How to Troubleshoot a 2S 8.4V BMS (HW-391): Complete Wiring & Charging Guide
🔧 Troubleshooting Guide: 2S 8.4V 20A BMS (HW-391)
If you’re using a 2-cell (2S) lithium-ion setup with a compact 8.4V 20A BMS, you may run into situations where the board seems “dead” and will not charge or discharge. This guide walks you through quick checks to confirm wiring, polarity, and charger setup for this specific style of BMS (often marked HW-391).
👉 Click here to buy this 2S 8.4V BMS (HW-391)
1. Confirm All Three Battery Wires Are Connected Correctly
This BMS has three battery pads:
- 0V → Battery negative
- 4.2V → Middle connection between the two cells (cell junction)
- 8.4V → Battery positive
Your 2 cells in series should be wired like this:
Cell 1 (-) → 0V Cell 1 (+) → 4.2V Cell 2 (-) → 4.2V Cell 2 (+) → 8.4V
If the middle wire (4.2V) is missing or connected wrong, the BMS will stay off and nothing will charge or discharge.
2. Check Battery Polarity in the Holder
Even if the wiring to the pads is correct, the BMS will completely shut down if one battery is inserted backwards in the holder.
- The negative end of the pack must go to the pad marked 0V.
- The positive end of the pack must go to the pad marked 8.4V.
A reversed cell will cause the BMS to disable itself. The board may look “dead,” but it is just protecting the cells.
3. Measure Battery Voltage at the Pads
Use a multimeter to make sure the pack voltage looks normal:
- Between 0V and 4.2V: about 3.0–4.2V
- Between 4.2V and 8.4V: about 3.0–4.2V
- Between 0V and 8.4V: about 6.0–8.4V
If these readings are reasonable, the BMS is getting power from the cells.
4. Measure the BMS Output Pads (+ and –)
On the other end of the board you’ll find the output pads:
- + → P+ (output positive)
- – → P– (output negative)
Measure across the + and – pads with your multimeter. You should see the full pack voltage (around 7–8.4V, depending on state of charge).
- If you see that full voltage, the MOSFETs are ON and the BMS itself is working.
- If you see 0V, the board is in protection mode or wired incorrectly.
5. Check How the Charger Is Connected
Charging must go through the output pads, not directly on the battery pads.
- Charger positive → + pad (P+)
- Charger negative → – pad (P–)
If the charger is accidentally connected to 0V and 8.4V instead, the BMS is bypassed and may not behave as expected. Reversed charger polarity (positive and negative swapped) will also prevent charging.
6. Test the Charger Output
Many “dead board” issues are actually charger issues. To verify your charger:
- Unplug the charger from the BMS.
- Measure the voltage directly on the charger’s output leads.
A proper 2S lithium-ion charger should output around 8.4V. If you see 0V, a very low voltage, or an unstable reading, the charger may be faulty.
When to Suspect a Bad BMS
After all of the following are confirmed:
- 0V, 4.2V, and 8.4V pads wired correctly
- Battery polarity correct in the holder
- Voltages at the pads look normal
- Output pads (+ / –) show the full pack voltage
- Charger polarity and output are correct
If the board still refuses to charge or discharge, then the BMS itself may be defective. This is rare compared to wiring, polarity, and charger problems, but it can happen.
Buy This BMS
If you need this exact 2S 8.4V HW-391 BMS, you can get it here:
https://renewspark.com/products/2s-8-4v-20a-bms-board-with-balance-lithium-battery-protection-18650-21700-packs
This small 2S 8.4V 20A BMS is reliable once wired correctly, but the voltage labels (0V / 4.2V / 8.4V) can be confusing if you’re expecting B– / B1 / B+. Follow the steps above and you’ll solve most issues quickly. If you still need help, feel free to contact us and include a clear photo of your wiring so we can help you troubleshoot.
