Best 18650 Battery at pulsed loads of 70W per battery (140W with two batteries)

After last week's test results on 0.2 ohm constant resistance testing (like that found in an e-cig mechanical mod), many readers wanted to know how a regulated device would compare.  Unlike mechanical mods, regulated devices use a constant power output instead of a constant resistance load.  This changes things some.  So here are some results for some of the best batteries.

The Contestants

Just like last time, the batteries under test are the Samsung 25R  (full part number INR18650-25R), the Sony VTC4 (also known as VCT4, full part number US18650VTC4), and the Sony VTC5 (also known as VCT5. full part number US18650VTC5).  The test will be at the top end of the rating of the Samsung and the VTC5 (they are rated 20A continuous), while being mid-way through the rating for the Sony VTC4 (pretty much the only battery that's truly rated for 30A continuous).  The Samsung 25R is the new green Samsung, the 25R5.  All batteries we tested at least one new one as well as a couple that had been lightly used and stored for a bit.  After verifying there wasn't a huge difference, the data for the new ones will be shown below.

So how do the 25R, VCT4, and VCT5 perform with 70 watts?

The Test:  3 second load pulses of 70 Watts

This test simulates a constant watts device with one lithium ion cell at 70W.  As you know, if you add multiple batteries in series or parallel in a regulated device then they share power loads equally so if you had two batteries this would be like having a 140W device (under ideal conditions of course).  The device is fired for 3 seconds, then turned off for 8 seconds, then repeated.  This is done from full charge (4.2V) until the voltage goes to 3.2V.  This will mean current levels will vary from 16.6-21.8A, per battery.  Regulated devices work backwards of the way mechanical devices do - whereas in a mechanical device power and current decrease as the voltage drops, in a regulated device power stays constant and current increases as voltage drops.  So in a mechanical mod you're being hardest on your batteries when they're fully charged, in a regulated device you're being hardest on them when they're low on charge.  

Here's an overview chart of how the test turned out:

The overall results are the same as the mechanical load test - all batteries handle it great but the 25R lasts the longest, followed by the VTC5, followed by the VTC4.  So let's look at the first part of the test in more detail:

Again we see that the VTC4 starts off stronger in the beginning while near full charge, but its lower mAh rating means that it can't keep that up for long.  By 200 seconds we see the trend that will hold for the whole test - the 25R just slightly better than the VTC5, and the VTC4 still performing strongly but at a little lower voltage.

Here's the 2nd pulse:

Below is a pulse pretty early in the test, about 2 minutes in.  This is an interesting one because the nominal battery voltages are all about equal so you can see the difference in voltage sag under load.  You can see that the VTC4 can handle these high currents a bit better than the others, giving a higher loaded voltage (less sag).  The VTC4 is a 30A battery while the other two are 20A batteries, and at this moment the current is around 16A so you can see that the VTC4 has an easier time with it.

Unfortunately the VTC4 has lower mAh so it doesn't have the staying power.  Here's the rest of the test:

The trend continues and the VTC4 is depleted first, followed by VTC5, and the 25R lasts the longest.  Here's how many pulses you get out of each battery to 3.2V.

Temperature

All the batteries stayed near ambient temp for the whole test.  At the end when the 20A batteries started to get taxed a little bit they went up just a few degrees, but still only lukewarm.

Conclusions

All three batteries are awesome.  The VTC4 has a little bit better performance in terms of pure voltage sag but it's a pretty small difference.  The 25R clearly wins in terms of being able to hold voltage the longest and provide the most pulses per discharge cycle.  The VTC5 lost voltage quicker than the 25R in the beginning of the test but after a while mostly ran the same.  It also gave a few less pulses before needing a recharge but not a huge difference.

So what would the best 18650 battery be for 140W regulated with two batteries, or any other constant load at 70W per battery?  I would say the 25R, although it is very much at the top end of where it should be used.  Any higher watts and a true 30A battery like the VTC4 should be used.  Also, technically the 25R is slightly above its CDR (constant discharge rating) at the very end of the test depending on where you set your voltage cutoff to, but the VTC4 is not.  For people who want to have a very large safety margin, the VTC4 would be the better choice for that reason.

How does this compare to a similar load level but with a constant resistance like a mechanical mod?  See our mechanical mod 0.2 ohm battery test here.

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