Converting a Cordless Drill to Lithium, Part Two: Charging It Properly

Last time, I explained how to properly rebuild a battery pack for a cordless power tool. I also said I’d cover the specifics of charging, and this time I’ll be reviewing a DC-DC converter board.
If you’re interested, come on in.

I originally planned to keep this to two parts: rebuilding the battery and modifying the charger. But while preparing the review, I came up with an idea for a more complex third part.
In this part, I’ll explain how you can modify the original transformer-based charger if it still works—or at least if its power transformer is still alive.

I ordered several of these converter boards quite a while ago as spares, specifically for this project. They have a few interesting features, but I won’t get ahead of myself; let’s take things in order.

First, I’ll divide chargers into three main types:
1. The simplest ones: a transformer, a bridge rectifier, and a few other components. These come with ultra-budget tools.
2. Branded chargers. Essentially the same thing, but with some basic «brains» added to automatically stop charging when the battery is full.
3. «Advanced» chargers: a switching power supply, a charge controller, and sometimes the ability to charge several batteries at once.

Tools in the first category rarely get modified, since it’s often easier (and cheaper) to buy a new one. The third category usually presents its own challenges. In principle, chargers in the third group can be modified too, but not as part of this article: there are so many different types, and each requires an individual approach.

This time I’ll be modifying a charger from the second group: a branded one, though a simple model. But the modification has a lot in common with the first group, so it should be useful to more readers.

To charge a battery, you can’t just connect it to a power supply; that experiment usually doesn’t end well. You need to connect it to a charger. This is where a bit of confusion comes in, because quite a lot of people are used to calling the small power supplies they use to charge their smartphones, tablets, and laptops «chargers». They’re not chargers; they’re power supplies.

So what’s the difference between a charger and a power supply?
A power supply is designed to provide a regulated voltage across its specified load-current range.
A charger is usually more complex, because its output voltage depends on the load current, which is limited in turn. A charger also has a circuit that stops charging when the battery is full, and sometimes protection against connecting the battery with the wrong polarity.

The simplest charger is just a power supply and a resistor (sometimes an incandescent bulb, which is actually better) in series with the battery. This circuit limits the charging current, but as you can imagine, it can’t do anything else.

A slightly more complex version adds a timer that stops charging after a set time, but this approach quickly «kills» batteries.
For example, that’s how one inexpensive screwdriver charger works (photo not mine).


Next come the «smarter» chargers, although in essence they’re not much better than the previous type.
For example, here’s a photo of a branded Bosch charger designed for NiCd batteries.


But all these chargers look very simple compared with modern chargers for lithium batteries.




Of course, the last option doesn’t quite fit our approach to this project, since we want our charger not only to charge properly, but also to cost as little as possible.

Chargers for Chinese screwdrivers are, of course, much simpler, but then again, I doubt anyone would want to build one from scratch. That’s exactly what I’m planning to do in part three, though—with a more correct design.


So, to start with, let's assume we have a charger that simply isn't suitable for the new type of battery, but still works. Or at least its transformer does.
As I wrote above, you could even just use a resistor or a light bulb, but that's “not our way.”

A typical inexpensive charger circuit looks roughly like this:
Transformer, bridge rectifier, thyristor, and control circuit. Sometimes, though, there's a relay instead of a thyristor, the current isn't limited at all, and there may be an overheat protection circuit (although that doesn't always help either).


But all we need from this circuit is the transformer and bridge rectifier. We'll have to add a capacitor, though. That gives us a basic, unchanging section, marked in red, which won't be modified further.


The bridge rectifier is usually on the circuit board and can be reused if needed (provided it's working). So, basically, you can desolder all the components from the board, leaving just the four diodes and the battery terminals, and use the board itself as a base.
The cathode on each diode is marked with a stripe. The point where the two striped leads connect is positive; accordingly, the point where the unmarked leads connect is negative. The transformer connects to the other two junctions.


However, when you open the charger, you may find something like this (ignore the missing transformer):
In this case, you'll have to desolder everything.


It's convenient to replace the diodes on the board with a ready-made bridge rectifier. Connect the transformer to the AC terminals; the + and − terminals then go on to the rest of the circuit.
I could explain how to choose a capacitor, but I recommend keeping it simple and using one like the one in the photo: 1000 µF, 35 V. You can use a higher capacitance, say 2200 µF, and a voltage rating of 50 or 63 V, but there's no point in going higher on either; it will only make the capacitor larger.
Any capacitor will do, even a no-name one. And yes, you need to install it in any case, whether or not the bridge rectifier is working.


