Why Does Battery Charger Say Fully Charged But Car Won’t Start?
When a battery charger indicates “fully charged” but the car won’t start, the issue often stems from voltage degradation, internal resistance spikes, or compromised cell integrity—even if surface charge appears normal. Common culprits include sulfation in lead-acid batteries (reducing active material), BMS misreporting due to cell imbalance in lithium systems, or cold-cranking amps (CCA) falling below the vehicle’s minimum threshold (typically 300–600A for gasoline engines). Pro Tip: Use a multimeter to verify resting voltage (12.6V for lead-acid; 13.3–13.4V for LiFePO4) and load-test under 50% charge to expose weak cells.
Why does voltage appear normal but cranking fails?
Voltage alone doesn’t reflect usable energy. A 12V battery might show 12.6V at rest but collapse to <9V under load due to high internal resistance or sulfated plates. For lithium variants, BMS disconnects during high-current draws if cells exceed temperature or voltage limits. Pro Tip: Test CCA with a dedicated analyzer—values below 80% of rated capacity indicate replacement.
Can a damaged BMS cause false “full charge” readings?
Yes. A faulty BMS may misreport state-of-charge (SOC) by ignoring cell voltage deviations or failing to balance. For example, a 3S LiFePO4 pack with one cell at 2.5V and others at 3.3V could still read 10.1V total (≈90% SOC), but the weak cell triggers shutdown during discharge. Pro Tip: Use a cell-level monitor like a LiFePO4 balancer to detect outliers.
| Issue | Lead-Acid | Lithium-Ion |
|---|---|---|
| False Full Charge | Sulfation | BMS Failure |
| Load Collapse | Plate Corrosion | High IR Cells |
How does temperature affect starting capability?
Cold temperatures (<0°C) increase electrolyte viscosity in lead-acid batteries, slowing ion transfer. Lithium batteries suffer reduced ionic conductivity, with LFP cells losing 30–40% capacity at -10°C. Pro Tip: Preheat lithium packs to 15°C before charging/discharging in freezing conditions.
Are charger compatibility issues a factor?
Absolutely. Lead-acid chargers apply 14.4–14.8V absorption phases, while lithium systems require precise 13.8–14.6V (LiFePO4). Using the wrong profile creates surface charge illusions—the battery reaches voltage cutoff without proper saturation. For example, a lithium pack charged with a lead-acid profile may hit 14.2V prematurely, leaving cells at 80% SOC.
| Charger Type | Absorption Voltage | Float Voltage |
|---|---|---|
| Lead-Acid | 14.4–14.8V | 13.2–13.8V |
| LiFePO4 | 14.2–14.6V | 13.6–13.8V |
Could parasitic drains deplete a “full” battery?
Yes. Modern vehicles have always-on systems (ECU, alarms) drawing 20–50mA. Over 14 days, this can drain 13.4Ah from a 100Ah battery. Lithium batteries self-discharge slower (1–3% monthly) but suffer permanent damage if drained below 10% SOC. Pro Tip: Install a disconnect switch for long-term storage.
Battery Expert Insight
FAQs
Yes—voltage reflects surface charge, not capacity. A sulfated lead-acid battery might show 12.6V but deliver <5% CCA due to internal resistance >100mΩ.
How do I test a battery’s true health?
Use a carbon pile load tester (applies ½ CCA for 15 seconds) or conductance analyzer. For lithium packs, check cell-level voltages under 20A discharge.
Why does my lithium battery disconnect when starting?
BMS overcurrent protection triggers if current exceeds rating (e.g., 100A BMS with 150A starter draw). Upgrade to a high-drain BMS or parallel cells.