I've been trying to figure out what the best way is to recharge my starting and house batteries while motoring for a significant amount of time. I obviously want ot get both fully charged up. Should I have the swithes to both batteries "on" at the same time? I wasn't sure if charging both batteries at the same time is a problem if say the house bank is is pretty low voltage and the starting battery is prtty full. Also, if I'm motoring for a long time is that bad for my batteries (I have flooded batteries) as I think this would overcharge them. I can see if that's the case that once the house bank is fully charged it would be better to overcharge the starting battery as it would be cheaper to replace than the house bank. Thanks!
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Battery charging
The way I do it is that I have two deep cycle batteries which are getting charged while motoring. Once the sails go up, I switch the battery switch from the All position to either battery 1 or battery 2. Prior to turning the engine back on, I place the battery switch back to the All position. This way I know that both batteries will be fully charged. Your concern about overcharging should not be an issue as your alternator should prevent overcharging. Hope this helps. Happy sailing!
The way I do it is that I have two deep cycle batteries which are getting charged while motoring. Once the sails go up, I switch the battery switch from the All position to either battery 1 or battery 2. Prior to turning the engine back on, I place the battery switch back to the All position. This way I know that both batteries will be fully charged. Your concern about overcharging should not be an issue as your alternator should prevent overcharging. Hope this helps. Happy sailing!
Bill w. Do you worry about this battery charging
problem with your car batteries. The voltage regulator limits the output voltage from the alernator and senses the battery voltage and charge rate. You can run a hundred miles in your car and not over charge the battery.
problem with your car batteries. The voltage regulator limits the output voltage from the alernator and senses the battery voltage and charge rate. You can run a hundred miles in your car and not over charge the battery.
Charging
Hi Bill, First you need to know if your battery switch is 'make before break' or 'break before make'. To test this, stop the engine, switch to battery 1, turn on the cabin lights. Now slowly switch to 'both' and then to battery 2. Do the cabin lights stay on, or do they go off and then back on? If they stay on the switch is 'make before break' and it's safe to turn from 1 to both to 2 when the engine is running (I assume you know to never turn the switch to 'off' while the engine is running). Now to answer your question, if the batteries have close to the same voltage it is fine to leave the switch on 'both' when the engine is on. If one of the batteries is much lower than other you should set the switch so that only that battery is charged. When it has been charged, you can get the switch back to both. If you don't to that you will overcharge the higher battery and undercharge the lower battery. Good luck, Barry
Hi Bill, First you need to know if your battery switch is 'make before break' or 'break before make'. To test this, stop the engine, switch to battery 1, turn on the cabin lights. Now slowly switch to 'both' and then to battery 2. Do the cabin lights stay on, or do they go off and then back on? If they stay on the switch is 'make before break' and it's safe to turn from 1 to both to 2 when the engine is running (I assume you know to never turn the switch to 'off' while the engine is running). Now to answer your question, if the batteries have close to the same voltage it is fine to leave the switch on 'both' when the engine is on. If one of the batteries is much lower than other you should set the switch so that only that battery is charged. When it has been charged, you can get the switch back to both. If you don't to that you will overcharge the higher battery and undercharge the lower battery. Good luck, Barry
Charging batteries
In the example of one battery being at a lower state of charge compared with the other when both are combined, you need not worry about over-charging . What happens in that case (when two unequally charged batteries are combined) is that the higher battery will begin to discharge into the lower one essentaily trying to create an equilibrium. When the alternator is charging both, each will receive an equivalent charge rate as the alternator can't differentiate between batteries. In other words, there is no need to recharge each seperately - as a practical matter, if, for some reason, you did want to recharge each seperately, you'd never know when the lower battery was up to the higher one. As Ross indicated, you can't overcharge regardless of what you do unless the alternator regulator is malfunctioning.
