Re: Battery Switch dilema.....
Here is the deal. There are basically two ways you can wire up the power system of a small, single engined dual-battery boat. The first, and maybe the most common (but certainly not the most desirable) is to use a typical automotive type "Starting" battery to get your engine running and then a "House" battery to run all of your electrical loads off of. The second (and far better for most boats) system has it wired with two identical batterys, usually hybird combined cycle starting/deep cycle, either of which can, and will, be used for the boat's entire electrical load.<br /><br />Now, as to the switch, its positons, and what you might want to be thining about when you use it, because you can use it any way you want, its just that some ways make more sense than others. The switch has two input lugs on its back and one output lug. The switch has 4 positions; Off, Battery 1, Battery 2, and Both. If its off you should not be getting power to anything on the boat (with just two exceptions, the float switche(s) for your bilge pump(s) and the memory for you music system and Flowscan, if you have them). If your boat is wired somehow that anything at all other than the bilge pump(s) come on when the switch is in the Off position you may safely assume your boat was wired by a moron and that you can't trust any of it.<br /><br />OK, each battery is connected to the switch independently by its own battery cable. If you have the switch set in Battery 1 postion the second battery is not conected in any way. If you have it in Battery 2 the same is true for the other battery. In the both position you are drawing from, and chargin to, both of the batterys at the same time and in exactly the same amount. By that I mean that if you are using more power than the engine is making it will come from both batterys equally and if your engine is making more power than it needs some of that may be used to charge both batterys - equally. That is in the Both position. Now here are the two basic problems there. The first and greatest probme is that by using the both positon you are absoltuly guaranteeing that one battery is gong to be over charged and one undercharged and the second thing is that if you are out on the water and the charging systmem fails your chances of being stranded are infinitly greater if you have the battery switch set in the both position. The simple reason is that if you have it in the both positon when out there running you will kill both batterys at the same time. So you run it, which requires almost no power, and then you stop to fish, sunbathe, or just take a leak, and when you go to restart the engine all you get is a click. Welcome to the world of running on both. Then there is the charging problem.<br /><br />Let's say you have a typical small starting battery, about what you'd find in a mid-sized SUV or plain or car. Its probably about a Group 26 or so and it probably has about 70 amp hour capacity with some fairly high cranking amps. Sitting beside it is probably a Group 27 battery of some type that is being used for a house battery, with its amp hour capacity of somewhere around 90~100. OK, here is a quick primer for you, the voltage of a battery is a function of its state of charge, so as you use up stored amps, as when running at idle speed or so with electronics turned on (most engine's alternators put out very little power at idle, although they all put out their maximum power by the time you hit somewhere aroud 1,500 RPM or so). The alternator's regulating scheme will adjust the output to match what it sees in the batterys, the lower the batterys the more of its available power it will send back into the cable. If you are running in the both position what is there for the engine to sense is the average voltage from both of the batterys. That means that as soon as you start your engine, using the "starting" battery that is is somewhat discharged. So you flip to the both positions and what you find is that the average voltage is decreased even though the house battery is still fully charged - so its gonna get charged some more, which is over charging, and the starting battery, which you no longer are using, is getting a nice slow charge. The starting battery recharges pretty quickly and all settles down to a nice even charge while you are running, but then you start fishing. Electroncis are running and the batterys are now being discharged, but not equally. While the power being drawn is comming from each battery equally the batterys themselves are not equal, with the starting battery being what you could think of as smaller (in capacity). Proportionatly the smaller starting battery is being discharged more than the house battery so t he decline in the average voltage in the boat is mostly attributable to it So when your chrank the engine back up it now, once again, run the engine up to speed and you over charge the house battery while undercharging the starting battery and at a rate that overcharges the house battery. This significantly decreases the life of the batterys, both of them.<br /><br />Keep this one thing in mind, you can do anything with a deep cycle battery you can do with a starting battery, but not the other way around.<br /><br />So what makes the most sense for most day boats with a single engine is two batterys of equal size and type wired such that each of them is capable of starting and running the boats entire systems. It is a very simple system and it relys on the use of just one battery at a time, holding the other completely in reserve. You pick which battery you are going to use in the morning and stick with it. If something happens to it you flip to the other one and go home and fix the problem. One battery at a time, either Battery 1 or Battery 2, but never both.<br /><br />By the way, I use a fool-proof systme for getting precisely the same use and charge out of each battery. When I go out in the boat I guess what the date is, it doesn't matter if I'm right or wrong. If its an even number day of the month I use battery 2, if its an odd number day I use Battery 1. If I get it wrong it all averages out over the 4 years or so I get out of my batterys.<br /><br />Thom
