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Author Topic:   Odyssey Battery; Peukert's Law
caddis posted 06-21-2012 10:21 PM ET (US)   Profile for caddis  
I'm going to install a new an Odyssey PC-625 battery in my little 13-footer. I've heard good things about these while searching this and other boating boards. And, at 13-lbs, I can't argue about the weight. Might not even have to move it under the console now. If I'm out of my mind here please let me know. Most difficult thing is finding a battery box for it.

Re a trolling motor battery for a dinghy I have: I am trying to figure out how long my kids can use [the trolling motor] before [its battery] dies out--leaving them stranded around the bay won't go over well with my trophy wife. I am looking at an Everstart Group-24 deep-cycle from Wal-Mart. From the label it is rated at 690 Marine Cranking Amperes, and 101-Ampere-hours at 1-Ampere. From my limited knowledge, I should be able to take the roughly 32-Ampere draw from my Minn Kota Endura Pro 32 trolling motor--that is amps per hour, correct--and divide the Ampere-hours by that. I come up with 101/32 = 3.15 hours. Does that sound correct?

A lot of the other information I've gotten from searching online talks about a 20-Ampere assumption on reserve capacity for most batteries, but this battery says nothing about reserve capacity and actually spells out the Ampere-hours. Three hours at full power just seems like a pipe dream to me, hence my question. I know I could just buy the battery and test it, but I'd like to see if I'm on the right track before I do.

Lots of other questions coming in the next week or so. Thanks for all your help.

jimh posted 06-21-2012 11:12 PM ET (US)     Profile for jimh  Send Email to jimh     
The specification of Ampere-hours for a storage battery must also be referenced to the rate of current flow. A rating at a flow of one-Ampere is going to make the battery look much better than if it were rated at 20-Amperes. If the rating is 101-Ampere-hours at 1-Ampere, the rating at 20-Amperes is going to be lower. This effect is generalized by Peukert's Law. It is well explained here: Battery_(electricity)#Battery_capacity_and_discharging

In your case, at 32-amperes you could expect the storage battery to provide

T = Qp/I^k

T = 101 / 32^1.3

T = 101 / 90.5

T = 1.1-hours

Amperes are a measure of current flow without regard to time. If one-Ampere flows for one hour, you have one Ampere-hour.

caddis posted 06-22-2012 12:32 AM ET (US)     Profile for caddis    
Thanks Jim! So can I assume for my future endeavors that I can use the formula:

T= (AMP hours on the battery label) / 90.5

to make it easy on an algebra challenged bug like me? If so, that's what I'll do. Of course, if something changes I may be asking for your help again.


gusgus posted 06-22-2012 03:17 AM ET (US)     Profile for gusgus  Send Email to gusgus     
I can't address the use in a boat, however I just replaced my first Odyssey PC-680 battery in my motorcycle.
My bike has a huge alternator, 40 amp.
The bike runs a great deal of electrical items, the four 100 watt head lights , all the tail lamps, blinkers and running lights being a good load and heated gear for me and my bride. GPS, heated seats and heated hand grips.
This battery lasted 7 years. It was dead a few times since it was in Alaska 5 of the 7 years, and sat a few winters.

I just bought another, PC-680 and couldn't be happier.

I would recommend this name battery for any use, if it is large enough to handle your loads.

jimh posted 06-22-2012 08:15 AM ET (US)     Profile for jimh  Send Email to jimh     
The formula is for scaling from the Ampere-hours at the 1-Ampere rate to a higher rate. Not all batteries specify their Ampere-hour rating at the 1-Ampere rate. It is also common to specify an Ampere-hour rating at a 20-hour discharge rate.
swist posted 06-22-2012 08:16 AM ET (US)     Profile for swist  Send Email to swist     
What is the technology of this battery? Lead-acid, or something more exotic?
gusgus posted 06-22-2012 02:38 PM ET (US)     Profile for gusgus  Send Email to gusgus     
PC 625 Specs:

YB16-CLB replacement
625 cranking amps for 5 seconds
545 cranking amps for 10 seconds
480 cranking amps for 20 seconds
Short circuit current over 1800A
27 minute reserve capacity with 25amp load
Length 6.7"
Width 3.90"
Height 6.89"
Weight 13.2 lbs

Odyssey design:

2 year full warranty
Rugged Drycell sealed design
Military grade
Vibration resistant
60% more starting power
Deep discharge reserve power
2 year storage life
8-12 year design life

Excellent Batteries!

jimh posted 06-22-2012 05:43 PM ET (US)     Profile for jimh  Send Email to jimh     
I believe the Oddyssey PC-625 is a sealed lead-acid battery employing absorbed glass mat construction and using pure lead. See

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