In the example of one battery being at a lower state of charge compared with the other when both are combined, you need not worry about over-charging . What happens in that case (when two unequally charged batteries are combined) is that the higher battery will begin to discharge into the lower one essentaily trying to create an equilibrium. When the alternator is charging both, each will receive an equivalent charge rate as the alternator can't differentiate between batteries. In other words, there is no need to recharge each seperately - as a practical matter, if, for some reason, you did want to recharge each seperately, you'd never know when the lower battery was up to the higher one. As Ross indicated, you can't overcharge regardless of what you do unless the alternator regulator is malfunctioning.
Doesn't battery type matter too?
Ex: I run on (1) 1 deep cycle for starting; and on (2) 4 golf carts for house. I never motor or even charge dockside on "all" for any period because of the mixed types (voltages?) Does this matter or is this just over caution?
Ex: I run on (1) 1 deep cycle for starting; and on (2) 4 golf carts for house. I never motor or even charge dockside on "all" for any period because of the mixed types (voltages?) Does this matter or is this just over caution?
It's doubtful you have a problem....
Flooded batteries don't actually 'overcharge'. Let's look at the charging process.... A charged battery has lead plates in a pool of sulphuric acid (H2SO4) mixed with water. H2SO4 has 2 'extra' electrons, so when you hook up a load to a battery, it breaks the bond into water (H2O) and sulphur dioxide (SO2). The SO4 is attracted to the plate and hands over its 2 electrons. As the amount of available acid in the battery decreases, so does the voltage, until the battery goes 'dead' at around 11.3 volts. The charging process involves adding electrons (from the alternator/charger) which entice the sulphur dioxide off the lead plate and to re-bond with the water, which gives us back our sulphuric acid (H2SO4). When ALL of the SO2 is off the plates, the battery is fully charged. The time it takes to get to fully charged is dependent on the amount of current that the battery can accept, as well as how much current your charger/alternator can put out. Continued charging won't 'overcharge' the battery, but if the voltage is high enough (above 14.3v), it will cause the water in the battery to 'boil' off over time - it's not really boiling, more like evaporation. This isn't going to happen in an hour or two or even 5 or 10 hours, (even if your battery is already fully charged) and there's another consideration. It takes a LONG time to get a flooded battery to FULL charge. Most 'smart' chargers use a voltage somewhere between 14.3 and 15.5 volts, because the last 10% of charge takes so long. If you have a 'dumb' alternator (one without a 'smart' regulator attached), it probably isn't putting out more than 14 volts - and 14 volts is going to take a really long time to get your batteries fully charged. It is highly doubtful that a sailboat would motor long enough to cause a problem. A 'dumb' alternator is basically a low-cost compromise - it doesn't put out enough voltage to efficiently charge the battery, and it doesn't 'know' how to vary its output voltage, but at the same time it doesn't put out enough voltage to do any damage. Having said all that, your batteries WILL last longer if you invest in a 'smart' alternator/regulator. You can get a 'smart' regulator for your existing alternator for about $300, but do you really need it? To answer that, you need to provide more information about your sailing/cruising habits, boat type, battery bank, charger, etc. At a guess, I'd say no. To answer your other question, you don't need to worry about charging one battery at a time - the battery that is more discharged will 'take' the current until it reaches the voltage level of the other, then they will share. On my boat, the alternator/charger are wired directly to both battery banks, so this happens automatically. It also means that I can leave that pesky orange switch in the 1 position (I'm always forgetting it) and not worry if both of my battery banks are getting charged.
