made)

Any thoughts on replacing the bq2031/using a different circuit since these are harder to come by?

 

thank you my friend very helpful

 

The BQ2031 is my favorite lead acid battery charger IC. Great choice, excellent circuit and very very good PCB layout.

 

Hello This .sch file is not getting opened in eagle.
Is it Single sided??

 

Easy enough just to supply an PDF if it is single sided.

Single sided using DIPs is the way to go for DIY hobby.

 

will it work for NiMH battery. 7.2V and 3000mAH.
you can suggest which can work

 

Hello,

No, a SLA charger can not be used for NiMH batteries.

Bertus

 

How much current does the 24VDC input need to supply, would it be the full 2.5A?