Battery protection board, or how to rebuild a cordless drill battery

Since I’ve reviewed quite a few batteries and also mentioned converting cordless tools, I often get private messages asking about various details of these conversions.
Different people ask, and the questions are often much the same, so I decided to put together a short overview and answer some common questions about choosing components and converting batteries.

Some people may find this overview incomplete, since I’m only converting the battery pack itself. But don’t worry: I’m planning a second part where I’ll try to answer questions about converting the charger. I’d also like to hear what people think: which is better—a combined power supply and controller board, a standalone board, DC-DC boards, or something else?

Cordless drills, along with other cordless tools, have been around for many years. As a result, users have accumulated quite a few old batteries, as well as tools that sometimes just sit around unused.
There are several ways to deal with this:
1. Simply repair the battery by replacing the old cells with new ones.
2. Convert it from battery power to mains power, even going so far as to install a power supply in the battery compartment.
3. Replace the nickel-cadmium or nickel-metal hydride cells with lithium cells.

As a brief aside, sometimes there’s simply no point in converting or repairing a tool. For example, if you have a really cheap drill you bought in a mega-sale for 5 bucks, you might be surprised to find that the conversion costs as much as several of those drills (I’m exaggerating). So first, weigh up the pros and cons and decide whether the conversion makes sense. Sometimes it’s easier to buy another tool.

Many of you have probably already tried the first option, as have I. It works, although with branded tools the result is often worse than it was originally. It costs a little less and is much easier and less labor-intensive.

The second option is also worth considering, especially if you work at home and don’t want to spend money replacing the batteries.

The third option takes the most work, but can significantly improve the tool’s performance. You get more battery capacity, no “memory effect,” and sometimes more power.
But besides the extra work, there’s a side effect: lithium batteries don’t perform quite as well in freezing temperatures. Still, given that many manufacturers make cordless tools like this without any problems, I think the issue is sometimes overstated, even though it is real.

Battery packs come in different designs, though they have a lot in common. So I’ll explain the process and show it using one example: the Bosch PSR 12 VE-2 drill. It belongs to a friend of mine, who also “sponsored” this overview by providing the drill, batteries, protection board, and consumables for the conversion.
It’s a pretty good drill, with a spindle lock and two speeds, so it’s worth converting.


As it turned out, there were actually three battery packs, but we’ll convert just one. I’ll save another for a future overview :)


By the way, the batteries are different, but both are rated at 12 V, with a capacity of 1.2 Ah, or 14.4 Wh.


Battery packs come apart in different ways, but most often the case is held together with a few self-tapping screws. I’ve also come across snap-fit and glued cases.


In any case, this is roughly what you’ll find inside. This pack contains 10 nickel-cadmium cells. Usually, cells of the same size are used, but their arrangement can vary. The photo shows one of the common layouts: 9 cells at the bottom and one in the vertical section.


First, we need to choose replacement batteries.

Power tools use batteries designed to deliver high discharge currents.
Not long ago, I put together a review of various batteries, and included a table at the end that may help with this. If you're not sure, simply look up the datasheets for the batteries you plan to buy. Fortunately, branded batteries usually have good documentation.


For example, I recommend these types:
For high-power tools — Samsung INR18650-25R, US18650VTC4, Liitokala INR26650-50A
For medium-power tools — LGDBHG21865, LGDBHE41865, Samsung INR18650-30Q

Keep in mind that a battery's stated capacity is often inversely proportional to its maximum discharge current. In other words, the higher the current a battery is designed for, the lower its capacity. This example is, of course, somewhat simplified, but it's very close to reality. For instance, the high-capacity Panasonic NCR18650B batteries aren't suitable for power tools because their maximum current is only 6.8 A, while a cordless drill draws 15–40 A.


And now, what not to use:
The batteries shown in the photo below, as well as all those UltraFire and MegaFire cells, and any 18650s claiming a capacity of 100,500 mAh.
Also, I strongly advise against using old batteries from laptop packs. First, they aren't designed for this kind of current. Second, they're likely to have widely varying characteristics—not just in capacity, but also in internal resistance. Put them to use somewhere else instead, for example in a power bank to charge your smartphone.


