Universal battery holder with four-wire connection

I can't say I work with batteries very often, but I do occasionally have to test whether they're "up to the job". Since I'm mostly dealing with high-current batteries, it was time to get a proper holder for them.
As always, the inspection, description, comparison of the different connection methods, and conclusions are below the fold.


I ordered this holder (I'm not sure what the correct name for it is) from the store of the well-known modder 100MHz, along with 48 V power supplies. Shipping to the forwarding service cost $1.88, split between the two items. The holder itself cost $21.56.

The rated current for this holder is 20 A, although I've seen it listed on some sites as 40 A or even 60 A. In this case, I'm inclined to trust the person who designed and manufactures it.

It was originally wrapped in a lot of bubble wrap, but since I received the package quite a while ago, that has gone missing somewhere. All that remains is the forwarding service's bag.


It comes as a kit for assembly.


The seller also has two other holders:
1. BF-3S, available in two versions rated for 5 A or 10 A - link
And the BF-1, available in 5 A, 10 A, and even 30 A versions - link.


Its size and weight are impressive: it's 135 mm long, and the holder alone weighs just 156 g. Definitely not a plastic toy.


Since the holder is sold with several types of contacts, they are supplied separately.


The positive terminal is 8 mm in diameter and consists of two contacts: a fixed high-current contact and a spring-loaded auxiliary contact. The inner contact is fully insulated from the high-current one; they connect through the battery's own terminal. Both contacts have grooves—or, more accurately, milled notches—about 0.5 mm deep.


The negative contact also has an insulated center contact, but it's larger in diameter at 14 mm and has a slightly different design.


The thing is, here not only the center contact is spring-loaded, but the entire contact assembly. The only thing I didn't like is the rather soft plastic that holds it all in place.


There are several versions of the holder. To start with, there are a couple of acrylic ones, but I wasn't interested in those.
They also come in different current ratings, determined by the type of contact assembly.
1. 10 A
2. 20 A
3. 30 A
4. The 20 A and 30 A versions differ only in the positive contact; the negative one is the same.

The usual question: is there one on AliExpress? Yes, but the cheapest one I found costs $43.

Of course, I originally wanted to buy the 30 A version, but there's a warning about it: don't use it with batteries 18 mm in diameter or smaller that have a flat positive terminal, as it could short against the battery casing. The 30 A version is suitable for 26650 batteries and possibly 21700, though I'm not sure about the latter.


There was also an extended version for 32800 and 32900 batteries, but I don't use those and don't see the point of a holder for them.


The so-called "chassis" is shared by almost all versions (except the extended one).


It has the model number BF-2A, as well as the manufacturer's logo and a measurement scale. The holder is designed for batteries up to 70 mm long.


There is an additional slot underneath with three flat-head screws threaded into it. The slot is intended to hide the wires running along one side of the holder; the screws clamp the wires in place so they don't fall out.


In my opinion, this is a pretty well-thought-out solution. The idea is that it should work as shown in the picture.


What's more, there's even a cutout for the power wire terminal—the lower red one in the photo.


The top has a toothed section that locks the movable part of the holder in place. The screw visible in the slot is only there to keep the movable part from falling out.


To move the sliding part, lift it, then lower it once it's in the desired position. The design is extremely simple, and since almost all the components are made of metal, hopefully it'll last a long time.


Naturally, both contacts are insulated from the chassis. The positive contact is insulated by a pair of plastic washers, like the ones used to mount power transistors to a heatsink, only much larger in diameter. The negative contact has a separate insulator that also serves as a retainer, since it is threaded. In my opinion, this is the least reliable part of the whole design, but in practice it isn't subjected to loads that could damage it.


The store page has an assembly drawing showing all the parts and how they fit together, although in my opinion everything is so intuitive that you'd have to try pretty hard to assemble it incorrectly :)


There's also an additional retainer inside the sliding part, in the form of a spring-loaded ball. It locks the sliding part in two positions, up and down, using two grooves in the main part. The grooves aren't visible; there are actually three of them: two for the pin and one for the ball (on the left).


Assembled, the whole thing looks a bit like a small lathe :)
The smallest battery you can install is 10 mm long. In the “relaxed” position, the contacts almost touch each other. You can actually move them even closer together; it's just awkward.
The maximum distance between the contacts is 68 mm without retracting the movable contact, and 75 mm with it retracted. So you can actually use batteries up to 75 mm long without any problem, with about 3 mm left over for installation.


