Does disconnecting the battery have negative side effects?

On my vehicles, the only “side effect” is I have to reset the clock, as it defaults to 12:00 when the battery is reconnected. I find this only a minor inconvenience, outweighed by the knowledge that I will return to a vehicle with a working battery. I have done this with a 2017 Chevy Trailblazer and not had any bad effects. One thing to consider though, is the keyless entry. You might need to lock/unlock the vehicle with the key, as the remote lock will be non-functional.

As far as drain protection, I have found that even small drains such as the indicator light on a phone charger left plugged in, can drain the battery after a couple of days. Also, there may be unknown shorts in the system that cause drain. During daylight, it can be hard to tell if an overhead light is accidentally left on, so disconnecting the battery is a nice way to make sure nothing will discharge it while you’re hiking. The 30 seconds it takes to disconnect/connect the battery is worth it if you are in the wilderness away from jump starts, roadside assistance, or cell service.

As far as theft protection, though it might not be necessary in modern cars due to the RFID chips in the keys, many people, especially hikers use “hide-a-keys”. Loosing your keys while hiking would be a disaster, so I usually hide a key under my car. Sure a thief who finds the key might be smart enough to pop the hood and re-connect the battery, but it does provide one additional layer of security. Or, perhaps I get robbed while hiking/camping, and the thief steals my keys. Again, my hope would be that he is too dumb to figure out that I disconnected the battery.

For theft-prevention, this doesn't make any sense on any modern car. They have immobilizers. Practically the only possibility to steal the car is with a key. If the thief has the key, it's possible to reconnect the battery. So, you are mainly looking at drain prevention.

For drain prevention, you need this only if you leave the car unused for many, many weeks (over a month or so), if the battery is in prime condition. The small continuous drain is optimized to be so small that it won't typically be a problem. Cars with smart key system have larger drain, but even they have optimizations (such as only the driver's door working after parking for week or two).

An alternative would be a trickle charger. Does your car have electrical outlets nearby? If so, I would use a trickle charger.

The drawback of disconnecting the battery is that you forget many settings in electrical devices such as the radio presets. The engine control system may also need to re-learn some parameters, so the car may run a bit rough after disconnecting the battery.

None of this of course is something that you absolutely must avoid. E.g. for airbag replacement (which I had due to a recall in my past car, a 2011 Toyota Yaris), the battery must be disconnected. So nothing is going to break by disconnecting the battery. It's just going to be a major inconvenience.

1- Installing a kill switch in the car is an alternative. A 300Amp Relay would do nicely.

2- Installing a separate 12V rechargeable battery to the ECU and other components would keep them going when the main battery is disconnected. Make sure to have an adequate diode on the power side of each desired component so that the power from the extra batteries doesn't back feed in to the system.

3- The best way to do this is installing the a relay in line to the starter solenoid. Install a hidden switch in the dash somewhere, therefore no one can start the car unless they get underneath and hot wire the starter solenoid... or find the switch

4- This is my favorite... I personally like reed switches hidden behind the dash. Reed switches complete the circuit to the relay and the only way you can start the car is by having a magnet near the reed. You can leave the magnet by the reed if you know you are going to start the car remotely, or take it off as a security measure. NOTE: If you have an "eco friendly start stop engine system that ticks people like me off, then you can't use this option because as you are driving and the magnet gets loose and you stop at a light, good luck starting the engine again.. or.. shut off the dang "eco-BS feature" all together.

Unlike relays, reed switches do not need power to close a connection, all they need is a magnetic field. However, the amperage draw is to be considered here as most reed switches are rated at low amperage.

This is the reed switch I use to connect to a low power starter solenoid. This reed switch can handle up to 3 amps. https://standexelectronics.com/products/ksk-1a83-series-reed-switch-3/

My name is Nitro Burp and I build race cars