FAQ
TL;DR: A 1-second crank at 800 A draws ≈0.22 Ah, "A warm engine needs 0.5 s of the starter operation" [Elektroda, jannaszek, post #9434691]; the 90 A alternator replaces that charge in ~10 s; keep batteries at 25.2-26.0 V float.
Why it matters: Knowing the real numbers prevents both expensive under-charging and plate-eating over-charge.
Quick Facts
• Alternator rating: 28 V, 90 A [Elektroda, arturs75, post #9484628]
• Battery bank: 24 V, 180 Ah (2 × 12 V / 180 Ah in series) [Elektroda, arturs75, post #9433796]
• Recommended float voltage: 27.2 – 27.8 V at 20 °C [BatteryUniversity, 2024]
• Minimum “safe-to-start” voltage: 22 – 23 V under no load [Elektroda, K2, post #9489975]
• Maintenance charger size: ≥ 24 V, 5 A constant-voltage [Elektroda, kkknc, post #9493681]
1. How much energy does one engine start really take?
Warm start: 800–1000 A for 0.5–1 s equals 0.11–0.28 Ah (2.6–6.7 Wh) [Elektroda, jannaszek, post #9434691] That is only 0.06–0.16 % of a 180 Ah pair.
2. How long must a 90 A alternator run to replace that charge?
Divide consumed amp-hours by alternator amps: 0.22 Ah / 90 A ≈ 9 s. Add 25 % for wiring losses; 12 s of charging restores the start draw.
3. What float and boost voltages keep 24 V lead-acid batteries healthy?
Float: 27.2-27.8 V at 20 °C; boost (cyclic) equalize: 28.8-29.4 V for ≤ 2 h [BatteryUniversity, 2024]. "Do not exceed 28 V if you want to avoid gassing" [Elektroda, kkknc, post #9490137]
4. Can an external rectifier stay connected while the engine runs?
Yes, if it is constant-voltage and isolated. Add a relay on the D+ line so the charger disconnects when the alternator is live [Elektroda, kkknc, post #9484843]
5. What current rating should the stationary charger have?
For maintenance, 5 A (≈ C/36) is enough [Elektroda, kkknc, post #9493681] For faster recovery after deep discharge, pick 10–15 A (C/12–C/18) to stay under the 0.2 C limit [BatteryCouncil, 2023].
6. How will a cab voltmeter help?
Before start: ≥ 25 V means “go”; 22–24 V suggests longer engine run or shore charge [Elektroda, jannaszek, post #9486433] After shutdown watch recovery: if it settles below 25 V within 5 min, cells are under-charged [Elektroda, arturs75, post #9486791]
7. What happens if the bank remains below 24.5 V for days?
Persistent under-charge induces sulfation, cutting capacity up to 40 % in three months [BatteryUniversity, 2024]. Plates corrode faster and cold-cranking amps fall.
8. Edge case: cold morning, 2 s crank at 1200 A – what then?
You would pull 0.67 Ah. The alternator needs ~30 s at idle to replace it. Voltage may sag below 22 V; if so, start the rectifier immediately to avoid deep discharge damage.
9. How do I wire the automatic disconnect?
- Connect relay coil between D+ (alternator) and chassis ground. 2. Feed rectifier output through relay contacts to battery. 3. Optionally add the starter’s 50 line in series so the charger opens during cranking. Result: charger off while engine or starter runs [Elektroda, kkknc, post #9486850]
10. Will the alternator alone cope if the crane cycles 4 min on / off all day?
Unlikely. Four starts per hour consume ~0.9 Ah; 4 min run at idle gives 6 Ah back, but alternators at 1 000 rpm deliver ~60 % of rating ≈ 54 A, so 3.6 Ah net/hour. Capacity will slowly drop [Elektroda, arturs75, post #9484628]
11. Is a light-bulb current limiter really acceptable?
Yes for occasional top-ups. Insert a 24 V, 50 W bulb; it limits current to ~2 A until voltage rises, preventing over-current on a simple rectifier [Elektroda, jannaszek, post #9494065]
12. How often should I cycle to boost-charge?
Give the pair a 29 V boost for two hours once each month or after any depth-of-discharge > 30 %. This dissolves early sulfation and balances cells [BatteryCouncil, 2023].
. I just want to count how much electricity the starter "swallows" for firing and then how much time the alternator has to work to recharge the batteries.
