Mystery SOT23 in battery management circuit  - Page 1
Forum section: electronics / repair

author: raphaeltube
author_rank: Contributor
timestamp: October 03, 2022, 08:49:53 pm
content: HiI'll first give some context. If you're only interested in the technical electronics, skip to the next post.I was near the top of the 3rd pitch of a rock climb, around 50 or 60m up a cliff face, when I dropped my handheld radio (Alinco DJ-MD5EGP) and watched as it hit the rocks at the bottom.It took me a while to finish the climb, wait for my buddy to come up behind me and walk down to look for the pieces. By this time another climber had picked them up, and I was told he had headed back to the camp.That evening, I found him and finally got to see the state of the radio. I was VERY surprised how little damage there was. The antenna was very damaged, and the battery was in pieces, but the radio itself looked OK. just a crack in the case that I was able to weld up. Not even the screen was damaged!When the guy gave me the pieces, he said to be careful, the battery was dangerous. He had tried to put the battery back together, and something went Pfft!I have subsequently tested the radio with another power source & it works 100%though I guess the waterproof rating has been compromised.So in the end, it seems the only actual damage was caused by the moron who decided to pick up the pieces & "fix" it

author: raphaeltube
author_rank: Contributor
timestamp: October 03, 2022, 09:01:27 pm
content: This is a battery management circuit inside the battery (EBP-88) for balancing 2x Lithium cells.I assume the guy touched some of the exposed ribbons across the cells, dumping the full current into whatever poor component is now a black smudge.It looks to me like it was a SOT23.I don't have the brains to know if it would have been a MOSFET, a BJT or a Diode package, and I couldn't find any documentation.Please help to suggest what component I could try put in there.I have tried to draw the circuit out as well.See the below pics:

author: Manul
author_rank: Super Contributor
timestamp: October 03, 2022, 09:58:24 pm
content: I believe it is not at all sot23. It will likely be two separate kinda "shared pad" smd passives. Like in the sections attached bellow as examples from your board picture. It is clear that they like such "layout".

author: fzabkar
author_rank: Super Contributor
timestamp: October 03, 2022, 11:58:14 pm
content: This IC seems to have a pinout which is similar to the "3X99":https://www.hycontek.com/wp-content/uploads/DS-HY2120_EN.pdf

author: fzabkar
author_rank: Super Contributor
timestamp: October 04, 2022, 01:43:56 am
content: Quote from: Manul on October 03, 2022, 09:58:24 pmI agree.I suspect CHG- was linked to RADIO- via a zero-ohm resistor. BAT- may have been coupled to CHG- via a filter capacitor, or perhaps that spot was unpopulated.Maybe an smt fuse would be a better proposition in place of the zero-ohm link?I would test for a short between RADIO+ and RADIO-.

author: raphaeltube
author_rank: Contributor
timestamp: October 04, 2022, 07:02:20 am
content: Ah, right. That would explain why the pads don't quite line up for SOT23.Thank you.

author: raphaeltube
author_rank: Contributor
timestamp: October 04, 2022, 07:23:19 am
content: Hm. Bridging CHG- to Radio- doesn't give power to the radio.Does that mean the FS8820 is damaged?

author: fzabkar
author_rank: Super Contributor
timestamp: October 04, 2022, 07:06:12 pm
content: Quote from: raphaeltube on October 04, 2022, 07:23:19 amThe charger should power the radio directly. The FS8820 only comes into play when the radio is battery powered.

author: raphaeltube
author_rank: Contributor
timestamp: October 05, 2022, 08:25:56 am
content: Correct, but the radio is not getting powered by the battery when out of the charger.