About 3 months ago, I purchased a deep cycle marine battery (12v) from Costco. It probably weighs 60 pounds. The application I intended for it is slowly proceeding, and I haven't used the battery at all. Any suggestions for charging it? I'll check the voltage tomorrow. It may not need it yet.
That reminds me, the June issue of Consumer Reports has an article about using an inverter with perhaps such a battery to keep the refrigerator, sump pump, lights and electronic chargers. They tested two inverters, Sears and WallMart. The arrangement is not going to be as useful as a generator, but it could be handy for short term power outages. One could use a car battery to act somewhat as a generator.
Charge it like any other lead-acid battery. It shouldn't need charging after only three months but it's not a good idea to leave L-A batteries to self-discharge. They prefer a float charge.
This is often done for sump pumps (a DC powered pump is a better idea), where a few hours can mean the difference between a dry and flooded basement. I don't think it's of much use for a refrigerator, though. With the door closed, they should stay cold for 24hrs or so, anyway. A car-battery-sized deep-cycle battery has a useful capacity of perhaps 50AH, so that's only
600WH. If you do this, be sure the inverter cuts out early enough to save the battery. They don't like to be run down to zero. A sump pump is a different deal because a flooded basement is worse than a ruined battery.
A typical car battery would be 80AH. That's 4 amps for 20 hours, not
80 amps for one hour. As the load increases (more amps) the Peukert factor comes into play and there are diminishing returns. Don't expect to get the battery's rated AH capacity with a large load.
There are also the losses associated with excessive discharge of lead-acid batteries. For maximum life, the battery should never be discharged below 50%. That means your 8AH battery can only deliver
40AH and have reasonable life (perhaps 3-5 years). You're now down to
4 amps for 10 hours. At a real-world load of say 5 amps to power a 60 watt laptop using a DC-DC converter to convert 12 volts to the 17-19 volts most laptops require (5 amps * 12 volts = 60 watts), the 80AH rating drops to 75AH, so the 50% level is about 37AH which would give a little over 7 hours (37AH/5amps = 7.4 hours). When you add an inverter to get AC, you're losing some power to the inefficiency of the inverter (depending on design, type, and load efficiencies can run 80-95%). For an example, I'll use 90%. Say you have an AC device that uses 400 watts. With a 90% efficient inverter, you would need 400/.9 = 444 watts from the battery. 444/12 = 37 amps. At that load, the battery's AH rating drops to about 40AH, so the 50% level is 20AH. 20AH/37amps = .54 hours or about 32 minutes. That's not much time for whatever appliance you were thinking about powering. Bigger loads (fridge, etc) require proportionally more current from the battery and deplete it even faster. There's a reason alternate energy homes have battery bans rated in the hundreds of amp-hours.
There is some rounding in the numbers, but they are realistic. I don't have a solar farm, just spent some time doing the calculations. If I cut enough trees to provide a full day of sun on the roof, I'd need to double the size of the solar farm to handle the additional heat load. The cost of tree removal plus doubling the number of solar panels keeps the project from being an economically sound propsition.
Want to do your own calculations? There's an Excel spreadsheet here:
I've had a motorcycle battery hooked up to an Optimate maintenance charger for about 3 years - it gets to deliver cranking current every blue moon when the one in the motorcycle is found to need charging.
There are various competing brands - do a web search.
That's good for an open battery, though it can be hard to tell.
A couple amp charger should do it. Don't leave it on all the time, though. A few hours a week should be fine. If you want to do a good job of keeping it topped off, put it on a cheap timer and charge it an hour a day. It'll probably last ten years float charging like that.
The chemistry is the same. The difference is only in the geometry of the plates. Deep cycle batteries have heavier plates so they don't warp (and short) during a deep discharge. "Starter" batteries have thinner plates, thus more surface area, given the same amount of lead, to deliver more current to the starter. Deep discharging them will warp the plates and they're toast. Neither likes to be completely discharged or self-discharged.