Greg,
The link that you provided stated that the sodium batteries are suited for battery banks larger than 20kWh. If we're talking about just that, a 20kWh battery bank, then 14% daily would be 2800Wh per day or 117 watts continuous (pretty close to Miles' guess). In Southern California, a properly aligned fixed solar panel will average across the year about 5 times its rated capacity in Wh per day. Taking 2800Wh divided by 5 indicates the need for 560W of solar panels to replenish the charge lost to the battery heater alone. You would need more panels at higher latitudes (less sunlight energy), and you would need more panels if you wanted to charge the boat after you use it.
These batteries sound like they count on an unlimited power source (like the grid) to keep them ready for use and they seem more suited for consistant (daily) duty. But even if the same 20kWh pack was run to 80% DoD every day, you would need to provide almost 19kWh each day to get 16kWh of propulsion. That means that you're taking a 17.5% hit on efficiency before you even turn on the drive. Less usage make the efficiency even worse, i.e. 50% DoD needs almost 13kWh for 10kWh of propulsion or only 72% efficient before your cast off. A day off means 2.8kWh burned with no propulsion at all. You can see where this is headed. And nobody has mentioned how these batteries measure up in Peukert's Effect.
That said, they do seem suited for certain use cases (not my boat). And like I've said many times before, ther is no single "best" answer. The more choices that we have means that we can pick the best answer for the problem at hand. So if electric is a good answer (ICE is better in many cases), then picking between the pros and cons of flooded, AGM, Lithium, Nickle Iron or Molten Sodium can lead to greater success for a project.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Greg Martin" <ffmagellan@...> wrote:
>
> Thanks Myles. Those numbers really put things in perspective. I think I also read somewhere that the ZEBRA batteries were mostly recommended for continuous and regular users. You're right it would not make any sense at all if you only used them a few times per month. Modec produces commercial EVs, like delivery trucks and for commercial fleets of taxis, delivery and utility trucks that work everyday and go back to the recharge station at night so it makes more sense for them, probably. For boats, maybe if you had a water taxi or a charter service or just really knew you were going to go sailing everyday- and- your grid power source was eco, then it would make sense. Where I'm at, I don't feel as bad using grid power since we get a lot of our power from wind turbines.
>
> I'm also very curious- and I admit, I'm no expert, I really do need to get smart on how to calculate this- how feasible would it be to supply that much power regularly to the ZEBRA batteries with a big enough solar array that you could have on a boat? Is that even a practical option, if you had say, a couple of 100 watt panels over a bimini? Presumably one of those big panels would have to be dedicated just to keeping the sodium hot. When they say a solar panel is rated at 100 watts, is that over 24 hrs or are you screwed at night?
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> -Greg
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