I recently got a second-hand Theragun Elite G3 (the one with DC 4017 powerwell input) for 350 RMB (about 50USD) from Xianyu (the largest second-hand/personal website from Alibaba), and the battery seems dead. And I tore it down according to the helpful guidelines above.
The battery uses three Samsung 25R 18650 lithium-ion batteries, which were connected in a serial way; in my case, they were tested using a multimeter; two of them had an open-circuit voltage of about 0 volts, which indicates their SOC(state of charge) is in a negative state. This is normally what is called extreme overdischarge. One of them is overdischarged, its open-circuit voltage is at about 2.5V. They were connected to the machine by a protective board, and the power cable was soldered to the control board. Three extra cables using a JST connector transmit the NTC signal (thermal information of each battery) from three electric thermocouples. The device won't boot or show anything if these 3 NTC signals are absent.
So I tried to use a digital-controlled-adjustable power source to charge the two dead ones, and closely monitor them using a thermo camera, unfortunately, that didn't do much help. *(This is a bit dangerous, and if you don't know what you are doing and do not have any experience with Lithium-ion batteries, please DO NOT try this at home).
The model for the battery is TG-G3-25S, and I was able to find them brand new at a cheap price on Xianyu for 59RMB (approximately 8.28 USD at this time). The seller was from Zhuhai, Guangdong, which is where the battery and the Theragun manufacturing factory are located. I assume there were spare parts from the factory. The battery's manufacturing date was 2020, and the seller promised me they were never used and were in a proper state.
I also noticed there were other models from other buyers in models such as Elite-QR001B-RBA (2023-11-28), TB-F1-01 ......, there were also swappable batteries for G3/G4 pro, and the latest model, which has a type-C port, not a DC 17 diameter port like the legacy one. But I don't need them, so I gave up buying them. I also bought some motors from Theragun because they look nice, and they might come in handy. The 12V motor can be purchased at Taobao at about 10 RMB - 20RMB, depending on the model. I got a BL3435, BL4539P-L, and a BL2827S for 40 RMB total.
The battery hasn't arrived, so I would wait and see how they are when they arrive.
The information above is to help someone who can't find an alternative, and their battery cell might be dead like mine. Personally, I won't recommend anyone buy a Theragun. They intentionally make it hard to open and repair things; it would come off easily if not the massage tips were designed like this
. I guess someone could use an electric grinder pen to remove the pressed metal, which prevents the nylon plastic cover from coming off, and then the plastic and stuff would come off.
The protective board uses the BM3451SMDC-T28A, which is a rare component. I can't wrap my head around how cells like Samsung 25R, which have a good reputation, would overdischarge like this. There must be something wrong with their battery management solution to have such a wide range of battery failures. According to the datasheet, BM3451SMDC-T28A has charge balance and overcharge protection. I never use components from BYD IC, and I think they really shouldn't cut corners on these things, or maybe they did this on purpose? Texas Instruments has far superior solutions for battery management. As a so-called California Brand, they really used as much cheap Chinese IC as possible to reduce costs. And I suspect that everything of their electrical design is outsourced to a not-so-great engineering team with little durability and longevity testing.
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