Cars have Batteries, everyone knows that, and neither your starting nor charging system will work very will without it. In fact, your whole Electrical system needs the battery, but not in the way you think...
The battery in your car is a lot like the defibrillator that emergency medics carry; it's a great big pack of electrical juice, all delivered in one solid punch to get the Starter moving. And that's pretty much it. It's NOT a long-term power supply like the batteries in your phone or watch. That's why a battery the size of a breezeblock can be drained by leaving your stereo on for more than a few hours.
The secondary job of your battery is to smooth out the current. Your charging system will pump out anything from 13.7V to 14.4V, depending on how fast the engine is turning at any one time, and this can change in milliseconds. But delicate stuff like your ECU or Instruments don't like dramatic voltage changes - they want a steady stream of electricity coming in, or they'll fry. So the battery absorbs the overcharge, softening the spikes and dips of the voltage into a more digestible 12.6V.
Batteries are measured in Ah, or Amp-Hours. This doesn't mean how many amps they deliver, but how LONG they can deliver one Amp for. So a 65Ah battery can give 1 Amp for 65 hours, or 65 Amps for one hour.
Big-engined E30s came fitted with a 096 battery as standard, meaning 72Ah and 680CCA (cold cranking amps). These are a good upgrade for smaller-engined cars, especially if you're planning on running a lot of extra kit without the engine running, such as heated seats and a massive stereo. However, the 4-cylinder wiring loom around the battery tray makes the larger battery a little difficult to drop into place.
The physical dimension is also crucial; too tall or wide and the battery won't fit into the car. The E30 uses an 096 style battery, meaning its dimensions are: 278 long x 175 wide x 190 high (mm)
On almost all E30s the battery is located under the bonnet, up against the bulkhead on the driver's side. However, a second option was to have the battery fitted in the boot, behind the fuel filler flap. This redistributes the 20kg to the rear of the car, making some models feel a bit more balanced. While not standard on any E30, it was a popular option for M20-engined cars, as well as Touring and Cabriolet models due to their heavier design.
Battery In Boot
Many owners bought the optional extra of having the battery fitted in the boot, and the kit to do that is very often available from online auctions or the Zone For Sale section. Any E30 can be converted to Battery In Boot simply enough, although it does require removing certain parts of the interior trim which are rather fragile and may break.
If you're scouting around for parts, it's worth looking for the main cable from the E36 battery-in-boot installation. Although the end is plug is different and will need to be changed, the entire cable is a few inches longer which makes a world of difference when it comes to installing.
If you can get your hands on the kit, learn more about fitting battery in boot.
You get in, turn the key and "clunk"; the motor doesn't start. You check further and see your headlights are very dim and your central locking sounds slow and heavy. You've got all the signs of a flat battery.
First thing to do is to get the battery tested. Even if you know you've left your lights on, there's a chance that you've over-drained the battery and killed it, so take it to a professional to have it Load Tested. Specialist battery technicians are better for this, as they're more likely to tell you the truth rather than force you to buy a new battery that you don't need.
Common drains include internal boot lights or interior lights staying on, and bad radio or alarm installations. But on a 20 year old car, any and all wiring will become suspect, so it's important to go through the car systematically.
If these all prove negative, open a window or make sure you have the car keys out of the car, then connect a milliamp meter between the negative pole of the battery and the battery earth lead, and remove fuses until you find where the drain is. Normal quiescent drain on an E30 is around 30mA.