How to install a battery:
1. Lift the sliding part.
2. Slide it to the required length. The start of the right-hand bevel roughly lines up with the battery length marked on the scale on the body.
3. Lower the sliding part.
4. Then press the battery's negative terminal against the holder's negative contact and insert the battery.

After the second or third try, you'll get the hang of installing the battery and applying the right amount of pressure.


And here's the battery installed—in this case, my “test subject,” a VTC4.


The positive contact is exactly the same size as the battery's positive terminal. I think it's now clear that if it were 14 mm in diameter, like the 30 A version, it would cover almost the entire battery and could short it out.
The negative contact is sized so that it just avoids the heat-shrink sleeve on 18650 batteries. The problem is that if the battery has a smaller diameter—even a 17650, let alone a 16340 or similar—you'll have to trim the sleeve.


As a test, I measured the contact resistance, but since it's difficult to measure directly, I used an indirect method.
1. First, I measured the battery's resistance: 13.88 mΩ.
2. Then I measured the total resistance including the entire assembly—that is, the metal nuts and everything else. It came to 16.53 mΩ.
3. This and the next test were done with the battery clamped as tightly as possible, with the edge of the movable part at about 65 mm.
4. The resistance measured between points closer to the battery terminals (excluding the nuts, etc.) was 15.37 mΩ.
5. After that, I moved the mechanism two marks to the right. This is the furthest position at which the battery is still held securely.
6. With this setup, I got 16.85 mΩ.

So, the results are:
Total resistance of the holder with maximum clamping force: 16.53-13.88=2.65 mΩ
Battery-to-terminal contact resistance with maximum clamping force: 15.37-13.88=1.49 mΩ, or 0.75 mΩ per contact.
Battery-to-terminal contact resistance with reduced clamping force: 16.85-13.88=2.97 mΩ, or 1.5 mΩ per contact.

There was one more position between the maximum and minimum test clamping forces. Its resistance can be taken as the average of the two test values, but I’d use the maximum clamping force setting.
The thing is, even with a contact resistance of 1.5 mΩ and a current of 20 A, 0.6 W will be dissipated at the contact, which is quite a lot. With maximum clamping force, the losses are half that, about 0.3 W. These losses won’t affect the measurement results, but they will heat both the battery and the terminals themselves, which seems entirely unnecessary to me.


Now for a quick explanation of what a four-wire connection is and what types there are.

The simplest option is a two-wire connection, used in the vast majority of devices, most often power supplies and basic chargers. It works perfectly well if you don’t need to discharge the battery under test or measure how much capacity it delivers. The drawback is that, because of the voltage drop across section A-B (I’m showing just one pole here; the other is exactly the same), the voltage measured by the power supply or electronic load will differ from the actual voltage.
This is almost irrelevant during charging, since charging ends at a low current. But if you want to measure capacity in Wh, you’ll get a result higher than the actual value, because the capacity will include the losses in the wires.
During discharge, you’ll get a lower capacity in both Ah and Wh. In the first case, discharge will simply stop earlier because of the voltage drop in the wire; in the second, because of the power lost as heat in the wires. For example, say we discharge a battery down to 2.8 V, and there’s a 0.1 V drop in the wires. If we don’t account for that drop, discharge will actually stop when the battery is at 2.9 V. It’s even worse for capacity in Wh: say the capacity is 10 Wh and the current is 10 A. With a 0.1 V drop in the wires, 10×0.1=1 W is dissipated, so we’ll calculate just 9 Wh instead of 10.
The solution is to use wires as thick as your finger to minimize losses.

A much more advanced option is a four-wire connection. In this case, the voltage drop across section A-B doesn’t matter at all, because measuring the voltage across section B-C requires very little current, so the error is minimal.
This approach is used in advanced power supplies, electronic loads, and measuring instruments. Voltage drop in the power wires is no longer critical. This circuit design is especially useful for high-power—or, more precisely, high-current—power supplies with low output voltages.


I use the first option in my power supplies and in my high-power electronic load, although, to be fair, I was simply too lazy to modify the load. It can be switched to three-wire mode without any trouble, and with a small modification, even to four-wire mode.
The second option was built into my second electronic load by the manufacturer from the start.



But as I mentioned above, this approach is also used in power supplies. For example, in a linear power supply with an output current of up to 20 A, where the wires are often longer than those in an electronic load and the current is high.


