Odyssey makes a small 8AH version of their AGM batteries, the PC310 (http://www.odysseybatteries.com/battery/pc310.htm). It weighs nearly 6 lbs, however. Keeping it fully charged might be accomplished with a DC-DC converter.

Of course all this might add up to be a complicated Rube Goldberg contraption too, but I'm a gear and wirehead so I like these kind of things and would consider it for my own plane.

Neal - not sure why you feel the need for a DC-DC convertor to charge the PC310. Am I missing something here? This battery has seen a fair amount of successful use being charged straight from the buss, provided the buss is well regulated. Several local folks are using the PC680 as primary cranking battery, again, connected straight to the DC buss. So far they seem to be holding up well, despite seeing heavy thermal cycling with some pretty brutal winter weather.

Panasonic Battery Weight
 

I'm also thinking about using a dual screen GRT panel, and was wondering if anyone knows how much the Panasonic 7.2 AH sealed lead-acid battery weighs. It was mentioned on the first page of this post. The model number is LC-R127R2P.

Thanks,

We recommend the use of small sealed lead acid batteries for our systems (Enigma, Odyssey, Voyager, Explorer...).

The reason is experience with various types of batteries over many years. New type Lithium Polymer batteries are quite good and are not that volatile anymore but still require carefully matched, intelligent chargers and are not simple "fit and forget" solutions.

Our profile is based on intermittened use with short charging intervals (at perhaps less than ideal charge voltage) followed by long periods of inactivity. Also, very important, the battery may be exposed to heat and extreme cold while just sitting there doing nothing.

Once you take all factors into account (and that includes price), sealed lead acid, despite the age of the basic battery design, wins - it's hardly even a contest.

We favour external backup batteries for our instruments as this provides flexibility. You can choose desired capacity for your particular needs and also locate the battery where it can be of advantage (weight distribution) - we use a lot of our instruments in motor gliders...

Rainier
CEO MGL Avionics

Apologies for the late response here, but since the original purpose of the additional smaller battery was to provide a robust source of backup power for the EFIS/EMS, isolated from the ship's main bus, the classical way of keeping such a backup battery charged has historically been via using a simple diode to prevent anything else in the ship from drawing juice out of that battery in the event of total failure of the main battery and charging system. A diode in series with the line that charges the backup battery from the main bus will give a forward voltage drop and the backup battery may never reach a full charge, which will give you greatly reduced runtime and possibly even shorten the useful service life of certain batteries. A DC-DC converter with a regulated output can be employed in the charging circuit for the backup battery to keep it charged at a constant full recommended float charge voltage level, even when the main bus voltage sags too much.

Expensive solution? Yup.
Complicated? Yup.
Necessary? Hmmmmm, probably not, but personally if I was going thru the ordeal of adding a separate backup battery for my avionics and since I'm an electronics geek, I would be inclined to try to conjure up some kind of solution to keep the backup battery fully charged instead of "mostly charged".

A Schottky diode will give less of a forward voltage drop than a regular silicon diode.

Battery isolators like these: http://www.hellroaring.com/nodiode.php are also an interesting possibility.

Yes, you are correct, a Shottky diode is what you should use. This gets peak charge voltage to around 0.3V less than bus voltage (the current is very small when the battery reaches this level of charge, so the normal ~0.5V drop goes down somewhat).
This has very little effect on reaching a state of charge that is as near 100% as is possible with a typical gel cell.
Our built in chargers draw from incomming bus and decouple via two Shottkies in parallel (to inclease current capability also at low drop out voltage). This is then fed to a zero drop current limit switch mode supply which limits maximum charge current but also limits maximum battery charge voltage to around 14V (13.8 volt float) in case bus voltage is higher - a frequent occurance in many aircraft supply systems which tend to be regulated somewhat "rough".

Have a look at the data sheets when you buy a Shottly diode - they come in different drop out voltage vs current ratings. Some of them are not much better than your average silicon diode but others are quite impressive.

Rainier