<<<<<<<<<<<<

 

Finally, I was able to answer my own questions.

 

No special regulator or system to maintain a float voltage need be constructed or utilized. 

 

There are at least three things that shorten the life of SLA batteries - multiple deep discharges, long term overcharging, multiple long periods of disuse (measured in several months).  By shortened life I mean lack of ability to deliver rated Amp Hours service including, for starting batteries, Cold Crank Amps and otherwise the delivery of voltage/current over usage time.   

 

There is one thing that extends the life of SLA batteries and that is maintaining a continuous constant float voltage at a small current level. Thus a standby battery maintenance scheme was considered. 

 

Normal life (several years) can be achieved by normal usage.  Discharging to a single digit percent of its rating, recharging at the proper voltage level and limiting the current as the battery becomes charged accomplish this end.  In an airplane, such management of the main battery is accomplished by the voltage regulator as long as it is matched to the battery. 

 

So, if I did construct a float voltage device for my standby battery, the fact that it is cycled at each Master Switch cycle and then operates only for the duration of each flight means that it would not necessarily extend its life.  The slight over voltage charge rate during each flight will probably not shorten the life to any less than the main battery. 

 

Conclusion:  Replace the standby battery on the same schedule that the main battery is replaced.  I.E. Every two to three years or when the preflight voltage tests indicate a drop of more than .2 volts from the previous trend (See Battery Tender FAQs re: How to tell if a battery is bad).

 

BTW, I considered modifying a Battery Tender Jr (120 VAC in) to provide the float voltage.  It was interesting to note that most of the weight and space inside the unit was taken up by a transformer that dropped the 120 VAC to 13.7 VAC.