Full battery, no crank

[deeclue]

Hello! I’m a longtime e30 owner but a brand new e30 m42 owner. I have a 1991 318i convertible and it’s not starting after being garaged for a couple weeks. Shortly after getting it I started having dead battery issues in the morning, the battery was pretty old so I swapped a new one and everything seemed fine while I was daily driving it for a bit.

I parked it in my garage and didn’t disconnect the battery as I would if I was leaving it for a long time but when I went back to it today it acted as if the battery was completely dead. Turned the key to nothing on the dash. Weird. I threw my charger on it figuring it that would work but the charger acted like the battery was full and would really charge. The volts said somewhere around 14 so I think the batter is still full.

While on the charger I got the dash lights to turn on but when I turned the key nothing happened, not even really a click. And the second the charger is removed it goes back to dead or the faintest dash lights you could imagine.

I’m thinking maybe grounding straps but I figured I make sure someone on here wasn’t like “oh that’s the classic blah blah blah that happens to m42s” before I started buying parts.

Thanks and happy to be a part of the m42 club - this car has way more pep than my old m20 even if it doesn’t ultimately have as much power. I’m excited to get it back on the road!

[roguetoaster]

Well, if the dash lights came on when charging and immediately go away once the charger is disconnected I'd immediately suspect the battery as faulty. Personally, I'd start with the simple thing and swap or otherwise connect another battery if you have one to see if it starts.

In future, a battery disconnect is a nice add on for older cars that sit for extended periods of time.

[deeclue]

Yeah I’ve been meaning to add a battery disconnect but I haven’t had a chance yet - I’ve only had the car a couple months. I suppose I can take the battery back and see if they can test it/trade it. It’s not even a month old. It’s possible it’s bad but I feel like the issue is somewhere else.

[deeclue]

Little update: I removed the battery and put it on the charger and it slowly trickle charged it for quite a while until it finally kicked into high gear and started to charge it at more full power. I left it charging for a really long time and then I put it into the car and it fired right up. Clearly I have a parasitic drain going on, so in the meantime I installed a battery disconnect and left it disconnected until I have time to troubleshoot and find the drain. I guess the battery was just so incredibly dead that the charger had a hard time charging it for a while.

[jsnppp]

Feels like a drain somewhere for sure. Check trunk light bulb and possibly if you have aftermarket stereo, with an amp thats always powered (and has a light). Or an alarm system. Never know what the PO installed.

And of course it also could be a crappy battery.

M42s are super fun!

[reelizmpro]

oh boy, there are so many variables. Drains are common on older cars especially with accessories. I'd start with the basics, check for any additional ring terminals attached to the battery terminals that are drawing constant power. Alternator output should be greater than 14v while charging the battery after start up and go down to about 13.7v after a while. If not, check the regulator, belt tension and connections. I've had accessories draw more power than normal and the trunk light stay on because the trunk adjustment had gotten loose over time. I've also had the aux fan relay get stuck on to the point of killing the battery. What battery size and type are you running? a week is a long time not to run your car and this will shorten battery life if you keep deep cycling a battery not designed for it. Put a tender on it but if you can't I recommend H6 (grp 48) AGM batteries. I can go a couple of weeks without driving and it will fire right up. A regular lead acid battery is pretty drained after a week and if you have a smaller battery it's even worse. I killed two German made T6 Duralast Gold's this way (slightly shorter than H6). Good luck.