Since LiPo batteries started becoming big in the RC hobby, all sorts of processes and procedures for proper battery care have popped up. The one I’d like to talk about today is the process of always charging your LiPo batteries to a “storage charge” voltage of 3.8V per cell before packing them away after a day of flying. Many folks claim that storing LiPos at full or empty charge for more than a week causes permanent damage to them – affecting their ability to discharge and reducing capacity.

In this article, I wanted to put this claim to the test. To do this, I took three near-identical batteries and stored them for a month in three different charge states:

  • First battery was stored at a “storage voltage” as determined by my charger – which was 3.8V per cell
  • Second battery was stored ’empty’ at 20% charge, which is roughly the charge you can expect a battery to have after a flying session.
  • Third battery was stored fully charged

After remaining stored for a month, all three batteries were put through performance tests to see if anything had changed in their internal chemistry.

The Batteries

We were given three identical Infinity Graphene 1500mAh 4S LiPo battery packs by our friends over at Banggood. These are excellent miniquad batteries and have been our top pick for their combination of performance and price since their release last summer. With their graphene oxide internal composition, they are an ideal candidate for this test, as most miniquad pilots are moving to this type of battery for it’s improved power output and durability.

To read more on graphene LiPo performance, you can check out a previous article we wrote on the subject here!

When the batteries were delivered, I immediately got to work performing tests on them to verify that all three were nearly identical to one another. The first thing I noticed is that Banggood had shipped the batteries at a storage charge – a little under 3.8V per cell. Therefore, the batteries were not given a chance to “degrade” during the shipping process. I charged all three batteries to full, and ran them through a test cycle with a 40A load until each battery hit a voltage reading of 13V.

Here are the results:

Battery Discharge Rate (A) Voltage @ 15secs (V) Discharge Time Ah Discharged (mAh) Cell IR (mOhm)*
1 (Storage) 34 14.45 2:24 1515 8.4-8.4-8.4-8.4
2 (Empty) 40 14.28 2:23 1545 9.2-9.2-9.2-8.2
3 (Full) 39 14.2 2:20 1500 9.4-8.3-9.4-8.3
* Cell IR was measured by my charger, not specialized equipment. Variance was over 30% between measurements even on the same battery. I only include this figure because some people have specifically asked for it in battery tests.

As you can see, these batteries are nearly identical. The variance on the load on battery one was due to a problem in my load cell. I did not want to re-test with the problem fixed for fear it might affect the battery. Of the three batteries, it seems that battery 3 is the weakest by a small margin, but all are within about 5% of each other in this test on all fronts.

The Tests

My good old, well-used Turnigy power meter was used to make most of the measurements for this test.

Capacity and Low-load Performance

The first test I performed on the batteries after a month of storage was very similar to the test I performed immediately after receiving them. I subjected each battery to a 25A load until their voltage dropped to 13V. The idea of this test was to see if the batteries had lost any capacity due to improper storage. Voltage was also measured at 15 seconds to show how the batteries held their voltage under load. If the theory behind “storage charge” had any weight, batteries two and three would show decreased relative capacity, and would have a bigger voltage drop at 15 seconds.

Battery Discharge Rate (A) Voltage @ 15secs (V) Discharge Time Ah Discharged Cell IR (mOhm)*
1 (Storage) 23 15.02 3:59 1543 4.1-3-4.1-6.1
2 (Empty) 23 14.97 4:03 1542 2.2-4.4-2.2-2.2
3 (Full) 23 14.92 3:58 1496 2.1-3.5-4.2-4.2
* Cell IR was measured by my charger, not specialized equipment. Variance was over 30% between measurements even on the same battery. I only include this figure because some people have specifically asked for it in battery tests.

As you can see, these batteries show almost no disparity among each other. Battery three performed noticeably worse than the others, but was also the weakest battery in the initial test. Not looking good for “storage charging”.

High-load Performance

Some people claim that there is a chemical process that degrades the electrodes when you store your battery improperly. Electrode degradation is a real phenomenon, and is the causes behind batteries “puffing” from charging or discharging too quickly. In the case of “puffing”, the electrodes in the battery lose some of their usable surface area, and the battery has a higher internal resistance.  In turn, this causes the battery to output less power for the same load.

To test if this type of degradation happened on our test batteries, I put them through a 15 second high load test. I “calibrated” my load cell to pull 80A from a fully charged 4S battery. I then applied this load to all three batteries in the test for a duration of 15 seconds, and measured the current the batteries were outputting at the end of that time period. If any of the batteries had degraded, we would expect to see them putting out significantly less Amps under load than the other batteries due to higher internal resistance.

1 (Storage) 2 (Empty) 3 (Full)
70A 71A 69A

Once again, there was practically no difference between the batteries in the high load test. Again, battery three performed the worst, but was the weakest in all tests.

I will say – I was pretty amazed by how well all three of these batteries performed in this last test. I was conservative on the load I applied to them because I didn’t want to ruin the batteries.  However, I suspect I didn’t have to do that. I let one battery go until the voltage dropped below 13 and it lasted almost 50 seconds, discharging more than 70% of it’s capacity. That is absolutely incredible for a battery this small and cheap.

Conclusion

The load cell used in this test. Thanks to Joshua Bardwell for the idea.

I’ll be the first to admit that these tests are fallible. We are not using professional lab-grade equipment to do the tests, and my load-cell is literally a bucket of water and some fence wiring. That being said, I think it is fair to come to the conclusion that “storage charging” your LiPo batteries is unnecessary in most cases. If it does affect your batteries, it will be by a minuscule amount over very long periods of time. If you are ever planning on leaving your LiPos unused for extremely long periods of time – perhaps because of a deployment or something similar – storage charging them can’t hurt. Otherwise, your best course of action is to simply charge your batteries before using them and just leaving them alone otherwise.

So, do I think that LiPo storage charge is a total myth? No, not really: While writing this article, I dug around for some good research into the effects of storage charge states on the degradation of Lithium batteries. There is a lot of conjecture out there, and not a lot of good published data, but I did find this gem. It’s a research article done at the University of Colorado by Anderson Hoke, who tried to figure out the effects of different charging strategies on battery degradation.

The paper specifically focuses on Lithium batteries in vehicles, which are generally expected to last 10 years or longer. His conclusion is that the primary cause of battery degradation is storage at a high state of charge (full). Also worth noting, is that the projected battery degradation over ten years in this state is still well under half the capacity / power rating of the battery. A similar conclusion was not drawn for batteries stored at a low state of charge (empty).

This bears repeating though: Hoke’s experiment is relevant for batteries expected to last for decades, not for quadcopter batteries. The lifetime of our flight batteries is measured in months, not years, and the primary cause of degradation is the throttle stick on our transmitter.

Thanks to our sponsor

You can buy the batteries we used in this article here:

There’s a reason we approached Banggood about using these batteries, and it’s because we think they are the best value on the market. I wanted to take one last chance to thank Banggood for providing us with these batteries for the test. It’s not often you have budget retailers also sponsor research articles like this and we think they deserve some credit for this. Thanks, Banggood!

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