T&T: large batteries

[Dennis OConnor]

Let me show my bias here, but first let me tout my credentials (such as they
may be)... In a wasted youth I was an industrial electrician and later a plant
engineer... As such my duties included maintaining and purchasing electric
lift trucks and their associated batteries/chargers... I speak deep discharge
battery in hundred thousand dollar increments... 

Now, AGM is a nice technology which claim to fame is non spillage and no
maintenance... Also, accepts much higher charge currents...  But since they
are not re-waterable, when overcharged they lose water out the pressure 
relief vent and their life is considerably shortened...  They do have their
place and I use them in my airplane because for that service their advantages
outweigh their disadvantages...

Let me opine that for boat service the most bang for the buck is the T105 type
battery (I get mine from Sams Club, @ $67.78 for my most recent purchase last
week)  Used with a 3 stage charge controller and a temperature sensor (most
important) plus a battery maintainer and an anti sulphate pulser, you will get
many, many years of service...
(my most recent set that went bad was 10 years in service - the set was only
down about 10% in total capacity and went bad only because the battery
maintainer shorted internally and discharged the battery set to zero before it
was found)... So, the moral of that story is don't trust cheap chinese battery
maintainers...
 And since the T105's cost me less than half of an AGM set, I don't even feel
bad over spending the money... 

Now, for engine starting, you want a starting battery not a deep discharge
battery...  I am partial to the 4EH series for starting diesel trucks and
tractors and excavators and bulldozers and ,even, boats... These are 6 volt
units used in pairs... They are large,,, They are heavy... And they just plain
work...
 Again, you want a battery maintainer and a pulse desulphator on this set... 
And if they are used only for starting then the usual auto style alternator
with built in regulator will work just fine for them.  Since you will have a
high output alternator for the house batteries and the auto style alternator
for the starting batteries you have redundancy...

Notice that a battery maintainer is not a trickle charger...  It is a 12.8
volt device that will keep your battery at the proper float voltage for
longevity... A trickle charger is over 13 volts and is not good for your
battery; manufacturers' claims notwithstanding... However, I will note that a
trickle charger is better than nothing..

Lastly, charge stratification in a battery standing idle is a problem, but is
easily handled...   Every 60 days, discharge the battery to 40% down then do a
max current recharge with your 3 stage controller and temperature sensor,
running things...  This will remix the electrolyte, loosen any sulphate that
has avoided the pulser, and lengthen battery life...

denny / k8do

Oh yeah, and the place I like the AGM in a boat is on the anchor winch...  No
maintenance, no leaks, high current capability... Makes sense for a battery
usually buried deep in the bow that is used in bursts...