"My battery is dead and I need a new one. Which one should I buy for my BMW?"
First off, here are the current stock BMW batteries:
The long-wheelbase airheads, K75, K100, and '93 K1100LT (with ABS1) will usually accept either the large (25AH) or small (19AH) BMW battery. These are your choices:
The short wheelbase /5 airheads, early R65, 4-valve K bikes with ABS2, and oilheads are designed to only accept the small (19AH) Mareg battery. Do not believe the old literature that says you should use the YB18! Even Yuasa has dropped this ill-fitting battery from their latest recommendations. It can be made to fit in some cases, but it will never fit well.
|BMW 19AH gel||6.8"||7.3"||3.2"||Slightly taller than flooded|
|BMW 19AH wet||6.7"||7.3"||3.2"|
|BMW 25AH wet||6.7"||7.3"||5.05"||Same as 19AH but wider|
|BMW 14AH wet||170||181||78||NLA, R11 non-ABS|
|BMW 14AH sealed||161||150||87||R1200RTW|
|BMW 12AH sealed||149||143||86||Gray case (was YTX14-BS)|
|Yuasa 53030||6.9"||7.4"||5.1"||Dims taken from literature|
|Yuasa YB18A-LA||6.4"||7.1" *||3.55"||Too wide for some small trays|
|Yuasa YTX14-BS||145||150||87||Replaces gray 12AH|
|Yuasa YTX16-BS||161||150||87||Same as stock for RTW|
|Odyssey PC-680||Replaces 19AH sizes|
*Plus end vent.
There are two main flavors of lead-acid batteries: flooded and sealed.
Flooded (which has free liquid electrolyte, like the OEM batteries) can be standard or low-maintenance.
Sealed (should not be opened, ever) are also in two flavors, AGM (absorptive glass mat) and gel.
Sealed batteries don't have loose, liquid electrolyte (acid) in them; instead, it's absorbed in a thick fiberglass mat and is thus immobilized. This mat looks like fluffy cardboard when it's not in a battery.
The good: there is nothing to spill when the battery tips over, and very little to spill if it cracks.
The bad: with the immobilized acid, the only ion movement is by diffusion. So you can get stratification which leaves part of the plate in a weak acid solution and part in a strong solution. Ideally, all areas would have the same concentration so the entire plate can work for you. Also, cold performance is theoretically worse because of the slow replenishment of ions at the plate surface (they get 'used up' and need to be replaced). IIRC I could start the K75 down to about 5°F with the flooded battery but the sealed one was only good to about 10 or 15.
Due to the chemistry involved, sealed batteries usually have an open-circuit voltage of 13.1 to 13.2 volts, which is probably why they are so famous for eliminating ABS faults (by keeping the voltage higher during cranking). Flooded batteries are usually designed to have a voltage of about 12.6 to 12.8. You can get a flooded battery to maintain a higher voltage but it's not really good for it and performance suffers. With higher voltages, the charging force (difference between the resting voltage and the applied voltage) is smaller and you can get into situations where the battery might not get fully charged. This is kind of complicated but the uncharged portion becomes permanently unusable over time.
So in summary, sealed batteries are great for what they do - or don't do. The don't leak, which is handy for a bike that might be upside down on short notice. They don't need watering (really, the BMW batteries hardly do either) and they'll keep your ABS faults at bay for a while, especially if you keep the bike on a tender. But the Westco specifically doesn't last very well, and Panasonic has openly stated that they're not recommended for vehicle use.
Sealed batteries are a good idea on the Oilheads, due to the location. The battery is difficult to access and check the electrolyte level on, and on the left side (where the vent tube is on a flooded battery) the main wiring harness runs just under the battery tray. I see a LOT of damage to the wiring harness here, from leaks in the vent hose or vent hoses which have been disconnected. I've even seen cracks in the fuel pipe there from acid leakage, which is a fairly expensive repair.