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hi youtubers jeff cote here with another  session of ask pys  we've got a fellow boater don asks  jeff i've been told not to switch  batteries while the engine is running  is there any truth to that i'm talking  about going from battery one to battery  two or going from battery  from both to battery one or battery two  don  great question  all right well first of all there's no  easy answer to this  because ultimately the way that a boat  is wired is going to influence how  the battery switch is used but that  being said  most of us the answer that i'm going to  give is going to apply to a lot of us  so this is commonly the battery switch  that most of the boats used to have  or even have i'd say 99 of all boats  have this battery switch  there's certainly been if an evolution  since this battery switch came out  and there are different ones but this is  common right you've got uh  you're basically saying which battery is  going to run all your loads you can  either choose battery one  you can choose battery two or you can  have both run all your loads  notice the back of the switch only has  three posts  the third post is what's called the  common post that means that's where all  the loads are going  right and that's the bottom post right  here and then you're gonna have battery  one and battery two  so the question as a boater is and i'm a  huge proponent of this  is always to start your engine off of  what you consider to be an engine  battery  or at least demonstrate that a battery  can start your engine  without having the help of another  battery bank  what does that mean is you want remember  when we're at the dock we have control  this is the best place for you before  you leave in your dock  your mooring ball for you to know that  your engine can start on  one battery bank and one battery bank  alone right because it gives you  confidence you're like hey listen  if i can start on one i can certainly  start on two right if you're gonna have  two batteries it's better than one  so the reason is always start your  engine off a battery that you think  should start your engine now in some  cases in rare cases some people are  treating their boats and saying you know  what  i can start on one and start on two as  well they're both equal  in that case alternate start one day on  one  start the other day on two constantly  confirming  that either battery or the battery you  think should start your engine  has enough juice to start the battery on  its own  now where's this advice of running a  battery switch on both  the challenge is unless you have a  battery combiner or battery isolator and  that would be only a very few subset of  us that have that device on our boat  most of us are going to have an engine  and an alternator now that alternator  outputs only to one battery bank  but you might have two battery banks on  your boat right a lot of us do  so how do you share the charging voltage  that is coming out from your alternator  to two batteries  well you do that by actually going to  both  and that's why a lot of boaters have  been told  that when they're underway and the  engine is running  to bring the battery switch basically  to both right here in this case they  just label one plus two  so that the alternator is not only  charging the engine battery but it's  also charging the other battery  caveat and i screwed up i made this  error  this battery switch and all of them are  going to have a little label  never just never turn off when engine is  running  if you ever move this battery switch and  you go from  instead of going from let's say one and  you turn the switch to off  to go to the other battery you'll blow  up your alternator so it's  very important that you always go from  either one  to both to two but never ever go  off if you turn your battery switch off  when your engine is running your  alternate will blow up  i did that i know it and so as a boat  owner you want to avoid losing your  alternator  right and so use caution when using this  battery switch the other thing  another little side note battery  switches do age  nothing lasts forever we know that if  your battery switch is 30 years old 40  years old the contacts  as you switch from one to both or from  both to two  might not be great and even though that  you might see the lights barely flicker  you're like i think i just lost power  even for a millisecond  that millisecond is enough to blow up an  alternator so if you are changing that  battery switch from one to both or two  make sure that you have a relatively new  battery switch because if the contacts  aren't perfect  you'll blow up your alternator as well  so dom that was a phenomenal question  and thanks for asking  thank you for watching this pys video if  you've got further questions  ask them down below or go on our website  to fill one of the forms  also support to keep this youtube  channel  ad free by donating on paypal or  purchasing some merchandise  on our store don't forget to subscribe  and thanks for watching  you