As a marine electrician, involved in electric conversions (EC's), all I can say is learn as much as you can and use REAL MATH... A lot of the proponents of EC's tend to mislead folks or tell partial truths in order to get in the door. I am on a number of electric boat forums and rarely do I ever hear someone mention the Peukert constant, battery temps or actual cycle life at 100% DOD which is where you wind up if you ignore Peukert etc......
Weight conversions are also most often misleading. They often pick the heaviest old school engines like the older Perkins 4-108 & the heaviest reversing gear, not a standard and considerably lighter Yanmar like the 3JH which is under 400 pounds with a Kanzaki or Hurth/Kanzaki.... They then entirely leave out the house bank when doing EC conversion math.. You can't really do that with a straight face.....
When we discuss battery weight we need to be cognizant of the fact that you will still need a 12V house bank. This was from an actual quote one of my customers received.
The 48V propulsion bank weighed 520 pounds but then the house bank may was another 130 or so pounds making for approx 650 pounds of batteries on a 28 foot boat. The house bank weight was left off any conversion calculations. If you are real energy conscious you could get away with a single 65 pound house bank but he wanted a minimum of 130Ah's to be able to easily power his windlass at 50% SOC....
With diesels you also have fuel capacity weight, which is not very light weight. A 20 gallon tank when full would weigh about 140 pounds +/-.. So in his case he had a 175 pound engine + 140 fuel = 315 pounds. With fuel filters and misc spares he was at about 360 pounds for engine and full fuel.
He was told the EC for his boat would weigh less? Really? Basic math often flies out the window when EC salesmen...
His quote was for an 8 battery bank which alone was approx 160 pounds heavier than the diesel engine & full fuel plus filters and spares. Of course the battery bank weight did not not the weight of the electric motor and associated gear which can be 100 pounds +/- with wire mounts, motor etc. This made his conversion nearly 260 pounds heavier not lighter as he was quoted.. You must be able to do accurate math for your own vessels conversion.
Not to mention he now had two battery systems, house/12V & electric / engine 48V at two different voltages but no large current source that could be directed at either. Sometimes an EC will weigh less but with lead acid batteries very often it does not.
On a 37 footer be sure you can physically fit enough solar and wind to charge two banks adequately in the time frame you would want in order to do 20 or 30 nautical mile runs.
E-power almost always requires a boat that is tied to a dock when done sailing or you wait a while between uses to recharge via solar & wind.....
By the time one gets done building a system like this fixing the diesel looks like a Kmart Blue Light special in comparison.
This is not to say e-power can't be done, it can be dove very suitably, if your use fits into the box.. Unfortunately "E" currently fits into a pretty tight box, range wise, for "E" only power.
To do these systems right, many are done very poorly using shoot from the hip math, can be very expensive and then you still need to replace expensive banks every few years. Or yearly, as one of my customer learned the hard way. He was mislead on his installation. Go figure, never seen that before.....
One must also keep in mind that the e conversion is only part of the expense. You also want wind and solar, and don't even think about the sub 1.5k wind gens as they are plain and simple
wallet drains. A good quality wind gen and pole will run over 3k and still won't give you any sort of Ah recovery
speed.
Also don't forget that you are charging two banks not one and at two different voltages.. Tapping off the propulsion bank for 12V is wrought with problems and I would not do that to any of my customers with cruising boats.. 48V to 12V converters are often quoted, cheap & dirty and okay for a day sailor but rarely a cruiser... Let us know how a converter works when your windlass, water maker or other high draw 12V motor load kicks in and toasts it.... This is yet one more way some companies try and get a foot in the door.. You really do not want to tap off a 48V bank for 12V use especially if solar and wind are your only means of recharging. This is another long involved discussion so suffice it to say keep your house bank.
In colder climates with cold water or early or late in the season you can literally cut your capacity by 1/3 or more due to battery temps.. Batteries are rated at 77F-80F and at colder temps, will not put up the same Ah capacity.
