General Fiat 500c not starting unless jumped - have replaced carplay and disconnected blue&me, any other ideas of what it could be?

[charlillie]

Super frustrating and have had it looked at by various mechanics but no one can figure out … any ideas?
- blue&me disconnected
- has been re-alligned with the thing you plug into computer
- battery charges when plugged into a mains and lasts for 3 days
- i can go for a massive drive (5+ hours) and it will still not work next time I try
- 2015 plate

 

[Zardo]

Parasitic drain

first make sure the battery isn't discharging itself

disconnect the battery and leave it disconnected
fully charge the battery
measure the voltage whilst useing a defined small dummy load
let it sit disconnected for three days
measure the voltage again whilst using a defined small dummy load
compare the two measured voltages

If there basically the same, fit the battery, connect it, with ignition on, make sure all optional consumers are off, switch ignition off, disconnect one terminal, use a multimeter with at least a 10 A setting (better 20 A), measure current with ignition off (you should know how to measure a current, otherwise you might blow the fuse in your multimeter). There should basically be no current, or only a few milliamperes (less than 15 mA, I guess)

A parasitic drain able to suck the power out of, for example, a 42 Ah battery over 72 hours would draw about 500 Milliamps with ignition off, such a finding would be very abnormal. If that's the case, systematically take out fusibles (with ignition off) until the multimeter doesn't show a parasitic drain anymore.

I guess the most common culprits are an ill-fitted aftermarket radio and the alternator itself, could also be the wiring (isolation gone for the "always on" wiring parts like those before ignition, before relais)

 

[John202020]

I guess the most common culprits are an ill-fitted aftermarket radio and the alternator

this