Versapak Smart Charger

Richard Spelling

I hope you enjoy this article. I enjoyed writing it. Please note that I still sell the boards to make this, as well as a "kit" to make them, both on Ebay and on my online store. The board is $10, an the kit is $25. I also have available the fully assembled and tested boards, a "drop in" replacement board, $43 at my store, link to left.

While doing research for the ebike project (yes, I have more projects than I have time. Who doesn’t?) I decided I needed to learn a little bit about charging NiCd and NiMH batteries. That was some very interesting reading, in particular it was mentioned by several sources that NiCd and NiMH batteries could be charged 500 to 1000 times, according to the manufacturers of the batteries.

Now wait a second here. Back in the day I was enamored by the thought of interchangeable battery backs, and I bought quiet a few Versapak tools. The batteries that come with them say they are good for up to 300 charges, not 500 to 1000. And my own experience, plus anecdotes from friends that use them, would lead one to think that they are good for MUCH less than that. In fact I have had the experience of buying new Versapak batteries, using them once, sticking them on the charger, then when I went to use the tool again the battery would be completely dead.

I slowly came to the realization that the chargers for the Versapaks burn up the batteries. If you leave them on the charger, which is the natural thing to do in order to keep them "topped off", you will ruin the batteries.

The current that battery chargers charge with is generally expressed as a fraction of the capacity of the battery, denoted as “C”. So, for instance, the standard NiCd Versapak battery packs are rated at a “C” of 1.2 amp hours, so a charge rate of C/10 would be a charge current of 120mA. Some people claim that continuously charging NiCd batteries at C/10 (which is what the stock chargers do) won't hurt them; others claim that it will shorten their lives, but does not cause other problems.

I would say, both from personal experience, and from talking to friends, that it will DRAMATICALLY reduce the life of the battery packs. So much so that you should never –ever– leave these batteries on these chargers after they are fully charged. (Which is probably in the instructions I didn't read...)

Of course NiCd batteries also “self discharge” by up to 10% per month. Which means when you go to use the batteries that you have taken off the charger to keep them from being killed, there is a very good chance that they will need charging. You can’t win! Or can you?

I decided that I was going to build a smart charger for the Versapak batteries. These are chargers that detect when NiCD or NiMH batteries are fully charged, and shut down the charging current. While some so-called smart chargers charge two cells in parallel, I didn’t think this was such a good idea, so I decided to build a smart charger for each individual Versapak battery pack, and do it all inside the standard holder/charger. Yes, I would be “voiding” any warranty on the charger and probably the batteries, but it would be fun (and useful) to build, so I didn't care. BTW, these battery packs are really just three sub-c 4/5 size batteries, but that could be a whole other article!

I wanted to be able to use the existing holders for the Versapaks, and after taking one apart I discovered that there is plenty of room inside for additional circuit boards, or even expanding the component count and size of the current board. The battery contacts need a board to hold them in place, so I would need to have one under the batteries where the original board was anyway. I decided to see if I could build a better charger (with two smart charger circuits) that would fit in exactly the same slot as the current PC board (see Figure 1).

Cramming all these components in this tiny space meant I would have to use surface- mount components. This is right up my alley anyway, as I use them exclusively for all the circuit boards I make. This has the drawback that I can’t prototype things as easily as if I was using through hole components and a breadboard, but there is the advantage that when I have a working product, I don’t have to convert it to the final form, I just have to stick it in a case and move on. I do my prototyping using software design and simulation tools, specifically NI Multisim (a descendant of Electronic Workbench), and minimize my “breadboard” costs by buying in bulk and making my own PCBs; SMD components are much less expensive than through hole parts!

There are basically two methods of smart-charging NiCd and NiMH batteries: one method detects the fall in voltage that happens when the battery is fully charged and is called delta V; the other method detects the rise in temperature of the fully charged battery (called delta T.) I decided to forgo the temperature change method of determining that the batteries are charged, and instead use the delta V method. This way I didn't have to fiddle with trying to sense the temperature of the batteries in the stock Versapak battery holder. Sensing the temperature of the batteries would require putting thermistors in physical contact with the batteries, and I didn't want to have the make the modifications to the holder that his would entail.

There are quite a few manufacturers of smart charger chips, but Mouser (my favorite electronics supplier) carries the MC33340DG made by ON Semiconductor, which uses the delta V method for primary charge termination. It also supports elapsed time, or even thermistor temperature sensing, as secondary methods for terminating charging. And at $1.45 each they are fairly inexpensive.

