Better off leaving the battery in the car and connecting the charger to that. That way any spikes are smoothed out by the battery.

43 minutes ago, Llanigraham said:

Better off leaving the battery in the car and connecting the charger to that. That way any spikes are smoothed out by the battery.

But the point is that the 'charger' will be used to keep the car circuits powered up while the battery is replaced?

16 minutes ago, muddyjim said:

But the point is that the 'charger' will be used to keep the car circuits powered up while the battery is replaced?

 

Yes, but as has been pointed out by me and others, battery chargers are often not rectified so can produce spikes which can cause expensive damage to things like the ECU, but when connected to the battery those spikes are smoothed out. Therefore it is better to leave the battery connected to the car and connect the charger to the battery.

Removing the battery though will then allow the 50hz ripple of a rectified but non smoothed charger to affect the car's system? 

If the supply is not smoothed then it could cause damage; the battery smooths it, as has been said several times,

Even with sophisticated battery chargers like the Ctek, they are designed to be connected to a battery and not a car without the battery connected. The smart chargers do things like try to assess the battery health, state of charge, Ah, CCA which involve short bursts of high current pumped in and out of the battery. If the battery is missing, it's possible those tests will spike the system. Anything over 16 volts could fry your ECU for instance.

 

There are plug in devices designed to maintain the system while the main battery is changed, they consist of a small 12v lead acid battery, and usually connect to the cigarette / 12 aux socket.

1 hour ago, xman said:

Even with sophisticated battery chargers like the Ctek, they are designed to be connected to a battery and not a car without the battery connected. The smart chargers do things like try to assess the battery health, state of charge, Ah, CCA which involve short bursts of high current pumped in and out of the battery. If the battery is missing, it's possible those tests will spike the system. Anything over 16 volts could fry your ECU for instance.

 

There are plug in devices designed to maintain the system while the main battery is changed, they consist of a small 12v lead acid battery, and usually connect to the cigarette / 12 aux socket.

Would/should such a device be used when using the ‘ship’s battery’ to power an oil extraction device?.

I’m somewhat concerned that the voltage level might be lowered too much during an oil change or is that considered extremely unlikely?.