Every-Third-Morning No-Crank on a 12-Volt Dual-Battery Bus
A 2019 40-foot bus with two group 31 AGM batteries (950 CCA, 100 Ah each), a 270-amp alternator, and 186,000 miles will not crank on roughly every third cool morning. The operator jump-starts it and it runs fine the rest of the day. Overnight shop checks: battery A rests at 12.68 V, battery B at 12.29 V. Both are 41 months old and were installed as a pair. Alternator output at 1,200 engine rpm with electrical loads on is 13.9 V at the battery posts. A clamp meter on the negative cable at the battery box shows 240 mA, 45 minutes after key-off with all accessories switched off; the OEM spec is 50 mA maximum after 60 minutes. A 500-amp carbon-pile load test on battery B drops it to 9.1 V in 12 seconds. Terminals are clean and torqued, and cranking voltage drop across the cables is 0.3 V.
1. Decide which battery or batteries to replace and justify the decision against the option of replacing only the weak one.
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Replace both as a pair.
- Battery B fails: 12.29 V rest is roughly 60 percent state of charge, and a load test that falls to 9.1 V in 12 seconds is a failed battery, not a discharged one.
- Battery A passes today but is not a match: 12.68 V rest is acceptable, yet it is 41 months old and has been sitting in parallel with a failing cell. Its reserve is reduced and its internal resistance is rising.
- The pair is a single electrical unit. In parallel, the weaker battery drags the stronger one down during rest and forces it to carry the cranking load, which accelerates its failure. Installing one new battery next to a 41-month-old battery means the new one is chronically undercharged and the bus is back in the shop within a season.
Check the charging system and parasitic draw before or during replacement, because a new pair will mask a real fault for a few months. Also verify the replacement batteries are the same group size, CCA, chemistry, and manufacture date code, and record the install date and voltage on the work order.
2. Is the alternator condemned? Explain the reasoning, including what the 13.9 V reading does and does not prove.
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Do not condemn the alternator on this evidence.
- 13.9 V at the posts with loads on is inside the normal regulated range for a 12-volt AGM system, typically 13.8 to 14.4 V depending on temperature and the regulator's charge strategy. Many modern regulators intentionally run below 14.2 V once the battery is warm.
- 0.3 V cranking drop across the cables is acceptable and rules out a corroded cable set as the cause of the no-crank.
- What the reading does not prove: it does not show the alternator's output in amperes, and it does not show whether the regulator is temperature-compensated correctly. A 270-amp alternator can show correct voltage and still be weak on current.
Proper verification: load the alternator with a carbon pile or an inductive amp clamp and confirm it delivers a substantial fraction of rated output at 1,500 rpm without voltage falling below about 13.5 V. Also check the regulator sense wire and the battery temperature sensor, since a skewed sensor will hold system voltage low and leave AGM batteries chronically undercharged. If output is confirmed and the sense circuit is intact, the alternator is good and the batteries plus the parasitic draw are the whole story.
3. Walk through a systematic diagnosis of the 240 mA parasitic draw, and state how much of the no-crank problem it actually explains.
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First, quantify it. 240 mA over 24 hours is about 5.8 Ah per day. Over three days that is roughly 17 Ah out of a 200 Ah bank, worsened by cold and age. That is meaningful but it is not, by itself, enough to prevent cranking on a healthy pair - so the draw is a contributing fault, while battery B is the primary cause. Fix both.
The draw is also out of spec: 50 mA after 60 minutes versus 240 mA at 45 minutes. Note the timing - modules legitimately draw more before they go to sleep, so repeat the measurement at 90 minutes before condemning anything.
Systematic isolation:
- Clamp the negative lead and confirm the reading is stable, with doors closed and all accessories off.
- Pull fuses or use a current-sensing fuse adaptor one circuit at a time, watching for the drop. Do not disconnect the battery to "reset" - that erases the evidence and can wake modules.
- On a transit bus, check the usual suspects: camera/DVR, farebox, APC counters, telematics, radio, door controllers, LED marker drivers, and any accessory left on a live circuit.
- Once isolated, verify the repair by re-measuring at 90 minutes and document both the before and after values on the repair order.