Electrical fault

Hey all, new here. My Jimny is a 2016 manual JB43.
I just parked it for about a day outside my house, and it was raining heavy. When I came back to start it electricity was dead, no response when turning the switch and not a single dash light on. I had no previous issues with the car battery which is about 2 years old, but I thought that maybe its just super dead somehow. But then I had some other weird symptoms:

- ​​​​​​I plugged an 8000mAh external battery (charged full) to the car battery, the car started for a few seconds, then the engine stalled and then I started it the second time (ext. battery lost 40% charge).
- I managed to drive for a few minutes, then engine suddenly stalled in the middle of a turn (low revs) and again electricity is completely dead. Emergency lights seemed to barely just flicker for a bit.
- I used the ext. battery again a couple of times to start because the power kept going off, but this time it seemed to barely lose any charge (about 1-2%), implying that the car battery might be ok and there's something else going on.
- I drove for a few more minutes and the engine stalled again and power died when trying to park (again low revs). 
 - A few seconds after I plug the ext. battery there's a slight knock and power comes back, but if I try to start without the battery there's a click when I turn the switch and power immediately dies, starter won't even spin.
 - When ext. battery is plugged in the car starts fine, but can't hold it for long. Somehow the power died twice exactly when I closed the hood. 
- Overall I started the car using the battery over 10 times, and still had about 25% charge remaining. 

To sum up it seems that something causes the entire electrical system to randomly go off, and the only way it comes back is with external power. Any ideas what could be the problem?
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How about you have the original battery charged, tested & if necessary replaced - the standard battery is going to be somewhere in the region of 30~40Ah, so 3~4 times the capacity of your jump starter.

Once you have a KNOWN, GOOD BATTERY in the vehicle, perform a basic charge system test - connect a DVM to the battery terminals, with the engine and all accessories off, record the terminal voltage and then start the engine, if the charge system is working the voltage should increase by 1~2 VDC (it will vary according to the batteries state of charge).

Me too thinks it is a battery failure by the description. Many batteries are manufactured in low cost parts of Asia or emerging markets these days and fail suddenly. As Fordem says use a well known brand to avoid this.

Do you connect the external power to the car battery terminals or the ground to the engine?

If any difference check wires:
First to check is to see if the ground wire from engine to chassi and chassi to battery is ok, and the ground from somewhere around the starter bolt on the engine and to the chassi or battery. check the positive wire from the battery and all the way to the starter and generator and the connections on the generator.
Best is to measure volts at these spots with the car running and without the car running to see if there is any difference.

Should be 12.5 Volts +/-0,5 without the engine running and 13.8V +/-0,5 with it running.

If any oxydation is visible, disconnect battery first, disconnect the oxidized parts , and apply contact spray or WD40 or something better before reconnecting

wrote:

Best is to measure volts at these spots with the car running and without the car running to see if there is any difference.

Should be 12.5 Volts +/-0,5 without the engine running and 13.8V +/-0,5 with it running.

 

Please be aware that battery terminal voltage on a lead acid battery varies with the "state of charge", whether the battery is being charged or discharged and the charge/discharge rate.  What you should be looking for is the change in voltage rather than the actual voltage (it can be well below that 12.5 +/- 0.5)

Is the battery you have the standard size and is the positive terminal fitted with a cover? Where I am going with this is you mentioned the car died when you closed the hood, I wonder if you are shorting the battery out on the hood?

Batteries can just die, so as already suggested, charge it up with a battery charger, it should get to about 13.5V when fully charged. Then it will fall back to around 12.5V when you take the charger off, especially if still connected to the vehicle.

If connected to the vehicle and it falls very quickly to 12V or below, then either your battery has had it or there is a high current draw on the car. Try disconnecting the battery from the car and recharge, if the battery voltage still falls quickly after removing the charger then it’s the battery.

If you have got a high current draw on the car it could be a number of things, a multimeter and/or an automotive electrical engineer might be required to trace the problem.

Robin

Battery is dead indeed. Turns out it was older than I thought, and the previous owner didn't invest in a good brand either. By the time I brought a DVM the terminals already read zero voltage. Didn't think batteries just die that quickly, so I feared there might be some other electrical fault. I'll have it replaced after the weekend and hopefully that'll do. 

Thank you all for the elaborate answers, glad to expand my knowledge at least.