Intersil 95538H or how to reuse Mi Power Bank 3 Pro board for 4.2V cells - Page 1
Forum section: electronics / beginners

author: herpderp
author_rank: Newbie
timestamp: March 26, 2024, 10:44:11 pm
content: Hi all,I'm a bit of a sucker for reusing otherwise perfectly functional pieces of electronics and today I finally opened my old xiaomi 20000mAh PD powerbank to investigate. It outputs 15V@3A and 20V@2A so it's 45W one. Good enough for everything from charging macbook air to powering TS100.Anyhow, otherwise good powerbank is plagued by shitty cells used inside. Those are pouch ATL 4.35V cells, 2x10000mAh in parallel. Haven't used the powerbank thaaat much, maybe couple of dozens cycles, 50 max. At "higher" power levels (20+ watts) it barely gave out half the capacity before turning off. Now, testing cells directly on a bench (but still two in parallel), I do see enormous voltage sag even at 0.25C (from 4.3V to 3.6...3.5V already at 5A). IR is 50+ mOhm too...Now I would have thrown them away and soldered 6 good 18650 cells as replacement without writing here if not the pesky high voltage cells originally used in this powerbank. Charging through the board I verified that it's configured to charge up to 4.3V. While there are some HV 18650 cells in existence, they are expensive and practically unobtainable in a shithole country I'm currently in.So, me, a total noob, started tracing components on the board and searching datasheets online. Found out that the board uses Intersil 95538H buck-boost NVDC charger chip. Didn't find proper pinout for that chip, justthis diagram(it's for 95538B, didn't find the H version at all). Seems that there are no external voltage dividers options for setting voltage levels as is sometimes done in that kind of circuits..So.. any ideas how to repurpose the board to charge regular li-ion cells and limit charge to 4.2V? It would be weird if that charger chip wasn't configurable, but if and how is it possible to poke it? Preferably without desoldering it from the board. Anyone done something similar before?I know it's not a lot of savings in terms of money, but I have some good ~3000mAh 18650 cells laying around and instead of buying another PD board and building a DIY powerbank I would really like to repurpose the old board and shell. 6x18650s would be exact fit there..PS. added some pics of the board as attachments if that helps anything

author: kevin.gibbs
author_rank: Regular Contributor
timestamp: March 27, 2024, 01:32:24 pm
content: The chip is controlled similarly to the ISL95338 over the I2C bus. Perhaps even the registers are the same.https://www.renesas.com/us/en/document/dst/isl95338-datasheet

author: LooseJunkHater
author_rank: Frequent Contributor
timestamp: March 28, 2024, 01:12:10 am
content: Can't find a datasheet for the 95538H specifically, nor the "TR5611"/"TR3611" (can't make out exactly what it says). My guess is that the other chip is a MCU which handles USB PD negotiations and sends out I2C communications to the Intercil chip for various things (maybe even charging of the battery?).This board is quite complex with various I.C's and I do notice two inductors on the PCB, with the large one likely being for the USB PD output, with the other likely being for the USB A ports, based on the statement of "Each USB-A can provide 5V/2.4A, 9V/2A, or 12V/1.5A output if used one at a time, or if used simultaneously can provide 5V/3A output." on the product page.Out of curiosity, did you look up the datasheets of the other I.C's on the board? I do wonder if it's a different I.C handling battery charging. Maybe you could share another image of all of the other I.C's on the board labelled.Another thing I noticed is that the datasheet of the ISL95338 says that for pin "PROG", "A resistor from the PROG pin to GND sets the default forward system output voltage.".I think you may need to reverse-engineer the powerbank a bit to find out what I.C's are doing what and whether the internal battery is being charged by the 95538H or a different I.C.