For boat purposes, *my* model (others will certainly vary) is that the batteries are charged on a charger at the dock while I'm away - typically several days at a time.
For $100, I can get an inexpensive 48V charger that will easily recharge the bank while I'm gone.
It's simpler and provides more redundancy to just get two chargers and have each one work on a separate bank.
Yes, I could use diodes to allow a single charger to feed multiple banks, but it seems better to just go with two chargers.
I realize things may be different if/when I go to a LI battery solution. But for now, two banks, two chargers, two switches seems like a really safe way to go.
John
From: weaversailor <jcacord@gmail.com>
To: electricboats@yahoogroups.com
Sent: Friday, October 19, 2012 2:00 PM
Subject: [Electric Boats] Re:Battery Buddy Pairing
If you are willing to pay attention to the consumption and state of charge of each bank, then, generally, keeping them separate and managing things makes sense. With two banks you will also want to manage the recharge as well. I would think this line of thinking is more appropriate where currents and discharge rates vary as in running a motor as compared to other uses where the discharge rate would be lower and over a longer period of time.
Paralleling batteries in general results in less voltage drop for a given current. But this depends much on battery chemistry; charge/discharge regimes are very dependent on that.
John Acord
--- In electricboats@yahoogroups.com, oak <oak_box@...> wrote:
>
> Hmmm... Â 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. Â 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? Â (if so, I don't have nearly enough batteries....)
>
> John
>
>
> ________________________________
> From: John Acord <jcacord@...>
> To: electricboats@yahoogroups.com
> Sent: Friday, October 19, 2012 12:38 PM
> Subject: [Electric Boats] Re:Battery Buddy Pairing
>
>
> Â
> 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.Â
>
> 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."
>
Paralleling batteries in general results in less voltage drop for a given current. But this depends much on battery chemistry; charge/discharge regimes are very dependent on that.
John Acord
--- In electricboats@yahoogroups.com, oak <oak_box@...> wrote:
>
> Hmmm... Â 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. Â 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? Â (if so, I don't have nearly enough batteries....)
>
> John
>
>
> ________________________________
> From: John Acord <jcacord@...>
> To: electricboats@yahoogroups.com
> Sent: Friday, October 19, 2012 12:38 PM
> Subject: [Electric Boats] Re:Battery Buddy Pairing
>
>
> Â
> 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.Â
>
> 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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