Now let's move on to the charger itself—or rather, its possible implementations. This section is marked with a rectangle in the last schematic.
The simplest and reasonably proper approach is to use an LM317 voltage regulator IC.


But as I wrote above, the charging current needs to be limited. Yes, many circuits can not only limit it but regulate it as well. But, by and large, batteries don't care whether the charging current is 1, 2, or 3 A, or whether it stays constant or fluctuates during charging. What matters is that it doesn't exceed the limit specified for the batteries. That said, it's hard to exceed that limit with the batteries used in cordless screwdrivers, since they can handle high discharge currents as well as high charging currents.
The simplest solution is to switch the LM317 from voltage regulation to current regulation—or, more precisely, to add current regulation.
You do this by adding a single resistor, as shown in the schematic.
The resistor value is easy to calculate: 1.25/I (current in amps) = R (resistance in ohms).
For example, if you need a current of 1.5 A, then 1.25/1.5 = 0.83 Ω.


Calculating the resistor values for the voltage divider is also quite simple, but I’d recommend putting a trimmer in series with the upper resistor so you can set the voltage precisely, since accuracy matters here, unlike with current.
You can use a special calculator, but it’s not very convenient, so I’ll suggest values without it. For 12.6 V (three 3.7 V batteries in series), use a 1.5 kΩ upper resistor, a 200 Ω trimmer in series with it, and a 13 kΩ lower resistor.

I specifically mentioned that the trimmer should be connected in series with the upper resistor. If it goes open circuit, the output voltage will be at its minimum. If the lower resistor goes open circuit, the output voltage will be at its maximum. Incidentally, common DC-DC converter boards do this the other way around: if the trimmer goes open circuit, they put the maximum voltage on the output.


The circuit above has plenty going for it: it’s simple and inexpensive. But the high power dissipation wipes out those advantages, since it needs a pretty substantial heatsink, so it’s not a great fit for high charging currents.

A better option is to use a step-down DC-DC converter. For example, this one:


Of course, as supplied, it won’t limit the current, but you can modify it if you want (if you already have one, for instance).
The modification is simple, and I’ve already described it in one of my reviews. I used it there as an LED driver at the end, but that doesn’t really matter.
You’ll need:
1 BC557 transistor or any equivalent (even the well-known KT361 or KT3107)
2 resistors rated 33–200 Ω, any power rating.
1 resistor to use as a current-sense shunt
1 0.1 µF ceramic capacitor.

The current-sense resistor is very easy to calculate, just as with the LM317, though the values are slightly different.
0.6/I (current in A) = R (resistance in Ω).
For example, if you need a current of 1.5 A, then 0.6/1.5 = 0.4 Ω.

Connect the output of the added circuit to pin 4 of the LM2596 IC. If you’re using a different IC, find the pin marked FB in its datasheet and connect it there.

With this setup, use the trimmer to set the output voltage (with no load). This circuit may slightly undercharge the batteries, though not by much—that’s the trade-off for simplicity. To charge them fully, connect the voltage-sense input (one of the voltage-divider resistors) to the output of the entire circuit.


All the charging methods described above work, but they’re not very convenient.
A better approach is to use a board that can regulate not only the output voltage but also the current.
For example, this board. It’s easy to tell suitable boards from the rest: the description should say “DC-DC StepDown,” and the board should have at least two trimmers.


Besides output-current adjustment, this board has an extra bonus: indicator lights.
1. The LED at the top indicates current-limit mode.
2. The pair of LEDs at the bottom indicate when charging is complete.

The battery charge indicator works very simply: the LEDs switch when the current drops below 1/10 of the initially set value. This operating mode is very common and is used in many basic chargers.
For example, if we set the charging current to 1.5 A and connect a battery, then once the charging current drops below 150 mA, one LED will turn off and the other will light up, indicating that charging is complete.



The circuit diagram for this board is also from the review mentioned above; it may come in handy.


So this board is quite well suited for charging batteries: first set the end-of-charge voltage to 4.2 V per cell, then set the charging current.
For those who want a little more, there’s a similar board with charging-current and battery-voltage readouts, but I think that’s unnecessary in this case.
I reviewed this board, and this is actually a photo from that review. I also showed how to build a switching power supply yourself there.


This is what this option looks like in the block diagram.


We’re gradually getting to the subject of this review, which caught my eye primarily because of its low price. I have serious doubts about whether the installed chip is genuine, but as long as you don’t push it to the full rated 3 A, it should work just fine.

