Eric,
It is a combination of lack of Peukert effect, the LiFePO4 industry giving a conservative a-hr rating as compared to lead, and acceptable DOD. Here are the numbers,
The lead battery would have been a group 31 at 60 lbs and 80 a-hr rating (12 volt). That 80 a-hr rating is awarded for an easy 20 hour rate, or in the case of this battery a 4 amp load. The LiFePO4 gets its a-hr rating at a brutal 1 hour rate, plus unlike the lead that was pulled flat at the easy 20 hour rate, the lithium is only pulled to 80% DOD. So you can see that comparing capacities based on a-hr ratings is apples vs oranges. In real use, the cruisers on the Cruiser's Forum have discovered for house banks, they can size a 400 a-hr LiFePO4 bank and it will give them the same usable a-hr as their old 800 a-hr lead bank. In my case I replaced a single 60 lb 12 volt lead battery for (2) 12 volt lithium banks at 32 lbs apiece or a total of 64 lbs. I cruise at 4 mph (top speed is 5.5+mph). The coast line of Lake Tahoe is 75 miles, which I completed in 2 days on a bit less than the total capacity of my two banks. When I stopped at the marina for dinner I recharged the bank I was using and switched to the 2nd bank. Spent the night camping and finished the last 38 miles on the 2nd day on the 2nd pack. The first pack that I recharged enroute, I never needed.
Bob
From: "ewdysar@yahoo.com" <ewdysar@yahoo.com>
To: electricboats@yahoogroups.com
Sent: Thursday, March 13, 2014 12:48 AM
Subject: Re: [Electric Boats] Electric Kayak
Hi Bob,
Doesn't 200Ah weigh about 250 pounds? Wait, maybe not, you didn't mention voltage. 200Ah@12V should weigh about 65 pounds, 24V close to 130 pounds, 36V about 190 pounds, and so on....
I'm guessing that your original FLA batteries were rated with lower capacity. Same weight? Lithium weighs about 40% as much as FLA or AGM for the same rated capacity and voltage. That generally leads to only 3 times the range for the same weight battery pack at the same speed (2.5 times on capacity alone, the rest from lithium's lower Peukert's coefficient). So a 5 times range increase sounds like 60% more battery weight and about twice the battery size (volume).
80 mile range? At what speed? We all know that speed has a huge influence on overall range. Should we assume that 80 miles means about 70 nautical miles? We are discussing boats after all....
All of this is an educated guess, but there's not much info to go on. Can you let us know if any of these guesses are correct? Providing more specific info is a bigger help to those who would follow in your footsteps.
Fair winds,
Eric
Marina del Rey, CA
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