Re: How to wire switch so I can charge 2nd battery from engine?
OK, back to the discussion. I will answer your question "thom- i run my boat in the both position. when it get to anchor and turn off the engine, i switch to position 1. before i start the engine again, i switch back to both and head home." in the last paragraph - so if you want to avoid my long-winded nonsense you can jump to the bottom. It might be the smart thing to do just for your sanity.<br /><br /><br />First lets think about what sort of boat we are talking about here, because it matters. I am not talking about a cruising sailboat with its solar array and days on end of rationed power consumption (the guys and gals who do that sort of thing tend to pay tremendous attention to the cataloging of power use that I described above) and minimal engine running for recharging. I'm not talking about the guy with a bass boat and a half a dozen battery and configurations of chargers to run trolling motors and such. I'm not talking about large house boats or the mega yacht Sportfishermen with their Killowatt generators. What I'm talking about here is the common day boat, maybe used for fishing, maybe for sking, maybe just for family cruises. Now and then it gets used for over night fishing or partying, mostly its a nice weekend boat. It might be on a trailer or it might be in the water much of the time, but its not a large boat, probably less than 28 feet or so. This is my boat, it is the boat I'm talking about, its probably the boat most of us have.<br /><br />OK, that's the background. Now, what is a perfect world for that boat?<br /><br />Well, it will have a battery who's size is such that it can start the engine. That is the most important thing it can do. Next it will act as a reserviour to store and release power when your ability to recharge is less than your demand for electricity. Of course you have to replace what you use in the recharging process but with our day boat, with everything working as it should, that is rarely a problem as long as the battery that we use is of large enough capacity in the first place. I'm going to get back to capacitys in a bit but for now keep that size question in mind. <br /><br />Now, what happens with such a boat on a perect day? Well, you start the day with a battery that is at 100% of its possible charge. Now no used battery has its full capability forever but go ahead and assume it does. When you start the engine it draws down some of that 100% power but in truth its very little. Everyone talkes about the demands of starting their engine as if it required a lightening strike. Even with todays high pressure injection systems it still doesn't take all that much to start an engine, a lot of amps in a hurry and good strong voltage but in truth you don't crank an engine more than for a few seconds (you do keep your engine tuned don't you?) and the power that is used in that startig process is replaced by the engine's alternator within mere minutes.<br /><br />So, you start the engine and get under way. Within a few minutes the power removed to start the engine has been replaced. You've got some equipment running but that's OK because the alternator's output is great enough that it not only recharges the little bit you took out of the battery but it also runs the engine and the electronic device you've got pumping back there. Now you stop to fish for a couple of hours or to watch the kidds splash around back by the swim platform while the CD player tells the world just what your taste in music is like. This is using up battery power, as discussed above, but the capacity of your battery is great enough that the depletion is not great compared to the reserve available. So the voltage at the battery doesn't drop much (look at the voltage numbers I gave you above for different states of discharge) because of your couple of hours of sitting and enjoying the boat. Now you decide to call it a day. You hit the key and because all is well with your connections and your battery and your engine it starts right up. Immediately the voltage regulation part of your charging systems notices that the power supply is low and it starts charging away. Depending on how much power you used, what the output of your alternator is, and how long you run the engine, it will, with luck, be fully charged by the time you return to the dock or trailer. This is a perfect world.<br /><br />Now, lets put a damper in our perfect world, let's 'make it real'. Let's say that you are a worry-wart and are constantly afraid of being stranded out on the water because of a dead battery. Maybe you did the arithmatic and felt that you were using a little more of the battery's power than you could replace with your run out and back or maybe you just don't trust your old battery or charging system. What would be the simplest thing you could do to avoid that stranding you are concerned about?<br /><br />Well, that's an easy one. Just bring alone a spare, and fully charged battery. Put it up in the cabin or under a seat somewhere and leave it alone. If you need it you can drag it out, hook up the jumpers and be on your way. I know guys who actually take the battery out of their tow vehicle and toss it in the boat for just this reason. It works.