GEN 4 Disconnecting the Battery Sensor

Thanks @Soeley for the photos -- always helpful for a thread of this type.

I was surprised to see this as a topic for discussion here, because I had not heard the Jimny had any kind of issue with charging.  

My own use case would seem to be prone to provoke flattening of the battery if the model were weak in that regard.  My Gen4 mostly sits, often for weeks at a time.  Looking at my notes, the longest it has sat between runs was 6 weeks, though 2-3 weeks is more typical.  Unlike some sportscars I have owned in the recent past, some models notorious for flattening batteries, I've never put the Jimny on a maintainer / trickle charger. The car's three years old now and (touch wood) no issues with the battery.  And it does get cold here, being at a latitude around the Orkneys'.

Anyway, good info to have, thanks to all who have contributed.

With or without smart charging, the ECU does not observe the small losses when it is dormant - in short, this is not a smart charge issue.

To give you a bit of background, I spent over two decades working with lead acid batteries, first in standalone (no grid input) solar power and then uninterruptible power supply systems - I'm trying to avoid getting into lengthy explanations, but, in a nutshell, the charge system does not need to make an assessment of the battery's state of charge in order to charge it fully, it just needs to provide a sufficient quantity of electrical energy within a specific voltage range and the battery itself will do the rest.

Moving onto your Land Cruiser - what is your definition of a dead battery? Is it a battery that does not have sufficient stored energy to start the vehicle (which I would term a flat or discharged battery) or one which is no longer capable of storing such a charge?

Lay people confuse these, which is not surprising since the first can quite rapidly become the second.

If you have a situation where the battery discharges to the point that it cannot crank the vehicle and this can be resolved by disconnecting the battery, there is a good chance that this is not a charge issue, but a discharge issue - when you shut the vehicle off there is sufficient charge stored to restart it, but over time with the vehicle not being used, the battery becomes discharged - the fact that disconnecting the battery "breaks" this pattern points to a constant slow discharge as the cause.

I don't know how old your Land Cruiser is, or what sort of electronics it has (both OE or aftermarket), but newer vehicles have more electronics that require "keep alive" energy and present a small but consistent drain on the battery, older vehicles, the ones of my youth could sit untouched for months and still start at the first turn of the key, my experience with newer models suggests that vehicles do not need to be daily or even weekly driven for any long distances to maintain a decent state of charge on the battery.

Parked at the side of my house is a 2019 Nissan turbo-diesel pickup, it's what I would describe as an "electronics heavy" truck, in that it has all the bells & whistles, keyless entry, central locking, alarm system, push button start, nav system, power windows, power mirrors - lots of OEM electronics to drain the battery whilst it's parked.

The truck sits more or less unused for months at a time, moving only to allow the space it is in to be cleaned weekly - start up, drive not more than 100 meters, shut off, and once the cleaning has been completed, the process is reversed, so two starts, average run time ten minutes, average distance 200 meters, and that is enough to keep the battery charged.

The truck was purchased new, and in the six years we've had it, the battery has been changed once.

It's a 2018 shorty 155. In It's short life it has managed to have 7 new batteries fitted because it's draining them to the point they won't hold charge. I'm not going to go into it further as the whole issue is now under the assessment of the motor ombudsman.