Slow cold cranking on a 24 V excavator
An 18-month-old 24 V hydraulic excavator cranks slowly on a 5 °C morning but starts once boosted. Both batteries are 12 V 110 Ah, 800 CCA, in series. After a 12-hour rest the open-circuit voltages are 12.62 V and 12.66 V. During a 15-second crank you record: 19.4 V across the battery pack terminals, 19.2 V at the starter B+ stud, 0.28 V from the starter case to the battery negative post, and a peak cranking current of 390 A against a nameplate figure of 350–450 A. At 1500 rpm with all loads on, the alternator holds 28.6 V with 0.3 V AC ripple. The customer wants only the 'bad' battery replaced.
1. State where the fault is, and explain what each reading proves and what it rules out.
Read the worked answer
The batteries cannot deliver normal cranking current - high internal resistance.
- OCV 12.62 / 12.66 V after 12 h: roughly 90–95 % state of charge, no shorted cell, no serious self-discharge. So charge level is not the problem.
- 19.4 V pack voltage during crank = 9.7 V per battery. With 390 A total, each battery is supplying about 195 A. An 800 CCA battery at 195 A should sag perhaps 1–1.5 V, not 2.9 V. That is the signature of aged, sulfated batteries.
- Positive cable drop = 19.4 − 19.2 = 0.2 V; negative drop = 0.28 V. Both well under the usual 0.5 V per leg, so cables, terminals and the earth strap are serviceable.
- 390 A sits inside the 350–450 A nameplate band, so the starter is not dragging and the engine is not abnormally tight - a seized starter or tight engine would show current above spec.
- 28.6 V with 0.3 V ripple rules out the charging system as the cause of the low state of charge.
Conclusion: batteries at end of useful life. OCV alone never proves capacity - it must be proven under load.
2. The customer insists on replacing only the weaker battery. Explain why that is the wrong call and what test you would run first to back up your advice.
Read the worked answer
Why one battery is not enough:
- In a series string the same current flows through both. The weaker battery reaches full discharge first and is then driven into reverse charge by the stronger one; the strong battery never reaches full charge because the weaker one limits the charge voltage it can accept before gassing/heat.
- A new battery paired with an 18-month-old one is a permanent mismatch: different capacity, different internal resistance, different age. The new one is over-worked and the old one is over-stressed. Both fail earlier, and the customer pays twice.
- Standard practice for series strings on heavy equipment: replace as a matched pair - same make, type, capacity and date code.
Test to prove it (before advising replacement):
- Charge both fully, then rest.
- Load-test each battery individually, disconnected from the string - a 15-second load at 50 % of CCA (400 A); a healthy battery holds above 9.6 V at 5 °C-ish.
- Alternatively a conductance test on each, recorded as a printout.
If one battery collapses and the other holds, the pair still gets replaced, but the printout justifies the invoice and shows the root cause. Then look for why they aged: long standing periods, a parasitic drain, or a maintainer never used.
3. You fit the new pair. Set out your post-repair verification and the one charging-system check you would still make before releasing the machine.
Read the worked answer
Reconnection and verification:
- Terminals clean and bright, correct stud torque, terminal grease, covers fitted; interlink and earth bond tight - always connect the earth/chassis side last.
- Crank and measure the pack voltage at 10–15 s: should now hold well above 19.4 V (typically 21–22 V at 5 °C) and cranking speed should be audibly and measurably faster.
- Repeat the cable drop tests: ≤0.5 V on the positive leg and ≤0.5 V on the negative leg under crank.
- Clear any stored undervoltage codes and check none return after a start cycle.
Charging-system check: run the engine at 1500 rpm with all loads on and confirm a stable 28.0–28.8 V at the battery posts - not just at the alternator B+ - and that ripple is low. Measuring at the alternator hides a volt drop in the charge cable, which would undercharge the new pair exactly as it undercharged the old one.
Also worth 10 minutes: key-off parasitic drain after the machine has gone to sleep (clamp meter, typically under 50 mA). An 18-month life on a machine that stands for weeks is often a drain plus sulfation, not bad luck.