<br /><br />What are the advantages to doing this spare battery trick? The answer should be obvious. The advantage is that no matter what you have a good battery to do the most important thing, get that engine started and get you back home. The integrity of that batter has not been placed in peril because its not hooked up to anything. What I'm saying is that the spare battery under the seat, presuming it was good and fully charged when you tossed it into the boat, will stay fully chared no matter how terrible the corrosion on your other battery's terminals, no matter if your alternator has given up the ghost or not, no matter what, its going to stay a good battery ready to get you home.<br /><br />Now lets look at a boat with the best wireing system, one in which two idential batterys are wired into a standard multip point battery swithc in which you may select one or the other batter, select the use of both batterys at once, or turn off the supply of power completely (which is a safety measure to avoid fire that EVERY BOAT NEEDS) to the electrical system. <br /><br />With the 2-battery system you can run the boat off of either battery independently or you may use them in combination. <br /><br />If you use them in combiation the voltage available to the system will be roughly the average of the voltages of each battery (not exactly but I'm not about to go into that here) and the amperage that is available to the system will be the sum of the amperages of both batterys. What I mean by that is if you have one battery who's age and state of charge gives it 12.3 volts and 95 amp hours of capacity available and the other (lets say its the same battery but a year newer) had 12.5 volts and 105 amp hours capacity left. So you put the switch in the BOTH position and now you have about 12.4 volts available (the average of 12.3 and 12.5) and the total amperage available to you is 200 amp hours, the combination of 95 out of one battery and 105 out of the other). There is nothing at all wrong with this - not one blessed thing. However you need to understand something here. What you have just done is make those two smaller batterys into one larger battery with the flip of a switch. Where is your spare? What is your equivelent to the one tossed under the seat? You don't have one. So, now what is the effect if for some reason your alternator crapped out on you? Well, back it up. First off the power you used when you started the engine wasn't replaced while you were running. Next the power you used when you stopped had no way to be replaced. After a day on the water, CD player or baitwell pump doing its thing, will there be enough power left in either battery, or even in both of them combined to restart the engine? Maybe. Usually actually. So you bring it back to the dock or trailer and you clean up the boat, but this time when you got back the batterys were not fully recharged from the alternator on the ride back. Now you have a boat sitting there with two batterys that are both partially discharged. <br /><br />Let me digress for a moment. Battery maintainence, or battery killers if you want to call it that. 1. The single worst thing you can do to a wet cell battery is leave it sit with low electrolite. 2. Next would be to leave it in a state of partial discharge, which would be accomplished by undercharging it or not charging it at all. 3. After that you can kill one very quickly by over charging one. 4. Heat is a battery killer too and is to be avoided. <br /><br />Now back to the boat that just came back with partially discharged batterys - 2 of them because someone felt it was a good idea to have that switch in the Both position at some time or another. Here we have a boat that has partially discharged batterys that are going to be left to sit for some time. See item 2, above. So the next time you go out on the water if you are lucky, or is that unlucky, it will start OK at the dock, might crank a little slow, you might notice or might not, but it will probably start and get you out there. Will it restart the engine after a couple of hours of Stevie Ray Vaughn? It might this time but it sure isn't going to the next time. Starting to see what I mean? Most battery failures don't take place the day the boat dies. Most of them started a trip or two before. What is really a shame is that between trips the battery, which were left in a partial state of discharge, are steadly going down hill (see item 2 again).<br /><br />So, tell me if this makes more sense. Remember, I start with good batterys. Today is the 22nd of August, I think. That is an even numbered day of the month. So I've decided that on days that I believe are even numbered I'll run my boat on battery 2. If I had thought today was the 21st I'd have run it on battery 1. I go out and start the engine, which my combined Starting/Deep-cycle has plenty of power to do, and then I got though my Perfect Day sequency that we started with. I have never put the switch in "Both" so if there is something wrong with my charging system, cables, terminals, or even with the battey itself I still have my second, fully charged battery that hasn't been touched by anything, still standing ready in reserve just as surely as the guy who took the battery out of his truck. The only difference between him and me is I don't have to break out the jumper cables, I can put mine into the system by simply flipping a lever on the switch.<br /><br />Is this starting to make sense? How about this, because I was only discharging my one battery when my system had a defect in it somewhere I will know a lot sooner that something is wrong because my engine may not start, which is OK because I still have my reserve battery. It will get me home and allow me to deal with the problem.