Here is a link to a good basic battery FAQ. Cheers, Bob s/v X SAIL R 8
Flooded batteries don't actually 'overcharge'. Let's look at the charging process.... A charged battery has lead plates in a pool of sulphuric acid (H2SO4) mixed with water. H2SO4 has 2 'extra' electrons, so when you hook up a load to a battery, it breaks the bond into water (H2O) and sulphur dioxide (SO2). The SO4 is attracted to the plate and hands over its 2 electrons. As the amount of available acid in the battery decreases, so does the voltage, until the battery goes 'dead' at around 11.3 volts. The charging process involves adding electrons (from the alternator/charger) which entice the sulphur dioxide off the lead plate and to re-bond with the water, which gives us back our sulphuric acid (H2SO4). When ALL of the SO2 is off the plates, the battery is fully charged. The time it takes to get to fully charged is dependent on the amount of current that the battery can accept, as well as how much current your charger/alternator can put out. Continued charging won't 'overcharge' the battery, but if the voltage is high enough (above 14.3v), it will cause the water in the battery to 'boil' off over time - it's not really boiling, more like evaporation. This isn't going to happen in an hour or two or even 5 or 10 hours, (even if your battery is already fully charged) and there's another consideration. It takes a LONG time to get a flooded battery to FULL charge. Most 'smart' chargers use a voltage somewhere between 14.3 and 15.5 volts, because the last 10% of charge takes so long. If you have a 'dumb' alternator (one without a 'smart' regulator attached), it probably isn't putting out more than 14 volts - and 14 volts is going to take a really long time to get your batteries fully charged. It is highly doubtful that a sailboat would motor long enough to cause a problem. A 'dumb' alternator is basically a low-cost compromise - it doesn't put out enough voltage to efficiently charge the battery, and it doesn't 'know' how to vary its output voltage, but at the same time it doesn't put out enough voltage to do any damage. Having said all that, your batteries WILL last longer if you invest in a 'smart' alternator/regulator. You can get a 'smart' regulator for your existing alternator for about $300, but do you really need it? To answer that, you need to provide more information about your sailing/cruising habits, boat type, battery bank, charger, etc. At a guess, I'd say no. To answer your other question, you don't need to worry about charging one battery at a time - the battery that is more discharged will 'take' the current until it reaches the voltage level of the other, then they will share. On my boat, the alternator/charger are wired directly to both battery banks, so this happens automatically. It also means that I can leave that pesky orange switch in the 1 position (I'm always forgetting it) and not worry if both of my battery banks are getting charged.
Here is a link to a good basic battery FAQ. Cheers, Bob s/v X SAIL R 8
Another View
Almost all batteries immediately rise to 13 volts or more when on charge. This applies unless the battery is dead flat (or dead) or on trickle charge. This 'on charge' voltage is higher than any battery will be at when not on charge. Thus, I reason that when charging in parallel, both batteries will receive a charge, but the most charged battery will only receive a minimal charging current until the lower of the two reaches the same state of charge as the higher one. Then the charging will continue normally until they both reach the cut off voltage of the alternator and then they will both go on to float charge - and come to no harm. So, when they are BOTH ON CHARGE, I do not subscribe to the theory that the better battery discharges into the worse one. This will only occur if connected together when not on charge - and only slightly then due again to the need to have 13 volts or more before charging of the lower battery can commence. This is not the conventional advice offered by other 'experts' and gives me cause to wonder who is wrong. After a lifetime of living with house and starter batteries, and using the above logic, I have never found it to be at fault. Anyone like to disagree? I would like to have this explored by the forum please in case I am wrong. Thanks. PS:- Of course both batteries need to be of the same chemistry.
Almost all batteries immediately rise to 13 volts or more when on charge. This applies unless the battery is dead flat (or dead) or on trickle charge. This 'on charge' voltage is higher than any battery will be at when not on charge. Thus, I reason that when charging in parallel, both batteries will receive a charge, but the most charged battery will only receive a minimal charging current until the lower of the two reaches the same state of charge as the higher one. Then the charging will continue normally until they both reach the cut off voltage of the alternator and then they will both go on to float charge - and come to no harm. So, when they are BOTH ON CHARGE, I do not subscribe to the theory that the better battery discharges into the worse one. This will only occur if connected together when not on charge - and only slightly then due again to the need to have 13 volts or more before charging of the lower battery can commence. This is not the conventional advice offered by other 'experts' and gives me cause to wonder who is wrong. After a lifetime of living with house and starter batteries, and using the above logic, I have never found it to be at fault. Anyone like to disagree? I would like to have this explored by the forum please in case I am wrong. Thanks. PS:- Of course both batteries need to be of the same chemistry.