As it turned out, I hadn’t originally planned to review this board. Although I bought four of them, only one was left at home, and that one already showed signs of my tinkering.
I desoldered the original LEDs and soldered in different ones.


In its original form, the board has three LEDs:
1. Charged.
2. Charging.
3. Current-limit indicator.

How the indicators work.
The Charging and Charged LEDs are wired so that only one lights at a time, so you can think of them as a single indicator. On boards without adjustable current, the indicator switches when the charging current drops below 1/10 of the value set by the resistor—Current Limit. On the board under review, you can set the switching threshold to any value; I’d recommend setting it to 1/5.

The current-limit indicator works a little differently: it lights when the current is being limited—that is, when the current at the set voltage tries to rise above the value set with the adjustment knob.
For example, suppose you set the current to 1 A and the voltage to 10 V (for the sake of argument), then connect a load that draws 0.5 A at 10 V. The output will be 10 V at 0.5 A. Now connect a load that would draw 1.5 A at 10 V: the output will be 1 A at 8 V (again, for the sake of argument). In other words, the board lowers the voltage until the output current no longer exceeds the set value, and the LED lights up.


The board also has three trimmer potentiometers:
1. Output-voltage adjustment.
2. Adjustment of the charge-complete indicator threshold.
3. Output-current limit adjustment.

The board is quite simple. It has an LM2596 chip, a 78L05 regulator, and an LM358 comparator.
The LM2596 is the PWM controller.
The 78L05 powers the comparator and provides a reference voltage.
The LM358 “monitors” the current and also controls the indicators.


A trace on the circuit board serves as the current-sense shunt.
This method of measuring current isn’t very good, since the current will “drift” with the board temperature. But since stable output current doesn’t matter to us, we can ignore that.


Pinout, controls, and indicators, as shown on the product page.


Boards with output current limiting are very well suited for charging batteries. And boards that indicate when charging is complete also offer a bit of convenience, letting you know when the battery is charged.
But all the methods listed above have one drawback: none of these options can disconnect the battery once charging is complete, i.e. stop the process entirely.
Of course, someone will ask: what about the batteries in uninterruptible power supplies? There is a special consideration here: some types of batteries have what is called cyclic charging and so-called standby charging. The same lead-acid battery is charged to 14.3-15 V in cyclic mode, but only to 13.8-13.9 V in standby mode.

If the battery isn't disconnected, a small charging current will always flow through it. Lithium batteries are a little «luckier» in this respect—their current drops significantly—but even so, leaving them in this mode is not recommended.
The thing is, cadmium or lead-acid batteries simply start to degrade and heat up, while lithium batteries can catch fire. Yes, lithium batteries have a safety valve, but extra protection never hurts.

A question that comes up very often is: what about the protection board? It can disconnect the battery when charging is complete. It can, and it will—but not at 4.2 V per cell. It will do so at 4.25-4.35 V, because disconnecting the battery is a protective function for the board, not its primary purpose. So this is strongly discouraged.

That's why I came up with a simple little circuit that disconnects the battery when charging is complete. The operating principle is very simple (which means it has some limitations). Connect the battery. Since capacitor C1 is discharged, current flows through it and turns on the transistor, which supplies current to the relay. The relay connects the battery to the charger, and then the relay is powered through an optocoupler connected to the charge indicator output of the converter board.


So I designed a small board, in a universal layout.


The rest is simple and familiar: print the board layout on paper, transfer it to the copper-clad board, and etch it.
If you're interested, the PCB fabrication process is shown in detail in this review.

When I came up with the circuit, I tried to simplify it as much as possible, using the fewest components I could.
1. Relay — any relay with a 12 V coil (for setups with 3-4 batteries) and contacts rated for at least twice the charging current.
2. Transistor — BC846, 847, or the well-known KT315, KT3102, or equivalents.
3. Diode — any low-power diode.
4. Resistors — any values in the 15-33 kΩ range.
5. Capacitor — 33-47 µF, 25-50 V.
6. Optocoupler — PC817, found on most power supply boards.


I assembled the board.


I made the board universal: you can use a MOSFET instead of the relay, and some of the components are the same as before. This version is also more versatile, since it works with cordless drills using 3, 4, or 5 batteries.
But this board has a drawback. There is a «parasitic» diode inside the transistor, and if you leave the battery connected to the charger but unplug the charger from the wall, the battery will discharge through the charging circuit. The version I showed above has a similar issue, but the current there is tiny, about 0.5 mA, so it would take around 4000 hours to fully discharge the battery.

