Say what?!? The output of the battery tender is 5 amps max in the charge stage. In the maintenance phase of the battery tender the amps are in the .5 to 1 amp range. Your alternator puts out 55 to 115 depending on model. The amperage is going to the same place with both. I use and have used these tenders in both 12 and 24 volt models for about 4 years on approx 300 + items of equipment. I have also used this on my 90 300E for extended periods of deployment (7 months) with zero problems. Sounds to me that someone has used the wrong coolant and/or the wrong cap. Just my experience of many years of batteries and chargers.
Chris