The manufacturer’s example circuit shows a trickle mode as well as a “fast charge” mode. Cool. So the charger I was building would be a fast charger, then when the batteries were done charging, I could leave the batteries on the blasted thing, and it would keep them fully topped off. Yes!

As you can see from Figure 1, the existing board has hardly any components on it at all.

I decided to use bi-color LEDs, so that I could get an indication of when the charger switched from fast to trickle mode, and so I could use the existing plastic case with virtually no modifications.

As these things tend to work out, the charger section worked the first time, but the circuit to light the LEDs didn’t work at all, the trickle LEDs just always stayed on. I had come up with the bright idea of detecting the forward voltage drop over the blocking diodes in order to light the corresponding trickle and fast charge LEDs. Testing showed this only sort-of works, I could do it with the fast charge LED, but the trickle LED shows 0.5v forward voltage across it just from various leakage currents, and, of course, the standard 0.7v(ish) forward drop when the circuit was trickle charging.

Also, I had originally thought to use comparators to trigger the LEDs. This worked fine for the fast-charge LED, as the inverting input would go higher than the non-inverting input when it switched from fast to trickle. However, the inverting input for the trickling LED would never be higher than the non-inverting input, so the trickle LED would always stay on.

After thinking about this for a bit I realized I could simply use differential amplifiers with a specific finite gain. (See Figure 2) As I was getting 0.5v and 0.8v across the trickle circuit diode, I decided to stick a current sensing resistor on the trickle side and measure that instead of the forward drop over the diode. This gave me 35mV when trickling, and 00.6mV when no battery was connected. I turned the gain down on the differential amps so that the amplified voltage in the “off” state would be less than the forward voltage drop on the green section of the bi-color led, and “Bob’s your Uncle”. (When I told this to the electrical engineering friend who helps me work out problems like this, he replied “Bob *IS* my Uncle… hehehe)

Don't have Versapaks? The charger circuit in Figure 2 can be used to charge any NiCd or NiMH 3.6v (three cell) battery pack without modifications, or any two cell pack (which I'm not sure anyone makes). It won't work beyond that on either end without changing the voltage sense divider. The voltage divider for the smart charger chips has to have the sense voltage in a certain range (1.0V to 2.0V for the chips I use). It is also more sensitive and works better if you use the high end of the range (mine is set for ~1.85V)

The commercial "universal" nicd/nimh chargers you can purchase work from 7.2-12v, they work on the same principal.

If you modify the circuit in Figure 2 so that the voltage divider formed by R1/R2 (and R11/R13) gives you around 1.85V when your battery packs are fully charged, you can use the circuit to charge any NiCd or NiMH battery pack up to 18V. This is the most the MC33340DG will handle. Of course you could isolate the MC33340DG and the LM224 behind voltage regulators, and use high voltage adjustable regulators like the Texas Instruments TL738 then go quite high with the voltage, but that is beyond the scope of this article.

<Toner transfer sidebar>
Toner transfer is a method (common in hobby PCB making) of printing your circuit board on paper, then using heat and pressure to de-laminate the plastic toner onto the clean, bare copper, untreated (and cheap) PCB board.
It can be done with ink jet photo paper and a clothes iron, but you get more consistent results if you use a laminator and glossy paper. I use Hammermill Laser Gloss, but others have had good results with the gloss paper used in magazine pages.
You print the resist pattern on the paper, use the iron or laminator to transfer it to the copper, then you soak the paper in water to make it easy to remove. The plastic toner stays on the copper, and makes an excellent etch resist.
It takes a little practice to get the technique down, but once you have it down you can knock out good prototype boards for essentially the cost of the copper board.
</toner transfer sidebar>

I used the toner transfer method to make the board pictured in Figure 3. I can reliably make boards with 7 mil traces and 10 mil spacing, though, truth be told, the toner does spread out a bit in the process, so the traces are slightly wider and the gaps narrower. I’m more comfortable using a 15 mil trace clearance, but in order to get this thing to route on the tiny board I had to bump the clearance down a bit.

Like I mentioned earlier, I’m fairly comfortable working with SMT components, which is why I use them for prototyping. And as my prototyping boards rarely work right the first time, or the second time, I like to tell people that I’m very good at making boards that don’t work…

I can have a few of these board professionally manufactured if any of you want to build your own Versapak smart chargers without making your own board. They would be available for a few dollars (costs would depend on order size). If you are interested just send me an email at: here to ask about them.