Alternator 14.4 volts
I wonder about charging two types of batteries at the same time. Electricity takes the path of least resistence. So your alternator is putting out 14.4 volts the battery with the least charge will have less resistence so most of the electricity will go that route. This is fine until they both approach fully charged. Supposedly the alternator is putting out enough voltage so that the charged battery doesn't discharge into the less charged battery. With mixed battery types and or mixed resistence in the wiring you might get a situation where you boil one battery dry . I mention this because I boiled my deep cycle battery dry but my starting battery was full of water. I usually start with the starting battery and then switch to both for motoring and then switch to the deep cycle to run the radio depth sounder etc. There may be some other explaination. I hope that I lucked out with the deep cycle. It was completely dead in the morning so we probably didn't have an anchor light part of the night. I refilled it with water and it seems OK. I might go back to using two deep cycle batteries. The deep cycle batteries seem to start the diesel just fine. Someday when I nothing better to do I will fully charge the deep cycle and starting batteries and then measure the voltage. If the voltage is different fully charged then there will be a difference in resistence. Overcharging isn't really a problem as long as you refill with distilled water. I don't think that an expensive "smart" regulator will extend a battery's life enough to justify the cost of the smart charger. My alternator is just a self exciting self regulating automobile type rated at 45 amps. It cost about 60 dollars at Autozone. The original alternator was fried when my wife turned the selector switch to off as we were motoring. Even a switch that allows you to switch batteries while motoring isn't idiot proof. Supposedly you can put a resister in the circuit to protect the alternator even if it has no load. I guess that instead of off leaving an open circuit it goes to ground via the resistor.
I wonder about charging two types of batteries at the same time. Electricity takes the path of least resistence. So your alternator is putting out 14.4 volts the battery with the least charge will have less resistence so most of the electricity will go that route. This is fine until they both approach fully charged. Supposedly the alternator is putting out enough voltage so that the charged battery doesn't discharge into the less charged battery. With mixed battery types and or mixed resistence in the wiring you might get a situation where you boil one battery dry . I mention this because I boiled my deep cycle battery dry but my starting battery was full of water. I usually start with the starting battery and then switch to both for motoring and then switch to the deep cycle to run the radio depth sounder etc. There may be some other explaination. I hope that I lucked out with the deep cycle. It was completely dead in the morning so we probably didn't have an anchor light part of the night. I refilled it with water and it seems OK. I might go back to using two deep cycle batteries. The deep cycle batteries seem to start the diesel just fine. Someday when I nothing better to do I will fully charge the deep cycle and starting batteries and then measure the voltage. If the voltage is different fully charged then there will be a difference in resistence. Overcharging isn't really a problem as long as you refill with distilled water. I don't think that an expensive "smart" regulator will extend a battery's life enough to justify the cost of the smart charger. My alternator is just a self exciting self regulating automobile type rated at 45 amps. It cost about 60 dollars at Autozone. The original alternator was fried when my wife turned the selector switch to off as we were motoring. Even a switch that allows you to switch batteries while motoring isn't idiot proof. Supposedly you can put a resister in the circuit to protect the alternator even if it has no load. I guess that instead of off leaving an open circuit it goes to ground via the resistor.
