John -
I have had two TMs over 15 years, and like you, I have had bad luck with batteries. In the process, I have learned a lot, I seem to be doing a bit better with batteries now. Perhaps some of it would be of help to you. What follows is my experience only, so take it for what it is worth.
The first thing I learned is that there is a mental hurdle. You need to distinguish between battery CHARGING and battery MAINTAINING. Charging means that you have pulled some energy out of the battery and need to replace it. Generally, you would like to replace it quickly and correctly. "Correctly" is defined by the chemical nature of batteries, and results in the stages of charging called bulk, absorption, and float. The problem is that the details of these stages are often defined by the charger manufacturer. No two "smart" chargers are the same.
Maintaining means that you start with a fully charged (or nearly fully charged) battery, one that you won't be using for a while, and you need to maintain its health during the down time.
These two operations are very different, and despite glowing words in the advertising, I am not aware of a unit that is good at both. Why? "Charging" is basically a brute force operation, though smart chargers handle it better than earlier units like the 6300 and 7300. But almost by definition, charging is expected to happen quickly, while "maintaining" is a finesse operation. It takes place over a long time.
I went through the 6300/7300 era, and bought a new battery every year. I upgraded to a Progressive Dynamics charger, and things improved. I could get 2 years out of a battery. I started disconnecting the PD when there was going to be a long down time, and connecting the Guest trolling-motor charger instead. Things improved a bit more.
My current approach, and one that seems to be working, is to charge the battery using the PD, and then maintain the battery using a small Battery Tenda. This was a hard pill for me to swallow, since I had spent years warning people against cheap trickle chargers. But someone gave me a B.T., and I finally discovered that it is not a cheap trickle charger. I'm an EE, so I figured out why and how it works, and I have been using it with good results for a few years now. It is not magic, and it will not charge your battery. But after several 3-to-6 month periods of down time using the B.T., my batteries appear to be unchanged, and that is good. No water loss, no increase or decrease in terminal voltage, no scum on the surface of the electrolyte, no corrosion on the posts, and so forth. Those are signs of improper maintenance, and I saw them all with earlier approaches. I'm happy that I haven't seen them lately.
As I said, this is my experience, and you should take it for what it is worth. And I am not connected with B.T. in any way.
Bill
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