Here is that the inside looks like with the top removed. The "+" and "-" are the polarity of the battery terminals on the top. ![[IMG_0345.jpeg]] # Charger ### USB Charging Profile This battery uses a CN3302 ([[CN3302.pdf]]) IC, which is not a LiFePO4 charger! It is a charger for "normal" lithium cells with a charging voltage of 4.2 volts, which is much higher than the charging voltage for LiFePO4. The only reason this doesn't dangerously overcharge LiFePO4 batteries is the BMS kicks in at 7.3 volts and immediately halts the charge (see the BMS section). This also explains why the batteries don't fully charge. There is no CV (constant voltage) phase. Usually, the final phase of charging is holding the battery at a constant voltage and trickling until the current falls below some threshold. As you can see below, the current suddenly plummets at 7.3 volts, which is the BMS over voltage protection threshold. The lack of a CV trickle means the batteries are not fully topped off. ![[Pasted image 20260706103325.png]] Some thoughts: - Slightly undercharging the batteries is probably good from a safety and longevity perspective. - Using the BMS as the charge cut off is not a best practice. The BMS is meant as an additional layer of safety, not the only layer. - I have no idea what's going on between 4 volts and 4.5 volts. To be fair, the batteries should never get below 5 volts, so we can probably ignore this part of the curve. But if the batteries were to get that discharged (maybe they were stored for a long time and self-discharged), it would be dangerous to charge them at 1.75 amps! ### Spring Terminal charging profile The manual also says you can charge through the spring terminals, but gives very few details on how to do this. What sort of charger should you use? The manual is silent. Here's the charging profile throught he spring terminals, using my bench supply with a current limit of 0.8 amps: ![[Pasted image 20260706103411.png]] Oh my! That's not good. It took the entire 0.8 amps for almost the entire test. I would not recommend charging through the spring terminals. There is no protection of any kind. Using a wrong or broken charger could subject the batteries to dangerous voltage or current levels. For example, connecting the spring terminals to a lithium charger meant for 4.2 volts (or 8.4 volts for two batteries in series) would dangerously overcharge the batteries. # BMS This uses some version of the HY2122 BMS IC ([[HY2122.PDF]]). ### Over/Under voltage | | USB | Spring Terminals | | --------------------------- | --- | ---------------- | | Over voltage (Charging) | ✅ | ❌⚠️ | | Under voltage (Discharging) | N/A | ✅ | | Short circuit (Discharging) | N/A | ✅ | This is why there is no charging protection through the spring terminals. The part of the BMS that protects charging (OC MOSFET) is simply not in the charge path when charging through the spring terminals. It does however have short circuit protection through the spring terminals. The battery can't be discharged through the USB port. ### Cell Balancing | Type | | | -------------------- | --- | | Balancing protection | ✅ | | Active balancing | ❌ | The BMS prevents charging if the cells are out of balance, but does not do anything to correct an imbalance. If the cells become too out of balance after many charging cycles, the battery will simply stop working. My cells are slightly imbalanced after a few cycles: 3.428 volts and 3.382 volts, a difference of about 46 mV. That's not a dangerous level of imbalance, but the BMS will do nothing to correct it if it gets worse.