Depends
on 1. how the batteries are wired - through the 1-2-B switch or directly to the house bank with a combiner or echo charger to the start bank 2. type of regulator - internal or external smart charging (I know Maine Sail disagrees with the difference, but for this discussion it would help to know because the question is overcharging). I agree with Donalex re his opinion, assuming both banks are in reasonably good shape. If you have an internal regulator, you almost can't overcharge your batteries. suggest you buy Nigel Calder's Boatowner's manual for Mech and Elec Systems or one of Don Casey's electrical boat system books. The downside of using the 1-2-B switch for alternator output is that the wiring from the batteries to the 1 and 2 posts of the switch are usually undersized for charging.
on 1. how the batteries are wired - through the 1-2-B switch or directly to the house bank with a combiner or echo charger to the start bank 2. type of regulator - internal or external smart charging (I know Maine Sail disagrees with the difference, but for this discussion it would help to know because the question is overcharging). I agree with Donalex re his opinion, assuming both banks are in reasonably good shape. If you have an internal regulator, you almost can't overcharge your batteries. suggest you buy Nigel Calder's Boatowner's manual for Mech and Elec Systems or one of Don Casey's electrical boat system books. The downside of using the 1-2-B switch for alternator output is that the wiring from the batteries to the 1 and 2 posts of the switch are usually undersized for charging.
I agree, Donalex......
I've seen the situation you described, where a battery can't be charged at greater than its full-charge voltage: A bank of 24 Exide GU-33 cells, delivered dry to Egypt. We filled them with electrolyte - 15 gallons each! - and even open-circuit they went through an immediate discharge cycle and got quite hot. (I never quite figured out the chemistry of that, but it was consistent behavior for more than 100 banks). After letting them cool down, we measured the voltage at about 45vdc and hooked up the chargers - 2 rated at 400 amps each at 48vdc. Then we started charging, and pumping 800 amps in would only get the voltage up to about 47. We backed off to 400 amps (C/8), but even then the temperature would rise to 120F in about 30 minutes, so it took weeks to get a full charge in. Did I mention it was summer in the Sahara desert and there was no cooling in the battery room? Certainly if a battery is fully-discharged, you should charge it in isolation, but in normal usage, you aren't going to run into any problems by charging all your batteries at once. For starters, it's a MUCH more efficient way to go, since most alternators can put out much more current than a single battery can accept. Tom, I can't figure out what happened to your battery, but it's hard to figure that it happened overnight. First off, loss of water isn't from boiling, it's a chemical reaction called electrolysis, which occurs when there is excess current being put through the battery over and above what is needed for charging - this is 'gassing'. Boiling would give you water vapor, but what is being released from the battery is actually hydrogen and oxygen - if your battery had clear acrylic sides, you could actually see tiny bubbles (Aloha, Don Ho!) of hydrogen and bigger bubbles of oxygen. Secondly, 'dry' is a mis-nomer, because more than 25% of the liquid in the battery is sulphuric acid, and that doesn't go anywhere. However, the level in the cels probably sank below where you could see it. Finally, your battery might have charged up and be acting like it's OK, but it's doubtful that it is. You certainly exposed at least part of the lead plates to the air, which is a bad thing, and it sounds like you discharged it down to dead flat zilch, which is worse. Before you get into a situation where you MUST depend on that battery, I strongly suggest that you load test it. Most auto parts stores can do that for you, or you can charge it full, then discharge it into a known amount of load and record the voltages over time, then compare your results to a nominal curve. In many cases like this, a battery will charge to the right voltage and appear to work fine for a while, but its amp/hour capacity is actually a fraction of what it should be. The fully-charged, open-circuit voltage of your two types of batteries shouldn't be different (assuming that they are both in good shape) by more than a few hundredths of a volt. Cheers, Bob s/v X SAIL R 8 Note: I made a mistake on the chemistry explanation (it's PBSO4, not SO2), but that doesn't effect the way the reaction works.
