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post #25 of Old 02-07-2014
Maine Sail
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Re: Repowering with an electric engine?

Originally Posted by whippet99 View Post
Stats at 80% Depth of Discharge

I have been discussing with those that have done this, and they are happy. Battery life is multiple years, not months. Maintenance considerably less.

Of course, it depends on your application. I need to get out of marina, motor in the calm, otherwise i'm sailing. I have shore power for recharge. That's why when the Atomic 4 finally gives in, I'm switching
I for one would love to see how they got to those numbers and at what voltage this bank is... It seems like someone forgot about our buddy Peukert in those equations......

You might get multiple year out of LifePO4 at 80% DOD but not a heck of a lot out of FLA or AGM... Even the best AGM's can only claim 400 cycles to 80% DOD but that IS IN A LABORATORY.. Real world, not lab derived under carefully controlled conditions, 80% DOD numbers barely break 200 cycles for lead acid...

To go 50nm takes 16.6 hours at 3 knots

A 5kW motor is pulling 104A at 5000W at 48V

If we figure you are using only 70% of that 5kW to do 3 knots that is a 73A load to the battery bank...

A 73A load on a 200Ah 48V bank with a 1.11 Peukert (AGM) gets you a 159Ah bank not 200Ah.. This must always be accounted for in good system design.

If you want to use 80% of the bank capacity at 73A you need to start at a Peukert corrected capacity of 159Ah's NOT at 200Ah's or you will be drawing the bank WAY below 80% DOD due to the high loads applied to it, which are well beyond the 20 hour rate for lead acid of 10A, which the bank was rated at.

So you have 127 (actual) Peukert corrected Ah's to use before you hit 80% DOD with a 73A load with an AGM bank (Lifeline).

If each hour of run time at a 70% load burns 73Ah you can't even go 5 miles at 3 knots before hitting 80% DOD....

Even if we figure the motor at just 50% load/52A/48V you have a 165Ah bank and 132 usable Ah's before hitting 80% DOD with a Peukert of 1.11...

Even a 50% load still barely gets to 7nm....

At 1000 watts or just 20% motor load you are still burning 21A at 48V.

21A X 16.6 hours/50nm = -348.6 Ah's consummed...

I see no way you can get 16.6 hours out of a 200Ah bank even at just a 20% load....????

It is late in the day and I am assuming a 48V pack but can someone please double check my math....

-Maine Sail / CS-36T

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