I've been looking at the various "build it yourself" options and ran across the LiFePO4 3.2v
"canister"s from Headway. My question is for the battery guru's concerning their 3810HP 8Ah @
$15.00. I notice that is has advertised 10C max charging (80A) and 20C (160A) max continuous
discharge current with max momentary discharge of 25C (200A).
see http://rightbikeonline.com/index.php?route=product/product&product_id=73
My project is typical, 33' sailboat, 3500 lbs, Cushman GE motor w/ brushes, original trans from the
diesel and a folding 12 x 12 prop. Mostly dry sailing on a lake (lauched from a trailer each
visit), so the boat only needs the aux elec motor less than 20 min on an ideal day (I go to sail).
But could be used for more than a hour or 2 when there is no wind, or outrunning a rainstorm.
I have the following table of estimated power requirements;
Knots HP/ice HP/elec watts Amps@24v Amps@36v Amps@48v
3.4 1 0.3 213 9 6 4
4.3 2 0.6 426 18 12 9
4.9 3 0.9 639 27 18 13
5.4 4 1.1 853 36 24 18
5.9 5 1.4 1066 44 30 22
6.3 6 1.7 1279 53 36 27
6.6 7 2.0 1492 62 41 31
Hull speed is about 6.7 knots
If I was on the coast I would definitely go with 50% more power but many boats this size have 6HP diesel motors.
What I think I see is these canisters provide quick charge capacity and ample heavy use capacity in
a 3/4 lbs package so that 15 in series gets you to 48v @ $225 for about 2 hours use at 3.5 knots, 1
hr use at 4.5 knots or only 20 mins at 6 knots (2 miles across the lake to outrun a storm). At
only 11.25 lbs so that if made modular [(3 x 1.6) x (5 x 1.6) x 4.75] or 5" x 8" x 5" 48v 8A
package. Obviously I would want to build two or three but at around twelve pounds you could carry
them from the boat to be charged better than a 62 lbs 6 volt Trojan.
Also they show the cell dropping to 7 amp after 500 cycles of 10C charge and 10C discharge. So my
question is does it make sense to buy these and probably only discharge at an absolute max of 4C
compared to their 38120L/S 10Ah with 3C max charge and recommended 1C (10A) discharge (3C 30A max
discharge) their chart shows 7.7A @ 1440 cycles 1C.
see http://rightbikeonline.com/index.php?route=product/product&path=62&product_id=68
Actually, after writing all this down it looks to me like the 38120S/L 10Ah is a better match to
the 4 to 30 amps required to go 3.5 to max hull speed with only one 15 series string. (To be
improved with a second pack down the road.) One thing that hit me was trying to actually get 1440
cycles means every day at the lake for 3.94 years. So I can afford the battery pack I just can't
get that much time at the lake???
Posted by: raytclark@netzero.net
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