Going Electric: Will the batteries explode ?

[Screwdiver]

Hi everyone,

Last year the indenor 50 HP on my Reinke Secura (35ft steel sailer) quit and i decided to switch to electric propulsion.

This is my setup:
16kw (max 30kw) Motor from a mechanic specialised in converting cars to electric propulsion. This Motor comes with a control unit that requires 84 V DC (as usual it converts it to AC and controls the Motor that way).

The Motor connects directly to the shaft, the three blade prop will turn approx. with the same revs as with the diesel.
Energy is stored in 7 Batteries (230AH AGM) connected in line to provide 84 Volts. Everyone tells me that 50 mm2 Wires are sufficent. The Batteries are dedicated Deep Cycle.
The shore charger ist rated 30 Amps at 84 Volts, a 5 kw Diesel Generator (240V) acts as a Range Extender, i will try whether the Charger will provide enough power to decrease battery discharge significantly.

By now my biggest concern is: If i open the throttle fully, the motor will ask for 16 kw (16,000 W), ignoring losses on the controlboard this means nearly 200 Amps on 84 Volts. Given the battery size of 230 AH this means a time to discharge the batteries to empty (roughly half the capacity) of half an hour.

(I speculate that in normal conditions i will need only a fraction of the available power, so that i will be going on batts for at least two hours and with the range extender around 8 hours)

Now how will the batteries response to these conditions? Means drawing that much energy from them effectivly to produce a short circuit and they will explode ?

And yes i know, battery capacity is defined as possible discharge in 20 h

Opinions welcome ;-)

[Pete7]

Can't answer your questions but what would be interesting is a graph showing kw of power or amps, against speed and therefore range/time.

Is the boat in Berlin? perhaps some speed tests up and down a quite river or canal.

Pete

[Screwdiver]

The installation is scheduled for next week, i will keep you updated. And yes, the boat is in Berlin, first testing will be on a quiet river (The Havel).

[dlymn]

I thought that discharging Lead acid batteries beyond 50% will lead to premature failure. I try to never go below 60%. Lithium batteries can go down lower (20%?) To flatten your battery in less than hour might lead to overheating and expansion of the cells. Your battery might start to bulge as the plates distort. I would guess that your batteries might quickly lose their storage capacity let alone their ability to store electricity. I think that lithium iron batteries might cope better with rapid discharging.

[Screwdiver]

Thank you for your opinion. LiXX-Cells are literally swept off the market by the growing demand of the car industry. I expect to reconsider my battery choice in a few years. I am afraid that the complex management of LiXX-Cells will lead to a higher integration of battery management in the car electronics so that for us, who want to expose those systems to the harsh marine environment (even with enough mai tais) it will be a complex task to use them on board.

Besides: AGM are expected to withstand a DoD of 80% (resulting in shorter lifetime) and have a very low internal resistance, so i think i can give it a try.

(Battery temperature will be monitored)

[ranger58sb]

Originally Posted by Screwdiver

Energy is stored in 7 Batteries (230AH AGM) connected in line to provide 84 Volts. Everyone tells me that 50 mm2 Wires are sufficent. The Batteries are dedicated Deep Cycle.

That would be seven 12V AGMs? Others have said 12V "deep cycle" batteries are often not really deep cycle, but actually closer to dual purpose. (Which might be fine for your application anyway.)

But... what's the footprint of your 12V 230Ah batteries? 8D, maybe?

If so, maybe you'd have physical space for fourteen 6V deep cycle golf cart batteries in series/parallel to 84V? (Lifeline makes AGM versions, if that's critical.) Not sure if that approach would offer any advantage or not, so I only mention it for your consideration... especially if some of the taller versions (more capacity) might also be an option for you.

-Chris

[DotDun]

Your experience will be interesting.....just looking at the high level numbers.

Batteries:
230ah @ 84v @ 80% DoD = ~15kwh, less than 1 hour at full power

Diesel generator w/shorepower charger:
30a * 84v = ~2.5kw, ~16% of full power

I would guess you will have to be very conservative with the throttle, normal cruise speed will need to be no more than 15% of full power. Be sure to keep some reserve in case of unforeseen issues, i.e. weather changes etc. requiring higher than expected power to make port.

