Just to be clear, the benefit isn't just longer life. In fact, that's more intangible and as you say, long term than the real daily benefit: You would get 18-20% more range in the scenario I described. In your words, that's a "REAL difference". So if you care about the short term, this suggests paralleling the strings. I ran my boat with 2 strings of 6 T-105 batteries for several years with absolutely no issues. I currently run with just 1 string, but wouldn't for an instant consider 2 separate strings of high-Peukert batteries. The perceived benefit of having a spare string is really not that useful if you are aware of the state of your pack and plan your trips well.
-Myles
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of oak
Sent: Saturday, October 20, 2012 1:28 PM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Re:Battery Buddy Pairing
OK, this makes a little more sense. By paralleling the batteries instead of running them in series, you're reducing the current drawn from the battery under load. Batteries seem to have a longer life when discharged at a slower rate - so you get a longer battery life by draining the batteries more slowly. Right?
Long term, that might be the right thing to do.
Short term (while I'm more likely to run myself dry), I'd probably leave them as two separate strings till I have more data on how far I can go on how much power.
I agree that running a little slower is probably a good thing to do. And motor-sailing definitely seems to be a good way to go.
Thanks for the response!
From: Myles Twete <matwete@comcast.net>
To: electricboats@yahoogroups.com
Sent: Saturday, October 20, 2012 1:37 PM
Subject: RE: [Electric Boats] Re:Battery Buddy Pairing
The answer: It depends.
It depends on the cruising current you expect to draw, the Peukert coefficient for the batteries you're using and the 2 battery strings' capacities.
Say you expect to draw 75amps of current while cruising and have 2 strings of T105's (Peukert coefficient: 1.240).
In one test, you charge the 2 strings up, then run with one string, then the other.
In the second test, you parallel the strings, charge them up, then run with the parallel combination.
In each case, you cruise at 75amps.
How far can you go with each?
A T105 is rated for 2 hours at 75amps --- i.e. 150amp-hours.
For 80%DOD, the discharge time: 1.53 hours
For 60%: 1.15 hours
Multiplying by 2 strings: 2.3 to 3.0 hours
In the second case, each string will be delivering 37.5amps.
For 80%DOD, discharge time: 3.61 hours
For 60%: 2.71 hours
Net: 2.71 to 3.61 hours
Difference: 17.8 to 20.3%
Academic: 2%
Real Diff: 20%
I would suggest that the difference is so close to your definition of REAL that it should make you think twice about separating the strings.
OTOH, how important is an extra 0.4 to 0.6 hours of cruising when you could slow the boat down by 0.1knot and probably make up the difference.
In that case, the difference would be academic.
Another thing to consider however is that battery life will be much greater for the case where discharge rates are 37.5amps instead of 75amps.
See Neon John's Peukert calculator XLS file to play with the numbers yourself:
-Myles Twete, Portland
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of oak
Sent: Friday, October 19, 2012 4:11 PM
To: electricboats@yahoogroups.com
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."
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