I’ll also mention three-wire connections, just as an extra. This option performs worse than a four-wire connection, but with a sufficiently thick ground wire it can still be useful. You’ll most often find this setup in a standard ATX computer power supply, where an extra feedback wire is connected to one of the 3.3 V wires (orange).
By the way, this method can easily improve voltage regulation accuracy in almost any power supply by bringing the feedback wire out externally. But be careful: if it breaks, the output will rise to whatever the power supply can deliver. This can be prevented by installing an additional resistor between the power supply output and the feedback input inside the unit.


But in reality, although the four-wire connection shown above provides good operating and measurement accuracy, it isn’t perfect either.
The thing is, when the component under test is connected through a connector—for example, the battery itself—there’s such a thing as contact resistance at the connection (holder, alligator clips, probes, etc.). For simplicity, we can treat these as two resistances, one on the positive side and one on the negative side, as shown in diagram 3. Above, I measured these resistance values for the holder under review and got 0.75-1.5 mΩ.
The effect on the result is, of course, much smaller than with a two-wire setup, but at currents of around 20 A and above, it should still be taken into account. For example, with a contact resistance of just 1 mΩ and a current of 20 A, 0.02 V is lost at each contact, or 0.04 V across both contacts. At a current of 40 A, these values will be 0.04 and 0.08, respectively. The difference will also be noticeable in Wh: the result will be 0.8 W lower in the first case and 3.2 W lower in the second (at 40 A).

To compensate for this error as well, a so-called full four-wire connection is used. The power (A-B) and sense (C-D) contacts are connected directly to the component terminals, so even the voltage drop across the power contacts doesn’t matter—except, perhaps, for the heating.
This type of connection is almost never used in power supplies, since a break in the feedback circuit most often causes the full voltage to appear on the power wires. The exception is when the power supply manufacturer has accounted for this, as in the high-power linear power supply shown above.


This method is used more often in measuring instruments, such as low-resistance meters, but there’s nothing to stop you using it for battery capacity tests, too.
I have at least two instruments built around this circuit design: a milliohm meter and an LCR meter.


And what would a review be without some tests? Of course, this setup is mainly for demonstration rather than practical use, since the load is connected with two wires, but I’ll show the key feature anyway.
The photo shows two multimeters connected: the one on the left is connected to the sense terminals, and the one on the right to the load input. So the reading includes the terminals and wires, but not the losses at the load’s own terminals.


1. First, the voltage at the battery and the load input. The difference is due only to the measurement error of the meters themselves.
2. Set the load current to 20 A, the maximum specified for this holder.
3. I deliberately chose a point where the difference is easy to see. In my tests, I pause at voltages of 2.5, 2.8, and 3.0 V. Here, the battery is at about 3 V, but the load is already receiving just 2.8 V—in other words, the next test stage has effectively begun.
4. And so the test wouldn’t go to waste, I also measured the capacity of an old VTC4 battery at 20 A.

The 0.2 V drop across these fairly thick wires is quite striking. At 20 A, that works out to about 4 W. Since the test lasted around 6 minutes, the final result in Wh would have been about 0.4 Wh lower. That’s not a huge difference, of course, but it all depends on the wire gauge and the condition of the contacts. With a full four-wire connection, it doesn’t matter.


Battery temperature after about 3 minutes (1 Ah discharged) and at the very end of the test.


Comparison with a standard acrylic holder.


Next up for me is another round of tests with a new batch of batteries from AliExpress: four VTC6 cells and more than a dozen protected Litokala cells. I’m sure this holder will come in handy.



In conclusion, I’ll mostly be praising this holder, because it’s sturdy, convenient, and, most importantly, properly designed.
But there are a couple of things to criticize. As I mentioned, the plastic on the negative terminal seems a bit flimsy to me, though that’s tolerable since it isn’t subjected to much stress. The second issue is that it’s difficult to use with batteries under 18 mm in diameter; you’ll probably have to trim the heat-shrink tubing around the negative terminal.

Unfortunately, this version of the holder is rated for only 20 A, even though my load can already handle currents up to 30 A without any trouble. In this case, however, the current is limited by the design of the holder’s terminals. There is a 30 A version, but its positive terminal is too large in diameter, so it’s completely unsuitable for me. In my opinion, the manufacturer could have included two types of terminals, so you could swap them as needed.

The seller is happy to communicate, but when I tried to arrange a sale of another device without the case and other extras I didn’t need, they flatly refused. What a shame.

This review was sponsored by the purchasing agent yoybuy.com, which covered the shipping costs. Affiliate link for registration; new users get a 10-off-50 coupon.
The holder cost $21.56 and weighed 186 grams, according to the agent. Shipping costs through the agent depend on various factors, such as the quantity and whether there are other items in the order.