Battery rotation ...like Tires rotation

Oct 29, 2005
2,355
Hunter Marine 326 303 Singapore
I've 3 batteries in Bank #1 and a single in Bank #2. Bank #1 drives everything and #2 is essential for emergency use. Both Banks are connected via Battery Combiner. All 4 batteries are identical Bosch SMF 105D31. Does it make sense to periodically rotate the Bank #2 battery with one from Bank #1? Just like tires rotation to balance out the wearing? Thus each battery has a chance to be an emergency battery.
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
I have my bank on a 1/2/Both switch. #1 is my old battery. I use it most of the time & only switch to #2 in the rare instance when #1 gets real low. This keeps #2 in reserve & in gently used condition for future use. When #1 gets to be in poor condition due to old age & numerous recharge cycles, I then rotate battery #2 into the #1 position & put a new battery in the #2 position. This is not the best way to do it for everyone. There are advantages to running all batteries in a house bank together. This system works for me. My battery use is modest. I am protecting myself against having all batteries dead in the event that I forget & leave an unattended load on the bank.

As was already indicated, mainsail is the best source of comprehensive information on battery banks.
 
Jul 7, 2004
8,402
Hunter 30T Cheney, KS
Maybe there's some logic to rotation. I'm guessing if one particular battery gets drained (cycled) more often than the others it will be the first to need replacement. Is that wise or should the burden be shared? I now have a portable jumpstart battery just in case.
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
Jim. As you will learn in MaineSail's info batteries are killed with kindness and bad charging systems. What are you using to charge the batteries. You indicate you are using a "combiner". And that you have a 1/2Both switch. Have you read and understand the info on this forum and in MaineSail's site about the switch and it's use?
Of note is how batteries in a bank respond when one goes bad. In non technical terms... If a good and bad battery are connected the good one tries to heal the bad one and they both die. Bad battery in your bank. Surgery is the answer.

Having 2 banks makes good sense. Think of them as House Bank and Starter.
My application of this is:
  • Starter gets used to start the boat.
  • House Bank for everything else.
  • Have the switches set up so that you can isolate the House Bank from the Starter.
  • Charge systems go to House Bank
  • Starter is charged by combiner s always maintained but isolated from the risk of leaving on a DC load that will risk no way of starting the engine.
  • 1/2Both switch is an on off switch, not a way of controlling charging.
All of this is outlined in many messages on this site.
https://forums.sailboatowners.com/index.php?threads/1-both-2-off-switches-thoughts-musings.137615/
or
https://forums.sailboatowners.com/i...-banks-vs-one-house-bank.184137/#post-1381791
the general belief, not backed up by any critical thinking of course, was to alternate bank use. In doing so boat owners quickly found battery life suffering and charging of multiple banks quite inefficient due to the amount of time needed to properly absorb multiple banks.
Now this refers to charging multiple house banks. But it may affect you in the the switching of batteries as well. Set up the charging to do the right job then use the system.​
Best of luck...
 
Jan 22, 2008
8,050
Beneteau 323 Annapolis MD
I bought 3 x 31 for my B323, and they were all manufactured in the same month. My use/charging rotates them so they would all die around the same time (not all at once, though) so I can replace all 3 at the same time- with all from the same date..
 
