T&T: Batteries

[bob Austin]

There have been a number of points made about batteries which are somewhat
confusing.  First look at the construction:  For example here are some of the
thicknesses of battery plates:

typical starting battery:      .040"
golf cart "deep cycle:        .07-.11"
Concord AGM                     .115"
Rolls//surette L 16            .150"
Rolls/surette traction         .265"

Consider that the longivity and capacity of a battery is proportinate to the
amount of lead, thickness of the plates and construction of the battery. Just
because a battery says "marine" or "deep cycle" does not mean it is the best
battery for your application.

 The "traction"  which means a battery which runs a fork lift or some
appliance which is taken to 80% discharge has thickness of plates which are 6
times thicker than the automotive starting battery.

Now lets look at some of the manufacturer's curves on depth of discharge:

Concordia AGM Lifeline:  20% discharge   3000 cycle lifetime
                                            50% discharge   1000 cycle
lifetime
                                            80% discharge    500 cycle
lifetime

The L 16 by Discover:      50% discharge    1100 cycle lifetime
                                            80% discharge    700 cycle
lifetime
                                            100% discharge  550 cycle
lifetime

Typical Golf cart:             50% discharge        750 cycle lifetime
                                          80% discharge        225 cycle
lifetime

Typical start battery        20% discharge            2000 cycle lifetime
                                         50% discharge               200 cycle
lifetime
                                         80% discharge                  25
cycle lifetime

Although the AGM will take a higher rate of recharge (within some limits) ,
does not require water to be added, is spill proof etc, unless it is built
heavier it does not guarantee that it will have that much longer a life span.
It still depends on the construction of the battery.   The more lead, the
thicker the plates and the better the construction the longer the battery will
last, if cared for properly.

How do you know what quality of battery you are getting?  If you check you
will find that not all L 16 batteries (susposedly "traction") batteries are
the same.  Steve Dashew comments on this with an amendum to his web site.
Trojan used to make a high capacity L 16--but when the company was sold, they
cut back the capacity, weight and thickness of plates.

Charging:  When we were crossing oceans in sailboats we had about 1200 amp
hours of golf cart batteries in the house bank. We had the demand of full time
autopilot, nav instruments, lights and occasional inverter and radio use.   We
had a150 amp alternator on the main engine and a 150 amp alternator belt
driven off the generator and we had a 150 amp charger on the inverter.  We
were able to get by with a couple of hours of running the generator twice a
day.  But we were running the battery banks between 20% and 50% discharged.
You never get the last 10 to 20% of the charge in the short high capacity
charging, even with that robust a system.  Most trawlers have much less
charging capacity that that.  But most trawlers spend much of their time at
the dock and the relitatively low capacity 3 stage  battery charger keeps the
battery "topped off" that last 10 to 20% of the charge.  The average user is
not running their boat away from the dock 95% of the time.

Summary:  Check the weight, capacity and curves on the batteries.  Buy the
most "lead" for the $$.  If you need the battery to be spill proof, in a
remote area, or in an inhabited compartment, then definately go with AGM.
Check the charger, alternator and regulator in your boat.  Be sure that these
are matched to the battery (including type, temperature compensation, and
charging curve).  Batteries require a "balanced" system and some good way of
monitering the usage, such as an amp consumption meter.

Bob Austin