At 18:08 16 September 2012, wrote:
Which Optimate you choose is indeed dependent on battery size as a
function
of the C/10 formula. I use a 12 volt 14 ah battery and am contemplating
a
12 volt 18 ah. The Optimate-5 is correctly sized for these batteries I
believe.

The web site indicates a wide range of amp-hour sizes for each chargers.
Does the charger detect the battery size and adjust the current
accordingly?

I don't think so - from the Optimate 4 manual:

"The minimum time required for the OptiMate 4 to complete a charge and test
cycle and provide a result, even on a fully charged
battery, is 45 minutes. Charge time on a fl at but otherwise undamaged
battery is roughly equal to the battery’s Ah rating, so a 20Ah
battery should take no more than about 20 hours to progress to the
self-discharge check (§ 5). Deep-discharged batteries may take
signifi cantly longer.
If using the OptiMate 4 on a severely discharged automobile battery of
larger capacity, a full charge may not be achieved within the
48 hour charge safety limit. In this case follow the reset procedure
below."

I am not an expert, but personally I use 2 old Optimate III's and 4 new
Optimate 4's for glider batteries, and an Accumate for a car that doesn't
leaves the garage. Never had a problem with any of them. I used the III's
on an 18amp turbo battery when I had one, it was fine.

I would be inclined to use an Optimate 4 for your batteries, they will
charge and 4 does a lot of other things for you if your batteries have had
a bad day.

Mike