Another option is RC batteries, for example, ones for boats, quadcopters, cars, and so on.
You can certainly use them, but I'd prefer the familiar 18650 or 26650 cells because they have a sturdy casing and are easier to replace in the future. 18650 and 26650 cells are easy to find, while RC batteries may be discontinued and replaced with batteries in a different form factor.


Above all, remember not to use batteries with different capacities. Ideally, use batteries from the same batch, buying the required number all at once (plus one spare, just in case you end up with a mismatched cell). So if you have two batteries sitting on a shelf for a year, then buy a couple of new ones and connect them in series, you're increasing the risk of problems. Balancing may not help at that point, and that's not even considering batteries that had different capacities to begin with.

For rebuilding this cordless drill's battery pack, I chose LGDBHG21865 batteries.
The drill isn't very powerful, so I don't think there should be any problems. These batteries are rated for a continuous discharge current of 20 A. When choosing batteries, find the corresponding line in the datasheet and check the current rating listed there.


Lithium batteries have noticeably higher capacity in a smaller package than cadmium batteries. In the photo, the pack on the left is 10.8 V, 3 Ah (32 Wh); the original pack on the right is 12 V, 1.2 Ah (14.4 Wh).

When choosing how many replacement batteries you need, a good rule of thumb is that one lithium battery (Li-Ion, LiPol) replaces three conventional cells. A 12 V battery pack contains 10 cells, so it is usually replaced with three lithium cells. You can install four, but the tool will be overloaded and could be damaged.
If you have an 18 V battery pack, it most likely contains 15 conventional cells, which are replaced with five lithium cells, but tools like that are less common.
Or, to put it simply:
2–3 NiCd = 1 lithium cell,
5–6–7 NiCd = 2 lithium cells,
8–9–10 NiCd = 3 lithium cells,
11–12–13 NiCd = 4 lithium cells
and so on.


Before assembling the pack, check the capacity of the batteries, because even cells from the same batch can vary—and the less reputable the manufacturer, the greater the variation.
For example, here's a table from one of my reviews, where I tested and, while I was at it, sorted batteries into matched sets for converting radio packs.


After that, fully charge all the batteries to bring them to the same state of charge.


Connecting the batteries.
There are several ways to connect the batteries:
1. Holders
2. Soldering
3. Spot welding.

1. A holder is simple and readily available, but absolutely not recommended for high currents because of its high contact resistance.
2. Soldering. It can certainly work—I do it myself sometimes—but there are a few things to keep in mind.
At the very least, you need to know how to solder. And not just solder, but do it properly—and, most importantly, quickly.
You also need a suitable soldering iron.
Here's how to solder the connection: Clean the contact area and apply flux (I use F3). Take a tinned wire (not too thick; 0.75 mm² is enough), load plenty of solder onto the iron tip, touch the wire, and press it against the battery contact. Alternatively, place the wire on the joint and touch the point between the wire and battery with a soldering iron carrying a large blob of solder.
But as I mentioned above, there are a few things to keep in mind: you need a powerful soldering iron with a large, heavy tip. A battery has a high thermal mass, and a small tip will simply cool down so much that the solder “freezes” in place—sometimes along with the tip (depending on the soldering iron). You'll end up trying to heat the joint for a long time and eventually overheat the battery.
That's why people use an old soldering iron with a large copper tip, preferably well heated. That way, only the joint heats up; the heat then spreads out, and the overall temperature stays fairly low.
The negative terminal is the problem; soldering the positive terminal is usually straightforward because it's lighter, but I still don't recommend overheating it.

In any case, if you don't have soldering experience, I strongly recommend against this method.

3. The best method is spot welding: it's instant and doesn't overheat the battery. But the welder must be set up correctly so it doesn't punch a hole right through the bottom of the cell, so it's better to leave this to the professionals. You can have your battery pack welded at a market for a small fee.
Another option: some online stores offer a service (or rather, listings with or without tabs) to have contact tabs welded on. It's not very expensive, and it's much safer than soldering.

This pack was “welded” by the same guy who lent me the drill for review.
The photo shows a piece of cloth insulation between the tab and the battery casing. This is important: without it, you could overheat the tab and melt through the battery insulation. I think the consequences are obvious.


Attentive readers have probably noticed the odd plastic spacers between the cells.
This is one of those things that falls into the “how to do it properly” category.
The tool is subject to vibration during use, so there is a chance the insulation between the cells could be damaged (I’ve never seen this happen, but it’s theoretically possible). The spacers prevent this. You can leave them out, but using them is the better way to do it. I can’t tell you where to buy them, but you could try looking at battery stalls in markets.


Next, we need to run wires for connecting to the protection board and the terminal block.
For the power wires, I use wire with a cross-sectional area of at least 1.5 mm², and 0.5 mm² for less heavily loaded circuits.
Of course, you may ask why use 0.5 mm² wire when there’s no current flowing through it and much thinner wire would do. Wire with a larger cross-section has thicker insulation and is mechanically stronger, so it’s harder to damage. You can use any wire you like; I’m just showing the option I consider the better one.
Ideally, tin the wires at both ends first and insulate the free ends, but that’s only practical when rebuilding the same battery for a second time, once you know the required wire lengths. For the first rebuild, I usually leave some extra length.


If you look closely, you can see holes in the outer battery terminals in the photo above. This is also done to make the connection more reliable. The untinned wire is inserted into the hole and soldered in place, which reduces the risk of a poor connection.
So, solder the wires in place, and while you’re at it, it’s a good idea to add heat-shrink tubing to insulate the terminals.


This is what the assembly looks like when finished. Two wires run from the positive terminal because of the way the protection board is connected.


The final step in preparing the assembly is recommended rather than essential. Since the assembly is “live,” the cells need to be secured relative to one another. I use heat-shrink tubing for this, although in this case “sleeve” would be more accurate. It’s fairly thin but quite strong; its purpose is to compress the whole assembly.


Slide on the heat-shrink tubing and shrink it with a heat gun. The usual lighter method probably won’t work here, since it’s best to heat it evenly.
The result is a battery assembly that looks pretty much factory-made.


Test-fit the assembled pack in the case. Of course, this is usually done first; I somehow overlooked that step, but I think it’s pretty obvious :)


Installation.

Next comes installing the pack in the battery compartment. This seemingly trivial task has a few pitfalls.
First, wash the dust and dirt out of the compartment. I made the mistake of wiping only the bottom; I had to clean the rest with a brush and cotton swab afterward. So it’s easier to wash it with soap and let it dry.

Next, attach the pack. In the original setup, the batteries were simply held in place by the two halves of the case, but that rarely works in our situation, so packs are usually glued in.
As before, there are several options. Let’s take a look at them.
1. Double-sided tape
2. Hot glue
3. Silicone sealant
4. Nail it right through with 150 nails, then bend them over on the other side. :)

Since the last option is better suited to thrill-seekers, I’ll go over the more “down-to-earth” ones.
1. Very simple and convenient, but the contact area is small, so it doesn’t hold very well. You also need to use good-quality tape.

2. A good option; I use it myself sometimes (I use black hot glue, by the way). But I wouldn’t recommend it here. The thing is, hot glue tends to soften and flow when heated. All it takes is leaving your drill outdoors in summer, and you could end up with a battery rattling around inside. I’m not saying that will definitely happen, but the glue does have this property—that’s a fact. Also, hot glue doesn’t adhere very well to large components and may simply come loose under load.

3. In my opinion, the most convenient option. Sealant can withstand heat, doesn’t run over time, and adheres well to most materials. It’s also quite flexible and barely loses its flexibility over time.

I used Ceresit sanitary sealant. In the photo it may look like I’ve applied only a thin smear, but that’s not the case—there’s quite a lot of sealant. By the way, keep in mind that most sealants won’t bond to a previous layer of sealant.
You can also use a similar construction adhesive in the same kind of tube, such as Moment, but silicone seems more suitable to me.

Anyway, apply the sealant, insert the pack, press it down, and leave it to dry.


Protection board.

We’ve finally reached the subject of this review: the protection board. I ordered these back in the spring, but the package got lost. They sent them again, and eventually they did arrive.
I can’t remember why I ordered these particular boards, but they’d been sitting patiently, waiting for their moment. And now it’s here :)


This board is designed for three batteries and has a rated operating current of 20 A.
Only now did I notice that the board’s overvoltage protection threshold is rather high: 4.325 V. I may be wrong, but I think 4.25–4.27 V would be better.
It also says that 20 A is the maximum continuous current, while the overcurrent trip point is 52 A.

The table looks very much like the ones for other boards, so I’ll highlight a few key points.
1. Balancing current. Since this board doesn’t support balancing, this is marked with a dash.
2. Maximum continuous current. For most applications, you need 20–25 A. For less powerful tools, 15–20 A is enough; more powerful ones will need 25–35 A or more.
3. Maximum cell voltage at which the board disconnects the battery. This depends on the type of batteries used.
4. Minimum cell voltage at which the board disconnects the load. 2.5 V is quite low; it’s better to choose a value that matches the battery datasheet.
5. The current at which the overcurrent protection trips. There’s no need to aim for extreme values. Although this current is directly related to the maximum operating current, so there are usually no problems here. Even if the protection trips, most of the time you just need to release the drill trigger and press it again.
6. This item controls automatic reset after the protection trips.
7. On-resistance of the switching transistors: the lower, the better.


The board looks fine; the assembly quality is quite neat.


There’s nothing on the bottom, which is just as well—no problems gluing the board in place :)


I’ll explain protection boards in a little more detail.
First, let me answer the question: can you do without a protection board? No.
At a minimum, the protection board shuts things down in the event of an overload, which is harmful to both the batteries and the tool.
The board also protects against overcharging and over-discharging. You could argue that over-discharging is noticeable from the drop in power, but that’s not true of every tool. Besides, you can end up in a situation where one cell is badly “worn out” and its voltage drops very sharply. In that case, it’s easy to get reverse polarity: the battery doesn’t just drop to “zero”—current flows through it in the opposite direction. This can only happen when the cells are connected in series, and for some reason people often forget about it.
Lithium batteries are quite dangerous, so a protection board is essential!

Protection boards are mainly divided into two types (though there are actually more): with and without balancing.

Let me explain what balancing is and why it’s needed.
First, the “passive” balancing option.
This is used on the vast majority of boards because it’s the simplest to implement.
As a cell reaches its threshold voltage, it starts dumping current through a resistor, which takes some of the charging current. While that cell is “fighting,” the others have time to charge to their maximum.
Here are a few images from this article.

1. One of the cells is either more charged than the others or has slightly lower capacity.
2. With a simple charge, its voltage will be higher than the others.
3. The balancer diverts some of the charging current, preventing the voltage from rising above the maximum.
4. As a result, all the cells are charged evenly.


The second type of balancer is “active.” It works quite differently and isn’t suitable for high charging currents. Its job is to keep the voltage across the cells equal at all times. It works by “transferring” energy from the cell with the higher voltage to the one with the lower voltage. In one of my reviews, I built one of these balancers. If you’re interested, you can read a little more about it.
And in this review, I built a proper charging setup with an active balancer. The chart below shows the balancing process without connecting the battery and board to a charger… Yes, it’s slow, but it runs continuously, not just while charging.


But I digress.
A protection board with balancing usually has several large SMD resistors, with the number being a multiple of the number of channels. For 3 channels, that’s 3 or 6. They’re usually marked with something like 470, 510, 101, and so on.
The 4-channel board is on the left, the 3-channel board on the right.


There’s no balancer here, but there are current-sense shunts in the form of low-resistance SMD resistors. They’re usually marked R010 or R005. So you can tell whether a board has a balancer just by looking at it.
By the way, some boards don’t have a current-sense shunt. That doesn’t always mean the board can’t measure current. Sometimes the controller can use the MOSFETs as a “shunt.”


There are also standalone balancer boards, as well as balancer + protection board kits.
This is a perfectly viable option if the price works for you, but you’ll have more wires.


Along the way, I often come across the misconception that these boards can be used as a charger. People are usually misled by the word Charge in the listing title.
These boards can't control charging; they only protect the batteries. But sellers' lack of knowledge or a bad translation does its work, and people keep getting it wrong.
There are also all-in-one boards, though they aren't designed for high currents and aren't suitable for power tools.


This board has eight power MOSFETs installed, or more precisely, four pairs.
The transistors used are AOD514 and AOD4132, with an on-resistance and maximum current of 5.9 mΩ and 46 A, and 4 mΩ and 85 A, respectively.
The current-sense shunt is visible on the left. This option is preferable to SMD resistors, which sometimes tend to burn out due to high pulse currents.


The MOSFETs on these boards are paired for a reason. Powerful MOSFETs have a parasitic diode built in, so you need two of them to block current in both directions.
In this case, the two transistors have a combined resistance of 10 mΩ (5.9+4) and a maximum current of 46 A (based on the weaker one). But since there are four pairs of transistors, that works out to 2.5 mΩ and 180 A, which is very good for a board like this.


The board has no central controller and uses a fairly primitive circuit: per-cell voltage monitors, followed by a circuit that combines their outputs to control the MOSFETs. It's simple, but it works. Though if I were choosing now, I'd probably go for something more sophisticated.
The board also has no balancer. You might ask, how can that be, when I just went over the advantages of a balancer?
A balancer is a good thing, and I recommend buying boards that have one. But I also think that well-matched batteries don't really need a balancer. It won't save them from severe voltage sag, and it can cause problems. I've seen cases where a faulty balancer drained the battery.
Besides, most power tool manufacturers don't put balancers in their battery packs. Of course, that's part of the principle of planned obsolescence, so I'm still more in favor of a balancer than against one.


The board also has terminals for connecting a temperature sensor (the photo above from another store shows an example of a board with a temperature sensor). A temperature sensor is a good thing, and one of my plans is to figure out how to connect the original temperature sensor from the cordless drill battery.
Presumably, you need to desolder resistor RT, replace resistor RY with one whose value matches the new sensor, and solder the new sensor to terminals RK.


I think we've covered the boards enough; let's get back to the conversion.

Since the board can heat up during operation (though not much), I decided to make a spacer to protect the batteries from excess heat. It will also protect them if the MOSFETs fail and the board burns through (this can happen, but very rarely, so it's mostly a theoretical concern).
I took a scrap piece of fiberglass board and removed the copper foil.


Then, using the same silicone sealant, I glued the gasket to the battery pack, and then glued the board itself in place.
The setup looks pretty ugly, of course, but in this case it’s the simplest and fairly reliable solution.
I didn’t just stick the board on at random; first I worked out the most convenient way to connect it afterward.


The wiring diagram was on the store page, but in fact it’s almost identical to the diagrams for other boards. Connect the batteries in series, the negative terminal to the board, then the first midpoint from the negative end to B1+, the second to B2+, and the third to B3+. Since there are only three batteries, B3+ is the positive terminal of the whole pack.
The second wire from the positive terminal goes to the load.
The load’s negative wire (and the charger’s, too) connects to a separate terminal on the board.


