Tamiya L6.2, connectors for connecting batteries

In my reviews, I often mention that I try to use plug-in connections.
Of course, soldering is very reliable, but it's not always convenient. Today's review is about one type of connector.
But since I probably don't have anything new to say, this is one of those reviews where I just show you what arrived :)

A quick aside before we get started. The title lists prices of 4.99 and 3.99. I bought 4 lots, and the seller offers a discount when you buy more than two.
I actually bought the connectors for a friend, not for myself. He needs them to connect battery packs for various toys. But I kept one set for myself to play around with :)

We chose the seller based on the lowest price. There may have been a cheaper option, but I didn't come across one at the time.
The package was shipped with tracking via ePacket. It was dispatched in a day or two, and another 10 days later the order was in my hands. So, credit to the seller for that.


Since the bags were identical, I opened the first one I picked up.
Each lot contains 10 complete male-female sets. As you can see in the photo, everything arrived as ordered.


I gave the connectors to my friend, but kept one set for myself to experiment with a little.
As far as I understand, these connectors were developed by Tamiya and are most commonly used to connect batteries in various RC toys.
At least, that's what I found on Wikipedia.


According to the same source, they come in two sizes: standard and compact.
I ordered the standard size. Here's a dimensioned drawing to give you an idea of what they are.


The connector has a latch. A very similar type of latch is used on connectors on motherboards for connecting the power supply. The latch is fairly soft, but holds well; the connector itself won't come loose.
The connectors are also keyed to prevent incorrect connection, thanks to the shape of the housing. I tried plugging one in the wrong way around, and it would probably break before it connected incorrectly—though I suppose you could manage it if you really tried :)


To help me find the right ones, my friend gave me a sample of the connector he uses.
I had no compatibility issues; the connectors are identical.


When I took a close look at the contacts, I was a little surprised by their design.
The part that faces the wire and the actual contact are joined by small bridges.
Given that the connector is rated for 20 A (30 A according to other sources), this seemed a little odd, since those bridges aren't very large in cross-section.


According to information I found online, this type of connector is designed for wires up to 14 AWG, about 1.63 mm²
But in practice, you can use 2.5 mm² wire without any particular problems.
In the photo, 2.5 mm wire is inserted on the left and 1.0 mm on the right.
Mechanical strength probably matters more: 2.5 mm wire is usually less flexible than 1–1.5 mm wire.


When soldering wires to this connector's contacts, it's convenient to strip about 5 mm of insulation.


Yes, I know there are special crimping tools for these connectors. But buying a crimper for occasional use seems a bit expensive, and besides, I personally think a soldered connection is much more reliable.
The contacts are very easy to solder. I tried soldering without any additional flux; I only used the flux in the solder.
The contact material is unknown, but at least it's definitely non-magnetic, which is a good sign.


And of course, what would a review be without some tests?
Ideally, for a proper test, I should have used the connector for a while with periodic loads, but I simplified things a bit.
First, I left my set on the table for about a week, then put it on the balcony for almost a month.
Since a new connector performs better because its contacts still have good spring properties, I mated and unmated it a couple dozen times before starting the test.
For the test, I measured the contact resistance from the solder point on one contact to the solder point on the other. I think we can ignore the resistance of the solder joints, since the cross-sectional area there is very large compared with the contact itself.


Next, I tested it over a current range of 1–22 A. There were six tests, each lasting five minutes, while I monitored the voltage across the contacts and the temperature.
The total resistance of the contact pair came out to about 2.8 mΩ.
It's easy to calculate. For example, at 10 A, the test showed 28 mV across the contacts. Divide 0.028 V by 10 A and you get 0.0028 Ω, or 2.8 mΩ.
Or, in the last test, divide 0.063 V by 22 A and you get the same 2.8 mΩ.
The most interesting thing was that the contacts barely heated up. The IR thermometer showed 36.6 degrees at the maximum current of 22 A.
I think this was because some of the heat was carried away by the wires—in other words, they acted as heat sinks.


The next test has nothing to do with the review, but lately I’ve often come across tests of various USB cables, and they’re quite often done without taking cable length and voltage into account.
The right way to measure voltage drop is to measure it across the cable and then normalize it to a given length—for example, 0.1 V per meter.
Also, it’s important to look at how much voltage is dropped across the cable, not at the voltage after the cable. Those aren’t the same thing.

For this example, I used a 6 m length of 2×2.5 mm² ShVVP cable made by Interkabel (the specific cable doesn’t really matter; this is just an example).
I tested only one conductor in the cable. Another option is to connect the conductors together at one end and test the cable end to end. This can improve measurement accuracy by increasing the voltage drop across the cable.

So, I connected one 6 m conductor, applied a current of 10 A, and got a voltage drop of 0.53 V.
Divide 0.53 V by 10 A and you get 0.053 Ω. Since the cable is 6 m long, divide that by 6 to get 0.0088 Ω per meter.
Or, 0.53/6 = 0.088 V per meter of wire, which means 0.176 V per meter of cable at a current of 10 A.

What can we do with this information?
For example, take 0.0088 Ω and multiply it by 1,000 to get 8.8 Ω. Then go to this page and see what gauge of standard copper wire has an equivalent resistance.
To do that, find the Ω/km column in the table, look for the closest resistance value, and check the corresponding cross-sectional area of the copper wire.

I found that my rather nice-looking, thick cable has an actual cross-sectional area of about 2 mm².
Of course, you could say it’s easier to measure things like this with a caliper and a calculator, but the cross-sectional area might be correct while the strands themselves are made from a mix of foil and cardboard. Besides, measuring the cross-section of stranded wire with a caliper isn’t very convenient anyway.


And here’s what the assembled set looks like.


And that’s about it.
The seller is reliable, ships quickly, packs things well, and delivery was fast too.
The connectors seemed a little flimsy to me, but they work without any problems and are easy to solder.

Overall, if the price works for you, I think they’re a perfectly good buy. I’m happy with them.

I looked for a datasheet for these connectors because I wanted to find out what the contact resistance should be for a set like this, but couldn’t find that information. So the measurements above are mainly for reference, to have something to compare against later.