Understanding Internal Resistance – A “Must Read”

I have discussed many of the aspects and terminology associated with lipo battery packs recently but in reality, the most important characteristic required to really grasp the health of a pack requires developing an understanding of the battery’s internal resistance, or IR. So, as a follow-on to the series of articles I wrote previously I’d like to offer a simple summary of this very important topic regarding the health and safety of lithium polymer battery packs.

As lipo battery packs age (or are mistreated) their ability to deliver the power they had when they were new decreases over time. You may have noticed this effect in one of you models… that even when the battery pack is fully charged the model just doesn’t fly with the same “zip” that it used to. Also, you may have noticed that it takes longer to charge your battery than it used to and sometimes, even when fully charged some or all of the cells never reach 4.2 volts.

Why is this?

Hopefully, you will recall that heat is the number one enemy of lipo batteries. During use this heat is generated by the battery itself and is perfectly normal during its operation due to the chemical reactions occurring within the cells. Chemical reactions produce heat. However, the way we use the battery (and oftentimes misuse) will greatly affect the amount of heat it produces. In addition, the chemical reactions themselves produce byproducts which act as contaminants and collect on the cells’ internal parts. These byproducts serve to restrict the flow of current through the battery, producing even more heat. Keep in mind that the wire and connectors in your system also provide resistance to the flow of current. In short, as the heat generated by a battery increases through all of these components and mechanisms, the pack’s resistance to the flow of current also increases. So in effect, the battery itself begins to act as a giant resistor within an electrical circuit, which in this case is our model’s power system!

The internal resistance of a lipo pack is measured in milliohms, which is a thousandth of an Ohm, the basic unit. You may be familiar with Ohm’s Law which quantifies the effect that resistance has on an electrical circuit. The IR of a lipo pack is not a number you will find printed on the case like the others we have discussed, because it changes over time. To get a reading the the pack’s IR we must measure it directly. Some battery chargers can measure the IR of the pack or individual cells (or both) during the charge process but the accuracy of the reading will depend on the quality of your charger. The most accurate measurements come from meters specifically designed for this task and they can be expensive.

Let’s look at this IR effect in a little more detail. Ohm’s Law shows us that the reduced efficiency (amount of current being delivered through an applied voltage) of the battery, results in less power being delivered to the motor and therefore less RPMs (remember Kv?) to the propeller. Again, recall that watts of power is the result of amps times volts, so if you have fewer volts you have fewer watts and fewer watts due to fewer volts means that the battery has to try and deliver more amps to make up for the loss in power. This increased demand for more current results in the battery pack producing more heat. More heat produced increases the IR of the cells and pack overall reducing its usefulness, health and longevity.

With this in mind it also stands to reason that a pack’s “C” rating will be greatly reduced as a result of increased internal resistance, so an already over-inflated number provided by the manufacturer will be further reduced, yielding less power available for the model. In addition, increased resistance will also become an issue during the charging process and will most likely result in a pack that will no longer fully charge.

IR should be expected to increase as the pack ages but can be minimized through proper care. Here are a few extremely important tips to keep in mind as you use your lipo packs:

  1. NEVER leave lipo packs in direct sunlight or in a car where the temperature can get very high. Your car may burn up and I have seen it happen.
  2. NEVER leave your lipo packs fully charged! This is the number two killer of all lipo packs. Always keep your packs at storage voltage (3.85 volts/cell) and get them to this voltage as soon as possible after a day’s flying. Buy a good quality charger that offers this storage charge feature.
  3. If you can, store your packs at a temperature of 40-50 degrees (nice and cool). Your garage might be a great place to store your packs during the winter months. It probably isn’t a good idea the expose them to freezing temperatures, but since the packs contain no water (very bad reaction between water and lithium!) there currently is no data that I am aware of which would suggest that temps this low will damage the pack.
  4. The number one killer of lipo packs is OVER DISCHARGING them. If you are demanding power from a pack that it cannot possibly deliver, you will not only greatly increase the pack’s internal resistance but you also risk the pack swelling (puffing) and in the worse case, a FIRE. Use a watt meter to measure the actual current draw in your power system.
  5. If possible, begin recording you pack’s IR when it is new and throughout its life. A normal IR reading of 3 or less milliohms when a cell is new is typical but if you notice a cell with a much higher reading, you probably have a defective pack and should contact the supplier for an exchange or refund.
  6. There are no exact guidelines in terms of what is acceptable IR but in general, it is widely accepted that a cell with an IR reading (at full charge) of less than 8 milliohms is still OK. A reading of 10-12 milliohms is showing signs of deterioration and should be monitored closely. A reading of 20 or more milliohms definitely suggests that the pack should be retired and most likely discharged and discarded. Just how to do this is the subject for another article.
  7. If a pack begins to “puff” or what I call, “get squishy” after dozens of cycles it probably isn’t cause for major concern but it IS an indication that the pack is on its way south (it is beginning to “outgas”) and should be monitored carefully. Be sure to treat these packs with extra care. Only charge them at 1C and never expect them to deliver their advertised power in your model. This could easily lead to an over discharged condition and possibly result in a FIRE.

If you follow the above simple steps and now have a bit of an understanding of what internal resistance is and how it affects a battery’s health and longevity, then I will consider this series of articles complete.


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