Slow 24 V Crank on a Wheel Loader: Batteries or Cables?
An operator reports that a 24 V wheel loader cranks slowly and occasionally fails to start on the first attempt; the problem appeared with cold weather (-5 °C). The machine carries two 12 V, 1,150 CCA batteries in series, both 26 months old. Your measurements: at rest, 12.62 V and 12.51 V; during a 10-second crank, 9.4 V and 10.4 V at the individual battery posts and 19.8 V across the series pair; 1.8 V drop from the battery positive post to the starter B+ terminal while cranking; 0.15 V from the starter case to the battery negative post. At 1,500 rpm the alternator B+ reads 27.6 V while the battery positive reads 25.1 V, where 0.2 V is expected. The starter solenoid pulls in and the engine turns at about 110 rpm. The drawings show a 20 m round-trip run of 70 mm² positive cable and a chassis ground strap.
1. Interpret the readings. Which faults are proven, which are suspected, and what is the single most cost-effective next action?
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What is proven:
- The positive cable circuit is a real fault. 1.8 V drop from the battery positive post to the starter B+ during cranking is far beyond the usual acceptance limit (roughly 0.5 V for the cranking circuit). At 400–600 A of cranking current that is several hundred watts of heat in a cable and connections that should be delivering nearly the full 24 V to the starter. That alone will produce the slow crank.
- The charging circuit is a real fault. The alternator is doing its job at 27.6 V, but the battery only sees 25.1 V because 2.5 V is lost in the B+ cable and connections. A 24 V battery that never sees above about 25 V sits chronically undercharged, sulfates and loses capacity - which explains why two 26-month-old batteries are already marginal. The charging fault is the root cause; the cranking fault is the presenting symptom.
- The ground side is acceptable - 0.15 V from starter case to battery negative is within limits, so do not start there.
Suspected: the 9.4 V battery is the weaker of the pair. Both are below the roughly 10.5 V expected at the end of a 10 s crank at -5 °C, but a 1.0 V spread means the 9.4 V unit has significantly higher internal resistance or less capacity.
Most cost-effective next action: do not buy batteries yet. Clean and torque the battery terminals, the starter B+ stud and the alternator output stud, then segment the 1.8 V drop - battery+ to solenoid terminal, across the solenoid stud, solenoid to starter, and along the cable by length - to find where it actually is. A corroded terminal and a chafed cable look identical on a voltmeter until you segment. Then recharge, conductance-test both batteries and re-run the crank test.
2. The customer wants only the weak battery replaced. What do you do, and why?
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No - this is a series string, not two independent batteries.
In a 24 V series pair the same current flows through both, so the string's capacity is set by the weaker battery. Fit one new 1,150 CCA battery next to a 26-month-old one and:
- The old battery reaches full charge first and then sits at lower capacity, so the pair delivers less cranking current than either rating suggests.
- During charging, the new battery can be over-driven while the old one lags, causing gassing, water loss and grid corrosion in the unit you just paid for.
- The imbalance you measured (9.4 V vs 10.4 V) is evidence this is already happening.
What I would do instead: repair the charging circuit and the positive cable first, then fully charge and conductance-test each battery against its CCA rating. If one fails, replace both with the same make, model, CCA and date code, and record the install date. Give the customer the honest version: the batteries are the casualty, the charging cable is the cause. Replacing batteries without fixing the 2.5 V drop means they will be back in 12–18 months.
Also check parasitic draw - key off, modules asleep, typically under 50 mA after the sleep delay - because the machine also fails to start occasionally rather than only cranking slowly.
3. Describe the verification sequence and acceptance criteria after the repair is complete.
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Each step must pass before the next.
- Mechanical: terminals clean and bright, correct torque, anti-corrosion applied after torque, cable secured away from heat and chafe, ground strap intact.
- State of charge: each battery charged to full and rested; 12.6–12.8 V open circuit per battery after surface charge is removed. A pair that will not rest within about 0.05 V of each other is suspect.
- Capacity: conductance test against 1,150 CCA, plus a load test at half the CCA rating for 15 s, corrected for temperature at -5 °C.
- Cranking: 10 s crank, at least about 21–22 V at the starter B+, no more than 0.5 V drop on the positive side and 0.2 V on the ground side. Cranking speed should be noticeably up.
- Charging: at 1,500 rpm with electrical load on, 27.0–28.5 V at the alternator B+ and within about 0.3 V of that at the battery posts. Confirming only at the alternator is how the original fault was missed.
- Parasitic draw: key off, modules asleep, clamp meter, compared to spec.
- Handover: record all readings on the work order and note the -5 °C ambient so the next technician has a baseline; recommend a battery load test at every PM.