P0B29: How to Diagnose & Fix — Hybrid Battery "B" Voltage Low
7 vehicles · 2 makes · $200-$6,500 repair
How to diagnose and fix P0B29: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
Mercedes-Benz S-Class · 2021–2025
The 48V EQ Boost mild hybrid system battery degrades or fails prematurely, triggering multiple warning messages and disabling the start-stop, electric boost, and regenerative braking functions. Software glitches also cause false battery fault warnings.
Common Symptoms
- Multiple electrical warning messages on dashboard
- Start-stop system disabled
How to Fix
Have the dealer diagnose with XENTRY to differentiate between a software fault and genuine battery failure. Update to the latest software calibration. If the 48V lithium-ion battery has failed, replacement runs $1,500-3,000 at the dealer.
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Owner tips & cautions
- TipRequest the latest XENTRY software update before approving a battery replacement. Many 48V fault codes are resolved by software calibration alone.
Sources for this issue (2)
Mercedes-Benz GLS · 2020–2025
The 48V EQ Boost mild hybrid system causes excessive battery drain when parked. A software bug in the 48V battery management module fails to properly enter sleep mode, draining both the 48V lithium-ion and 12V lead-acid batteries.
Common Symptoms
- Dead battery after sitting for 3-5 days
- Multiple electrical warnings on startup
How to Fix
Update the 48V battery management software at the dealer. If both batteries are deeply discharged, they may need replacement. The 12V battery is $200-400, the 48V battery is $1,500-3,000.
Owner tips & cautions
- TipIf the GLS sits for more than a week, connect a battery tender to the 12V battery under the hood. The 48V system draws from the 12V to maintain its state of charge.
Mercedes-Benz AMG GT 4-Door · 2019–2025
The 48V EQ Boost mild-hybrid system battery in AMG GT 43 and GT 53 models degrades prematurely, causing reduced electric assist, ISG (Integrated Starter-Generator) faults, and inability to use electric auxiliary functions. The E Performance plug-in models can also experience 48V system faults separate from the main high-voltage battery.
Common Symptoms
- EQ Boost system fault warning
- Reduced electric assist during acceleration
- ISG not restarting engine smoothly
- Start-stop system disabled
- Multiple electrical warning messages
How to Fix
Diagnose the 48V battery state of health with Mercedes Star Diagnostics. If capacity has dropped below threshold, replace the 48V lithium-ion battery pack located in the engine bay. Software calibration is required after battery replacement.
Owner tips & cautions
- WarningIf the 48V battery fails out of warranty, expect $2,000-$3,000 for replacement. The battery is separate from the main 12V battery and requires dealer-level diagnostics to properly calibrate.
Mercedes-Benz CLS-Class · 2019–2023
Mercedes-Benz C-Class · 2022–2025
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P0B29 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0B29 mean?
P0B29 stands for "Hybrid Battery "B" Voltage Low." The battery energy control module monitors every cell block in the high-voltage or 48V battery pack to confirm each is contributing equally. P0B29 sets when the voltage measured across the block designated "B" falls below the calibrated threshold, or falls too far below the other blocks. In practice it means one section of the pack is weak, degraded, or not being sensed correctly - it is a cell-group-level complaint, not a whole-pack complaint.
What are the most common causes of P0B29?
The most common causes of P0B29 are: Degraded or failing cell block within the battery pack, Poor connection at a cell-group busbar or voltage-sense connector, Faulty voltage-sense wiring harness inside the pack, Failed battery energy control module / cell supervision circuit, Battery pack imbalance from long storage or repeated deep discharge, On 48V mild-hybrid systems: failing 48V lithium battery or its DC/DC converter. The specific cause varies by vehicle.
How much does it cost to fix P0B29?
Repair costs for P0B29 range from $200 to $6,500, depending on the vehicle and root cause.
Which vehicles are affected by P0B29?
Au7o has documented P0B29 across 7 vehicle models from 2 manufacturers: Audi, Mercedes-Benz.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.