Kelly KBS24101X on a 24 V boat: won't run, or cuts out, once the engine or charger is on. Usually it isn't broken.

Measure the battery voltage at the controller with the engine running before you buy anything. On a 24 V boat a KBS24101X is often just shutting itself down because charging batteries sit at the very top of what it can take. A multimeter tells you in ten seconds; a new controller of the same model won't help.

Applies to: KBS24101X, KBS24121X, KBS24051X, KBS24 series

The ten-second version: start the engine (or plug in the shore charger). Set your multimeter to DC volts — the 200 V range if it isn’t auto-ranging — and touch the red probe to the metal where the controller’s thick red B+ wire bolts on, and the black probe to where the thick black B− wire bolts on (don’t let a probe bridge the two). 29 V or more: the controller isn’t broken — it’s shutting itself down because your batteries on charge are at the very top of what it can take. The rest of this note explains why and what to do about it. Well under 29 V — say 27 V — and the winch still won’t go: it’s something else; skip to If it isn’t this at the bottom.

Symptom: fine with the engine off, won’t run or cuts out with it running — or worked on 12 V, not on 24 V

On a boat it usually looks like this: the winch works at the dock with the engine off, but once the engine is running or the shore charger is on it won’t run, or runs a few seconds and stops, and the small red light on the controller blinks a pattern. The other version is a controller that came off a 12 V boat: a Kelly KBS24-series controller (KBS24101X is the one I see most) drives the winch or motor perfectly on a 12 V system. Fitted to a 24 V system it refuses to run, runs briefly then locks out, or shows a red LED fault code at power-up — on this family red 1,2 (over-voltage error) or 4,1 (start-up or regen over-voltage) code numbering is from the KBS-X manual; the KBS-E manual numbers over-voltage 1,1 — check yours. Swap it back to 12 V: fine again. The natural conclusion is “wrong part” or “faulty controller”. Usually it’s neither.

Cause

The KBS24101X is a native 12–24 V controller. Its operating window is 8–30 V and it’s rated 45 A continuous / 100 A for 30 s (Kelly KBS-X user manual V4.2, model table and §2.3; the 2023 V4.3 manual no longer lists the 24101X and quotes a one-minute peak). 24 V is the top of its range: Kelly’s maximum operating voltage for a 24 V-rated KBS-X is 30 V (1.25 × nominal), and the over-voltage parameter can only be set between 8 and 30 V. Kelly rates the detection at ±2 % of the setting, so a 30 V cutoff can trip from about 29.4 V, and the fault clears itself after 5 seconds once the bus drops — which is the runs-then-locks-out pattern.

Now look at what a “24 V” pack actually is at the terminals:

On a boat the alternator is a charger: with the engine running, the bus sits at 28–28.8 V before you touch the winch. And paying out under load regenerates into an already-full pack; the KBS-X progressively cuts regen current as bus voltage rises and disables drive on a start-up or regen over-voltage (manual §2.1 and LED 4,1). So the controller sees a bus at or above its threshold and does what it’s designed to do. On a 12 V system the bus sits at 12.6–14.7 V, miles inside the window, so everything works. There is no fault.

Two other things produce the same symptom and get ruled out in the same session:

1. A voltage-sense fault inside the controller — it thinks the bus is higher than it is. You catch this by comparing the live reading in the config tool against your meter on B+.
2. A lowered over-voltage parameter — someone set it below 30 V, or loaded a parameter file from a different unit.

How to confirm it in ten minutes

You need a multimeter, Kelly’s configuration program for this family, and the right cable. No cable or program? Step 1 is still worth doing — the meter reading alone settles most cases — and if it doesn’t settle yours, post the controller in rather than buying the kit: the bench fee is AUD 175, credited against any repair.

1. Meter the real pack voltage at the controller’s B+ and B− terminals — engine off and rested, then with the engine running or charger connected. Not at the battery; at the controller. Write both numbers down.
2. Disconnect the motor phase leads, then connect to the controller over its serial port. Kelly’s manual insists on the motor being disconnected before you run the configuration software — and it means the winch can’t move while you’re leaning over it.
   • For KBS-X and KVD controllers the tool is Kelly’s Configuration Program KVD/KBS, downloaded from the support page at kellycontroller.com (the executable inside is called Motor ETS; it’s portable, no installer). Older KBS-L units (L suffix) need a different program — the manual is explicit that the KBS-X program can’t talk to a KBS-L. The KBL/KEB “User Program” is a different tool for a different family and won’t talk to a KBS either.
   • The KBS-X’s serial port is a 4-pin JST SM connector, not a DB9. Kelly’s manual says the KBS-X needs Kelly’s RS232-to-SM-4P converter between the controller and your serial port (it ships with Kelly’s programming cable, or costs about US$9 on its own). On the PC side any USB-to-RS232 adapter works; I use an FTDI one, and for that the FTDI driver is all you need. Kelly’s downloads include Prolific and CH341 drivers for their USB cable — they do nothing for an FTDI adapter, so if the tool can’t see the port, check Device Manager for the COM number the adapter actually got before you install anything.
   • If the tool won’t connect at all, it’s the converter, the COM port or the baud rate before it’s the controller.
3. Open the Monitor tab and read the live bus voltage and the fault flags.
   • Live voltage matches your meter → the controller is reading correctly; go to step 4.
   • Live voltage is well above your meter → voltage-sense fault. That one is a bench repair.
4. Read the over-voltage parameter and compare it with the charged-pack voltage from step 1. If it’s been set below 30 V, put it back to 30. If the charged pack is at or above the cutoff, you’ve found it.

Fix

In order of preference:

• Fit a controller that has 24 V in the middle of its range, not at the top. That means a different model, not another KBS24101X: a 36 V- or 48 V-class part with a low-voltage cutoff that suits a 24 V bank. If someone else will choose it, tell your marine electrician or winch supplier that the batteries reach 29–30 V on charge and the current controller trips on over-voltage; check the motor’s rating first.
• Keep the bus below the ceiling while the winch runs. Don’t run it with a shore charger in absorption; on a boat the alternator is the charger, so this may mean engine off for the winch, or a lower absorption setting if the battery chemistry allows. These are workarounds, not fixes.
• Check the over-voltage parameter hasn’t been lowered. On a 24 V-rated KBS the parameter only goes up to 30 V (manual Figure 4.2), so 30 V is a hard ceiling you can’t configure past. If someone set it lower, put it back. If it’s already at 30 V, the parameter isn’t your problem and the two options above are what’s left.

What not to do

Don’t replace the controller with an identical KBS24101X. It will do exactly the same thing, because it’s doing the right thing.

If it isn’t this

If your meter says comfortably below 29 V with the engine running and the winch still won’t go — or you have no cable and program to look inside and the meter didn’t settle it — the controller needs a bench. Mail-in diagnosis is AUD 175, credited against the repair. If you have a fleet of these with the same fault, that’s a diagnostic week, not a repair.

Numbers in this note are from Kelly’s KBS-X user manual V4.2 (the last revision listing the KBS24101X) and V4.3, and from the KBS24101X units on my bench. Other KBS24 variants and the KBS-E family differ in places. The KBL family is a different part with its own ratings and parameter ranges — check the KBL manual rather than carrying these numbers across.