Maximum Charge Level Exceeded — Charge Level Reduced by the Battery Management System
The TESLA-BMS_A079 (Tesla BMS (Battery Management System)) EV fault code means: Maximum Charge Level Exceeded — Charge Level Reduced by the Battery Management System. 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
- Touchscreen shows "Unable to charge — Maximum charge level reached" or "Reduced maximum charge level — Schedule service"
- Charging stops, or will not begin, once the battery is at or above the reduced cap
- Usable range is lower than it was, sometimes dramatically so
- The car otherwise drives normally
Probable Causes (Ranked by Likelihood)
- The battery management system has reduced the pack's maximum permitted charge level in response to a condition it detected, and the car is at or above that reduced ceiling. How far it is reduced VARIES by vehicle — owner reports range from a modest cut to only a few dozen miles of usable range, so there is no single percentage this alert corresponds to. High
- An underlying high-voltage pack condition that the BMS is protecting against. BMS_a079 is the symptom of that decision, not the root fault — companion alerts logged at the same time usually point at the cause. High
- High mileage or an ageing pack. Several reported cases cluster past 100,000 miles. Medium
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?
Tesla's canonical name for it is BMS_a079_Max_Charge_Level_Exceeded. The battery management system has reduced the maximum level the pack may be charged to, and the car is at or above that reduced ceiling, so charging will not proceed.
Is my Tesla safe to drive with BMS_a079?
Generally yes — this alert limits charging, not driving, and Tesla's guidance on the related reduced-charge alerts is to schedule service rather than stop. Expect less range until it is resolved.
Does BMS_a079 always cap charging at 50%?
No, and it is worth correcting: there is no fixed percentage. The cap is whatever the BMS has reduced it to, and reported cases vary widely — one owner reported being unable to charge past roughly 56 miles of range. A different alert, BMS_a029, is the one documented as limiting to a 50% state-of-charge equivalent.
Can a generic OBD2 scanner read or clear this?
No. Tesla alerts are manufacturer-specific and appear in no SAE standard, so a generic OBD-II tool cannot read or clear them.
Mechanics Guide Step-by-step diagnosis
Step-by-Step Diagnostic Procedure
Read the exact alert text on the touchscreen. Tesla's own wording is more specific than any third-party list, this page included.
Check what OTHER alerts the car logged at the same time. BMS_a079 reports that the ceiling was lowered; a companion alert usually says why.
Note your actual maximum charge in miles or kilometres rather than percent — the reduced cap is a pack-voltage limit, so the percentage displayed can be misleading.
Book Tesla service. This is a pack-level decision by the BMS and cannot be cleared with a generic scan tool; Tesla alerts are not OBD-II codes.
If the vehicle is in battery warranty, raise it promptly — capacity-related coverage has both time and mileage limits.
Sources 3
This page is built from documented references. Verify against your own service info before repair work.
- Tesla Model 3 Owner's ManualTroubleshooting Alerts (official Tesla alert text for BMS_a079)
- Tesla Model Y Owner's ManualTroubleshooting Alerts (confirms BMS_a079 is documented across models)
- Tessie Tesla alert statisticsthird-party Tesla alert reference with owner-telemetry frequency — not an official Tesla document
Commonly Affected Vehicles
- Tesla Model 3
- Tesla Model Y
- Tesla Model S
- Tesla Model X
- Tesla Cybertruck
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