Eric--
When I looked at the discharge curve in the link provided below, the LFP battery doesn't fall to the recommended cutoff voltage of 2.8V at a discharge rate of less than 1C until more than 100% capacity is reached.
Since I would be operating at 30 amps (0.33C for the 90AH battery), shouldn't I be able to discharge beyond 80% without damaging the battery, provided I stop prior to reaching 2.8V?
John Casperson
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> Here's a quick comparison that I recently wrote: "As a practical comparison, the 160Ah 48V lithium battery bank in my recently converted 1964 Cheoy Lee Bermuda 30 ketch is 60% lighter, 50% smaller and cost 50% more than an high quality 200Ah 48V AGM battery bank that would deliver a similar range under normal 4-5kt operation."
>
> So here's what I would do if I was aiming for the performance equivalent of your battery. I have no idea what the Ah capacity of your battery is, so I'll have to focus on the weight.
>
> Thundersky 90Ah LiFeYPO4 3.2V cells weigh 7.05 pounds, so 4 cells with strapping would be right around 30 pounds. The cells cost $112.50 a piece for a total cost of $450 dollars. Her's a vendor that sells them. http://currentevtech.com/Lithium-Batteries/Thundersky/Thundersky-90ah-cell-p23.html
>
> So what does that buy you? Your battery would have a total capacity of 1125Wh and a usable capacity of 900Wh to 80% depth of discharge. With a 30A load, the battery will last 141 minutes.
>
> How big of a flooded deep cycle battery would you need to match that duration? I'll assume a Peukert's exponent of 1.25 (the same as a Trojan T-105). You would need 1965Wh total capacity to support the required 1180 usable capacity to 60% depth of discharge. That works out to a 157Ah 12V battery. Because high quality deep cycle batteries hold about 22.5Wh per pound, that battery would weigh about 87 pounds.
>
> So for your application a $450, 30lb, 90Ah 12V Lithium battery would replace an 87lb, 157Ah 12V deep cycle battery. I would bet that your 65lb battery will not nearly as long under that 30A load, it should be at 60% depth of discharge in about 97 minutes.
>
> That's the math. You get to figure out if you think that it's worth it.
>
> Fair winds,
> Eric
> Marina del Rey, CA
> 1964 Cheoy Lee Bermuda 30 ketch
> 5.5kW Propulsion Marine drive, 160Ah 48V Thundersky LiFePO batteries
>
> --- In electricboats@yahoogroups.com, "john_casperson" <john_casperson@> wrote:
> >
> > Hi Chris--
> >
> > I took a pair of partly waterlogged and partly sand filled 16 ft Prindle hulls.... on which I placed a group 29 Wal-mart deep cycle battery (say 65 lbs).... I mounted a T-34 motorguide trolling motor with modifications to the underwater part and weighing say 15 pounds. Total weight--I estimate about 455 lbs. With just me on it, it ran 4.7 mph drawing 30 amps at a total displacement estimated at 635 pounds. Without the battery or motor, I estimate weight was about 375 pounds, and I could move it around on level ground easily by teetering it on 2 inch PVC pipe.
> >
> > ... I intend to power it with a pair of modified T-34 motors and shafts, but will discard the mounting brackets to save weight and replace the heads with remotely mounted Motorguide Varimax heads for 45 pound thrust motors for efficiency and to enable skid steering. I will use the rest of the Varimax motors if I cannot make the heads work with the T-34 motors, but they are much heavier and not as streamlined. I estimate the powertrain will weigh about 20 pounds.
> >
> > With a 30 pound battery, estimated weight would be about 310 pounds.... With a 130 pound battery, I think it would be over that line.
> >
> > John Casperson
> >
>
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