Charging accessory battery before combiner

[Tinman]

I need to trickle charge a 12 volt 14 amp/hr AGM battery before the combiner box and very close to an array. The array is series wired so the voltage is quite a bit higher. Without making the impractical possible, there is no way to use a 12 volt panel to charge this battery, I'm hoping to somehow bleed off power from the array.

The array is made up if 3 12 volt panels in series, each panel is:

Voltage at Maximum Power Point Vmpp
17.7 V
Current at Maximum Power Point Impp
5.70 A
Open Circuit Voltage Voc
21.2 V
Short Circuit Current Isc
6.10 A

I can't add a small accessory panel to do the job because of space considerations.

The battery would be used in float service.

Any recommended ways to keep this battery charged?

[Westbranch]

what loads do you expect on this agm? frequency?

[Tinman]

1-3 amps at 12 volts, for 1-2 seconds. 5-10 times per day. Basically I just want to maintain the charge since it doesn't have enough load on it to ever get to a bulk or even absorb stage.

[niel]

for only float charging this would look to be about 1-2% of the battery ah rating. to be safe lean toward the 2% area. .02 x 14ah = .28a with a vmp of say 17.5v for 4.9w so about a 5w pv would be my guess.

[Westbranch]

what about sacrificing the battery, period with replacement every 6 months or? however long it takes to kill it?
Otherwise all i can think of is a cheapo  1.8W ~ 12 inch x 3 inch panel...like this http://www.canadiantire.ca/AST/browse/Green/EnergyConservation/RenewableEnergy/PRD~0112001P/Blue+Planet+1.8W+Solar+Battery+Maintainer.jsp?locale=en

[Tinman]

A bit more info which might make a difference.

Battery (insulated)  is located on a metal frame to which the array negative is connected. The array negative then goes to combine via wire. Array positive is to combiner. (Midnight MPV6)

The battery and frame are physically disconnected from each other.

Can I use the frame as a common negative for the battery to be charged? In other words, both charger and battery would use the metal frame for connectivity.

Positive from Charger and battery would then be using the metal frame for negative along with negative from array. Positive from charger and battery are physically separated from array positive.

It is impossible to to run wire from charger to battery. Battery is not being charged by array but via 12 volt panel also separate from array.