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Electric propulsion sailing cat – Decentralised power system Mark 2

Hi All

Many thanks to the community for comments on my first post.

We have started building our 52’ ocean cat and looking to finalize electrics. Have taken on board previous comments and researched options. Our goal is to achieve a fully electric sailing boat with minimal weight.

We are keen to decentralize batteries into two banks, one each hull, adjacent the highest loads - electric motors. This design reduces overall weight by reducing heavy cabling, improves weight distribution & provides redundancy in event one bank fails.

Was previously concerned about managing and balancing individual banks. I believe the solution is to link the banks with a common charge cable. I understand that electricity, like water, will find its way to the lowest point (voltage) and flow until all voltages on system are balanced. With some extra switches, this charge “tie” cable could also be used to power up equipment on a failed bank from the working bank.

Main parameters are listed below. We have sized banks and gen set to provide short term (say 30mins) power for max. elec propulsion after which we are happy with 50% continuous via 16kw gen set.

Main parameters & equipment:

  1. Propulsion and galley services all electric.
  2. Motors 2 x 15kw (DC 48V), Input Voltage range: 25.2v – 60v
  3. 2 x 48V 10kwh battery banks - (HE Lithium 24V 200A x 2S) one in each hull adjacent elec motors. HE batteries preferred as highest energy/kg density. Although note voltage discharge profile comment below.
  4. 2 x Lynx Ion 400A BMS,
  5. House loads DC - 24V (peak 2.3kw) – Orion-Tr 48/24-16A (380W) x 6P. Input Voltage range: 40v – 70v
  6. House Loads AC 240V (peak 4.8kw) - Phoenix Inverter 48/5000w. Input Voltage range: 38v – 66v
  7. Gen set 48V DC 16kw (continuous, water cooled alternator) – located in port hull.

Have attached a Schematic showing concept.

Have attached a Victron HE Lithium Battery – 24V 200A discharge profile that I stumbled on.

My questions for the community:

  1. Will the charge cable from charge bus linking Charge inputs of each bank’s BMS also balance voltages between banks as I hope?
  2. “Allow to charge” outputs from Ion BMS’s can be used to manage charge input circuits to charge BUS ie one to solar & one to gen set. 2 BMS signals to one solar input should work if balance principle in 1. applies.
  3. “”Allow to discharge” outputs from BMS’s can be used to manage loads. Not yet sure of configuration but guess a NO contactor in load circuits.
  4. Elec motors have regen function which can be activated while sailing. This comes back to batteries via motor load cable. Not sure how BMS handles this – maybe a manual monitor situation when regen operating or is BMS smart enough to read this?
  5. HE Lithium discharge profile. I understood Lithium battery voltages stay relatively constant during discharge except at the tail ends. The attached graph which purports to be a Victron HE 24V discharge profile shows a fall in voltage proportional to SOC%. Our planned equipment appears to be fairly tolerant on DC Input side so maybe a non-issue?
  6. Orion-Tr 48/24-16A (380W). This appears to be the largest 48v DC:24v DC converter available from Victron range and can be a paralleled up. It is isolated which is good for voltage spikes in event of primary/secondary break down but not as efficient (around 89%) cp to non-isolated at 93%.
  7. Inverter function of Phoenix & Multiplus are equivalent, weight same and cost difference marginal. While not planning to use 240V grid hook up maybe worth the investment to use a Multiplus & have this option in future?
  8. Assume DVCC via CCGX / Venus GX will be able to monitor and manage all this great equipment?

Would be truly grateful for any comments, suggestions & feedback.

Regards Rob

GF520 Elect schematic V3.pdfHE 24V 200A lithium battery discharge profile.pdf

Lithium BatteryPhoenix InverterBMSorion converter
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Not an answer, I'd be interested to know what DC genset you're looking at. Fischer-Panda, Polar, Eniquest, or...?

3 Answers
John Rushworth avatar image
John Rushworth answered ·

Hi Rob,

As you may know I keep the Victron blog. Nice project btw which I’m keen to follow.

I use LiFePO4 12V Victron’s in series in my electric sailing yacht and the HE battery in my motorhome.

One comment re battery choice is that whilst the HE is very good in terms of volume and weight it’s rated at 0.3C continuous and on the basis you plan 2 x 200Ah HE in series for each hull then that is only 200 x 0.3 x 48 = 2.88kW continuous discharge if I have understood you correctly. Not sure where you got voltage profiles but I note that is 0.35C. HE also has fans and is more exposed to the marine environment than say:

https://www.victronenergy.com/batteries/lithium-battery-12-8v

Say you had 4 of these in series in each hull or 2 of the 25.6V ones depending on the Ah and kWh useable you need for range, then say you had 2 x 25.6 200Ah in series (call it 48V as you’ll have some voltage drop via cables and not quite 100% voltage at motor on full throttle) then you have 48 x 200 = 9.6 x 0.8 for max useable energy gives 7.68kWh useable energy per hull/motor - and the rated continuous discharge is use less than 200A so at you have 48 x 200 = 9.6kW continuous power which is closer to your motor rating. Maybe you would in that case be better off with 4 x 12.8 x 300Ah in series with 300A continuous discharge at 48V for more continuous power, increased range and longer life due to a better load.

Also note greater cycle life of these Smart batteries compared to HE.

John



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Hi John

Thanks for your answer and apology for not responding sooner, