Milwaukee M18 Power Tool Battery

I bought this battery mostly out of curiosity for a review, though that doesn’t rule out using it for its intended purpose. But, as often happens with products in certain categories, “something went wrong.” Still, you can find out all about it in the review.


So, I’ve already posted a couple of reviews of Milwaukee tools. They’re great, and I use them quite often, but convenience was somewhat limited by the fact that I have only three original batteries, with capacities of 1.5 and 2 Ah. That’s a bit on the low side, especially when using an oscillating tool.

The question of optimal battery capacity is pretty debatable. In some situations, it’s more convenient to have several small batteries (when the charger is nearby), while in others, a maximum-capacity battery makes more sense. I wanted to extend the runtime and avoid carrying several batteries around.

Delivery took about three weeks, and a courier brought the package to my door, which was especially convenient. It came in a cardboard box that was clearly the wrong size, so it had been wrapped in tape. Inside were the battery and another item, which I’ll review a little later.
The package includes the battery itself and a manual, but there’s nothing interesting in it, so I haven’t included any photos.


It looks very neat; I’d even give it five stars if not for a few minor details. On the other hand, this battery isn’t a counterfeit, since it doesn’t completely copy the original’s appearance and doesn’t mention Milwaukee anywhere. I’ll compare it with an original battery later.


The case, latches, contacts, and even the bottom of the battery look very much like the original. By the way, the bottom isn’t plastic; it’s made of something like very hard rubber. The original’s is a little softer.
The screws are for a Torx screwdriver. You can see that the plastic has been pushed out a little around them, but they hold just fine.


The battery weighs 655 g. For comparison, the original 2 Ah battery weighs only about two-thirds as much.
And here I’ll make my first digression.
“Single-row” batteries, i.e. those built in a 1P configuration, are the least efficient in terms of weight-to-capacity ratio. On the other hand, if you work a lot, even an extra 200–300 g can make a difference. And if you’re working close to the charger, smaller batteries are more convenient, although of course they need charging more often and wear out faster.



Like the original, it has a charge indicator with a button to turn it on. Functionally, it’s almost an exact copy: when you press the button, the segments don’t all light up at once but gradually fill the scale. It also turns off like the original, with a delay, and when the battery is fully discharged, the first segment flashes.
One thing to note: the indicator on the original battery is very easy to see, while on the one under review it’s easier to see when looking down from above.
Otherwise, the batteries are very similar. I had no complaints about the latching mechanism, guides, and so on.


The product page had an interesting table explaining how the indicator works.


The voltage at the battery terminals was over 18 V.


But of course, I was more interested in what was inside. To get to it, you need to remove four screws; the ones on the button side are noticeably longer.


The internals are built as a module like this. I’d say everything is neat and well put together, if it weren’t for one “but.”


The thing is, the case was apparently originally designed for the connections between the cell groups to be made using a specially formed strip. Here, though, they used wires, and one of them has already been pinched a little. I later routed all the wires properly, and even had to cut a small groove in the plastic for one of them.

The circuit board is noticeably simpler than the one in the original battery. Actually, “simpler” doesn’t begin to cover it—the difference is huge. On top of that, the board isn’t protected by a conformal coating, which is obviously a bad thing.
Under the board, you can see a temperature sensor and what looks like a busbar that serves as a fuse.


For comparison, here’s what the original controller board looks like. By the way, I noticed that the plastic inside the case has been almost completely copied from the original.


As expected, the pack is configured as 5S2P, with 10 cells in the 18650 size. The welds are a little rough, but the positive terminals are insulated, and the cells are separated from each other by parts of the case, so everything is fine here. There’s a little play, but I can’t remember how firmly they fit in the genuine battery.
The cells are from a manufacturer I don’t recognize, most likely generic from the start.

And now we get to the sticking point with Chinese batteries: not only are the cells rated at 2 Ah instead of the specified 3 Ah, but at least one cell has a different marking as well....
So even without testing, we could say that this is really a 4 Ah battery, not 6 Ah. That means the 9 Ah version should be expected to deliver 6 Ah.