The component values here are slightly different, although in practice only the values of resistors R4 and R5 matter. R5 should be at least 2 times lower in value than R4.


Let's pick out the components for the board. Unfortunately, you'll probably have to buy the transistor, since it's rarely used in off-the-shelf devices. You might find one on a motherboard, but that's very uncommon.




The board is universal: you can use a relay and follow the previous circuit, or use a MOSFET.


The charger block diagram will now look like this:
Transformer, then a bridge rectifier and filter capacitor, followed by the DC-DC converter board, and finally the cutoff board.
I haven't marked the polarity of the charge indicator connections, since it can vary from board to board. If something doesn't work, just swap the wires to reverse the polarity.


Now let's get to the conversion itself.
First, I cut the traces from the bridge rectifier output, the battery connection terminals, and the charge indicator LED. The goal is to disconnect them from the rest of the circuit so it won't get in the way of the “operation.” Of course, I could simply desolder all the components except the bridge diodes; it would amount to the same thing, but cutting the traces was easier for me.


Next, solder in the filter capacitor. I soldered it directly to the diode leads, but you can install a separate bridge rectifier, as I showed above.
Remember: the lead with the stripe is positive, and the one without it is negative. The capacitor's longer lead is positive.


The boards wouldn't fit on top at all, constantly hitting the upper cover, so I had to put them underneath. That wasn't entirely straightforward either: I had to cut off one support and file down the plastic a little, but they fit much better there anyway.
They even have some extra clearance in height.


Now for the electrical connections. First, solder on the wires. I initially wanted to use thicker ones, but then realized I wouldn't be able to maneuver them in the cramped enclosure, so I used ordinary stranded wire with a cross-sectional area of 0.22 mm².
I soldered these wires to the upper board:
1. On the left: the converter board's power input, connected to the bridge rectifier.
2. On the right: white and blue wires—the converter board output. If you're using a cutoff board, connect them to it; otherwise, connect them to the battery terminals.
3. Red and blue wires: the charge-in-progress indicator output. Connect them to the cutoff board if you're using one; otherwise, connect them to the indicator LED.
4. Black and green wires: charge-complete indicator. If you're using a cutoff board, connect them to the LED; otherwise, leave them unconnected.

For now, only the battery wires are soldered to the lower board.


Oh, I almost forgot: you can see an LED on the board on the left. I completely forgot and desoldered all the LEDs on the board, but if you desolder the current-limit indicator LED, the current won't be limited. So leave it in place (it's marked CC/CV on the board). Be careful.

Anyway, connect everything as shown; the photo is clickable.


Then stick double-sided tape to the bottom of the case. Since the undersides of the boards aren't completely smooth, it's better to use thick tape. Basically, do this however you like: you can use hot glue, screw them in, or even nail them down :)


Stick the boards in place and tuck the wires away.
We should be left with 6 loose wires: 2 to the battery, 2 to the bridge rectifier, and 2 to the LED.


Ignore the yellow wire; that's a special case. The only relay I could find was rated for 24 V, so I powered it from the converter input.
When preparing the wires, always try to follow the color coding: red/white for positive, black/blue for negative.


Connect the wires to the original charger board. Of course, the details will vary from one setup to another, but I think the general idea is clear. Be especially careful to check that the battery terminals are connected correctly. It's best to use a multimeter first to identify positive and negative; the same goes for the power input.


After all these steps, be sure to check the converter board's output voltage and adjust it again if necessary. The setting can get knocked out of place during installation, leaving you with 12.79 V instead of 12.6 V (the voltage of three lithium batteries).
You can also adjust the charging current.

Since setting the charge-complete indicator threshold isn't very convenient, I recommend buying a board with two trimmers; it's easier. If you bought a board with three trimmers, you'll need to connect a load to the output for adjustment, drawing roughly 1/10–1/5 of the set charging current. For example, with a charging current of 1.5 A and a voltage of 12 V, you could use a 51–100 Ω resistor rated for about 1–2 W.

Once it's adjusted, check everything before reassembling.
If everything is wired correctly, connecting the battery should activate the relay and start charging. In my case, the indicator LED goes out while charging and turns on when charging is complete. If you'd rather have it work the other way around, connect the LED in series with the optocoupler input; then it will stay lit while charging.


