18 March 2018

Modding a DIY USB Power Bank case with HT4936S

Since I had some spare 18650 Li-Ion cells laying around, I decided to fit them in a couple of empty power bank shells sold online for few €/$. Today I will report on what looks like to be a copy of Xiaomi 10400 mAh power bank.

Knowing my pre-charged cells were relatively good, I suspected something was wrong when right after the first few seconds using the DIY USB power bank to charge my smartphone the status LEDs reported 75% or even 50% of charge remaining.

A quick inspection with the DVM after recharging the power bank revealed that my four cells were all measuring 4.0V, while I would and could charge them up to their nominal 4.2V voltage. Time to troubleshoot the electronics.

With the help of a magnifier I could identify the all-in-one controller chip as HOTCHIP HT4936S (Company website). Datasheet can be found online (7 pages in Chinese) and translated into English with an online service. The datasheet is far from being comprehensive, but the device isn't too complex either. The HT4936S chip takes care of charge and discharge control, voltage booster to 5V/1A, 4 status LEDs, a control button and, as every Respected Chinese product, a flashlight (press twice the button to activate - if the flashlight LED is present - twice again to turn off).

A note about the flashlight. This power bank does not mount the flashlight LED, but I tested the double click on another "shell" with unmarked controller chip and it works.

At first sight the circuit on the PCB is very similar to the datasheet sample application. Good. Some components are missing from their pads. Not so good.

Problem #1: cells are charged up to 4.0V only.
According to the datasheet, the HT4936S can be configured to stop charging either at 4.20V or 4.35V: while pin 1 is left open it should charge cells to 4.20V, if grounded it goes up to 4.35V. Why does it stop at 4.0V then? Shorting together R1 pads will ground pin 1 and set the target voltage to 4.35V. It works, and it keeps charging. I stopped it at 4.25V/cell so they don't get damaged.

Solution #1.
While this mod forces the IC to charge cells up to 4.35V, it moves "upwards" the discharge detection, therefore the chip considers them depleted at 3.6V thereabout. There is still the doubt why it stops at 4.00V instead of 4.20V in the default configuration. So this solution is not the final answer.


Problem #2: ground connection intrinsic resistance.
The ground connection is made of a single long wire that is wind-up to make the four contact springs: unless you insert a pre-made 1S4P pack, negative terminals of your 4x 18650 pieces won't be at the same potential (under load). I measured 100 mV drop between GND pad on PCB and the farthest cell during full current recharge (about 800 mA input). A solution could be to add a thick wire/strip that connects together all negative terminals.
In case you wonder, this extra resistance does not seem to be the culprit of Problem #1 since it keeps charging up to 4.35V.

Solution #2.
There is no much room in the power bank case to shorten the ground path. So ... live with it.


I wonder if off-the-shelf power banks suffer from the same design errors. Probably not. I will consider these as an energy source when no other solution is available. On the other hand I will not let "waste" 4 cells in there, but rather leave just a couple.

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