Unable to Charge Above 50% — Maximum Charge Level Reduced
The TESLA-BMS_A079 (Tesla BMS (Battery Management System)) EV fault code means: Unable to Charge Above 50% — Maximum Charge Level Reduced. This is a serious severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- expert
- Estimated cost
- Diagnosed and quoted by Tesla service; depends on the internal battery condition found. Battery-related repairs may be covered by the battery and drive unit warranty (8 years on current models, with mileage caps that vary by model) — check your coverage before paying out of pocket.
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Common Symptoms
- Alert on the touchscreen: "Unable to charge - Maximum charge level reached — Reduced maximum charge level - Schedule service"
- Charging stops around 50% state of charge, or will not start if the battery is already above 50%
- Reduced usable range
- Vehicle otherwise drives normally
Probable Causes (Ranked by Likelihood)
- A condition internal to the high-voltage battery pack that is limiting the battery's ability to charge beyond roughly 50% state of charge (Tesla's official description — the specific internal cause is determined at service) Likely
Step-by-Step Diagnostic Procedure
The battery management system has limited charging to about half the pack's capacity to protect it. Per Tesla's official text, the vehicle charges when below 50% state of charge, and charging will not start above 50%.
There is no owner-serviceable fix — high-voltage pack work is Tesla-service-only. Schedule service at your earliest convenience.
Without service, Tesla's guidance is that maximum charge level and range may be reduced further.
Plan trips around roughly half the normal range until the vehicle is serviced.
Common Fixes by Vehicle
What techs usually find when diagnosing TESLA-BMS_A079 on specific platforms — tap a platform for the fix and the exact part:
Tesla Model 3 (2017-present), Model Y (2020-present) Moderate DIY
Tesla 12V battery code BMS_a079 means the auxiliary 12V is at end of life. Newer Model 3 / Y use a 16V LiFePO4 battery (lithium iron phosphate) instead of lead-acid -- they last 8-12 years but cost more. Older Model 3 used a Group H6 lead-acid battery that fails at 3-4 years. Tesla Mobile Service typically charges $80-150 plus a $40-100 service call. DIY: pull the frunk floor, disconnect HV (CRITICAL safety step), swap battery. Aftermarket lead-acid LiFePO4 12V replacements are available on Amazon for ~$200-300. The car will run on lead-acid OR LiFePO4 -- but match what came in your specific car.
Labor: 30-60 min DIY (with safety knowledge)Tesla Model S (2012-2020), Model X (2015-2020) Moderate DIY
Older Model S/X use a Group 51R (or H6 in later years) lead-acid 12V. These typically fail at 3-5 years and cost $80-150 OEM. Aftermarket equivalents from Optima, Odyssey, or DieHard fit. Replacement requires lifting the frunk lid, removing the front trunk floor, and disconnecting the HV battery first for safety. Tesla Mobile Service is the cleanest path for owners not comfortable working near HV systems.
Labor: 45-90 min DIYFrequently Asked Questions
What does BMS_a079 mean on a Tesla?
Tesla's official meaning: the vehicle has detected a condition internal to the high-voltage battery that limits its ability to charge beyond about 50% state of charge. The vehicle is OK to drive, but service is needed to restore full charging capacity.
Why does my Tesla stop charging at 50% with BMS_a079?
The battery management system has capped charging at roughly half capacity to protect the pack from an internal condition. Charging works below 50% state of charge and will not start above it. This is the protection working as designed, not a charger problem.
Is BMS_a079 the same as BMS_a074?
They're closely related — both indicate an internal battery condition that reduces the maximum charge level, with the same 50% charging behavior and the same guidance: OK to drive, schedule service. BMS_a079 is the alert shown when the vehicle actively refuses a charge because the reduced maximum has been reached.
Sources
This page is built from documented references. Verify against your own service info before repair work.