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Email This Page to a FriendPreview: Boat Wiring, Part 1 – How Lead Acid Batteries Work – Wet Cell, Gel Cell, Absorbed Glass Mat
November 15, 2012
Electrical systems on boats can be scary, mysterious stuff. It's little wonder that batteries come in black boxes. In this video series, we asked Professor Don Eley to show us what we need to know to untangle our own boat wiring. We're starting here in Part 1 with the power source... batteries.
Professor Eley gives us a full walk through of lead acid batteries and how they work, including Wet Cell, Gel Cell and Absorbed Glass Mat technologies.
– [Male Host] For most of us, electricity on boats is scary, mysterious stuff. It’s little wonder, marine batteries come in black boxes. This stuff can be very intimidating, even for the very experienced. So we asked Don Eley, long-time expert, and professor of Maine Maritime Academy to show us what we need to know. So we have the confidence to untangle the electrical systems on our own boats. In this series, we’ll walk you through a simple, two-battery system. And, we’re going to start at the power source: batteries.
– Today, what I’d like to do is look at batteries. And, the typical battery that we find on board most vessels is, what’s called a lead acid battery. Meaning that it works with both a chemical reaction with lead and acid. And what we have here today is a cut-away of a typical 12-volt battery. We have six individual cells here. Each one of these cells produces about 2.2 volts of electricity. Combined, it’s actually a total of 13.2 volts. We call it a 12-volt battery, but in reality, it’s actually 13.2. The way this operates is this is typically filled with sulfuric acid. And, we’ve got two plates here. One has got lead on it and the other plate has lead dioxide on it. And these are little separators between those plates. And we’ve got a chemical reaction that occurs between the lead and the lead dioxide and the sulfuric acid and that produces electrons, or electrical pressure: voltage Now, this is called a flooded battery. Meaning that it’s flooded with liquid sulfuric acid. There’s also a couple of other ways that we can build lead acid batteries. One is called a gel cell where instead of having the sulfuric acid in a liquid form, we have it in a gel form. But the construction of the battery itself, internally, is very similar. In that we have the two plates and the sulfuric acid in a gel form for that chemical reaction. The third type of battery which is most popular, I guess now, or newest on the market now, is what we call a Absorbed Glass Mat battery, or AGM battery. Now the neat thing about the AGM battery is, instead of having the sulfuric acid either in a liquid or a gel form here, it’s actually in plates or on fiberglass mat between the plates. So here’s an Absorbed Glass Mat battery. And it’s perfectly sealed. We never have to get in to it for any reason. And this battery can be mounted in any position. We can mount it on its side, we can actually mount it upside-down, and that’s a pretty handy thing to be able to have a battery you can mount in any position that we would like it to be mounted in. So, let’s talk a little about the chemistry that takes place inside the battery. And again, whether it’s wet cell, gel cell, or AGM, the chemistry internally is the same. So, here’s what happens. We’ve got lead. Pb. And we’ve got lead dioxide. PbO2. And we have sulfuric acid which is H2SO4. So, when we have those in combination in the battery itself, we wind up with the two posts here, a positive and a negative post. And, then, when we obviously connect a load to it, this chemical reaction is going to take place. And what we wind up with then when this chemical reaction happens is we wind up with… PbSO4, which is lead sulfate, plus water, H2O. And in a simplistic fashion, we also then wind up with electrons. And this is where we get our electricity from this, from this chemical reaction. Now, the challenge is, that in a wet cell battery, or a flooded battery like this, when this chemical reaction takes place, the water and the sulfuric acid tend to stratify. So, the water is lighter than the sulfuric acid. It’s going to come up to the top of the cell here, the sulfuric acid is going to be down at the bottom and that’s going to give us less area of these plates that the chemical reaction can actually occur in. So there’s some challenges there with a flooded or a wet cell battery with this stratification and then the area that this chemical reaction can actually take place on. So that’s why they came up with gel cell batteries. In a gel cell battery, instead of having the sulfuric acid in a liquid form that could stratify, it was in a gel form and would not stratify. There were some challenges, well there are some challenges with a gel cell battery also and one of those is, if you overcharge it, you can actually break that gel down, create a pressure inside the battery cell here and then pop the vents off, and then the battery is no longer usable. So, there are some challenges around the gel cell battery with the charging. So the latest chemistry is the AGM battery, the Absorbed Glass Mat battery. Completely sealed and those… the glass mat that holds the sulfuric acid are actually similar to these dividers that we see here. And it’s actually fiberglass that’s impregnated with the sulfuric acid. And so when this reaction takes place, the only thing that happens is the reaction between the plate and the sulfuric acid directly connected to that, the plate and the absorbed glass mat. So, we don’t have this stratification that we talked about between the sulfuric acid and the water. It also produces a more efficient charge and discharge and the battery is completely sealed. And so we don’t have any venting going on. You know, we can mount it in any position and it turns out to be a pretty handy thing aboard a vessel, not having to maintain it. As opposed to our traditional flooded battery, most of you may be aware that, if the level goes down inside the flooded battery, then we would add just pure water to it. Usually distilled water to the battery. That’s the only thing we lose in this chemical reaction is H20 typically. And so, that’s all we want to add back to the battery. But, an Absorbed Glass Mat battery or a gel cell battery, both of those eliminate the need for any of that servicing of the battery. In fact, they’re both completely sealed. So, the other thing I’d like to talk about is battery size. And, basically there’s a couple things we need to understand about these batteries. One is, for an engine start battery, typically what we want to do is produce a lot of electricity a lot of electrons in a short period of time. The other type of battery is what we call a house battery or a deep-cycle battery. And that’s a battery we’re going to use smaller amounts of electricity for longer periods of time. For 10, 12, 24 hours. Whatever we need it to do before we recharge it. So, when we’re looking at batteries, it’s not just simply the physical size of the battery, we need to look at whether the battery was designed for engine starting or whether the batter was designed for deep-cycle use. And, physically, the size of the battery is not necessarily going to determine its voltage. It’s going to determine its capacity. Alright, so this battery here was originally a battery designed for engine starting. And if we look at the label on here, it actually has CCA 800 and MCA 1000. And what that’s telling us is that the Cold Cranking Amp capacity. The ability of this battery to produce amperage. It can produce 800 amps at actually zero degrees Fahrenheit. That’s what that Cold Cranking Amp rating is for. So, it can produce 800 amps for 30 seconds. So the hope is that you’d be able to start the engine within that period of time. And the engine manufacturer is going to determine what Cold Cranking Amp capacity is required of the battery. Now, most boats aren’t operating at zero degrees Fahrenheit, so there is a Marine Cranking Amp rating on here also. That’s at 32 degrees Fahrenhite. The battery is more efficient as it gets warmer. And this would be able to produce a thousand amps for 30 seconds at 32 degrees Fahrenhite. So, depending on the application of the battery and what the engine manufacturer is looking for, that’s the capacity that we’d be looking for in this particular battery. So that’s an engine start battery. On the other hand, this battery here is designed more for deep-cycle use. This would be a house battery. We don’t see a rating on this particular battery for Cold Cranking Amps, but we do have a rating on this particular battery that says 32 Amp Hours at 20 Hours. And, essentially, that’s telling us how many amps this can produce for 20 hours, times 20. So, 32. It looks like it’s producing a little more than one and a half amps for 20 hours is the capacity rating on this particular battery for deep-cycle use. And that’s what that design is for. So, essentially, we have two styles of batteries. We have engine start and deep-cycle. The chemistry’s the same but the structure internally is different in the number of plates and the thickness of plates. Even though they’re both 12-volt batteries, they’re designed for two different operations.