I've seen the situation you described, where a battery can't be charged at greater than its full-charge voltage: A bank of 24 Exide GU-33 cells, delivered dry to Egypt. We filled them with electrolyte - 15 gallons each! - and even open-circuit they went through an immediate discharge cycle and got quite hot. (I never quite figured out the chemistry of that, but it was consistent behavior for more than 100 banks). After letting them cool down, we measured the voltage at about 45vdc and hooked up the chargers - 2 rated at 400 amps each at 48vdc. Then we started charging, and pumping 800 amps in would only get the voltage up to about 47. We backed off to 400 amps (C/8), but even then the temperature would rise to 120F in about 30 minutes, so it took weeks to get a full charge in. Did I mention it was summer in the Sahara desert and there was no cooling in the battery room? Certainly if a battery is fully-discharged, you should charge it in isolation, but in normal usage, you aren't going to run into any problems by charging all your batteries at once. For starters, it's a MUCH more efficient way to go, since most alternators can put out much more current than a single battery can accept. Tom, I can't figure out what happened to your battery, but it's hard to figure that it happened overnight. First off, loss of water isn't from boiling, it's a chemical reaction called electrolysis, which occurs when there is excess current being put through the battery over and above what is needed for charging - this is 'gassing'. Boiling would give you water vapor, but what is being released from the battery is actually hydrogen and oxygen - if your battery had clear acrylic sides, you could actually see tiny bubbles (Aloha, Don Ho!) of hydrogen and bigger bubbles of oxygen. Secondly, 'dry' is a mis-nomer, because more than 25% of the liquid in the battery is sulphuric acid, and that doesn't go anywhere. However, the level in the cels probably sank below where you could see it. Finally, your battery might have charged up and be acting like it's OK, but it's doubtful that it is. You certainly exposed at least part of the lead plates to the air, which is a bad thing, and it sounds like you discharged it down to dead flat zilch, which is worse. Before you get into a situation where you MUST depend on that battery, I strongly suggest that you load test it. Most auto parts stores can do that for you, or you can charge it full, then discharge it into a known amount of load and record the voltages over time, then compare your results to a nominal curve. In many cases like this, a battery will charge to the right voltage and appear to work fine for a while, but its amp/hour capacity is actually a fraction of what it should be. The fully-charged, open-circuit voltage of your two types of batteries shouldn't be different (assuming that they are both in good shape) by more than a few hundredths of a volt. Cheers, Bob s/v X SAIL R 8 Note: I made a mistake on the chemistry explanation (it's PBSO4, not SO2), but that doesn't effect the way the reaction works.
Keep it in the both position....
The battery with the lowest charge will have the least resistance and accept the bulk of the charge at first. Then as the charge comes up the other battery will accept more charge until they are both charged. A fully charged battery after a short rest period will read 12.6 volts and your alternator should first put out about 14.2 to 14.5 volts, and gradually taper off as the batteries become charged. You could buy a battery combiner and eliminate the switching around.
The battery with the lowest charge will have the least resistance and accept the bulk of the charge at first. Then as the charge comes up the other battery will accept more charge until they are both charged. A fully charged battery after a short rest period will read 12.6 volts and your alternator should first put out about 14.2 to 14.5 volts, and gradually taper off as the batteries become charged. You could buy a battery combiner and eliminate the switching around.
It's important to know how
your boat is wired: Alternator Output: The older boats with only one 1-2-B switches are usually factory wired with the alternator output to the “C” post of the switch. This means that the switch was actually used for two purposes: (1) which battery bank(s) the alternator charge goes to when the engine is running; (2) which bank is chosen for use for DC power. In 1998, we changed that. We moved the original alternator output FROM the “C” TO the “2” post (our house bank) of the switch. The Combiner charged the start bank without having to use the 1-2-B switch for selecting alternator charging output ONLY because we moved that connection. The revised alternator charging path was from the alternator to the “2” (house bank) switch post to the house bank, using the #4 OEM red wiring. IMPORTANT NOTE: You cannot simply move the alternator output from the “C” to the “2” post without a Combiner or Echo Charger type relay equipment because the start bank would not get charged. Another way to put this is: If your alternator output is wired to the C post of the 1-2-B switch, and you do not have a combiner (or equivalent), then you have to start the engine with the switch on B (ALL) because this would be the only way to charge both of your banks from the alternator. A better way to wire the alternator and charger output is to the house bank with a combiner, replay, ARS or echo charge. You could also do it with a simple switch. Last year we installed a new alternator, and wired it to the house bank, so now all the 1-2-B switch does is choose which battery to use for starting and/or DC panel power. Very simple.