Nov 13, 2013
723
Catalina 34 Tacoma
Don't rotate them. Combined batteries should be of the same type and purchased at the same time. That way, they all are depleted at the same rate. If you add a newer or older battery to the bank, it will either be supplying power to the new bank all the time trying to bring the older bank up to it's charge, or, if older, drain the newer batteries. Like Jssailm said, I use my start battery to start and nothing else unless the house bank is depleted.
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
Jim. As you will learn in MaineSail's info batteries are killed with kindness and bad charging systems. What are you using to charge the batteries. You indicate you are using a "combiner". And that you have a 1/2Both switch. Have you read and understand the info on this forum and in MaineSail's site about the switch and it's use?
Of note is how batteries in a bank respond when one goes bad. In non technical terms... If a good and bad battery are connected the good one tries to heal the bad one and they both die. Bad battery in your bank. Surgery is the answer.
As I said in my previous post, the method I am using is not the best for everyone. My battery usage is moderate & one battery is enough for my needs 90+% of the time. When my #1 battery stops having enough capacity to do the job, then it is time for replacement. My second battery is mostly just carried as a back up. My charging is done either from a loop on a small outboard or, more often, from a ProMariner charger that plugs into 110vac. The shoreside charger has battery isolation. I use either #1 or #2, but I don't use Both on the battery switch, except on rare occasions, when charging from the outboard or a portable generator. #1 is my working battery. #2 is my emergency back up battery for the times when I goof & leave #1 loaded for too long & it gets drained to the point where I can't use it immediately. #2 is little more than a safeguard against my forgetfulness. My method works well within the narrow confines of my specific needs.

I have read some of what Main Sail has posted, but I have not had the time to read the vast majority of the information that he has been kind enough to provide. I find very little room for even mildly dissenting rebuttal in the posts that I have read from him so far. He seems to be the very best practical battery-information resource that I have encountered, bar none.
 
Last edited:
Dec 19, 2014
57
Tartan 30 Baltimore
As I said in my previous post, the method I am using is not the best for everyone. My battery usage is moderate & one battery is enough for my needs 90+% of the time. When my #1 battery stops having enough capacity to do the job, then it is time for replacement. My second battery is mostly just carried as a back up. My charging is done either from a loop on a small outboard or, more often, from a ProMariner charger that plugs into 110vac. The shoreside charger has battery isolation. I use either #1 or #2, but I don't use Both on the battery switch, except on rare occasions, when charging from the outboard or a portable generator. #1 is my working battery. #2 is my emergency back up battery for the times when I goof & leave #1 loaded for too long & it gets drained to the point where I can't use it immediately. #2 is little more than a safeguard against my forgetfulness. My method works well within the narrow confines of my specific needs.

I have read some of what Main Sail has posted, but I have not had the time to read the vast majority of the information that he has been kind enough to provide. I find very little room for even mildly dissenting rebuttal in the posts that I have read from him so far. He seems to be the very best practical battery-information resource that I have encountered, bar none.
Jim, I use my batteries very much like you do. #1 gets the use, #2 is reserve. I've yet to deplete #1 but we don't do too many continuous overnights at anchor and usually there will be some motoring in between to recharge. They are on a charge when at the dock. Because I'm a belt/suspender/whatever comes next kind of guy for our longer trips I always take my hot shot jump starter battery "just in case" something goes wrong and I deplete both batteries. At least this way I can get the motor started. I bought AGM's before I really understood the costs/benefits/tradeoffs. When I have to replace I'll do a lot more research.
 
Nov 13, 2013
723
Catalina 34 Tacoma
When my #1 battery stops having enough capacity to do the job, then it is time for replacement...#1 is my working battery. #2 is my emergency back up battery for the times when I goof & leave #1 loaded for too long & it gets drained to the point where I can't use it immediately. #2 is little more than a safeguard against my forgetfulness.
Sounds like you are killing your #1 bank by bringing it below 50% state of charge. If you read Mainsails recommendation you should not be depleting your house bank beyond 50% SOC.
 
Aug 22, 2017
1,609
Hunter 26.5 West Palm Beach
Sounds like you are killing your #1 bank by bringing it below 50% state of charge. If you read Mainsails recommendation you should not be depleting your house bank beyond 50% SOC.
I don't make a habit of bringing it down that far. I don't plan to bring it down that far. Occasionally, I make a mistake & it happens. My plan B (#2 battery in reserve) does not reverse the damage that my mistake did to battery #1, but it does prevent me from suffering immediately as a result of my mistake. That is the trade off that I have chosen.
 
Oct 22, 2014
20,995
CAL 35 Cruiser #21 moored EVERETT WA
Good for you Jim. You know it is not about following a crowd it is about knowing what you are doing. You have evaluated information did critical thinking and then identified a plan and are using your plan.
I think that is success. Well done.
Be safe out there on the water.