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Just to put my 2 pence worth in here, the DC from battery chargers can be dirty (it isn't always).
If you have a big capacitor (battery) across it it will absorb the peaks and some of the spikes (spikes from switch mode chargers, hummy higher volts DC from old tech 50hz transformer chargers.)
If you end up in a situation where that supply is connected to the vehicle electronics and there is no battery across the charger there is a possibility of voltage spikes or peaks damaging the vehicle electronics. I certainly wouldn't use a battery charger for that reason, unless there was always going to be a battery across it.
The radio itself only needs a few milliamps to keep it alive, and it can go way below 12V and still be OK, but the door might open, lights might come on and other electronics in the car might kick in so I'd use something a little more substantial to be on the safe side.
 

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Clio Mk IV 1.5 dCi MediaNav S/S 2013
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I cannot envisage a time when I would remove the battery and leave the charger connected. It’s not something you could really do accidentally.
But I take your point.
 

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I cannot envisage a time when I would remove the battery and leave the charger connected. It’s not something you could really do accidentally.
But I take your point.
I agree but in the context of using a charger to keep the radio alive while the battery is swapped it could very easily happen.
 

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Clio Mk IV 1.5 dCi MediaNav S/S 2013
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Indeed. 👍
 

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I am wondering what's the lowest AH battery we could get away with as a donor battery ..
Might want to consider one of these
30000mAh Car Jump Starter Pack Booster Battery Charger Emergency Power Bank UK | eBay
Though I am not sure I trust the claims or the ratings on the cheap ones...but will keep radio alive.

Or for a cheap 12v battery specifically as a saver... burglar alarm battery such as this....
YUASA NP2.1-12, 12V 2.1Ah (as 2.3Ah) BURGLAR ALARM Battery | eBay
 

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Then my 3 pence.

Shortage of time made me do it. That was the easiest and quickest procedure if I wanted the car to move and have a working radio in it. I would not dare do that on a car with AGM battery, they are far more complicated.
From that time, I find it easiest to disconnect the radio with 12 V to it's feed wires to keep the memory.

Every car generator is producing what a colleague calls "dirty".
That is why all of them are using car battery, to cut the edges of voltage spikes.

If you consider the procedure, charger is connected while old battery is still in the system.
Old out, new battery in. No major current users, ignition is off anyway, apart from radio memory and clock, charger voltage should be decently constant.
New battery connected into the loop, now you disconnect the charger and new battery is already there to serve as a buffer for voltage spikes as there will be some regardless of what I do.
 

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Apparently, laptop chargers, especially ones with about 15 Volts, are supposed to be excellent car chargers.
Lots of sophisticated electronics to keep voltage steady and it will gradually lower the voltage as the battery is close to full capacity. I have one, just have to fetch 2 clamps and then testing.
Thank God my old bucket uses normal battery.
 
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