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Since my car is currently parked outside in a Northern European winter climate, and only being used for short distances, the battery is sometimes just barely able to start the car, sometimes it can’t cope at all.

So my idea was to hard-wire my (existing) battery charger and run the mains cable to the outside, so I can quickly hook it up to mains power to top up the battery. (I’ve seen a power plug on the front of some vehicles operating in colder climates, such as Sweden or Canada—not sure if that was for a battery charger or something else.)

The charger I have has a 7.5A fuse—I am aware that this is just enough for charging the battery, not to run any major consumers and certainly not to start the car, so I would obviously disconnect it prior to starting the car.

Can I use a battery charger in this manner? Or will that cause issues if the charger is connected to the battery all the time, even when the mains side is not plugged in?

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I have a battery tender installed as you've described. It's not a charger so much as a device to maintain the desired voltage. It is hard-wired and the mains plug extends (for my convenience) to the area near the wiper motor pivots. It's possible to overcharge a battery using a standard charger, but not possible when using a battery tender. Your description of use appears to prevent that problem.

Having learned that I can overlook the power cord connection, I also draped the extension wire over one of the wing mirrors. Even better results are to be gained by placing it through or near your door handle!

Consider to install your device in a sheltered area in the engine bay, as most are not designed to be used while exposed to weather and also may be damaged if in airflow which contains precipitation. You may consider to add a shield of some sort, but ensure airflow for cooling.

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  • Even better, is parking the car inside a garage. Its a very hard life for a car living outside in all weathers.
    – Criggie
    Jan 17, 2021 at 11:52
  • Recommended reading: City of Darkness by Ben Bova, set in a dystopian future suburban settlement: The family’s electric car […] was always parked in the driveway so everybody could see how big and new it was. It consumed so much energy that it needed to be recharged every night. Once Mr Morgan had forgotten to unplug the cable before driving off, and the cable had hit the windshield like a whip, shattering it. Answers the question why some folks leave their cars out all the time, and got me to ensure leaving myself a reminder that the car is plugged in. Orange extension cord works best :-)
    – user149408
    Jan 17, 2021 at 14:01
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The plugs you saw were probably for a block heater, a gadget that is installed by cutting the bottom radiator hose and inserting inline to warm the engine coolant. I can't see how leaving a battery charger hooked to the battery all the time would make problems as long as it's mounted securely and the connections are solid. Maybe one of those solar chargers you plug into the cigarette lighter and leave on the dashboard where the sun can get at it would work.

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  • Quite likely what I saw was a block heater. As for the charger, I was thinking of installing it in the engine compartment near the battery. Plugging something into the lighter socket is not the way I’d want to go, as the lighter plug is a notoriously unreliable connection, and any overload (using too much on-board electricity with the charger connected and possibly a flat battery) might blow the fuse of the lighter circuit, rather than that of the charger.
    – user149408
    Jan 16, 2021 at 19:37
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Normal car battery chargers are plastic or metal boxes, and aren't really rated for the temperatures of the environment under-hood/bonnet.

While you could mount the charger inside a waterproofed and insulated box, its really a short term solution.

Instead, check your battery for a date code, or find out how old it is another way. 5 years used to be considered "normal" life for a battery, now with more modern smaller batteries they tend to die after 3~4. If your battery is older than 5 years just replace it with a new one

The charger you have might be vampiric, draining a small amount of starter battery if the mains supply is not there. Check that with a voltmeter, with the charger connected but not powered, every 24 hours for a few days.

You can also find ways to keep your car more protected too - I can't fathom people who store their vehicles in the driveway outside a perfectly good garage which is filled with not-cars. If you don't have access to a garage or even a car port then my apologies.

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    I didn’t check the manufacturing date, but I installed the battery just a few months ago. It is a quality brand and on the larger end of the spectrum. A garage would of course be the gold standard, but currently I don’t have access to one.
    – user149408
    Jan 17, 2021 at 14:19

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