Battery Flat Every Three Days on a Short-Commute 125 cc
A 125 cc commuter motorcycle, four years old with 26,000 km, arrives with a battery that goes flat every three days. The owner's commute is 4 km each way, six days a week, in stop-start traffic. Six months ago he fitted two 55 W halogen auxiliary lamps on the crash guard and a 2.1 A USB charger, both tapped into the ignition-switched line through a single relay whose coil he connected to a permanent live. The battery is a 12 V 5 Ah VRLA, 14 months old: open-circuit voltage 12.4 V after standing overnight, 9.8 V while cranking. Charging voltage at 3,000 rpm is 13.1 V at the battery terminals and 13.6 V at the regulator-rectifier output; the manual gives 13.8–14.8 V. The regulator-rectifier coupler has one browned, slightly melted terminal and the engine-earth strap bolt is painted over. With everything switched off and after ten minutes stabilisation, current draw at the battery negative is 0.19 A against a specified maximum of 0.05 A. The owner has already bought a new battery and wants it fitted.
1. The owner has already bought a new battery. Using the charging voltage, cranking voltage and parasitic drain, decide what to fix first and explain what each reading rules in or out.
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Read the three numbers as a chain
- 12.4 V open circuit after standing is roughly 60–70% state of charge. The battery is discharged, not proven dead.
- 9.8 V during cranking shows it can still deliver cranking current, so it is not fully sulphated. A failed battery would drop lower or fail to crank.
- 13.1 V at the battery and 13.6 V at the regulator-rectifier at 3,000 rpm, against 13.8–14.8 V. Both are low, and the 0.5 V gap points at the browned coupler and the painted earth strap, not only at the regulator-rectifier.
- 0.19 A drain against 0.05 A is about 2.3 W, roughly 2.7 Ah over a 14-hour night from a 5 Ah battery. That alone flattens it in two to three days and matches the three-day pattern exactly.
Order of work
- Fix the drain first: the relay coil should not sit on a permanent live. It is a certain, cheap fault.
- Clean and replace the browned terminal, scrape the earth strap to bare metal, then re-measure charging. If it reaches 13.8–14.8 V, the regulator-rectifier is serviceable.
- Only then charge and load-test the battery. Fitting the new battery first would simply discharge it the same way and the customer would return.
2. Show with numbers whether the owner's lamps and charger can be supported by the charging system on his 4 km commute, and state what you would recommend.
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Load side
- 2 × 55 W = 110 W, about 9.2 A at 12 V (about 7.9 A at 14 V).
- USB charger: 2.1 A at 5 V is about 10.5 W, so roughly 1 A at 12 V allowing for converter losses.
- Total about 10 A, around 125 W, whenever the lamps are on.
Supply side
A typical 125 cc commuter alternator is rated around 130–160 W, and only near 5,000 rpm. At the 20–35 km/h of his commute the output is well under half of that, while ignition, mandatory lighting and the charger already take 60–80 W.
Energy per day
8 km at an average 25 km/h is about 19 minutes of running. The lamps take roughly 3 Ah in that time, plus 0.5–1 Ah for cranking. The alternator cannot replace that within the same short trip, so the deficit accumulates daily.
Recommendation
Fit 2 × 10 W LED lamps (about 1.7 A total) and feed them and the charger through an ignition-switched relay from the battery via a 5 A fuse. Repair the drain and earth, then confirm 13.8–14.8 V at the battery at 3,000 rpm.
3. What would you record on the job card, and what would you tell the customer about the accessories?
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Job card
- Battery open-circuit and cranking voltage, before and after charging.
- Charging voltage at the battery and at the regulator-rectifier before and after repair.
- Measured drain before and after (0.19 A → target below 0.05 A).
- Earth-path voltage drop or resistance; terminals replaced; relay and fuse fitted.
- Accessories declared by the customer, with a note that they are outside warranty.
- Road-test result and the charging figure at 3,000 rpm.
What to tell him
The flat battery came from a permanent drain plus a lamp load that his short trips cannot support. The battery will be charged and load-tested rather than replaced outright. The accessory wiring is his responsibility and must be fused and ignition-switched. If he keeps the halogen lamps and the 4 km pattern, the problem will come back even with a new battery. Put that advice on the invoice, so the discussion is documented rather than remembered.