As mentioned in a previous question, I am looking to hardwire a battery charger/tender in my car, so I can just plug it into mains voltage when the battery is drained, or the car is sitting outside in a cold climate.

Requirements are:

  • Must be able to charge a near-flat battery sufficiently to start the engine, in a reasonable amout of time (as a reference, 1–2 hours on the €20 charger I have is usually sufficient)
  • Must be suitable for leaving plugged into mains voltage permanently (i.e. not cause damage to the battery, such as by overcharging, or drain it when finished charging)
  • Must detect the condition of the battery, and switch between charging and maintenance mode as needed (no manual switching between modes – plug in when coming home, unplug before driving off, and otherwise no need to worry about it)
  • Must be suitable for being connected to the vehicle’s circuit permanently, whether plugged in or not

I am planning to mount this near the fuse panel, i.e. inside the passenger compartment where I have access to 12V terminals suitable for the load (presumably no more than around 15A), and just run the cable to the bumper. Therefore, the device does not need to be waterproof.

Question: what features should I look for when buying a charger not specifically designed to be hardwired? Can I use any charger as long as it has some visual display of the charge state (charging vs. done), indicating it “knows” when to switch off? Or do I need to look for anything more specific in the product descriptions – and if so, what (short of the words “suitable for hardwiring”)?

I am in a non-English-speaking market, thus just asking for a “battery tender” in a shop will not work. Also, I don’t need a premium brand. Hence language-neutral and brand-neutral answers will be appreciated.

  • 1
    You listed the requirements in your bullet points.
    – Solar Mike
    Jul 17 '21 at 14:16
  • 1
    What SolarMike said ... as well, why hook it up inside of your passenger compartment? The most suitable place to attach something like this is at the battery, or if the vehicle is made with it, a positive jump point and a good ground. The closer to the battery, the better. Jul 17 '21 at 14:35
  • @SolarMike plenty of chargers are not specifically designed for this scenario (and those that are come with a higher price tag). I’d like to know how I can tell if the average charger from your local hardware store is suitable for that kind of setup, even if not specifically advertised as such. I’ve edited the question to reflect that, and would appreciate if votes to close were revised.
    – user149408
    Jul 17 '21 at 15:24
  • 1
    I think this question should be left open. It isn't a shopping question, as the information sought - noted in bold in the question text - asks about desirable features or specifications. The question does not seek a product recommendation. Jul 17 '21 at 16:23
  • @Pᴀᴜʟsᴛᴇʀ2 Placing it in the passenger compartment will protect it from moisture, as many chargers not intended for hardwiring have ventilation holes. The amperage of a charger is fairly low (the one I have has a 7.5A fuse, which will blow if I even connect a compressor while charging). Others I have seen go up as high as 15A. In contrast, the driver footwell has the connection points for the entire fuse panel, which goes well beyond 15A. Thus I figure it’s absolutely sufficient for a charger.
    – user149408
    Jul 17 '21 at 22:21

I doubt finding a combination charger/tender exists from your demands. True battery tenders output less than 2 amps that can be wired in permanently into an electrical system and plugged into an extension cord for days/weeks/months at a time as they trickle charge a battery overnight then maintain a fully charged battery without ever overcharging. Battery chargers output anywhere from 3-15 amps and automatically drop charging current to the lowest value specified but may overcharge a battery that can shorten a battery's life if left plugged in for day/weeks at a time. These battery chargers may not recharge a fully dead battery in 2 hours unless the battery is relatively new. A battery that's a few years old and dies from long periods of non use may require more than 2hrs of charging.

The best way to keep a battery fully charged is either removing it for storage in a garage and using a trickle charger/battery tender of less than 2 amps or installing a weather proof battery tender in the car permanently with the ac cord tucked away or the plug hung below the bumper or thru the grille for immediate connection to an extension cord. If there's room inside your car, wiring it to the cigarette/auxiliary power outlet would be another consideration. Be sure the aux outlet is hot at all times. Some outlets are turned off with the ignition switch.

I've used an inexpensive 12-15vdc transformer, normally used as a battery eliminator/charger. It outputs less than 2 amps and sits in the trunk. The dc wires are connected to an adapter to plug into the rear power outlet when needed, the ac cord let out of the trunk and fits neatly into trunk weather stripping without damage. I plug the end into an extension cord sometimes in winter when expecting sub freezing overnight temps to keep the battery fully charged.

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