After publishing the review, I finally got around to finding the datasheet for the cells used here. It turns out their continuous discharge current is just 3C. Yikes...



Second digression.
I’m often asked which batteries to use in a particular situation, without any details about the battery configuration or load current, even though those are pretty important.

Obviously, capacity is an important factor in a battery, but there are a few things to keep in mind.
The thing is, capacity doesn’t scale directly with the number of cells used, or rather with how they’re connected. The most common configurations are 1P, 2P, and 3P, where P is the number of parallel groups. Let me try to explain.
For example, take a powerful cordless drill with a working current of up to 30 A. That means the battery would be configured like this:
1P — required current: 30 A, so you need high-current cells (for example, Samsung 20S or 25S), giving a capacity of 2–2.5 Ah.
2P — the total current is unchanged, but each group has to supply half as much, or 15 A. You can use higher-capacity cells, 2.5–3 Ah, such as Samsung 25R or Murata VTC6.
3P — each group supplies 10 A, for the same total of 30 A, so you can use cells rated for 10–15 A, with a capacity of 3–3.2 Ah. LG HG2 and Samsung 30Q work well here, and even LJ MJ1 cells can be used.

At the same maximum current, the battery’s total capacity would be:
1P — 2 to 2.5 Ah
2P — 5 to 6 Ah
3P — 9 to 10 Ah

So tripling the number of cells increases the battery capacity by 4–5 times. This is, of course, a simplified calculation, and the currents are just examples, but I think the point is clear. Though 30 A at 18 V (5S) is already 540 W, which is quite a lot.


Back to the battery. It turns out the cells are very well balanced, which is surprising, but their internal resistance shows that they’re medium-current cells, with an impedance of about 15 mΩ per cell.


For testing, I charged the battery with the genuine charger, and that’s when the charge indicator’s shortcoming became apparent. On the genuine battery, it’s easy to see when the battery is on the charger, so you can get a rough idea of the charging progress. On the one under review, the indicator is almost impossible to see.


After that, the battery delivered 3.67 Ah, or 65 Wh. It was rated at 6 Ah, or 108 Wh. Getting less than 4 Ah is, to some extent, normal; even with good cells, it’s very difficult to get the full rated capacity. But delivering far less than the claimed 6 Ah is bad.
The battery was discharged in CP (constant power) mode at 150 W, so the current varied from 7.5 A at the start to 10 A at the end.

Third digression.
Like the genuine battery, and for example the Makita battery, this one has no built-in over-discharge protection. In other words, the controller measures the cell voltage and sends a signal to the tool, while the tool’s power stage handles the cutoff. Inside the battery, there is only an overcharge protection transistor, since the charging input is routed to separate terminals.
There are no issues when using it with the original tool, but lately I’ve often come across reviews of drills and other devices designed for Makita batteries. In those devices, the battery connects through just two contacts, so I suspect the over-discharge protection may not work.

Because there is no built-in over-discharge protection, I set the cutoff threshold to 14 V, or 2.8 V per cell.



Interestingly, the charge indicator still showed two bars after this, and the battery voltage was around 17 V.


The cell voltages also varied. Below are photos of the two extremes; the others were somewhere in between. The voltage of the weakest group was what mattered to me, and that turned out to be group 4.


After that, I measured the capacity of each group separately using this cycle: charge at 4 A, discharge at 10 A down to 2.5 V, then charge at 4 A. Here are the results (the screenshot is only for the first group).
1 - 4046 mAh 14.33 Wh
2 - 4054 mAh 14.41 Wh
3 - 4005 mAh 14.25 Wh
4 - 3966 mAh 14.11 Wh
5 - 4060 mAh 14.45 Wh

As expected, group 4 is the weakest, while group 5 has the highest capacity. Group 2 had been the highest before, but the difference is within the measurement error.



So I charged each group separately and ran the test again. The result was even a little over 4 Ah, which is somewhat odd, but I think we can put that down to measurement error.


Then I put the battery back on charge, this time disassembled so I could easily monitor the voltages.


It turned out that the original charger stops charging a little earlier. To be fair, though, I wasn’t monitoring the cell voltages during charging. Even so, after removing the battery, the highest cell voltage was 4.12 V.