A few words about working with surface mount components:

1206 form factor is about as small as you want to go, this is about the size of a match head. With the proper tools and good magnification you can work with considerably smaller SMT components, I have worked with 0603, which is about the size of two grains of salt. Others work with even smaller parts. However, the 1206 form factor is large enough to have a decent power rating, heavy enough to not stick to the tweezers, and almost solder-able without magnification. Plus it's about he same price as the smaller parts.

Use lots of flux. I like to coat the bare board with flux, let it dry, then give it another coat just before I solder on the components. Surface tension is your friend. I use a rosin core flux pin exclusively.

Use magnification and lots of light.

A temperature-controlled soldering iron is best (set to 350C), but you can get away with a small 15W iron with a fine tip.

Put everything on one face of the board before you try to solder the parts. Just get them on top of the pads and oriented close to correctly, you can fine tune as you solder. The wet flux will hold things in place until you are ready to start soldering.

I hold the tweezers in my left hand in such a way that the little finger of my hand is resting on the work surface, I then to move the components around by moving my arm, not my fingers. This is how the artists who work on the master dies for coins do it.

The method for SMT soldering I’ve found recommended on several websites, “put some solder on the pad, hold the component in place, then melt the solder” works OK, but I prefer the following. I just put a tiny dab of solder on the end of your iron, hold the component in place with the tweezers, and touch the iron to the joint between the component and the pad. Surface tension does the rest. Use a very fine lead free solder, .020" size works for me.

Don’t worry about having a little extra solder on the component, and don’t worry about the part being perfectly lined up. Hold the iron on the part for about ½ second then pull it off, and give it a second to cool before letting go with the tweezers. Solder one pad on all components, then go back and finish that side of the board. Check your work under high magnification (at least 10X), flip over and repeat.

You will need to pull the battery connectors off the old board that comes with your sacrificial charger. I’ve found that the best way to do this is to simply load up some solder wick with flux and get all the solder to wick-out. Then gently bend up the tabs that hold the clip in place and then heat the leg again while gently pulling on the part.

You will need a new power supply to run your fast charger, the ones that come with the Versapak chargers don’t put out enough voltage (or current). Anything that puts out approximately 12V at 1A should work. I found a deal on some scratch and dent linear supplies, but a switching supply would work OK as well. Most of the 12V "wall wart" supplies I have tested put out more than 12v, but you should be OK with a few extra volts. I wouldn’t go much higher than around 16V.

Once everything has been assembled and you place the “new and improved” board into the Versapak charger/holder (Figure 4) you can plug in the power supply. The LEDs should flicker briefly then go out. This is an unintentional "feature", but it lets you know you have hooked up the power, and everything is working, so I didn't try to fix it. When you plug a battery in you will get an orange light (red for fast charge plus green for trickle charge equals orange) showing that the charger is working and attempting to charge the batteries. When the batteries are almost fully charged you will see the light switch from orange to green and then back to orange periodically. This is the smart charger chip shutting off the fast charge circuit in order to get an accurate reading on the battery voltage. Given enough time, the LEDs will stay green, indicating that the batteries have been charged and the charger has changed to trickle-charge only mode.

Enjoy!

Parts List

Sacrificial (but to be improved greatly!), stock VersiPak charger

Power Supply: 12vdc @1A

Components:

(2) – MC33340DG (smart charger chips in SOIC-8 footprint)

(1) – LM224D (quad op amp in SOIC-16 footprint)

(2) - LM317LIPKG3 (any LM317 clone in the SOT-89 footprint should work)

(2) - LM317MDTX (any LM317 in DPAK footprint should work)

(2) - TLUV5300 bi-color LED

Resistors, 2010 surface mount

(2) 3.1 ohm

Resistors, 1206 surface mount, 5% OK:

(8) 1M

(12) 10k

(2) 7.5k

(2) 2k

(2) 1k

(2) 510 ohm

(2) 33 ohm

(2) 1 ohm

(2) 0 ohm

Misc

Caps, 1206 surface mount

2 – 10uF

1 – 100uF

Diodes:

(2) CD214A-B160

(2) FDLL4148_Q

Next project, a battery tab welder so I can fix all the dead Versapak batteries in my inventory of “junk”.