Good Luck!

[valhalla360]

Did you get the max RPM at the prop for a 20hp motor matching the 50hp motor in writing?

That makes no sense but is likely because you have a car guy doing a boat. With cars the use pattern is much different. A 120-200hp engine in a small car is typically putting out 50-70hp while running at freeway speeds. They put in the bigger engine so there is sufficient power at low RPM for brisk acceleration. Electric motors in cars can be sized significantly smaller because they have max torque from zero RPM. Thus they develop HP, much more quickly when you press the gas pedal.

With a cruising boat, acceleration rarely is a significant consideration. Engine HP is selected based on what you need to maintain hull speed against a strong headwind and/or waves. 0 to 6kt acceleration isn't typically a major consideration and with the prop able to slip in the water when first starting up, it is far less of an issue.

That said, 20hp for a 35' boat isn't drastically underpowered so it should be OK for most uses (in terms of engine power).

Supplying that power to the motor is a different issue. 16kw at 84V, is over 190amps. Way more than the 20hr rating of the batteries, so don't expect to get significantly less than the rated amp-hrs. The 5kw generator after conversion losses will be lucky to provide for more than 3-4kw.

If you are willing to accept significantly reduced performance (speed and/or range), it should work OK 95% of the time.

To determine if the cables are sufficient, look up voltage drop tables for 190amps and the length of the cable runs.

I would be very interested in seeing the amp draw vs speed given calm conditions and no current at various speeds (ideally, I would like to see them in other conditions also but it gets hard to account for several variables).

[Screwdiver]

I planned with these requirements in mind:

Hull-Speed is 6 knots, replacement near 5 tons
(The Diesel pushed her to hull-speed with less than 2000 rpm, which meant given the gearbox with 1.9:1 roughly 1000 rpm on the prop)

Motoring only in/out harbor 1h

Tight budget now, even tighter budget when en route

[ZULU40]

Originally Posted by valhalla360

Did you get the max RPM at the prop for a 20hp motor matching the 50hp motor in writing?

ought be about torque unless one intends to run flat out all the time
Significantly different power curves, where electric motors produce max torque at near zero RPM

[roland stockham]

I am wondering why you are using deep cycle cells? You are looking at C0.5 to about C3 discharge rates. Deep cycles and aimed at C10+ and start to really pay at C20 which is the opposite. If you go this route I think you may need a much higher capacity. My choice if you can find room to fit them would be NiCd, Commercial ones will to repeated 100% discharge and easily handle discharges in a few min (there are used as power backup for places like hospitals to fill in while the generators start and stabilize). Cost is similar but about 50% larger and 25% heavier. However because of the 100% discharge you can reduce bank size. Charger has to be current limited but you can charge to 80% in 20min. You need a good bat box as the alkaline electrolyte is more dangerous and if you short them they will dump 50% of their charge in a few seconds

[robbievardon]

Screwdriver, talk to the experts, http://electroprop.com. They are in Santa Barbara, but I am sure you can get some of the essential details from them. There used to be a Company in Europe that did electric drives for boats,sorry I can*t remember the name but it was 20 or 30 years ago.In their system the folding prop became a recharging generator when under sail. Try a web search. Best of luck

[bjymd]

Torqueedo in Germany has a lot of experience in this arena with very high end boats.

[ramblinrod]

Originally Posted by DotDun

Your experience will be interesting.....just looking at the high level numbers.

Batteries:
230ah @ 84v @ 80% DoD = ~15kwh, less than 1 hour at full power

Diesel generator w/shorepower charger:
30a * 84v = ~2.5kw, ~16% of full power

I would guess you will have to be very conservative with the throttle, normal cruise speed will need to be no more than 15% of full power. Be sure to keep some reserve in case of unforeseen issues, i.e. weather changes etc. requiring higher than expected power to make port.

Good Luck!

Ummm at 1C discharge, he is going to see no where near the .05C (20 hr A-hr rating).

[Dave22q]

Have you consulted any experts? It seems that you are way under resourced in reserves. One hour reserve is very thin.