A parasitic drain is the current a machine draws with the key off and every switch open. Every excavator has some — clock memory, radio presets, a controller on a timer, a telematics unit — and the manual states the figure, commonly a few tens of milliamps once the controllers have gone to sleep. A drain that flattens a charged pair overnight or in a week is far above that, and finding it is measurement work: charge and load-test the batteries, confirm the charging system, measure current at the battery negative with a clamp meter after the sleep delay, then pull fuses one at a time until the reading falls. The circuit that drops it holds the fault — a stuck relay, a lamp on an unfused live feed, water bridging a connector, or a leaking alternator diode.
What you will notice
| Symptom | What it usually points to |
|---|---|
| Starts fine after a shift, dead after a weekend | A drain in the hundreds of milliamps. A healthy machine sits at the manual figure; this one is an order of magnitude above it and the batteries are being asked to carry it. |
| One battery of the pair is flat and the other is fine | A load wired to one battery only, or a battery with an internal fault. Test them separately before blaming anything on the machine. |
| It goes flat faster in wet weather | Surface leakage across a dirty battery top or water bridging a connector. Clean and dry the case, then re-measure before chasing circuits. |
| A new battery dies in a few months, the same way | The drain was never found. The battery was replaced, the fault stayed, and the next battery is on the same schedule. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| A relay stuck closed on a circuit that should be dead | The load is small enough not to blow a fuse and large enough to flatten a pair of batteries. Feel the relays an hour after parking: a warm one is working when it should be asleep. |
| Telematics, radio memory and clock on permanent live feeds | Normal loads that add up. Each is inside the manual figure alone, and they push a machine over it the moment a second fault joins them. |
| Alternator diode failure leaking back through the charge circuit | A shorted diode lets current run from the battery into the windings with the engine stopped, and it degrades charging at the same time. Disconnect the alternator main lead and re-measure. |
| Water or dirt bridging a connector or a lamp holder | Leakage across a contaminated surface is a resistance rather than a short. It reads as a small steady drain that appears after rain or a washdown and disappears when things dry out. |
| Something left live by a field repair | An added work lamp, radio, camera or phone charger wired to a permanent live instead of a switched one. Additions are the single most common cause on older machines. |
Reading to meaning
| What the meter says | What it means |
|---|---|
| Draw within the manual figure after sleep | No drain. If the battery still goes flat, the battery or the charging system is the fault. |
| Draw many times the figure, gone when one fuse is pulled | That circuit. Chase its relay, connectors and loads rather than the fuse. |
| Draw falls only when the alternator main lead comes off | Alternator diode leakage. Replace or rebuild the alternator. |
| Draw appears after rain or a washdown | Water bridging a connector or a dirty battery top. Dry, clean and reseal. |
| Draw normal and one battery still flat | Battery fault, or a load wired to one battery only. Test the pair separately. |
How to confirm it
- Charge and load-test the batteries first
A battery that will not hold a charge imitates a drain on every test that follows. Charge both, let them rest, and load-test them separately so a weak cell is ruled out before the meter comes out.
- Confirm the charging system is doing its job
Measure voltage at the terminals at half throttle: a healthy 24-volt system sits in the high 27s or low 28s. If it does not, the fault is charging and not drain, and the diagnosis takes a different route.
- Let the machine sleep, then measure at the negative post
Key off, doors closed, every switch open, and wait the interval the manual gives because controllers can stay awake for minutes. Clamp the meter around the negative cable and read the current after the wait, not before it.
- Compare with the specification, then pull fuses one at a time
Above the manual figure, pull one fuse at a time and watch the meter. The circuit that brings the reading back to normal is the one to chase, and the before-and-after for each fuse belongs in the notes.
- Chase the circuit rather than the fuse
With the circuit named, check its relay for a stuck contact, its connectors for water, and its lamps for a feed that should be switched. A spray bottle of water reproduces the weather-dependent versions in a few minutes.
What to do about it
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Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Batteries | OEM-quality at the correct cold cranking rating and terminal layout. Matching a pair by age and rating matters more than the name on the lid. |
| Alternators and regulators | Genuine or OEM-quality at the machine's output rating. A rebuilt unit with the wrong pulley ratio charges poorly at low idle and mimics a drain. |
| Relays, fuses and connectors | Correct rating and sealed connectors. An unsealed repair becomes next season's water-bridging drain. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
How much current is normal with the key off?
The figure in the manual, commonly a few tens of milliamps once the controllers have gone to sleep, which can take several minutes. Hundreds of milliamps is a fault worth finding, and anything over one amp will flatten a pair of batteries in days.
Can I just fit a battery isolator switch?
It stops the symptom and hides the fault, and it also wipes the controller's learned values and the clock every time it is used. On a machine with a known drain the switch is a workaround; finding the circuit is the repair.
Why does the drain only show up in winter?
Cold cuts the capacity a battery can deliver and slows the charge it accepts, so a drain the machine tolerated in summer becomes a no-start in January. The current did not change; the margin did.