Every spring, thousands of classic car owners in Belgium open their garage, lift the cover and realise that their sleeping beauty no longer feels like starting. The battery gave up during winter storage, or almost. Before you panic or do something silly, here is what you really need to know.
Because jump-starting a classic car with a flat battery is not done in quite the same way as a modern Golf. The electrical systems are different, sometimes fragile, and some operations that cause no problem at all on a recent vehicle can destroy a vintage component that is impossible to find as a spare part.
Why a classic car battery runs down during winter storage
A battery that drains completely over a few months of winter storage is often the result of a build-up of small things. Classic cars generally have no electronic battery management system, no smart standby mode, no automatic cut-off. This means that any residual drain, however small, can quietly empty the battery over a period of three or four months.
The usual culprits are an interior light accidentally left on, a radio with a preset memory that draws power all the time, or simply a battery that was already nearing the end of its life and could no longer cope with a long discharge. Add to that the cold of the Belgian winter: a battery loses up to 30 to 40% of its effective capacity in cold weather, which speeds up the discharge and makes starting harder even when it is not completely flat.
💡 To avoid going through this again: if your classic car is going to stand still for more than 4 to 6 weeks, disconnect the earth cable or fit a battery maintainer (a smart charger such as a CTEK). These devices keep the battery at its optimum level with no risk of overcharging, for around fifty euros. It is the best investment you can make for laying up a classic car.
The classic mistakes that can ruin everything
Before talking about what to do, here is what you must not do. These mistakes come back every spring, and some cost far more to repair than simply replacing a battery.
Mistake no. 1: keeping on cranking the starter
When the engine refuses to start, the natural reflex is to keep trying. That is exactly what you should not do. Every start attempt puts heavy strain on the starter, and on a classic car whose electrical equipment may not have been serviced for years, this can burn out the starter motor or damage the flywheel ring gear. The rule is simple: two or three attempts at most, then stop and look for the cause.
Mistake no. 2: reversing the polarity of the jump leads
The most common mistake and often the most expensive. Red on positive, black on negative, in the right order. On a modern car, reversing them blows the main fuse and sometimes the alternator. On a classic car, the consequences can be even more serious if the original wiring is ageing or the electrical protection is inadequate. It is not complicated, but it is always done in a hurry and in the poor light of a garage.
Mistake no. 3: using jump leads that are too thin or of poor quality
Cheap jump leads with a cross-section of 10 mm² do not deliver enough current to start a big engine at the end of winter, especially if the weather is cool. The result: overheating leads, a connection that fails at the wrong moment, and sometimes not much benefit for your battery. For a classic car with a big engine, leads of at least 16 to 25 mm² are recommended.
Mistake no. 4: connecting to a modern battery with stop-start or AGM
If you use a recent car as a “donor” to restart your classic, be careful. Modern vehicles fitted with AGM batteries or stop-start technology are not recommended as power donors: the charge peaks during starting can disrupt the donor vehicle’s electronics. Use a portable booster instead, or a donor car that is also a bit older.
Mistake no. 5: neglecting the condition of the battery terminals and cables
On a classic car, the battery terminals often have years of accumulated corrosion. A loose earth cable or oxidised terminals stop the current from flowing properly, even if the battery is charged. Before concluding that the battery is dead, always check that the connections are clean and tight. A quick clean with steel wool or bicarbonate of soda sometimes solves the problem without spending anything.
Mistake no. 6: forgetting that 6-volt classic cars are in a different category
Many cars from before the 1960s still run on 6 volts. Connecting a 12-volt battery or using a 12-volt charger on a 6-volt circuit can destroy the entire electrical system. If your classic runs on 6 volts, this is non-negotiable: you need specific 6-volt equipment, full stop.
The right procedure to jump-start a classic car
If you have checked the terminals, the battery really seems to be flat and you want to try a jump start, here is the order to follow carefully.
1: Park the two vehicles close enough for the leads to reach the terminals, without the bodywork touching. Switch off the donor vehicle’s engine.
2: Connect the red clamp to the positive (+) terminal of your classic car’s battery, then the other red clamp to the positive (+) terminal of the donor battery.
3: Connect one black clamp to the negative (-) terminal of the donor battery. Then connect the second black clamp not to the negative of your classic car, but to an unpainted metal part of the engine or chassis, away from the battery. This avoids a spark near the battery, which can give off a little hydrogen.
4: Start the donor vehicle and let it run for 2 to 3 minutes. This lets your battery recover a minimum of charge before the attempt.
5: Try to start your classic car. If the engine fires, perfect. If nothing happens after two or three attempts, stop and move on to a more thorough diagnosis.
6: Once your classic is running, disconnect in reverse order: black clamp from the chassis, black clamp from the donor, red clamp from the donor, red clamp from your classic car. Do not let the clamps touch each other once connected.
7: Let your engine idle for at least 20 to 30 minutes, or better still, go for a drive. The alternator will gradually recharge the battery. Do not switch the engine off straight away if you can avoid it.
⚠️ If the battery is completely dead: jump leads will not be enough to revive a deeply discharged battery. In that case, only a slow charger with a regeneration mode can try to recover it, charging gently over 12 to 24 hours. If it still does not hold a charge after that, it needs replacing. On a classic car, choose a battery with the same dimensions as the original to avoid fitting problems in the battery tray.
The portable jump starter: the practical alternative for classic car owners
More and more classic car owners are switching to a portable booster, and frankly it is hard to argue with them. This small unit the size of a thick book contains its own lithium battery and delivers enough current to start most older engines, with no need for a second vehicle. You connect it directly to the terminals, turn the key, and the engine starts.
Decent entry-level models cost from €60 to €70. For a big engine with 6 cylinders or more, go for models with a cranking current above 400 A. It is more practical than jump leads, less risky for the donor vehicle’s electronics, and it fits easily in the boot for outings.
When the battery no longer recharges: how to tell it needs replacing
A battery that has been deeply discharged repeatedly often cannot be saved. The internal plates gradually sulphate and lose their ability to accept a charge. You can recognise a battery at the end of its life by a few fairly clear signs: it recharges quickly but runs down just as fast, it gets unusually hot when charging, or it simply no longer passes the load test at a specialist.
On a classic car, replacing the battery is generally not complicated. Dimensions are often standard and batteries with conventional terminals are easy to find. Be wary, however, of modern AGM or EFB batteries which, even if they physically fit in the tray, are not always compatible with old voltage regulators.
✅ The right habit before every winter lay-up: have your battery tested by a specialist in September or October. A load test costs a few euros and tells you exactly whether your battery will survive the winter or whether it is better to replace it now rather than in spring, in a rush, two hours before a classic car rally.
FAQ
It depends on the condition of the battery and the vehicle’s residual power drain. In general, a battery in good condition in a vehicle with no parasitic drain lasts 6 to 8 weeks without being started. Beyond that, the risk of a complete discharge increases significantly. For a winter lay-up of 3 to 5 months, a battery maintainer is essential if you want a working battery in spring.
In theory yes, but be careful with modern cars fitted with AGM batteries or stop-start technology. These systems manage charging differently and can be disrupted by the current peaks when starting a classic car. Ideally, use either a standalone portable booster or a donor vehicle that is also from a slightly older generation, or at least one without an AGM battery.
No, you cannot use a 12-volt charger on a 6-volt circuit. The consequences can be catastrophic for the entire electrical system. Check your battery voltage first: it should be marked on the battery or in the vehicle documentation. If in doubt, ask a specialist. There are chargers and boosters compatible with 6 volts, usually from specialist suppliers of classic car parts.