Chris,
Enticing idea. The cool part about prismatic LiFePO4 batteries is that they will take more than 2CA in charging current. "CA" = the numeric value of the Ah capacity, so an 80Ah cell can take a 160A charge or 200Ah cell can take a 400A charge. For anything but the smallest cells, I don't think that this is much of a limitation.
How much does a marinized diesel generator cost and/or weigh? And you'll also need a 4kW charger to convert the generator output to match your drive system's requirements. Alternatively, you could develop a controlling interface that manages the generator output into the proper voltage and current to match the motoring/charging demand.
From a purely math perspective, assuming 4kW continuous output from the generator, a 90% efficient controller/charger will give 3600W of charging current. If we're still talking about a 48V system, the average charge voltage is 55V which translates to about 65A. (3600W/55V = 65A)
For me, the last consideration is the fact that I just spent way too much time taking a diesel motor out of my boat, I'm not sure that I want to put another one back in (and I really don't have the room). So if I installed a diesel back in my boat, I'd skip the complications and expense of the electronics, batteries and energy conversion inefficiencies and drive the shaft directly, just like the old engine did.
My Yamaha 2000 stores in the lazerette and weighs less than 52 pounds with a full tank. It will live at home for day sails and will only come aboard for longer excursions. The onboard charger that is used for charging from shore power will run directly from the generator if/when needed. For me, it's the best of both worlds.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Chris Witzgall <chris@...> wrote:
>
> Consider a 4000 watt diesel generator what kind of sustained charging into
> a 48v LIFEPO4 pack do you think we could manage?
>
> Chris
>
>
> On 8/27/10 1:40 PM, "Eric" <ewdysar@...> wrote:
> >
> > Myles,
> >
> > While I'm not disputing your observations, I'd like to clarify some of the
> > math. Assuming a maximum of 1600W in and 1440W out, the charger has to
> > operate with at least 90% efficiency. There are some chargers that will do
> > this, but many chargers like Zivans, Dual Pros and ProMariners are not that
> > efficient.
> >
> > Since charging voltages are higher than resting voltages, I think that a
> > conservative average would be 13.8V per 12V battery (charging voltages are
> > often higher), so 1440W divided by 55V is closer to 26A. I know that it is
> > only 4A difference, but that is 13% less than quoted.
> >
> > That said, I'm on your side. I think that a generator like a Honda EU2000i
> > (mine is a Yamaha EF2000iS), matched with an appropriately sized efficient
> > charger, is a reliable way to deliver between 20A and 25A into the batteries
> > or drive system for as long as you've got fuel. Like you said, this can be
> > used to "limp" home at slower speeds, or to extend the range of your existing
> > battery capacity at somewhat higher speeds.
> >
> > For me, that is my backup to my auxilliary.
> >
> > Fair winds,
> > Eric
> > Marina del Rey, CA
> >
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