10 Things To Know About Lithiuim Ion Battery Internal Resistance

One of the most revealing attributes of a Li-ion battery’s health is its internal resistance. IR plays a vital role to make the best performance of your Li-ion batteries. Many users try to test the batteries’ IR via using smart chargers by themselves. In this article, we have collected ten frequently asked questions about the internal resistance of the lithium ion batteries. 

Q: How does internal resistance affect batteries’ performance?

A: Ohm’s Law states that V=IR . A battery with low internal resistance delivers high current on demand. High resistance causes the battery to heat up and the voltage to drop. The equipment cuts off, leaving energy behind. A battery’s State of Health (SoH) is 100% when new, and decreases from there, with its internal resistance increasing and capacity decreasing.

Q: What is the normal internal resistance of an 18650 battery?

A: It will depend on your batteries’ different conditions and usages, also on how you measure the IR. You may test some of your new 18650 cells measuring around 50-80 milliohms. And there is reference list online about 18650 cells’ IR and ranking:

Q: How to measure the batteries’ internal resistance?

A: The main reasons we want to test for a battery’s internal resistance is to understand its condition, as a higher than normal reading will indicate that the battery is near the end of its working life. And there are different ways to check the IR of a battery, and it’s by looking at the Direct Current (DC) and Alternating Current (AC):

*Direct Current – measures the voltage drop at a given current, looks only at pure resistance.

*Alternating Current – takes reactance into account and provides the impedance, which may be different from the DC reading but both are correct.

Q: I have a 18650 battery charger that can measure batteries’ internal resistance, while I notice that the resistance levels changes sometime I test it. Is this normal? How do I determine how much resistance is good or bad?

A: The resistance levels will change depending on a lot of factors, such as temperature, the contact between the battery and positive, negative plates of the charger. Please try looking up the manufacturers datasheet to check whether it is within margins. If it’s with the range, the battery should be good.

Q: Why is charging current determined by internal resistance? Does IR affect charging time?

A: The internal resistance basically tells how is the battery health so based on that the charger can select the appropriate charging current to make the best out of the battery. The lower the internal resistance, the faster for the battery to charge. For example, the batteries tend to lose capacity as they age, also the internal resistance increasing, then it may need more time for charging.

Q: I got a 18650 battery fully charged on the VC4SL, then tried on mode IR, but the charger didn’t test the battery’ internal resistance. Why does a fully charged battery not work for an IR check? How about the procedures of the charger testing IR?

A: The VC4SL charger adopts the direct current load method to test battery internal resistance. It means to apply a little bit larger current to the battery for a short period of time. Based on the voltage’s change of the battery, also together with the applied current, it can calculate the DC internal resistance of the battery. If the battery gets fully charged, in case of over-charging, the charger won’t test internal resistance in order to protect the battery.

Q: Will low charging current affect IR testing function on xtar chargers?

A: When the adapter input current is lower than 700mA via the charger, which is insufficient for internal resistance testing, the charger doesn’t test IR either. 

Q: For testing a same one battery’s IR on my xtar VC8, why do I sometimes get the different IR numbers on the charger?

A: For some chargers testing the batteries’ IR, it needs to put the batteries into the slots first. So it’s important to make sure whether the contact between the battery and the positive, negative plates is well. If there is poor contact, it may have a big deviation for the IR testing. 

In addition, some chargers adopt current load method to test the IR. When the battery voltage is particularly low, due to the large variation of the battery voltage, the measured IR will also be relatively high. The suitable testing voltage range is 3.6 V-3.8V for IR testing. If the battery voltage is higher than 4.16V, in order to prevent overcharging, the IR test will be interrupted, which may also cause the deviation of the displayed IR value.

Q: The VP4 plus tests the resistance results that are pretty consistent with a real cell IR tester. Is it because it uses separate probes for measuring IR? Then, it’s better than the typical in slot measurement on some chargers?

A: Yes, the VP4 plus probes are four terminal, and the 4 terminal connection means that it will automatic compensate for any wire resistance and connection resistance at the charger. It can precisely measure the batteries’ IR as a multi meter. For some chargers with the spring loaded holders’ design, which it self causes variable contact/resistance.

Q: For DIYing Li-ion battery packs, such as e-bikes battery pack, why it’s necessary to test cells’ IR and assemble identical IR cells into series?

A: You know the lower the resistance, the less voltage drop the cell will experience under load. This leads to less heat build up, which provides for a longer total cell life and, therefore, a longer battery pack life. If one of the cell has a much higher IR than the other cells, it will experience more voltage drop than the other cells. This causes energy to flow from the good cells into the bad one, which only exacerbates the issue. And remember, as a cell’s temperature increases, so does its resistance. This creates a potentially hazardous positive feedback loop that will make your battery pack fail a whole lot sooner than it otherwise would have. So, when you are learning how to make a Li-ion battery pack by yourself, the IR testing is one of the important steps.

You can download this document in Facebook Group: Lithium Ion Battery & Charger Talk (18650/21700/26650 & more)