Deep-cycle lead-acid battery charging

Hi, everyone, I have a virtually unused (7 months old) Marine/RV battery that I use as the supply to a very infreqently used DC to AC inverter (12 VDC to
120 VAC) Thus, it sits permanently charged (intermittantly) and unloaded with the invertor OFF, or loaded at 0.6 amp with the invertor ON but unloaded on the AC side. However, I'd rather trickle charge it continuously: A) keep it charged and B) preserve it "forever" as a stand-by unit. Question: at what NET continuous charge current to the battery?
The battery is marked: MEGA-TRON Marine-RV Deep-cycle SRM-24 550 CCA 690 MCA SPR I don't know the small current amp.hour rating. Might it be 100 amp.hours? 200? Other? Size is: (LxDxH) 10" x 6 1/2" x 7 3/4"
My guess is that a 0.5 amp NET charge rate is too high (would dry it up?) but 10 mA is too low (less than the charge leakage?), but I'll admit I just don't know (I am not a lead-acid battery expert) but I can design a fixed NET current charger at any given level.
Any other tips for the use of a Marine/RV battery in this service?
Thanks for all replies. Cheers, Roger PS. Above email is false (anti-spam.) Feel free to email me at "analogdino at(ta-boy) rogers dot(ty-as-they-) com(e)", suitably decoded.
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Engineer wrote:

Experience tells me that it is not terribly critical. The A/Hr rating of a typical car battery sized deep cycle battery is somewhere around 80 AHr. I've used a small low voltage incandescent lamp in series with a surplus "wall wart" transformer that had an output of 18VDC as a maintenance charger. Something like an LM317 wired as a constant current source would work as well.
If you want to get fancy, there are automatic smart chargers out there, everything from schematics through kits to turnkey products if you do some searching. I've not yet had a problem with the wall wart solution though.
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In article

If I understand you correctly you want to permanently "float" your battery. This is common with a lot of standby power plant but it's not current you concern yourself with, it's voltage. You need a constant voltage regulated power supply set at (IIRC) 13.45V. This is the figure that gives maximum energy storage with maximum battery life. Whatever the battery size this will maintain charge with just sufficient current and avoid excessive "gassing"
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Thanks, an interesting idea... I'll admit I'd not thought of that. Self-regulating, of course,so that if there's a lot of use the charge current would be higher. From memory, 13.6 volts was a benchmark figure (not sure for what!) but the battery goes to over 14.5 volts after charging at an amp or two for a while. Choice of the fixed voltage would determine how much maintenance charging but you'd need a current limit in the charger so it does not overload if you pull over 20 amps to get 250 watts from the inverter - a small internal resistance in the charge circuit should solve that one. Cheers, Roger
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In article

When I built mine some years ago I used a standard voltage regulator circuit. A 723 voltage regulator IC with a 2N3055 pass transitor. I think there was a protection diode on the output to prevent reverse current flow when the supply was connected to the battery with no mains. The 723 circuit has the option for a current limit; you need to limit the current anyway to protect your transformer.
You need to check manufacturers data for the best voltage to work at and there are probably more modern circuits you could use.
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PS. The above has been confirmed by an EE friend who was in the battery business, i.e. the maintenance charge should be to voltage (13.6 and 13.8 VDC seem to be the contenders) with current limiting on the float charger (as above, to protect the charger.) Long term charge current will then converge to the self-discharge current of the battery, perhaps around 10 mA for a typical 100 AH deep cycle battery (actual self-discharge current not checked yet.) Regular fast charge can be at a higher fixed voltage, say 14.5 VDC, with a "several amps" charge rate being fine, but not held beyond the full-charge state. Or just charge at "several amps" until fully charged. Thanks for all replies, most helpful. Cheers, Roger
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Wayback when I was a boy we used to use 1mA per amp hour fixed current if thats any help. It seemed to keep the battery up and water use was minimal.
Regards ............... Rheilly P
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