Power supply for ECU programming - including 24 V machines

Ask any tool manufacturer what to do before writing to a controller and the answer is some version of the same sentence: use a stable power supply. It is good advice and it is universally given. What is almost never given is the detail – how much current, at what voltage, for how long, and what changes when the machine in front of you runs on 24 volts instead of 12.

That gap matters, because trucks, buses and a large share of construction machinery are 24 V systems, and the published guidance, the recommended chargers and the supplied bench power supplies are overwhelmingly specified for 12 V. This page collects what is actually established, and is explicit about what is not.

Why the supply is the thing that fails

During a write, the controller is erasing and reprogramming its own memory. For the duration of that operation it must not lose power and it must not see the supply sag below its programming threshold. If either happens partway through, the controller is left holding an incomplete image – and depending on the platform, that is anything from an inconvenience to a controller that needs recovering.

The trap is that the vehicle’s own battery looks fine. A battery at rest measures a perfectly respectable voltage. Then the programming operation begins, other systems on the machine wake up and draw current, and the voltage the controller actually sees drops.

Published figures give a sense of the scale. Autotuner documents current demand exceeding 80 A during programming on some vehicles and notes that controllers commonly expect above 12.5 V – with manufacturers frequently specifying 13 V. Alientech’s New Zealand distributor states a minimum of 20 A for a charger and notes that some vehicles draw over 70 A, with the charger required to stay connected for both reading and writing. The numbers differ because the situations differ; the direction does not.

The 24 V question

Here is the honest state of the published information, because it is worth knowing how thin it is.

Searching the public documentation of the two tool manufacturers most relevant to agricultural and construction work, we found no published voltage specification addressing 24 V machines. Autotuner’s guidance on supply is written around 12 V and the charger it recommends is a 12 V unit. Alientech supplies a bench power supply as part of the kit, and its voltage range and current rating are not published anywhere we could find. Neither manufacturer publicly addresses what changes on a 24 V machine.

That is not a claim that the tools cannot be used on 24 V machines – they plainly are, every day. It is a statement about what is documented, and it is precisely why this question needs answering with your dealer rather than assumed.

What is generally true about system voltage

The word doing the work in that table is typically. System voltage is a property of the machine, not of the category, and the only reliable method is to establish it on the machine in front of you.

Why two 12 V batteries in series is not two 12 V systems

On a 24 V machine built from two 12 V batteries in series, it is tempting to treat the midpoint as a 12 V source. It is not a clean one. Loads taken across one half of a series pair unbalance the pair, and on a machine with a 24 V alternator and charging system that imbalance is not corrected in the way people expect. This is standard practice knowledge in truck and plant electrics and it applies here for the same reasons.

The practical position: establish what your tool and your bench supply are rated to accept, and supply the controller the way the platform requires – rather than improvising from whatever terminals are convenient.

Bench work has a different supply problem

On the bench the machine is not supplying anything; you are. That removes the variable of other systems waking up and drawing current, and it introduces a different one: everything the controller needs now comes from your equipment and your wiring.

• Correct voltage for that controller family, held steady for the duration.
• Adequate current capability – a laboratory supply with a low current limit will simply fold under the demand and produce a failure that looks like a communication error.
• Clean ground. A marginal ground connection produces intermittent, confusing behaviour that is easy to misattribute to the tool.
• Correct wiring of the specific connector. Some connections require a separate power feed alongside the data connection – Alientech’s Fendt CAN cable is documented as requiring exactly that.

The checks that prevent most of these failures

1. Confirm the machine’s system voltage before anything is connected. Not the category – the machine.
2. Confirm what your tool and bench supply are rated for. If it is not in your documentation, ask your dealer and write the answer down.
3. Connect the supply before starting and leave it connected through reading and writing both. Not just the write.
4. Watch the voltage during the operation rather than measuring it before and assuming. The sag happens under load.
5. Take the backup first – and know whether your tool actually produced one. On some tools and some access methods it does not. See when a write is interrupted.
6. Do not start a write you cannot finish. Not before a shift change, not with a machine that has to move, not on a supply you are unsure of.

Why this page exists

Because the published guidance for this industry was written for cars, and the machines we work on are not cars. A 12 V recommendation, a 12 V charger and a supply specification that stops at the workshop door are perfectly reasonable for a workshop doing hatchbacks. For a dealer with a 24 V excavator on the yard and a customer waiting, they are not an answer.

If you have established figures for your own equipment on 24 V machines, we would genuinely like to compare notes – this is an area where the industry is working on shared assumptions rather than published data.

Frequently asked questions

Can I just use a good battery instead of a charger?

Not reliably. A battery at rest measures well and still sags under the current demand of a programming operation – published figures put that demand above 70 A on some vehicles. What matters is voltage held under load for the whole operation, which is a charger or supply specification rather than a battery condition.

What voltage should I set on a bench supply?

That depends on the controller family and on what your tool and supply are specified for, and it is one of the few things genuinely worth confirming with your tool dealer rather than reading online. What is documented publicly is that many controllers expect above 12.5 V and that manufacturers frequently specify around 13 V for 12 V systems. For 24 V machines there is no equivalent published figure.

Is it safe to take 12 V from one battery of a 24 V machine?

It is common practice and it is not a clean solution. Loads taken across one half of a series pair unbalance the pair, and a 24 V charging system does not correct that the way people assume. Supply the controller the way the platform requires instead of improvising from convenient terminals.

What happens if the voltage drops during a write?

The controller can be left holding an incomplete image. What that means in practice ranges from a retry to a controller that needs recovering, depending on the platform and on how far the operation had progressed.