<br /><br />OK, I said I'd get back to your question so here goes:<br /><br />"thom- i run my boat in the both position. when it get to anchor and turn off the engine, i switch to position 1. before i start the engine again, i switch back to both and head home."<br /><br />How do you know that when you ran out there your batterys were being recharged at all? You don't. So if they weren't charging you have arrived at your fishing spot with two batterys that are now in worse shape than when you left the dock. You then flip over to one or the other of the two, which you are now surely going to kill if they are already discharged and you are placing your hopes on the notion that you haven't discharged got the other battery discharged to a point where it won't start the engine either. This would not have been the case if you had run out one one battery and just left the switch on that position. Had you done that you would still have a second fresh battery ready available to get you home when you found out your first battery had gone dead.<br /><br />That dear friend is the reason you NEVER run the boat on the Both position. There is absolutly no reason to. You do not need to recharge both batterys if you only discharged one of them when you initially started the engine or when you subsequently use the battery to power our stuff while you were enjoying the boat. Your spare battery is in just as good a condition as when you left the dock. Alternate the battery you use by whatever scheme you choose and you'll be much better off.<br /><br />Whoops! This isn't the end. I forgot that I had promissed a fellow that I would mention something else here. Stick around if you like, it may be interesting to you.<br /><br />Let's go back to the "Both" position and look at charging for a moment. Remember that I said that in that position what you had effectively done was average one big battery out of two smaller ones? Well that's not exactly true. What you have done is average the power availabiltiy from two batterys into one. However the batterys themselves, although now wired together via the switch, act as one only from the power of view of how much power they can supply to the system. This does not mean that they will act as as concerns the ability of each of them, individually, to recharge.<br /><br />No two batterys in any boat have exactly the same output voltage nor the same capacity even if you bought them in the same place on the same day. It doesn't matter if you installed them at the same time, if they were in the same storage area (heat and moisture considerations), maintained the same way, or even discharged and rechared at the same time. There will always be some differences. Not only that but the differences will become greater over time.<br /><br />Now, think about this. Your power regulation system really only 'knows' one thing about your batterys. That is the system, inessense, measures voltage. If it senses that you have high voltage, because your battery is good, it puts out less power. If it senses low voltage it puts out more power until the voltage comes up at which time it then decreases its rate of charge. Now in truth, before someone comes in here and calls me a mindless idiot (people will do that to you on the Internet even if they don't know a damned thing about you), what the regulator senses is not actually the voltage but the resistance to voltage, but I'm going to gloss over that because it really doesn't matter.<br /><br />Go back to my numbers for the two batterys, the one that had 12.3 (95 amps available) and the one that had 12.5 (105 amps available) volts. Remember that as far as the charging systme is concerened its just dealing with one battery, but that the battery it thinks is on the end of the wire is not a 12.3 or a 12.5 battery, its a 12.4 battery. What does that tell you? Simple, that it is overcharging one battery and undercharging the other. Go back up the page a bit and see what the 2nd and 3rd worse things you can do to a battery are. You are doing them both whenever you run a battery switch in the "Both" position.<br /><br />OK, fingers are tired now. Time to quit for a bit. <br /><br />Let me say this - its is important. I am not an engineer, not by a very very long shot. There are thousands of guys around who know more about this than I do. I have fond out what I could and found what I have found out to seem to prove true. That does not mean that what I have told you is the gosphel of boat electroncis. Look around, ask around, see if you can find better information. More importantly do what I asked you to last night. Think! Think about your system and what is really going on. Think about the alternatives and think about your comfort level with you own understanding of the system. Then act in the way that makes the most sense to you - don't necessairly listen to me.<br /><br />Ready for questions.<br /><br />Thom
<br /><br />Someone asked the question how you know your battery is being charged when you are underway. Voltmeter. You can use anything from a $25 analog in-dash to a $125 digital voltmeter with high and low voltage alarms, etc. Add an el-cheapo toggle switch and you can monitor the voltage of either battery. Cool thing about the inexpensive analog meter (and toggle switch) is with identical batteries you can flip between Battery 1 and Battery 2 and have a graphic indication of where your voltage will be if you (a) equalize the two batteries, or (b) when you kick over the motor and start charging the batteries in parallel. With a digital voltmeter you have to think numbers and figure it out.