My concern has always been the additional connection points that could also be failure points. It also means additional maintenance on those connections. Also think it would be much easier to monitor four batteries in a string (48 volt) than a mix of series and parallel batteries in strings with the same voltage. But, that's just my feeling.
Capt. Mike
From: oak <oak_box@yahoo.com>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Friday, October 19, 2012 7:11 PM
Subject: Re: [Electric Boats] Re:Battery Buddy Pairing
Ok, I'll bite:
Are we talking about a "academic difference" (2%), or a REAL difference (20%?
I can believe that someone could work out a proof that there is a lower internal resistance, and therefore improved output.
But I would have to question if the "improvement" is so small that it would get lost in the noise of all the other imperfect variables on my real world boat.
I guess to ask a different way: Would I really notice a difference?
From: Arby bernt <arbybernt@yahoo.com>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Friday, October 19, 2012 1:59 PM
Subject: Re: [Electric Boats] Re:Battery Buddy Pairing
Paralleled batteries will deliver more power as a result of a decreased Peukert's Effect.
A Rawson 30 we've repowered has two strings, 48v @ 100Ah, and 48v @ 170Ah (NSB100's and NSB170's).
We have measured more power out of the combined strings then the individual strings when sequentially drained.
Chances are a member would be willing to work it out as a proof...
Arby
From: oak <oak_box@yahoo.com>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Friday, October 19, 2012 11:22 AM
Subject: Re: [Electric Boats] Re:Battery Buddy Pairing
Hmmm... I know I'm just an odd duck, but....
It seems to me that I would MUCH rather have two separate banks, and have a switch for each bank, rather than putting them together.
I don't have much experience in electric boating, but I'm suspicious that I might be putting along, forget what I'm doing, or not paying attention when a well meaning friend has turned up the juice, or for whatever reason run my batteries down.
It would be really nice to be able to flip the switch over to the other bank, and still be able to get home.
I realize that out of "convenience", it might be nicer to have them tied together and just have a really large capacity to begin with, and never have to flip the switch. But given the choice, I think I'd rather KNOW when I've hit my limit, than be wondering.
Is battery life generally so long that I really don't have to worry about this? (if so, I don't have nearly enough batteries....)
John
From: John Acord <jcacord@gmail.com>
To: electricboats@yahoogroups.com
Sent: Friday, October 19, 2012 12:38 PM
Subject: [Electric Boats] Re:Battery Buddy Pairing
Chris,
I would be interested in the reason for paralleling each pair in the string. No two batteries have exactly equal chemical/electrical characteristics, so there will be slightly different sourcing/acceptance of currents. You want the battery stack to source/accept current so that each battery shares the "load" as equally as it can. By paralleling each pair you woll be forcing each pair to the terminal voltage of the other, where when you make a series string each battery can attain the voltage per it's charge state.
So if you make up a series string for the voltage you want, then parallel another string of the same voltage/capacity then differences between the two strings will be the overall terminal voltage and each battery in the string will charge/discharge according to it's individual terminal voltage and not to the terminal voltage of it's pair. With equal size/condition batteries the terminal voltage of the composite stack will be the source voltage but each individual battery will balance to it's own characteristic.
I have been running a house bank in a sailboat for many years that consists of a pair of 6v batteries wired in series for 12v, paralleled with another of the same. There will be usually about 10mv or less difference between the terminal voltage of each 6v battery pair, independent of charge state, which to me suggests that the batteries are sharing the load according to their internal resistance. As the batteries age (I get about 10 years out of a stack of flooded cell golf cart type) they seem to have the same characteristics except that the capacity is diminished over time.
Note that when combining batteries in series/parallel strings it is important that they be all the same age and condition.
You can probably convince yourself by drawing a comparable circuit of series parallel resistors, applying a voltage to the string then calculating the current in each resistor (Kirchoff's law) . Make small differences in the resistor values. Then draw it as two resistor strings paralleled and see what the difference is.
John
Flatwater Electronics
www.flatwaterfarm.com
"Neurosurgery for computer looms."
I would be interested in the reason for paralleling each pair in the string. No two batteries have exactly equal chemical/electrical characteristics, so there will be slightly different sourcing/acceptance of currents. You want the battery stack to source/accept current so that each battery shares the "load" as equally as it can. By paralleling each pair you woll be forcing each pair to the terminal voltage of the other, where when you make a series string each battery can attain the voltage per it's charge state.
So if you make up a series string for the voltage you want, then parallel another string of the same voltage/capacity then differences between the two strings will be the overall terminal voltage and each battery in the string will charge/discharge according to it's individual terminal voltage and not to the terminal voltage of it's pair. With equal size/condition batteries the terminal voltage of the composite stack will be the source voltage but each individual battery will balance to it's own characteristic.
I have been running a house bank in a sailboat for many years that consists of a pair of 6v batteries wired in series for 12v, paralleled with another of the same. There will be usually about 10mv or less difference between the terminal voltage of each 6v battery pair, independent of charge state, which to me suggests that the batteries are sharing the load according to their internal resistance. As the batteries age (I get about 10 years out of a stack of flooded cell golf cart type) they seem to have the same characteristics except that the capacity is diminished over time.
Note that when combining batteries in series/parallel strings it is important that they be all the same age and condition.
You can probably convince yourself by drawing a comparable circuit of series parallel resistors, applying a voltage to the string then calculating the current in each resistor (Kirchoff's law) . Make small differences in the resistor values. Then draw it as two resistor strings paralleled and see what the difference is.
John
Flatwater Electronics
www.flatwaterfarm.com
"Neurosurgery for computer looms."
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