Brand new battery indicating to ‘replace’ 2 days, Car goes haywire when trying to crank. 1998 C70.

[scot850]

We always appreciate when members take the time to close out the issue as it is a great help for others trying to diagnose these issues.

Thanks,

Neil.

[volvolugnut]

Note to future people who find this thread: Simple check to voltage at battery while engine running will often indicate if you have a bad charging system. You need to have about 13.5- 14 volts if charging.
volvolugnut

[abscate]

Usually this battery drain is caused by diodes in the alternator that have shorted to low ohms, the battery just runs current through them until they are dead. You can diagnose this but playing with unfused , high current stuff isn’t electrical 101 - you can start a fire and/or hurt yourself doing this

You can safely symptom diagnose by removing the hot cable from the alternator amd seeing if that stops the drain.

That means…

Charge battery
Disconnect negative terminal
Disconnect positive at alternator. Cover with heavy rag and tape
Reconnect negative
Leave overnight.

If you are battery drain stops, voila.