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Originally Posted by Brian S.
Thanks for the info. I am stil researching different brands.
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Over the last 30 years I've designed some battery chargers, and reverse engineered others...
The term "battery tender" has become a generic, like Kleenex for facial tissue.
"Battery Tender" is a trademark of Deltran company, and they make some really, really nice maintenance chargers that absolutely will not damage your batteries by overcharging or undercharging.
A while back I had the opportunity to reverse engineer the circuit board of a 3/4amp Battery Tender and was impressed with the design. Start with a true laminated core transformer, a 1,000 volt diode where a 200 volt unit would do, a 100 volt capacitor where a 20 volt would be sufficient, and more over-engineering. A circuit analysis revealed a microprocessor that measures the battery voltage, and won't turn on the charger unless there is 3 to 4 volts of DC - enough voltage to ensure that you're not trying to reverse charge the battery with reversed battery clips. Once the charger is enabled by the correct polarity it slowly ramps the voltage up and then switches to supply just enough current to maintain the charge level (it switches off the charger and measures the voltage periodically).
It's a known fact that your average lead-acid battery loses about 5% a month in self-discharge. The Battery Tender keeps up with that, and can also keep up with some low amperage loads like some LED lighting, the channel memory in the dash AM/FM broadcast radio, a propane / carbon monoxide sniffer, or an alarm system.
Over the last 25 years I have purchased a number of real Battery Tenders... I have seven of the 3/4 amp "Junior" units that live in some grab-and-go 2-way radio base stations (each has a 7ah AGM battery in it), a 3 amp unit that lives connected to my RV house battery, a 1.25 amp unit that lives on the RV chassis battery, a second 3 amp unit lives under the hood of a 4x4 that gets used occasionally.
I chose to use one of my 3 amp units on my RV house battery as it was available and the RV is used as a bedroom a few nights a week - all that is used is an AC powered electric space heater on a time clock and a few 12v LED lights for a hour or two each night. The 3 amp tender can keep up with that usage.
The reason I have so many of the 3/4 amp "Juniors" is because I found a box of them at a electronic surplus house for $5 each (they are normally $25 each). I bought every single one! They were filthy and a few had damaged cords but they all cleaned up nice... The 1.25 amp one I found at a garage sale several years ago for $5, and the 3 amp units were on sale at Wal-Mart one weekend a couple of years ago for $40 (normally $55). Since then Deltran has started making a 4 amp unit you can find for $65.
Note that a 3 amp charger is NOT appropriate on anything less than a 30ah battery, and a 4 amp on a on anything less than a 40ah battery. The data sheet on any flooded lead acid battery says that for maximum life you never charge at over a 10% rate.
Anyway, to provide a useful answer to the original posters question will require asking if the TT will be in storage, or actively used? A 3/4 amp or 1.25 amp unit is appropriate for storage but will not keep up with a dehumidifier or a propane refrigerator, a 4 amp unit might keep up with one of those but not both. If it's going to be in active use (i.e. lived in) then a regular multi-stage converter is more appropriate.