A simple upgrade for an 8×18650 power bank

About a month and a half ago, I did a review of an inexpensive power bank case—a handy, affordable model that can deliver up to 22 W and supports fast-charging protocols. But ever since, I’ve wanted not only to fit it with the highest-capacity batteries I could find, but also to make a few improvements.

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Just yesterday, I finished testing the self-discharge rate of Vapcell N40 batteries—18650 cells with a record capacity of 4000 mAh—so now I can put them to use and, while I’m at it, improve the power bank.

The modification is extremely simple, and just about anyone who can hold a soldering iron can do it (assuming you have one, of course).
For the modification, I used 1 mm² wire. I stripped the insulation, tinned it, and straightened it out. You could use thicker wire, but there’s no point, and it would be harder to solder.


Next, I tinned the current-carrying bus bars. No acid is needed; I just used solder with “built-in” flux, and it tins very easily. If you’re not sure of your soldering skills, you can remove the board from the case so you don’t accidentally melt the plastic. I couldn’t be bothered.

In fact, you can tin about half as many spots; it won’t make any difference.



Measure the wire so it’s long enough to reach from the far end to the corresponding contact on the circuit board.
Then solder it in parallel with the bus bar.


Do the same with the positive bus bar, though there’s a little less room here.


Before the modification, the bus bar resistance was 45 mΩ. Afterward, it dropped by more than a factor of 20, to about 2 mΩ.


There’s plenty of room for the wire; the 1 mm² wire practically “falls into place.” But you can see that the positive bus bar has a little less clearance.


First, I wanted to see what I could get out of the batteries I had on hand. Sure, 8 × 4000 mAh would have looked better, but 4 × 4000 + 2 × 3650 + 2 × 3500 mAh isn’t bad either.


The measurement showed that I got almost 87 Wh from this configuration. As you can see in the screenshot, the test was run with an 18 W load, so the operating conditions were far from optimal.


By the way, the modification affects charging as well as discharging—or, more precisely, the charging speed. With high resistance in the conductors to the batteries, the charger switches to CV mode earlier and charges at a lower current, so it takes longer.
Here, even after the energy delivered during charging passed 100 Wh, charging continued at the maximum rate.

Charging took almost 118 Wh in total.

A few calculations: the total capacity of all the batteries is 57.2 + 25.96 + 25.2 = 108.36 Wh. I got 87 Wh out, and 118 Wh went in during charging. That gives an efficiency of at least 80% when discharging and over 90% when charging. Not bad, especially since the actual discharge efficiency was probably higher: the batteries weren’t discharged down to 2.5 V, only to 2.95 V, so some of their energy remained unused.


But wait—did the modification actually do anything? Let’s check. For comparison, here’s a screenshot from the previous review, where the power bank delivered 77.85 Wh.


“But those are different batteries,” you might say. And you’d be absolutely right: that wouldn’t be a valid comparison. So I used the same batteries as before, charged them the same way, and ran the same load.

In the end, I got 79.34 Wh. The difference was 79.34 − 77.85 = 1.49 Wh. Honestly, I expected a bigger difference, but it’s worth remembering that the bus bars had high resistance mainly on the far side; the cells closer to the board were operating under more favorable conditions.

Only afterward did I notice that the output voltage gradually dropped toward the end of the discharge. The power bank was lying “upside down,” and it seems to have heated up more, so it limited the output voltage near the end.


Next, I wanted to repeat the experiment where, after discharging the power bank, I gave it a little extra charge (or rather, a kick) so I could discharge it further at a lower current. But no, that trick doesn't work this time. I disconnected all the batteries, and without any additional discharge I got the same result: about 80–81 Wh.



Conclusions.
Well, an extra couple of percent of capacity isn't much, but since the modification is simple and essentially free, why not? You never know—those extra two percent might come in handy someday.
By the way, what does 1.5 Wh amount to? Let's think about it. At 3.6 V, that's about 415 mAh, or roughly 10% of a typical smartphone's charge—or a couple of hours of use.

All in all, it's worth making this modification. Your smartphone could certainly use a couple of extra hours of runtime before you have to charge it again.

That's all for today. I hope you found this useful.