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Captnlen
Lots to consider here Part of issue is space and weight and as a open boat where to put anything
Captnlen
On 9/15/2018 2:58 PM, king_of_neworleans wrote:
My suggestion: triple the solar. 40lb thrust is a bit weak but will push your little cat around okayish. Two Sams Club 6v golf cart batteries in series gives you nominal 220ah at 12v, a respectable sized bank for only about $170 plus tax and core charge. And these are extremely rugged batteries. Extremely. 2640 total watt/hours rated but actually you only want to discharge down to half the rated capacity as a general rule. Certainly no deeper than 30% SOC. So call it 1848 watt/hours. Run the motor at 10a or less and you will actually get a slight bonus capacity due to Peukert Effect. Higher voltage would of course be better... lower current needed for the same power. So a 24v or 36v trolling motor would be much, much better. Just use more golf cart batteries in series to make up the correct voltage.
Energy usage with electric propulsion varies WILDLY with speed required. Not quite as important with a cat as a monohull but still if you want to electrocruise at 8kts or at 2kts the difference in power consumption will be unbelievable. So what is your target speed and range?
You will need some instrumentation. You MUST know the voltage and current at any given time so you can record it and calculate state of charge. A tachometer would be nice, too. You can't simply look up the SOC for the voltage you read at the batteries, because the batteries must rest about 24 hours before this gives you accurate results. Rest means without charging or discharging. So you count amps and hours, and correct for Peukert Effect. Unless you operate right at the 20 hour discharge rate, which is where the rated capacity is based. In this case, 11 amps. Discharging at 11 amps for 20 hours takes the batteries down to 0% SOC. 11a for 10 hours takes them down to 50%. So if you operate at 11 amps you can get a fairly accurate SOC from the time you ran the motor. Rest the batteries for 24 hours and compare the SOC from voltage reading to the calculated SOC for a correction factor. Then maybe run at 20a for 3-1/2 hours and do the same thing. You will see that you do not have as much juice left as you would think, and this is due to Peukert Effect. Try a higher power level. Then try at 5a. Construct a graph. This will be very useful in estimating time left in the bank, assuming no underway charging.
But in fact you WILL have underway charging ability. Well, you can estimate the energy going back into the batteries from solar charge controller voltage and current fairly well as long as the batteries are below about 85% SOC. At higher SOC, charging efficiency drops and not every coulomb pumped into the batteries is actually stored for use.
Another thing to do since you are recording current and voltage while running is to figure out how to currect underway voltage readings to approximate SOC without resting the batteries. This is not very accurate but once you have a table of corrections at a particular power level, you will be close enough at least not to trash your batteries by discharging too deeply.
There are State Of Charge meters, some work better than others. The socalled "SmartGuage" is probably the best. Google it. It counts power both into and out from the batteries and according to reviews does a fair job of estimating SOC and displaying it as sort of an electrical gas guage.
If you just run until the motor stops working due to low battery voltage, even golf cart batteries will eventually fail. By fail, I mean they will need to be replaced. So don't just go until the batteries are dead. This is terrible on the batts.
300w of so of solar you would think would allow you to run at 25a during the day, right? But no that's not right. Peak rated output can only be obtained in optimum conditions. On a sunny day when the day is 12 hours long, you can figure you will get total output equal to about 4 hours rated output. So your solar harvest would be about 1200 watt/hours under very good conditions over the course of a day. So with near perfect conditions, you could operate at about 8a without drawing from the batteries, sun up to sun down. That is at 100w output power. Less than 1/7 HP. If you only install 100w of solar, the panel will eventually recharge your bank after about 3 days at the dock or anchored. It can extend your range a bit, but not drive the boat very well without drawing from the batteries. This is why I suggest you install 300w or more of solar panels, along with a good MPPT charge controller.
Or double the bank size and skip the solar if you want to do this with no instrumentation, stay at 11a and count hours of run time.
On my website there are spreadsheets for calculating Peukert Constant and SOC/runtime. http://www.growleymonster.com/eboat/index.html
If you learn about batteries and electricity, instead of just hook stuff up and go for it, you will increase your usage and enjoyment of your electric boat, and reduce your cost for battery replacements.
-- Capt.Len Susman Rtd Trikini Trimarans
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