Free 12-volt diagnostic guide

How StartVitals reaches a result

Transparent thresholds are part of the product. This page explains what each reading can support—and what it cannot prove.

Updated August 28, 2026Reviewed against cited manufacturer and safety guidance

Purpose and scope

StartVitals is a free educational screening tool for conventional 12-volt lead-acid starting and charging systems. It combines measurements supplied by the user; it does not connect to the vehicle, read diagnostic codes or measure current.

Decision order

Safety gate: stop for physical battery damage or hybrid, mild-hybrid or EV high-voltage context.
Baseline gate: remove possible surface charge before comparing engine-off and running readings.
Charge gate: do not diagnose deeper from a substantially discharged resting battery.
Cranking screen: compare minimum voltage with charge state, symptoms and temperature context.
Charging screen: compare usable running/loaded readings with resting voltage and known system type.
Symptom cross-check: distinguish no-crank from crank-no-start and jump-start behavior.

Published screening bands

Input	StartVitals band	Interpretation
Resting voltage	>13.2 V	Possible surface charge or active charger; establish a valid baseline first
Resting voltage	≥12.6 V	Well-charged state supported
Resting voltage	12.4–12.59 V	Partly charged
Resting voltage	12.2–12.39 V	Low charge
Resting voltage	<12.2 V	Very low charge / possible fault
Cranking, moderate/hot	≥9.6 V	Reassuring basic screen
Cranking, moderate/hot	9.0–9.59 V	Borderline; multiple causes possible
Cranking, moderate/hot	<9.0 V	Heavy voltage sag; stop repeated attempts
Cranking, below 32°F / 0°C	8.5–9.59 V	Cold-context retest; not a battery failure verdict by itself
Running, conventional system	13.5–14.8 V	Common charging range
Running	>15.3 V	Confirm possible overcharging promptly

These are conservative screening bands rather than universal OEM specifications. The cranking band borrows context from the traditional 9.6 V load-test reference at moderate temperature, but starter cranking is not identical to a standardized load test. Cold conditions therefore lower confidence rather than pretending a load-test temperature chart is an OEM cranking specification.

Vehicle and battery context

Stop-start and smart-charging vehicles can intentionally vary terminal voltage. When the charging strategy is unknown, StartVitals records the reading but will not label a low one as an alternator failure. A reading above the conservative high-output gate remains important in every context. AGM and EFB selections add chemistry context; they do not create a replacement verdict from resting voltage alone.

Measurement-quality rules

A loaded-running reading is cleared and ignored if the main running measurement was skipped.
A possible surface-charge baseline blocks ordinary charging-rise conclusions until the resting test is repeated.
Cold cranking, unknown charging strategy, smart charging and unknown battery type reduce confidence.
Confidence depends on usable measurements and context—not simply the number of fields completed.

Retest comparison

The browser can compare the latest result with a new test on the same vehicle. It reports voltage changes and recognizes a useful pattern when resting and cranking performance improve after charging while running voltage remains broadly stable. This comparison still does not replace a CCA/load test.

What the tool deliberately does not diagnose

Battery CCA, reserve capacity or internal condition.
Alternator amperage or AC ripple.
Individual cable voltage drop.
Parasitic current draw.
Hybrid/EV high-voltage systems.
Fuel, ignition, mechanical or security-system faults.

Sources and review process

The editorial team compares the logic with manufacturer, industry and safety guidance and publishes material limitations beside the result. No ASE certification or hands-on inspection is claimed.

Last methodology review: August 28, 2026. Material changes update the date; cosmetic changes do not.