I could search around for multiple opinions, but I want some experienced answers. I have a deep-cycle marine battery I use to run radios off of, up to 100W output. I have a 5A, 13.8V power supply to provide supplimental power and charge the battery. However, the battery loses charge over a period of 2-3 days (down to 9V), so I think I need to float charge it. I have heard that you’re not supposed to float charge deep-cycle batteries, but I don’t want to have to worry about charging it every few days. So, 1) can I float-charge it, 2) do I need to buy a battery charger, or could I hook up a 12V/1A wall wart to float it, and 3) how often should I check the specific gravity? The cells have plenty of water.
(1) I’m not an expert on battery chemistry but I don’t see why not. A deep cycle battery just has thicker plates, which means less surface area (less cranking amps) but a longer discharge time. It’s still the same basic type of cell as a regular lead acid battery and should charge the same way. (2) You will need a charger that’s designed for this though. A wall wart isn’t going to do the trick. (3) Depends on the age of the batteries but I wouldn’t think you should need to check them all that often, as long as your float charger is capable of supplying enough current to keep the batteries topped off. If your charger doesn’t supply enough current to keep up with the load and you are discharging the batteries quite a bit then you’ll want to check the batteries for cell damage more often.
Here’s a description of the different types of battery charging systems, complete with cool pictures and stuff (warning, PDF): www.commutercars.com/downloads/ batteryCharging/PanasonicSLAHandbook.pdf
I used to work at a battery charger manufacturer, and I can say with authority, “Sure, you can float/trickle charge it.” There is a problem, though. Most wet batteries (car, boat, etc.) rely on the motion of the vehicle they ride in to “stir the juices.” You see, the acid and water will separate (to a degree) over time without some movement. This separation is called “stratification” for all you old timers that think the ground kills batteries (it’s actually stratification which is exacerbated by the temperature differential between the ground and the air). Any way, wet batteries which are typically in non-movement type installations normally will have special chargers that are designed to actually overcharge the batteries (occasionally). This overcharging causes the electrolyte to boil, which causes the juices to stir. So, with this in mind, any wet battery, deep cycle or not, can be trickle/float charged. The trick is to make sure it doesn’t stratify. (That’s a bad thing as crystals begin to form and they short the plates).
Also, you can use whatever you can get your hands on to float charge it as long as you stay under 14 volts. There’s an upper current limitation, too, but I seriously doubt your going to go over that. If you’re float charger won’t bring your battery up to 13.5 to 14 volts in a day, get a bigger charger. As a rule of thumb, always keep a battery at full charge if possible. That means, don’t let it run down to 9 volts for a couple days at a time. Deep cycle means you can discharge your battery to a lower level, normally under higher load, but it doesn’t mean you should leave it in that state for a long time. Bad battery juju.
Thanks for all the replies. The battery is there to provide backup power if and when the main power fails during a storm. It also acts as a reserve so that I can run 100 watts of radio power without damaging the power supply. I wasn’t aware of stratification, so I’ll come up with a way of mixing the cells (proabaly using a pipette squeegee).
