How Much Capacity Do 4.35 Volt Cells Lose at 4.20 Volts?

In an effort to wring the most power density out of the 18650 form factor, a few years ago some manufacturers began slightly modifying the chemistries of their batteries. This allowed them to increase voltage and therefore squeeze out a few more watt-hours.

These higher-capacity cells are shown below. For your reference, we’ve also compiled a non-exhaustive list of popular higher-voltage 18650 batteries, including photos, here. As always, check our Data Sheets page to verify.

Some of our testers can charge to 4.35v, but our analyzers use the popular TC4056A chip and are only able to handle a maximum of 4.25v. Thus, we charge all cells to 4.20v and then de-rate the higher-capacity cells to give a more accurate estimate of health. But how do we determine how much capacity is lost with that extra 0.15 volt?

Fortunately, the Sanyo UR18650ZTA data sheet provides capacity at both 4.35v and 4.25v. That shows that the rated capacity is 2850mAh at 4.35v and 2580mAh at 4.25v; a difference of 270mAh, or 9%. Yet even that is still a higher voltage than our typical charge of 4.2v. To dig deeper, we setup a little experiment.

Unbeknownst to many users, the popular Opus BT-C3100 v2.2 can do both charging and analysis at 4.35v. There is a small switch on the back side that allows users to select between 4.2v, 4.35v and even 3.7v (LiFePO4). Unfortunately, the rear cover blocks the switch, but for our test we drilled a hole to allow us to switch between voltages.

Test Setup:

1. We used 8 known good Samsung ICR18650-30B cells. These are rated at 2950mAh at 4.35v (data sheet here).
2. We used two separate Opus charger/analyzers.
3. Cells were labeled according to charger and bay, i.e., 11-14, 21-24
4. All testing was performed at 500mA both charge and discharge, which is approximately C/6 for these cells
5. We tested every cell eight times total: 4 times at 4.20v and 4 times at 4.35v. We alternated between the voltages each cycle to minimize the effects of degradation

Test Results:

As you can see in the test results below, the -30B cells averaged 8.7% capacity loss from 4.35 to 4.20 volts. Now, this test was highly unscientific: the ambient temperature wasn’t held constant, we used cells with unknown number of cycles on them, nor did we ensure the cells were charged to exactly the same voltage or allowed to rest the same amount between cycles.

A member of the community performed similar testing using a Sanyo UR18650ZTA 2900mAh cell, and found a 13-14% capacity loss, quite a bit more than we did! Check it out here.

Even unscientific tests like these provide valuable insight to allow us to effectively de-rate 4.35v models and better understand real-life capacity for most users. So when you buy higher-capacity cells, don’t be surprised if they’re capacity isn’t up to spec; that 3000mAh-rated cell is actually 2700mAh at 4.20v!