Explore the mystery of batteries!

I don't think any piece of equipment causes more heartache or bafflement than the battery on a boat, but perhaps we can unravel some of its mysteries here and it may prove useful at some later date. How to come to terms with the biggest problem of modern boating, that is that when we take the boat out to get away from it all, we want to take it all with us. Conveniences that is. Well the first part of the dilemma is that living on land we have access to almost unlimited energy at an extraordinarily cheap price.

Energy management
But stay out on the boat for a few days and the picture starts to change. Batteries go flat, generators are run for hours but the batteries never seem to come up to charge. Normally placid people become tyrants, nagging the long- suffering crew about lights and waste. Misery sets in, dissension in the ranks, even mutiny! Well it doesn't have to be like that. Energy management is the key, and for that to be effective an understanding of battery fundamentals is necessary, or "How many slices of toast can a battery make?"

All about "House" batteries
So what is a battery? Well, w'e all know that they are medium sized boxes, very heavy, mostly black, and most of us have at least a couple of them in the engine room. Generally we have them arranged in two banks, one for cranking the engines, and the other for the "house" , and it's the house batteries that I want to talk about. Batteries are boxes, we've agreed to that, and into these boxes we tip energy. How they manage to do this we'll leave for another day. Sufficient for this article that they can achieve this. But there is more that it would be beneficial to understand. The way a battery is manufactured affects the way in which it stores this energy, so let's take the top off one and have a look.

A look inside your battery
The first thing we would notice is that the box was packed with plates. Exactly like you would pack a box of dinner plates, except that these are square and flat and all standing vertical like a deck of cards. The next thing our observations would reveal is that the plates are arranged in sections, the same as the deck of cards would be arranged if all the spades were together, and all the hearts and so forth, except that there would be six sections and these would be linked together with heavy lead bars. Now each section of these plates can produce approximately two volts and so together this adds up to twelve volts, which is a familiar value to us all.

Fat and thin plates
Depending on the use that the manufacturer designed his battery for, these plates can be either thick or thin. Thin plates can give very high levels of energy for a very short period of time, exactly what we want when we crank an engine. Fat plates are good at delivering modest energy over a longer time span, and are much more efficient at this than the thin plate design, so this exactly fits the bill for house batteries. ßut beware, and this is the point of looking inside our battery; try and hurry things along by demanding large amounts of energy for a long period from our fat plated friend, and rebellion will be in the wind. That rebellion will be heat, and heat is the absolute enemy of our battery. Heat will cause our plates to expand beyond their design capacity, they will buckle, they will warp and they will die!

How much energy in the black box?
Put the lid back on the battery and let's have a closer look at the box. I mean, this is starting to get interesting. For the first time we notice a label on the battery, and one of the things this might say is "100 AH". This figure is an attempt by the manufacturer to teIl you how much energy can be stored in his black box, and this we need to know for efficient energy management. For lots of reasons - age, advertising agencies, spin doctors, snake oil salesmen and just good old fashioned lying, take seventy per cent of this figure and use this as the capacity of the battery which brings us back to 70 AH. What this means is that this box of energy can deliver 70 amps for one hour at twelve volts or seven amps for ten hours, the latter is called the ten hour rate, more of this later. Amps are a unit of electrical flow, volts are the measurement of pressure. So an analogy would be: if this were a water tank, seventy litres for one hour or seven litres for ten at a fixed pressure.

Watt: the energy reserve unit
If I didn't mention it before, the unit of power is the watt, and amps (flow) multiplied by pressure (volts) gives us watts. So our battery we have been discussing has an energy reserve of 70 x 12 which equals 840 watts. Or, if we were to take the ten hour rate, 7 x 12 x 10 which also equals 840 watts. Now because there are losses, and because the conversion of chemical energy to electrical energy, which is exactly what is occurring inside the battery, is a time-dependent thing, we are much more likely to extract 840 watts of stored energy at the ten hour rate than at the one hour. The rate of discharge is important, as we shall see later.

Have a toast!
Let's leave the battery to its restful place in the engine room and go to the galley. Since the advent of high output inverters, we can now run all those household conveniences we bring on board, or can we? Let's take a closer look. Grab the toaster and look at the label on the bottom which we've never bothered with before. Somewhere on this piece of information will be a figure with a big W after it. Mine says 800 W What this means is that if you leave this toaster on for an hour it will dissipate 800 W of energy, or 13.3 watts per minute. Plug in the toaster, make two slices of toast and time how long it takes. For me, it is exactly three minutes. So the total energy to make that toast was 13.3 times 3 which equals which is 40 watts approximately. This means that our battery has the potential to make two slices of toast every three minutes using 40 watts each time, until it is completely discharged. We know that the total amount of energy stored in the battery is 840 watts, so the battery can make twenty-one lots of toast. (840 divided by 40).

The discharge rate
Possibly. Unfortunately, a lot of other factors come into the equation, which nibble away at the efficiency of the toaster and the battery. One is how continuous is the demand on the battery's capacity. Remember we said that the rate of discharge affected the efficiency of the battery? Well for house batteries with fat plates, the rate of discharge should not exceed ten per cent of the total capacity. We have already established that capacity as 70 amp hours, so it would seem that the discharge limit is seven amps. If we go too much above that then heat will build up inside the plates and irreversible damage can result.

Don't kill your battery
Don't be surprised at this - everything has design limits. You couldn't expect to operate engines at maximum RPM all the time, so neither can you work a battery too hard. When making the toast, the battery is delivering 66 amps for three minutes, nearly ten times the recommended discharge rate! For the three minute burst it can handle that OK, but it will benefit from a rest for a while. If we continued to make toast willy nilly, the battery would suffer internal damage from heat and have a short life. It would still look the same on the outside. All that you would notice is that it would seem to go flat very quickly and never seem to charge up. The fact is you probably killed the poor thing the morning when you made all that toast that didn't get eaten.

Source: the Riviera Magazine, Issue 17.