My experiance with marine use and diesel truck use has shown that slapping any battery in works fine as long as they both are good. I've done it for 20 years and so do a lot of farmers with twin battery diesel tractors. If they are both 4-5 years old and one dies then replace them both for sure. As far as my battery that died in 6 months it was the high dollar Sears battery that did that. It was at 6.7 volts after sitting in an E350 for a month. The Yellow top that I had to drop to take a charge is 5 years old. It sits for months on end and is in the Arizona heat. It got good and dead this year and it even sat dead for a while. That will kill any battery and this was not the first time it's been abused in this way. I'd say it's been a great battery considering. I followed the advise on your site years ago and bought a 10amp charger with the AGM setting. It was just too far gone and after days of fooling with it I did the drop test. It worked. Go figure? That was two months ago and it worked fine today. Not bad for a 5 year old battery that has been run below 10 volts at least a dozen times. Thats the worst thing you can do to a battery and I'd say Optima handles that better than any battery..
Stumpalump, I can't speak for any battery manufacturers but Optima, but I will respectfully disagree with the suggestion that batteries dissimilar in size, age or type should be used in parallel or series applications. While this can be done, it is not recommended, especially on larger banks of batteries. On this point, I speak not only at the advice of our engineers, but from personal experience. My family owned an electric car in the 1980s and when my dad started replacing batteries (16 in total), he tried to save money by only replacing the failing batteries. When he started replacing batteries he had just replaced a few month prior, he decided to listen to the manufacturer, who originally advised him to replace them all at once. After doing that, the new bank of batteries ran successfully for several years.
While a 10-amp charge for up to two hours can help break up some sulfation in our batteries, we do not recommend intentionally dropping them from any height. Stumpalump, I'm sorry to hear about the problem you had with your previous battery after six months of use. Do you recall what the voltage of the battery was and the circumstances surrounding your issues?
eCare Manager, OPTIMA Batteries, Inc.