Here's a great 20 Amp battery charger/maintainer that can be left on indefinitely. It's internal circuitry monitors charging voltage and maintains a safe voltage.

IMHO, the Guest Marine Model 2620 (Now made under Marinco Model #28220, see more detail below) is an excellent 24v choice for maintaining your batteries. It will not boil or overcharge your battery and has a 20 amp capacity (10 amps per battery). And it's a great battery assist for doing gear swings. See videos of my gear swings with the Model 2620A Charger/Maintainer by scrolling down.

I had been using my Model 2620 for nearly 6 years before it died. I was so impressed with this unit that I replaced it with their Model 2620A. As of April 2013, I have about 8 years behind these Guest Marine units keeping my Concorde batteries in top maintained shape. By the way, my Concorde batteries are still going strong!

UPDATE: 12/2013 - My CB25XC batteries have been retired after almost 9 years of service. Their positive terminals swelled and I felt that was a sign that they had given all they could give.

Check out the article  HERE on why AGM battery technology is the way to go.

Here is the verbatim guidance from AC23-27, page 12, regarding batteries: "BATTERIES. If one type of series 35 battery (e.g. a Gill 35) is approved as original equipment or a PMA part, and another type of series 35 battery (e.g. a Concord 35) is PMA approved for some aircraft models but not your model aircraft, you may install the alternate type of series 35 battery and document the installation with only a logbook entry. This applies to all series 35 batteries. This philosophy also applies to other series, such as series 25 batteries. However, this part substitution is permissible only as long as the batteries are the same weight, within plus or minus one pound, and have such similar physical characteristics as to enable the use of the same securing or attaching devices, mechanisms, or containers.

Approval: This is a minor alteration and you may document it by a logbook entry. The logbook entry must reference the original and replacement manufacturer's identification or the performance specifications of the original and replacement battery."

With the new Part 23 rules AC23-27 (signed into law by Congress and the President) are enacted by the FAA some may choose to use the ETX30L or YTX30L-BS AGM technology battery options @ $105 or $80, respectively, w/Free Shipping and a capacity of 26 Ah or 30 Ah, respectively, and 400 or 385 Cold Cranking Amps, respectively. With weights of about 23 or 22 pounds, respectively. Read about AGM basics HERE.

Dimensionally, either will fit where a Concorde RG25 or Gil G25 battery would go with some wood spacers to take up a little of the slack. See the Gil vs. Concorde weight comparison HERE.

ETX30L Dimensions: Length: 6.65" x Width: 5.16" x Height: 6.89"

Here's another AGM candidate that some may choose to use the MTX30L AGM technology battery option @ an incredible $95 w/Free Ground Shipping from Battery Mart with great 30 Ah, and a whopping 420 Cold Cranking Amps performance specs and a weight of about 24 pounds. However, NOT made in the USA. Read about AGM basics HERE.

MTX30L Dimensions: Length: 6.55" x Width: 5.16" x Height: 6.89"

Here's a phenomenal AGM candidate that some might consider, the Chrome Battery YTX30L -BS @ an incredible $80 w/Free Ground Shipping from Chrome Battery with a great 30 Ah capacity and a whopping 385 Cold Cranking Amps performance with a weight of 22 pounds (PS: this is within the +/- 1 pound limit of the original Gil G25). Read about AGM basics HERE.

YTX30L-BS Dimensions: Length: 6.54" x Width: 4.96" x Height: 6.89"

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WOW! Check out this 35AH AGM battery @ $79 35AH "Chrome Battery"

Dimensions: Length: 7.68" x Width: 5.12" x Height: 6.46"

Weight: 22 lbs

See the, now discontinued, Guest 2620A powering my roughly 4 second gear swings below:

In my opinion, the Guest is a good modest priced choice for augmenting your battery during gear swings (because the amount of free play in the hand crank is very dependent on system voltage) or when at the avionics shop, or when doing heater testing on the ground or in cold weather climates for that extra "edge" in cranking power!

Here is the latest Guest Model 2720A, 20A Two Battery Bank unit, available for $163 on 11/29/2023 at Amazon

Here is another option popular with 28 volt Beechcraft owners, the Samlex Charger/Power Supply/Maintainer Model SEC-2425UL which offers 25Amps. See the Technical Specs HERE

12V 10-Amp and 24V 5-Amp 7-stage quick battery charger, charging speed is much faster than 8-Amp or 5-Amp automatic chargers. Charge or repair all 12-volt and 24volt lead-acid automotive, marine and deep-cycle batteries including AGM, GEL, SLA, Flooded in cars, trucks, SUVs, motorcycles, lawn mowers, marine, etc.