One of my customers was quoted an EC with AGM batteries using 80% DOD as average use. The math was also totally bogus and Peukert and water temp/battery case temp for the location chosen was completely and utterly ignored.
Sure you can get multiple years out of LifePO4 at 80% DOD but not a heck of a lot out of FLA or AGM unless you get back to 100% as fast as is humanly possible.... Even the best AGM's can only claim 400 cycles to 80% DOD (LiFePO4 2000+ cycles) but that is in a LABORATORY..
Real world, not lab derived numbers done under carefully controlled conditions, converted to 80% DOD numbers in the real world barely break 200 cycles for lead acid and this is with immediate and full recharges done quickly. Dilly dally on recharging from 80% DOD, using solar or wind, and sulfation will kill these banks faster than the cycles.
Here is some math from one of my customers quotes:
He was told he could easily go 50nm, the system was lighter and solar and wind could recharge him in 24 hours. None of it was true and it bordered on criminal, IMHO...
The only part that was true was that he was told he just had to run his boat slower. Sill the 3 knot figure they threw out made zero sense mathematically.......
To go 50nm takes 16.6 hours at 3 knots SOG
A 5kW motor is pulling 104A at 5000W at 48V
If we figure he was 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.
They also told him the batteries were best located in the keel sump (long full keel). At mid summer his keel sump temps peak at about 62F and off season in the 40's & 50's. This can bring usable bank capacity down to the 120Ah range. 80% of 120Ah's = 96 usable Ah's...
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. This is the most often ignored math EC conversion guys opt not to tell you about. The temp should also be accounted for.
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 he could not even go 5 miles at 3 knots before hitting 80% DOD.... How did they get to that lofty 50nm claim...?????? I have no idea......
Even if we figure the motor at just 50% load/52A/48V you have a 165Ah bank (Peukert corrected/excluding temp) and 132 usable Ah's before hitting 80% DOD with a Peukert of 1.11... I am completely ignoring battery temp in this math which would make it even worse here in Maine....
Even a 50% load still barely gets to 7nm.... Again, how on Earth did they do the math that shows his boat getting to 50nm..????
At 1000 watts or just 20% motor load you are still burning 21A at 48V.
21A X 16.6 hours @ 3 knots/50nm = -348.6 Ah's consumed...
There is no physical way he could get 16.6 hours out of a 200Ah bank even at just a 20% load....???? Thus the lofty, and completely bogus, 50nm range was a complete and utter lie based on the
actual quoted system...
The only thing I can say to help you keep everyone around you honest in this research is to:
Trust Ohm's Law - Many will try to re-wire it, but as of yet no one has been successful..
Convert Watts to Amps at fixed voltage:
Watts ÷ Volts = Amps
eg:
5000W ÷ 12V = 417A
5000W ÷ 48V = 104A
Never Forget Peukert - When dealing with high load applications such as e-boats one simply can not ignore Peukerts Law when dealing with lead acid batteries..
A 100Ah lead acid battery is only a 100Ah battery when used at, and loaded at, 5A at 75-80F, when new or just broken in....
Every other load & temp range either above or below the 20 hour rate at 80F results in CHANGES the batteries 20 hour rated Ah capacity! Cycle, DOD, sulfation and plate erosion also impact Ah capacity.. Anyone telling you otherwise is really not qualified to be doing so.
LiFePO4 cells have minimal Peukert's corrections but they are still not a Peukert of 1.0..... A 100Ah battery with a 1.0 Peukert would provide 100Ah at a 300A load and conversely 100Ah at a 0.5A load. This simply does not exist in the real world... The reality is that a 100Ah LFP pack might provide about 80Ah at 300A (3C) and 115Ah at 0.5A..
Do that with a flooded lead acid 100Ah battery:
100Ah Battery - 300A Load = 33Ah
Caveat emptor, and please do your research....
Hybrid systems have a whole other set of considerations and a number of them are available Nani, Beta, Steyer etc... The above is for "E" only...
