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Battery Tech - Battery selection - Sealed batteries - Charging

Battery selection

"My battery is dead and I need a new one.  Which one should I buy for my BMW?"

First off, here are the stock BMW batteries:

Size: large vs. small battery tray

Large battery tray

The long-wheelbase airheads (late /5 onward), K75, K100, and '93 K1100LT (with ABS1) will usually accept either the large (25AH) or small (19AH) BMW battery.  These are your choices:

  1. BMW battery.  A good performer, made by Mareg/Exide.  Usually costs around $90 from the dealer.
  2. Yuasa Y60N28AL-B, to replace the 25AH BMW battery. This is also a very good battery, but because it is imported (unlike many Yuasa batteries) the cost is similar to the Mareg.
  3. Yuasa 53030, to replace the 19AH BMW battery. Not always easy to find.
  4. U-1 garden tractor battery (a very common size). These may or may not fit well, and the vapors might corrode your seat pan.  These are very cheap batteries and are not designed to withstand vibration as well as the others, but sometimes people can get many years of service out of them. Within the same overall case size, the internals can vary from 120CCA  to 280CCA, so be careful what you buy. Not recommended!

Small battery tray

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.

Battery dimensions

Battery Height Length Width Notes
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       R1100S non-ABS
Yuasa YTX14-BS 145mm 150mm 87mm R1200GS/ST, K1200R/S
Yuasa YTZ12S 110mm 150mm 87mm HP2
Yuasa Y60-N24AL-B 6.7" 7.2" 4.9" Good match for BMW 25AH
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
Panasonic       Sealed
Panasonic 1220 6.6" 7.1" 3.0" Sealed
Odyssey PC-545       Sealed
Odyssey PC-680       Sealed

*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.

How sealed batteries work

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.

- big snip of outdated material -

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.