I discharged the battery for a third time and got around 3.8 Ah. So in this case, the difference between fully charging each cell separately and using the original charger is about 250 mAh, or 7–8%.

When I was testing the capacity of each group, the top-up charge cycle after charging with the original charger took 200–500 mAh, but I ran that test some time later.



During discharge, I monitored the voltage of the weakest cell. When the total voltage dropped to 14 V, the weakest cell was down to 2.55 V.
Later, I ran one more test without taking photos: I installed the battery in a drill and measured the voltage at which the battery tells the tool to shut down. It was exactly 2.5 V. After the over-discharge cutoff, the battery indicator flashed when I pressed the button to signal over-discharge, but the tool could not be switched on. So in this case, both the protection and the indicator worked correctly.


Temperatures during testing; discharge current was 7.5–10 A throughout. Measurements were taken after a full discharge.
1. Battery case
2. Internals and circuit board, top view.
3. Cells.

The last two tests were run with the case removed, and the temperature seems high to me: 20 degrees above ambient, even though the current was only around 2–2.5C. This also suggests that medium-current cells are used.


At the very beginning, I was also a little surprised by how accurately the cells were balanced, and by the large imbalance after a charge cycle. That gave me reason to suspect there was a balancer on the circuit board.
To test this, I decided to measure the cell voltages a few days later. Before that, the battery had been charged with the original charger, and you can see the approximate imbalance in the photo above.
I won’t claim that there is balancing, but the cell voltages definitely look as though they’ve been equalized. I tried again: topped up the charge, measured almost immediately and then again after 8 hours. The readings looked like this:
4.1203 - 4.1174
4.1019 - 4.0998
4.1032 - 4.1013
4.1028 - 4.1003
4.1139 - 4.1114

This time, there’s no such effect, but I removed the battery as soon as charging finished, whereas last time (if I remember correctly) it was a few hours later. I could investigate this in more detail, but I don’t see the point given the battery’s lower-than-rated capacity. Though I’ll probably check it later anyway, mostly out of curiosity.
I tried looking with an oscilloscope, but oscillation is present only on one of the battery’s outer contacts. I looked for oscillation because there’s clearly no passive balancer on the circuit board, and an active one would have pulses on the switched capacitors.
Although I later checked the resistor values in the cell circuit (01E = 1 MΩ) and realized there probably isn't a balancing circuit here.

Also, right at the beginning I noticed a quiet buzzing sound when pressing the indicator button, even though there are no inductors on the board.
In case it's useful, here's the approximate pinout of the battery:
1 - common negative
2 - control input; the indicator sometimes turns on even when a multimeter is connected
3 - charging input
4 - data output to the tool or charger
5 - power output for connecting the tool

The charger uses pins 1–4; the tool uses pins 1, 2, 4, and 5.



And of course, a couple of photos of the battery installed in the tool it's meant to power.


So, what can I say in conclusion? On the one hand, the battery has a case that's very close to the original, both in appearance and build quality. The original is certainly better made, but still.
It has both cell-voltage monitoring and protection against overheating, as well as “presumably” overload protection. The indicator works just like the one on the original battery, and there are no problems with the tool or charger either.
Ideally, I’d also test it with a powerful FUEL tool, but I don’t have one. The thing is, it looks like some kind of data is transmitted through one of the contacts, so it’s possible that a powerful tool won’t “recognize” it as a high-capacity battery.

As for the drawbacks, first of all, the battery capacity: it’s actually 4 Ah, not 6 as claimed. The cells are also mediocre, rated for just 3C. I also have a complaint about the indicator: it’s noticeably harder to read than the original.

In short, the manufacturer used a decent case, installed a good protection and monitoring board, and assembled everything neatly, but then ruined spoiled it all by fitting lower-capacity cells. The price difference between decent cells and the ones installed here probably isn't that great. It could have been an excellent battery—what a shame, such a shame...

I don't see the point in buying it as-is at this price. Better to look for a case without cells and install your own, or use a dead genuine battery as a case donor, although it may have a protection lockout.

That's all from me. I hope this information was useful.