Since the review title mentions the board, but the article is about modifying the charger, I decided to test the board itself too. After half an hour of operation at a charging current of 1 A, the IC temperature was about 60°C, so I'd say this board can handle up to 1.5 A. I suspected as much from the start: at 3 A, the board would most likely fail due to overheating. The maximum current at which the board can still be used relatively safely is 2 A, but since it's inside a case and cooling isn't very good, I recommend keeping it to 1.5 A.


That's it. Screw the case back together and let it run through a full charge. I did have to discharge the battery first, since I'd charged it while preparing the previous part.
If you connect a charged battery to the charger, the relay activates for 1.5–2 seconds, then switches off again because the current is low and the lockout doesn't engage.


Now, the good news and the not-so-good news.
The good news: the modification worked, charging starts, and the board disconnects the battery. Overall, it's simple, convenient, and practical.
The bad news: if you switch off the charger's power during charging and then switch it back on, charging won't resume automatically.
But there's a much bigger problem. While preparing this, I used the board from the previous review, where I mentioned that it has no controller, so it can't fully shut down. More «intelligent» boards, however, completely disconnect the output in a critical situation. Since that output also serves as the input, connecting one to the charger I modified above means it won't start. It needs voltage to start, and the board needs voltage to start too :(

There are several ways to solve this problem.
1. Put a resistor between the input and output of the protection board, allowing current to reach the terminals and start the charger. But I don't know how the protection board will behave, and I have nothing to test it with.
2. Bring the charger's input out to a separate battery terminal. This is often done with cordless tools that use lithium batteries: charge through one set of contacts and discharge through another.
3. Don't install a disconnect board at all.
4. Replace the automatic circuit with a button, as in this schematic.

The top option is without a protection board; the bottom one uses just a relay, an optocoupler, and a button. The principle is simple: insert the battery into the charger, press the button, charging starts, and you can go relax. Once charging is complete, the relay fully disconnects the battery from the charger.

Standard chargers keep trying to apply voltage to the output if it's below a certain level, but this modification is inconvenient, and using a relay doesn't really work either. I'm still thinking it over, though; maybe I can come up with a neat solution.


Here are a few recommendations for choosing a battery charging option:
1. Simply use a board with two trimmer resistors (it's covered in the review). It's straightforward and works reasonably well, but remember that the charger is on. Leaving it for a day or two probably won't cause problems, but I wouldn't recommend going on vacation and forgetting to switch off the charger.
2. Do it as shown in the review. It's complicated and has limitations, but it's the better approach.
3. Use a dedicated charger, such as the well-known Imax.
4. If your battery pack consists of two or three cells, you can use a B3.
It's quite simple and convenient.


5. Get a power supply and modify it a little. I did something similar in this review.


6. Build a completely custom charger, with all the automatic shutoffs, proper charging, and expanded status indicators. This is the most difficult option. But that’s a topic for the third part of the review; I’ll probably also cover converting a power supply into a charger there.

I also often get questions about balancing cells in a battery pack. Personally, I think it’s unnecessary, since good-quality, well-matched cells are not that easy to throw out of balance. If you want something simple and reliable, it’s much easier to buy a protection board with a balancing function.

Recently, someone asked whether it’s possible to make a charger that can charge both lithium and nickel-cadmium batteries. Yes, it’s possible, but it’s better not to, because the batteries have different voltages as well as different chemistries. For example, a pack of 10 nickel-cadmium cells needs 14.3–15 V, while a pack of three lithium cells needs 12.6 V. That means you need a switch, and it’s easy to forget to flip it. A universal option is possible only if the number of nickel-cadmium cells is a multiple of three—9, 12, or 15—so they can be charged like 3-, 4-, or 5-cell lithium packs. But common power-tool batteries use 10-cell packs.

That seems to be everything. I’ve tried to answer some of the questions people send me privately. I’ll probably also add answers to your next questions to the review.

The boards I bought work, but the ICs are probably counterfeit, so it’s best not to load them beyond 50–60% of the rated value.

Meanwhile, I’m thinking about what a proper charger, built from scratch, should have. Here’s what I have planned so far:
1. Charging starts automatically when a battery is inserted
2. Restarts after a power outage.
3. Several charging-status indicator levels
4. Select the number and type of cells using jumpers on the board.
5. Microprocessor control

I’d also like to know what you’d like to see in the third part of the review (you can message me privately).

I wanted to use a dedicated IC (I think you can even order a free sample), but it only works in linear mode, which means heat :((((

This might be useful: link to an archive with the PCB layouts and schematics. But as I mentioned above, the add-on board probably won’t work with boards that completely disconnect the batteries.