Jonyjoe101 - Goal Zero is out! I am going to use a 12 VDC battery and charge with solar panels. This past weekend I was looking for a battery and heard that I would need a marine deep cycle battery. I was looking at Sams Club and just looking at the batteries they have available the choices were any where from $60 to $200. What do I need? How do I choose? What is an agm battery?
Hi Leever2000, and apologies to Jonyjoe101 for answering a question posed to him/her. Essentially, a car battery is designed to deliver a lot of energy in a short period of time and that energy is advertised in CCA (Cold Cranking Amps) - this is how they tell you how much energy (power, actually...) the battery can send in a spurt to crank your vehicle. For RV-type use, you want a battery with thicker plates inside that can be drawn down deeper than a car battery can tolerate. Marine deep cycle batteries aren't true deep cycle, in the sense of an off-grid home, but they're better than a car battery for this use. Generally, if a battery advertises a CCA rating it's not an ideal deep-discharge battery. But, they can work for budget systems. A true deep discharge battery will show an amp-hour rating, which is the energy it can store (amps over a period of time). You really can't use more than 50% of that rating. For most of us, marine "deep cycle" batteries or dual purpose batteries work just fine and they don't cost a ton. If you want to go one step up, you can use two 6V golf cart batteries in series to give you 12V. Sams carries those.
An AGM (absorbed glass mat) battery is basically a standard flooded lead acid (FLA) battery with the addition of fiberglass mat between the plates. The electrolyte (acid solution) inside a regular FLA battery can spill. In an AGM battery, the mat is almost fully saturated with the electrolyte. Think of a sponge that's soaking wet, but not so wet that it drips. AGM batteries are "sealed" and, in theory, don't need the addition of distilled water. And, AGM batteries can be jostled or used on their sided without leaking acid.
Hope this helps!
Edit to clarify the above "power, actually" comment I made: I work in renewable energy and have taught system and battery bank design classes worldwide, including doing lots of village and commercial installations. So, I guess, this is my inner teacher coming out... sorry! But, I've seen lots of folks confuse power and energy (as used in these systems). Power is an instantaneous measurement and energy is power used or delivered over time. So, a heater or light bulb has a power consumption rating. Let's go with a 100W light bulb... that 100W (Watts being a power measurement, the result of multiplying volts times amps) being the power that bulb uses in any given instant. But, what we use is normally energy. We want our system to power that bulb for some period of time. So, a 100W bulb burning for one hour uses 100 Watt-hours. And, if you burn it for 10 hours, you use 1000 Watt-hours or 1kWh. That's what we buy from our utility company, kWhs (kilo-Watt-hours). But, I've seen lots of folks talk about how many kilo-Watts they use. kW is a power rating, which you use for sizing your inverter. But, for sizing a system to provide that power for some period of time, you need to talk in terms of energy. And, going up a few lines to the volts x amps = Watts comment, if you know how many Watts an appliance or bulb draws you can figure out how many amps you need to feed it... a 100W bulb in a 12V system is drawing 8.3 amps. Actually, the amperage will be a bit lower because your voltage should be higher than 12V. But, let's go with 12V for simplicity. So, if you want to run that bulb for 10 hours you need to feed it 8.3 amps for 10 hours, or 83 amp-hours. Assuming no losses for simplicity, you need a battery that has double that capacity of AH so that you don't draw the battery down below 50%. So, you'd need a battery bank that could store 166 amp-hours or more. I'm totally glossing over some other factors, like capacity changes at different temps and draw rate, but this gives you an idea. So, if you know the current (amp) draw and your voltage you can estimate the battery bank size you need pretty closely.
So, a battery that tells you a CCA rating is telling you how much power it can send out... but that doesn't tell you for how long, so you don't know how much energy (amp-hours) it can store.
