Thanks for your question. Thats some pretty cool trips you have in mind. With any of these kind of projects its always worth running what my friend Marv always called “the drudgery of the numbers.” With 400 watts of solar panels to charge you battery I think you could expect about hald this, say 200 watts, as the peak output that you might see from these panels under the usual conditions you get on a boat. That is, not ideal orientation, and constantly varying orientation, regular shading from rigging and sails, and so on. One way or another you can probably expect to get 3 times the namelate capacity of the panels on a nice day. Less on cloudy, rainy days, and maybe occasionally better on ideal days.

You could expect to get about 1200 watthrs of charging on a nice day. Your LiFepo4 has a capacity of 120x 12 = 1440 watthrs. Say you allow to discharge it to a maximum of 90%, so you could count on a usable capacity of around 1300 watthrs. This is pretty close to the capacity of the Torqeedo battery, so it would be possible to completely charge the Torqeedo battery from empty if you do have the litium battery fully charged.

Where I am going with this is to ask the question how often would you be in the position where you need to charge at 120 watts? And would the 50 watts you could expect by charging directly from you lithium battery be enough anyway?

Your lithium battery would normally be around 13 volts and so would deliver around 5o watts to the Torqeedo battery simply by plugging it in directly without needing a dc-dc converter. (thats 13v x 4 amps charge rate)

If you can keep the Torqeedo battery attached to the lithium battery continuously in 24 hours it would transfer the 1200 watthrs that you could hope to get from the solar to the Torqeedo battery. Any faster than this would not be possible because there is not enough energy harvested each day to do better than this.

Suppose you are on your adventure and arrive at the campsite for the day with the Torqeedo battery empty, using the last bit of power to arrive at camp, and the lithium battery is full from the solar charging, say 6pm. I think this is optimistic but lets go with this to start getting a sense of the numbers. Overnight you can keep the Torqeedo battery attached to the lithium and proceed to charge it. Lets say you are ready to leave camp at 8am, that is after 14 hours, and you would have added 700 watthrs to the Torqeedo battery. Not fully charged, but about three quarters full. Maybe there is a little sun hitting your solar panels to begin charging the lithium battery. It this point it is close to empty (or even completely empty of you intend to use this battery for charging your phone and other uses at the campsite). But anyway, even just supplying the TQ battery it would be down to 700 watthrs, or about half full.

During the day until 6pm you will harvest another 1200 watthours from the sun, and be able to deliver 500 watthrs to the Torqeedo battery. (thats in the 10 hours from 8am to 6pm). If you used all this energy to get to camp the Torqeedo battery would be empty, and the Lithium battery would be just full. (it started at 700 watthrs and you added another 700 bringing it full again).

As it turns out your size of solar panels, and lithium battery capacity, and charge rate, are all in balance on a nice day. I’m not sure that having a dc-dc converter would deliver much advantage for multiday trips. On rainy days you would run a deficit, and if you used any of the charge from the lithium battery to run other devices you would also have less power available than you could deliver to the TQ battery in 24 hours.

How you intend to use your setup may be different to what I describe and you may indeed need the faster charge rate. I have used Snaptec inverters http://www.snaptec.com.au/gb30-1230.html

the output voltage of these units is adjustable by about 10% from nominal. I have used them in series to get the power I needed. I have a couple here that I had on my yacht for 6 years and now in the shed but still working fine. I bought them in 2008.

There are many more options available these days and you can probably find a unit that will deliver what you want.

I have just ordered the new ultralight 915w battery from you guys to use with the Hobie Evolve set up. Very excited to try this out. Thanks to your replies on this forum almost all of my questions have been answered. I have a Hobie Tandem Island (sail, pedal, paddle or motor trimaran) set up with 2 flexible 200w solar panels (400w in total) charging a 12v 120ah Lifepo4 lithium battery. This has been powering a Watersnake brushless 12v 70lb trolling motor. Seems to work ok however the Watersnake at 600w is slower than the Torqeedo at 400w. Plus the motor alone must be 5kgs heaver than the entire Torqeedo system. I plan on doing some big trips (Murray River, Fraser Coast, Whitsundays) where i could be out on boat for weeks. I plan on retaining my current solar battery set up & will use the Lifepo4 battery to charge the Torqeedo battery. I will use a small 240v inverter to power the standard 90w charger as a back up only. I would like the main charging duties to come from a dc-dc converter 12v to 36v at 4\5 amps delivering the max 120w. Which leads me to my question. Do you know of a dc-dc converter or similar product that will do the job? Cheers ]]>

the V9 prop is 9km/h at 1200rpm

if you turn the V8 at 1200rpm you get 13Km/h ]]>

–chris

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