your boat is wired: Alternator Output: The older boats with only one 1-2-B switches are usually factory wired with the alternator output to the “C” post of the switch. This means that the switch was actually used for two purposes: (1) which battery bank(s) the alternator charge goes to when the engine is running; (2) which bank is chosen for use for DC power. In 1998, we changed that. We moved the original alternator output FROM the “C” TO the “2” post (our house bank) of the switch. The Combiner charged the start bank without having to use the 1-2-B switch for selecting alternator charging output ONLY because we moved that connection. The revised alternator charging path was from the alternator to the “2” (house bank) switch post to the house bank, using the #4 OEM red wiring. IMPORTANT NOTE: You cannot simply move the alternator output from the “C” to the “2” post without a Combiner or Echo Charger type relay equipment because the start bank would not get charged. Another way to put this is: If your alternator output is wired to the C post of the 1-2-B switch, and you do not have a combiner (or equivalent), then you have to start the engine with the switch on B (ALL) because this would be the only way to charge both of your banks from the alternator. A better way to wire the alternator and charger output is to the house bank with a combiner, replay, ARS or echo charge. You could also do it with a simple switch. Last year we installed a new alternator, and wired it to the house bank, so now all the 1-2-B switch does is choose which battery to use for starting and/or DC panel power. Very simple.
Charging and Safety
With both charged up leaving the selector switch in "both" is fine. If one is discharged and one charged then when you switch to both the charged battery will begin to charge the discharged one. That is not that big a deal except a battery can deliver a wopping amount of current in a condition like that. This is a potential fire hazard as this current could be on the order of 500+ amps for a minute or so. So charge the discharged battery for 15-20 minutes and then switch to both. If you don't believe me try it and as you switch to both feel the battery neg cables connecting the batteries. I did it once and smelled something hot. Checking the batteries I saw nothing unusual but felt the cables anyway. They where hot to the touch! I'll admit that I had an extreme case with one bank being almost completely discharged and on completely charged so perhaps I'm over reacting but it is probably better to know than to be caught unawares.
With both charged up leaving the selector switch in "both" is fine. If one is discharged and one charged then when you switch to both the charged battery will begin to charge the discharged one. That is not that big a deal except a battery can deliver a wopping amount of current in a condition like that. This is a potential fire hazard as this current could be on the order of 500+ amps for a minute or so. So charge the discharged battery for 15-20 minutes and then switch to both. If you don't believe me try it and as you switch to both feel the battery neg cables connecting the batteries. I did it once and smelled something hot. Checking the batteries I saw nothing unusual but felt the cables anyway. They where hot to the touch! I'll admit that I had an extreme case with one bank being almost completely discharged and on completely charged so perhaps I'm over reacting but it is probably better to know than to be caught unawares.
Stu...
on my 27 I did something similar but in a different way. I had two 1-B-2 battery switches. One was used to direct charging current from the outboard to either battery. The other 1-B-2 switch was put on a custom DC panel and selected which battery was used for house power. One battery could be charging while the other one powered house use. I also had rigged a DPDT switch on the DC panel to direct 12v AC from a 300 watt Malibu outdoor lighting transformer to power (JUST) the interior lights (all new halogen types) while at the dock, taking the lighting load off the batteries. The batteries were charged at the dock as needed with a portable multiple stage charger.
on my 27 I did something similar but in a different way. I had two 1-B-2 battery switches. One was used to direct charging current from the outboard to either battery. The other 1-B-2 switch was put on a custom DC panel and selected which battery was used for house power. One battery could be charging while the other one powered house use. I also had rigged a DPDT switch on the DC panel to direct 12v AC from a 300 watt Malibu outdoor lighting transformer to power (JUST) the interior lights (all new halogen types) while at the dock, taking the lighting load off the batteries. The batteries were charged at the dock as needed with a portable multiple stage charger.
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