Advice before tomorrow 11am v much appreciated... ?battery knackered?

[Dr Firefly]

Right, you may have seen that I recently managed to buy back my beloved 325i convertible after 8 or 9 years.

First problem is that the previous owner has been leaving her with the battery disconnected between starts as there is "a slow drain".

Now what I've found is - drop test of battery shows it is at "52%" according to my local battery dealer. They recommend changing battery at below 70% but obviously they would as they want to sell batteries. Most times (if I've left it disconnected) it works fine.

But twice - once after leaving the headlights on accidentally for 3 minutes, and another time after a really short run and windows going up and down without engine running - the battery has been completely utterly dead needing jump starting.

Does the drop test not test overall battery capacity?

Understand I know nothing about electrics. But I've flattened a couple of E30 batteries over the years by leaving the lights on and the battery is acting like a battery that is screwed from repeated flattening. Surely it should have enough charge to leave lights for a couple of min and not be completely dead?

Or could it be to do with it not charging because of a fault in the (alternator? charging circuit? Absolute novice so I don't know proper terms and rather than bullshit would rather profess my ignorance to learn).

All electrics work perfectly aside from bad earth in rear light clusters somewhere which I haven 't hunted down yet.

My plan (and where I need help):

1. Go to battery place tomorrow and get a new battery.
2. Drive to my dad's (ex mining engineer and tractor mechanic who is generally awesome at anything mechanical/electrical - I'm good at repairing people in A&E, less good at machines) Get him to check whatever needs checking to see if it's the car or the old battery that's at fault.

Here is where you come in (Brian?). What does he check using his voltmeter magic and where? And what readings should he get?

Thanks in advance and it's so lovely being in contact with you lot again
Ian

[Contours]

The reason they advise you to change a battery that is only at 70% or less state of health is to save the alternator. The alternator has to work hard to keep this battery charged up and this can cause it to overheat. People who try to get by with a bad battery for long usually end up having to replace the alternator as well within a short time. Your multimeter should show 13.8 to 14.2 volts if the battery is being properly charged by the alternator with the engine running. It does seem like your battery cannot hold a charge as well as having lost a considerable number of cranking amps since it was new.

[flybynite]

Lot of info in the WIKI
https://www.e30zone.net/e30wiki/index.php?title=Battery

Thread on batteries little while back

[Blanca]

If your battery is playing up and going dead after so little use it would indicate that it has a fault, just buy a new one, The battery is the heart of the car, everything relies on it so do not mess about with an old one, any battery over 4 years old or subjected to long periods of standing is suspect.
I have a kit car and I leave the battery disconnected for months at a time and so far have bought three new ones when the 'old ones' look unused. Just will not turn the starter even after all day at 4A charging.

Also make sure all your cables are good with good clean connections including chassis to engine strap.