Next, connect the wires.
The order in which you connect the wires can be critical. I usually connect the pack negative first, then the positive, and only then the midpoint wires, starting from the negative terminal (B1, B2, and so on).
I’ve read that connecting them in the wrong order can burn out the controller. I wanted to include that in the review, but couldn’t find any references.
Also, solder very carefully to avoid shorting the contacts, or things could end badly. This is probably one of the trickiest steps for a beginner when converting the battery… I tin the board pads first, then solder the wires; it’s easier that way.

Ideally, secure the wires with sealant afterward, too, so they don’t flop around.


At the very beginning, I showed the battery block I took out of the battery compartment.
The terminal block is visible at the top. Don’t throw it away, because it’s essential for the conversion. Terminal blocks come in different types, but they all serve the same purpose: quickly connecting the tool or charger.
When I first started the conversion, I thought the resistor here set the charging voltage (the charger is rated for 7.2-14.4 V), but testing showed that the charger doesn’t even have a corresponding contact for it, and neither does the drill :(
One of the other contacts is connected to a thermistor that monitors the battery temperature. That didn’t help much, though: one of the battery blocks shows clear signs of overheating and deformed plastic.


Before connecting it, though, you need to think about how to secure the terminal block. It was originally held in place by the batteries, but since they’re gone now, I’ll have to improvise.
To secure it, I measured the inside width of the protruding section, then cut a piece of plastic to match. I still got it slightly wrong and cut it a little too small, so I had to wrap it with electrical tape :)


Usually, both wires are desoldered, but in my case the negative wire was long enough, so I left it in place and replaced only the positive wire.
By the way, since the terminal block is made of plastic and the terminals themselves are fairly substantial, you can either use the same approach as when soldering batteries, or simply cut the old wire 7-10 mm from the end of the terminal and solder the new wire to it. The second option works just as well and is noticeably easier.


1. Solder the pack's positive wire to the terminal block. Heat-shrink tubing is really just perfectionism here—there's not much it could short against, but I wanted it to look neat.
2. Put the terminal block back in its original place and tap in (or press in very firmly) the plastic retainer I cut out earlier.


Solder the negative wire from the terminal block to the board, then coat the board with conformal coating. The latter isn't just perfectionism; it's genuinely useful, since the board is live and may be used in very humid conditions. Without a coating, the exposed parts of the traces and component leads may corrode.
I use Plastik 70 coating.


That's it for the battery. Put the springs and retainers back, then reassemble everything.
It's best to turn the whole assembly upside down first and shake out anything that might have accidentally fallen inside. In my case, it was a scrap of wire insulation.
You can also wipe down and lubricate the battery latch mechanism on the drill.


The minimum is done: the battery works. But since I haven't modified the original charger yet, I've connected it to a power supply for now.


The charger modification (and more) probably won't fit into this review, and I'd like to do it properly and make it look good, so I'm planning another post on the subject. I'll cover possible upgrades, modifying the charger, and ways to charge the battery correctly.

Of course, you can use a common charger like an Imax to charge it, but I don't think that's a convenient option.
Some people also add a balancing connector for the drill battery. It's certainly useful, but in my opinion it's a bit unnecessary—and not always safe. I think it's enough to match the cells once and then charge the battery without balancing. Alternatively, you can buy a protection board with a balancer. Exposed connectors increase the risk of shorting or breaking them, and this setup is more suited to home use anyway.


For more practical use, it's better either to modify the original charger or replace its entire internals.
The first option is technically challenging because the charging algorithm for a lithium battery is quite different from that for a nickel-cadmium one. Also, some original chargers barely deserve to be called chargers: inside, there's just a transformer, a bridge rectifier, and a handful of components, with no control circuitry at all.
Bosch, for example, also has a more advanced version with a controller.


For the second option, you can use the original charger's transformer and bridge rectifier, plus a piece of circuit board as a terminal block.

To modify it, you'll need to buy a board like the one in the photo.
Or any other board that can regulate voltage and current. These boards usually have at least two trimmer potentiometers. This one has three: the third adjusts the threshold for the charge-complete indicator.

Looking at the photo, the first adjusts voltage, the second the indicator, and the third the charging current.


