Here is yet another (I hope) relevant posting from the other list:

Subject: Battery Chargers

What charger, amp size, etc., would you recommend to slightly
charge or maintain a std lead acid 35 series aircraft battery?

By "maintain", I assume you are concerned about what happens to
the typical G35 aircraft battery during times when the aircraft
is not flown regularly (say bi-weekly).

The answer is complicated. As long as the aircraft is flown
once every couple of weeks, assuming that the generating
capacity in the aircraft exceeds the current being drawn by the
instruments, avionics and lights, (so that the reserve capacity
is available for charging the battery), and that the voltage
regulator gets the battery voltage up to 14.2+ Volts for the
duration of a flight, then you dont need to do anything
involving external chargers.

If the aircraft generation capacity or regulation is suspect,
then instead of tinkering with chargers, you really should fix
the root cause (which is install enough generating capacity,
adjust the voltage regulator to produce said 14.2+ V, fly the
airplane an hour or two every two weeks, etc.)

The reason for the "every two weeks" statements above are based
on the physics of flooded-cell batteries. There are two parts to
prevent degradation of a battery which is sitting unused for
periods of weeks to months. Lets call this "battery
maintenance". Note that "maintenence" only needs to be done when
the aircraft is not regularly flown, as in the winter months.

First, you need to keep the battery from discharging itself even
after disconnecting anything that draws current, which should be
the case if you turn off your Master. Second, you need to
periodically stir the acid in battery to prevent the acid from
stratifying with the lowest specific gravity on top.

All lead acid batteries loose charge just by sitting around. at
100 deg F, a G35 will loose 10%-20% of its capacity per month.
In cold weather, the self discharge rate is much lower. So if
you want to maintain a battery near full capacity, you have to
replace the charge lost due to self discharge. It requires only
~ 0.25A to make up for the leakage.

Preventing self discharge is usually accomplished by "floating"
the battery using a voltage-limited charger, which puts out 13.5
+-0.1 V open-circuit. Most commercial automotive chargers such
as you would get at Sears or Checker are not voltage limited
accurately enough to leave them connected to a battery for more
than 24 hours!!! They are suitable for recharging a run-down
battery, but they must be manually disconnected after a few
hours.

The better models of the automotive chargers have a time clock
shutoff which means you dont have to drive back to the airport
to shut off the charger. If you have the disipline, you can
"maintain" an unused battery through the winter months by giving
it an 4-6 hour charge every 30 days or so, relying on the clock
in the charger to shut it off, otherwise you have to drive back
to airport to disconnect the charger.

One of the primary causes of loss of capacity of batteries is
sulfation of the lead plates; Sulfation happens when the battery
is allowed to sit around in a partially or totally discharged
state for long periods; sulfation is minimised by keeping the
Specific Gravity of the acid high (fully charged) see:

http://www.sierrasolar.com/design/b_leadbattery.htm

The acid in a stationary battery (not being sloshed around in a
car, boat, motorhome, airplane) will stratify, meaning that the
Specific gravity at the top of the battery will be much less
that at the bottom. Due to low SG at the top, the top parts of
the plates will sulfate first, reducing the capacity and cold
cranking amperage of the battery.

The solution to stratification is to mechanically stir the acid,
which happens normally if you take your battery for a ride in
the car, or for a flight in the airplane. However, if the
airplane/car is parked for six months, what then?

A good substitute for mechanical agitation is to periodically
(once every two weeks or so) connect the battery to a charger
whose voltage is set to 14.4+ V for a few hours. In the industry
this is sometimes called "equalization", but the primary benefit
comes from charging the battery hard enough to evolve gas
(hydrogen at one plate, oxygen at the other). The bubbles rise
through the acid, stirring it as they go, mixing the weaker acid
with the stronger.

So, for total automatic unattended maintenance of batteries, the
charger has to float at 13.5V continuously, and every two weeks
or so, it needs to bump up the voltage to about 14.4V while
delivering a few amps for a few hours, and then revert back to
the float mode. If the aircraft is flown regularly, then you
dont have to bother with equalization.

Is it feasible to leave the master switch "on" to activate the
solenoid and charge/maintain the battery thru the cigar
lighter circuit?
Do you think this might create any unforeseen
problems such as overheating the solenoid if left unattended
for several days at a time etc. Or is this just a plain bad
idea?

Its just a bad idea. Overheating of the solenoid shouldn't be a
problem, because the solenoid is rated for it. However, your
charger has to deliver approximately 1.5A to the coil in the
solenoid, plus whatever unswitched loads there are in the aircraft (like
the fuel gauges)

Suppose someone unplugs your charger? Now the solenoid/gauges
will kill your battery. Its not hard to wire in a fused
connector which will allow direct access to the battery with the
master off.

Is it possible to overcharge a battery with today's low
amp chargers?

Yes.

You have to buy a charger specifically made for "float
charging", like the BatteryTender

http://www.batterytender.com/

If in doubt, connect your charger to a battery for 6 to 12
hours. Use an accurate digital voltmeter to measure the voltage
between the battery terminals with the charger still charging.
If the measured battery voltage after a few hours exceeds 13.6V,
the charger is not suitable for long term float charging.

In summary, if all you own is the typical Sears 10A charger with
the time clock, then while the aircraft/motor home/boat is not
being used, hook the charger to the battery with the time clock
set to 6 hours. Go back and do it each month. That is the best
you can do to prevent the battery from loosing capacity while
inactive. During the six hours, the charger will put back the
self discharge, and then it will bubble the battery just enough
to stir the acid. This regemin is way better than just letting
the battery sit idle for six months.

MikeM (Batteries r us)
Pacer '00Z
Skylane '1MM