What is an IR meter #

IR stands for Internal Resistance and it is a property of a battery that shows its imperfections in providing the electrical current. It is measured in Ohms (in practice milliohms) and gives us the possibility to predict the voltage drop on a battery on a given load. More details will be explained in a separate article soon. You should just know that IR is much better to represent the battery performance than the C-rating.

IR meter is a tool for measuring this property. Although many advanced chargers have a way to measure it, I have reasons not to trust them so I decided to create a simple and cheap way to measure this for anyone.

Building instructions #

What you need #

You need to own a multimeter, a load for your battery, and a way to plug the load and plug multimeter to the battery. You also need a way to connect all of those in a way that will survive the 10A current so soldering is your best bet. That’s all.

The multimeter #

Any digital multimeter will do the job. You can even use an analog voltmeter if you feel adventurous. All you need is a way to measure the voltage with 0.01V resolution.

If you can get a pair of probes with alligator clips at the and that will make the measurements easier. You can use any clip or tape to hold the measurement probes to the resistor leads, however.

The load #

For a load, we need some resistors that can dissipate a lot of power. We are looking at the currents of the 10A, so we need around 1 Ohm resistance which means 10101 = 100Wof power dissipated! The best resistors for this job are the cement ones but they come with a max 20W rating. For this reason, you should use 5 of them connected in parallel. They should be less than 0.5 EUR each. To get around 1 Ohm of resistance, buy ones with resistance between 4.7 and 5.1 Ohm.

Note, you want the cement (the one in the top) resistor, not the wire-wound (the on the bottom):

The wire-wound one, even though it is rated for 100W, has very bad resistance stability over temperature. If I connect a 10W of the load to it, the resistance triple in just a few seconds. They are meant to be connected to some beefy heat radiators.

As you can see in my photo, I use 10W resistors as they are much easier to get from Aliexpress. In theory, is too small for 100W of power but since we use it only for few seconds, it’s good as well.

The connectors and wires #

Just get one that suits your batteries. Most likely it is Deans (so-called T-connect) but if your battery has Tamiya-mini or XT60, use them. You can even connect all of those in parallel and be ready for each battery.

Be sure to use some thick wires, 16 AWG or more is best. You may want to use a heat shrink tube at the end of the connector but it’s up to you.

Measuring voltage #

We have to measure the voltage of a battery during the test. The easiest way would be to measure it on resistor leads but you also need to read to voltage when the resistor is disconnected. For this reason, the balancer port can be used. The simplest way to do this is to cut a male to male Dupont wire in half and connect one side to the balance connector and the other side wrap around the multimeter probes.

Warning: be extra careful to never short both battery poles as this may be dangerous, especially to your battery health. It’s best to insulate the multimeter probes with electrical tape after connecting them to the Dupont wires.

Build the load #

Just connect the resistors to form at least 50W 1Ohm resistor and connect your battery plug at the end, it should like this:

Here you can see how it all looks when it’s done:

How to use it #

We will use Ohm’s law to calculate the IR. The law states that Voltage (V in Volts) is current (I in Amperes) times resistance (R in Ohms). We can transform it to calculate resistance or current:

So to calculate Internal Resistance (IR) we have to know the current flowing through our resistor (I) and the voltage drop on the battery.

Calculating the current #

You could measure the current with your multimeter but this requires connecting it in series with the load which is a little tedious. Instead, we will just use the Ohms law. Since our resistor has a resistance of 1 Ohm, and I = V / R, our current should have the same numerical value as the voltage on the battery under load. If you want to be more precise, measure the actual resistance of your resistors and use it in the equation.

Measuring the voltage drop #

All you have to do is to connect your voltmeter to the battery, plug the resistors to the battery, read the voltage for the first time and note it down as V1, then disconnect the battery and note the voltage again as V2. Then just calculate the voltage drop V = V2 - V1.

Warning: The resistor will get hot very quickly. You have to be careful not to burn yourself or your desk. It’s best to put it on some metal pad that will dissipate the heat better and be careful when touching it after the measurements.

Note: You want to keep your load connected for as short as possible to avoid overheating the resistors and the drain of the battery.

Note: You also want to measure the higher voltage V2 after disconnecting the load as it might be a little smaller than before (due to battery drain).

Put it all together #

If you have your current I and your voltage drop V, all you have to do is to divide them:

IR = V / I

You will get the results in Ohms so it will be a small number much less than 1. We typically use milliohms for IR, so to get it, just multiply the result by 1000.

Practical measurements #

Now let’s check if it works by doing some real-life measurements.

With a scope #

Let’s start by having some base measurements done with my oscilloscope method.

The inrush current is 42A, and the voltage drop at this point is 0.52V. The R = V / I = 0.52 / 42 = 0.012Ohm = 12mOhm. That is the internal resistance of this battery at this point.

I didn’t do a screenshot of the cursors for the max current during the auto phase so you should either trust me or check this by counting pixels. The current is 17A with the voltage drop of 0.21V so the R = 0.21 / 17 = 0.012Ohm = 12mOhm - the same.

With my DIY meter #

Now let’s compare it to my cheap DIY method:

Note: The clamp multimeter is not needed, you just need the regular one. I have also connected the multimeter to a balancer port using a simple helper board but you can do it in any other way, it doesn’t really matter.

Warning: be extra careful to never short both battery poles as this may be dangerous, especially to your battery health.

I did the measurement 4 times to should you the variations I get (in the order of taking them):

While the clamp meter on a photo does show a slightly bigger current than in the table above, I have used the current computed based on the 0.96 Ohm resistance my resistors network have as I trust it more :)

The short tests were done with a load connected for as short as I could which is probably around 1s. The long tests were done with a load connected for more than 10s.

We can see that both short tests are pretty consistent - between 12.6 and 13.5 mOhm and it is just about the same we’ve seen in a real shooting test with an oscilloscope.

Two long ones are also pretty similar to each other - 19.4 to 21.3 mOhm, but they are much larger than the previous ones. They are not incorrect, though, this is what you would get if you keep auto-shooting for more than 10s as the IR raises due to the battery heating up.

Conclusion #

If you own a multimeter, you can build the IR meter for just a few EUR. If you don’t own one, you should buy one anyways - it is such a useful diagnosis device and you can have it for as low as 5 EUR and it will do the job just fine!

Since the battery IR meter is a useful tool to have, and it costs so little, there is no real excuse not to build one!