In this setup, the board replaces the original one, and all you need to add is a 1000–2200 µF electrolytic capacitor.

But this solution has its drawbacks too. The charger board only indicates that charging is complete; it doesn't disconnect the battery. It's not exactly terrible, but it's not ideal either.
The simplest way to solve this is to disconnect the output when charging is complete.
To do this, you'll need to add four components: a 24 V relay, a PC817 optocoupler, a diode, and a button.
The optocoupler's LED is connected in place of the LED that indicates charging, and its transistor controls the relay.
But with this setup, the relay can't switch on by itself, so you need a button in parallel with the contacts (as I said, it's a very simple solution). In other words, insert the battery, press the button, and charging starts. Once charging is complete, the relay switches off and cuts power to the battery.
You can connect the button in parallel with the optocoupler transistor's contacts, in which case a regular momentary pushbutton will work. Naturally, in either setup, the button must be non-latching.


The optocoupler and relay.


You can also use other boards; many of you have probably seen them on AliExpress.
The first one is simpler: you can adjust only the current and voltage. The charge indicator is fixed, and the LED goes out when the current drops below 1/10 of the set charging current (the standard lithium charging algorithm).
The second is essentially the same as the first, but in a more “advanced” version: it displays the battery voltage and charging current.
Review link


By the way, you can even use a board without current regulation for charging, but you'll need to modify it a little. I even showed how to do it.

All the options shown use the charger's original transformer. If you don't have one, simply add a power supply to the converter, for example, one like this.
Keep in mind that the power supply voltage must be higher than the battery's charge termination voltage, by about 3–5 V or more.
In this case, a 15 V power supply won't work, but these power supplies usually have an output voltage adjustment range of ±20%, so you can raise it a little. Or you can simply buy a 24 V power supply and leave it as is.


If all you have is a 12 V power supply, but you need to charge a battery like the one in the review, you can use a universal converter, for example, this one, though it costs more.


Possible modifications.
You can add a battery charge indicator, for example, an audible one or an audible and visual one.


Or you can measure the voltage with a small voltmeter, or even install a voltmeter-and-buzzer combo.


Personally, though, I prefer simpler options: measuring the voltage and displaying it with a few LEDs.


I’ve already built the last option and described the circuit and how I made it.


A very similar setup is used in one of my cordless drills, or rather, in its batteries.


A short video showing the results of the conversion. You can see the protection kicking in under heavy load. The battery was already a little low, so in ratchet mode at the second speed setting, the protection didn’t always trip. With a fully charged battery, it happens more often. You can also see that the protection works as it should: the load is disconnected. Then I release the trigger, press it again, and the drill works.


That’s about it, in broad strokes. I think I’ve covered everything I can remember about converting the batteries. I’ll go into more detail about the charger another time, since I have plenty of ideas.

Oh, I almost forgot to talk about the subject of the review itself: the protection board.
The board works, and works great. At least, I haven’t found any problems with it.
With the chuck locked, the ratchet set to maximum (I think level 5), and the drill in second gear, the board trips about half the time. In first gear, the current isn’t high enough to trigger the protection. Overall, that’s perfectly normal behavior. You could use a lower-value shunt so the protection trips later, but I don’t see the point.

Now, about the cost of the conversion. Three batteries cost about $15, plus $5–8 for the protection board and a dollar for miscellaneous bits, bringing the total to around $20–25 per battery.
Expensive? I think it’s pretty expensive, so there’s no point converting a cheap tool. But in any case, the conversion isn’t as difficult as it might seem at first—the main thing is to get started.

I didn’t cover LiFe batteries in the review. For the most part, they work exactly the same way, except that they need special boards because their voltage is a little lower than that of the familiar Li-Ion batteries. They’re excellent batteries and should be more reliable, but the pack capacity will be lower.

I hope you found the review useful. As always, I look forward to your questions in the comments.
Of course, there are other ways to do this, and I may be wrong about some things, so what I’ve written above is just my take on the process.