BYD Qin Plus DMi: Charging power drops, lack of power? Don't rush to replace the OBC – check this first and save hundreds of dollars
A real-world case: The owner plugs in the charging gun, and the charging power drops from 2.8kW to 1.5kW. At the same time, the car feels sluggish when accelerating. A dealership quoted over 15.**
1. Two typical symptoms
If your BYD Qin Plus DMi shows the following signs, read on carefully:
| Symptom | What you'll notice |
|---|---|
| Charging abnormal | After connecting the charging gun, the displayed charging power continuously drops from the normal 2.8kW down to 1.5kW or even lower. |
| Lack of power | The car feels weak when accelerating; in hybrid mode, the engine kicks in more often than usual. |
Why do these two happen together?
Because the On-Board Charger (OBC) and the DC-DC converter share the same liquid cooling system. When coolant level is too low, both components will actively reduce their power output – slower charging and reduced power. This is the vehicle's self-protection mechanism, not a hardware failure.
2. Root cause: a hidden legacy from previous collision repair
Inspection revealed that the coolant expansion tank was almost empty, with only dried residue inside. Further investigation found:
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The vehicle had been repaired at an independent shop after a front-end collision, including replacement of the radiator support frame.
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After the repair, the coolant was not refilled and bled according to the proper procedure.
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As a result, the cooling system contained a large amount of air, and was actually short of about 2.5 liters (0.66 gallons) of coolant.
In short: The OBC wasn't broken – it was overheating and couldn't work at full capacity.
3. Diagnosis: Check codes, then check coolant – avoid wasting money
If you experience similar issues, follow these steps in order. Do not start replacing parts immediately.
✅ Step 1 – Confirm the symptom
Plug in the charging gun and watch the dashboard. Does the charging power steadily drop? (Normal behavior is stable or slowly rising power.)
✅ Step 2 – Read fault codes
Use a diagnostic tool (a basic OBD reader may work) to check for codes. Pay special attention to:
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OBC coolant temperature too high
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DC-DC converter coolant temperature too high
If both appear together, the problem is almost certainly in the cooling system, not a failed component.
✅ Step 3 – Visually inspect the coolant expansion tank
Open the hood and locate the coolant expansion tank (a translucent plastic tank with MAX/MIN marks).
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Level below MIN → low on coolant.
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No visible liquid, only dried stains → severely low.
✅ Step 4 – Look for collision repair evidence
Check the cooling fan, radiator support frame, and coolant hoses for signs of removal or replacement. If the vehicle has a collision repair history, the chance of incorrect coolant fill is very high.
4. Repair procedure: Add coolant – but never just pour it in
⚠️ Important: The water pump in BYD's DMi hybrid system cannot self-bleed by just idling or normal circulation. You must use a diagnostic tool to force the pump to run. Simply adding coolant without bleeding will cause the problem to return quickly and may damage the pump.
Tools and supplies needed
| Category | Item |
|---|---|
| Diagnostics | Charging gun, diagnostic tool with water pump activation function |
| Supplies | Coolant, approx. 2.5 liters (0.66 gal) – use BYD genuine or equivalent ethylene-glycol type |
Step-by-step procedure
Step 1: Make sure the vehicle has cooled down (to avoid burns). Open the hood.
Step 2: Add a small amount of coolant to the expansion tank – just enough to cover the bottom.
Step 3: Using the diagnostic tool, enter the water pump activation test mode to force the pump to run.
Step 4: While the pump is running, slowly and continuously add more coolant. You will see the coolant level fluctuate and bubbles rising – that's trapped air being pushed out of the system.
Step 5: Keep adding coolant until:
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Bubbles are no longer visible
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The coolant level stabilizes between the MIN and MAX marks
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Total added coolant reaches approximately 2.5 liters (0.66 gal)
Step 6: Inspect all hose connections and the engine side for any leaks. If no leaks are found, close the tank cap and complete the repair.
5. Safety warnings (must read)
| Hazard | Explanation |
|---|---|
| 🔥 Burn risk | Fault codes indicate high coolant temperature. Wait at least 30 minutes for the system to cool before opening the tank cap – hot coolant can spray out and cause severe burns. |
| ⚡ Electrical safety | Always disconnect the charging gun before working on the charging system. |
| 💨 Incomplete bleeding | The water pump in hybrid/electric vehicles must be bled using a diagnostic tool – do not rely on natural circulation or idling. Incomplete bleeding will damage the OBC, DC-DC converter, and even the pump. |
6. Summary – remember this logic
| Item | Summary |
|---|---|
| Root cause | After collision repair, insufficient coolant refill and no bleeding → coolant system low by ~2.5L → OBC/DC-DC overheat → reduced charging power & poor acceleration |
| Solution | Top up coolant + forced circulation bleeding using diagnostic tool → fault resolved |
| Repair cost | Coolant ~$14 (2.5L), labor varies by shop |
| Key advice | Check coolant first, then decide on parts replacement. Before agreeing to an expensive OBC replacement at a dealership, ask them to verify coolant level and check for temperature-related fault codes. |
Final words
After simply adding coolant and bleeding the system, this vehicle returned to normal – charging power back to 2.8kW, acceleration restored. The owner said: "I almost spent over $700 to replace the OBC for nothing."
If you're facing similar symptoms, try the steps in this article first. Of course, if your car has no collision repair history and the coolant level is normal, the cause may be different – then it's time to visit a professional shop.
Found this useful? Share it with fellow Qin Plus DMi owners – you might save them a lot of money.
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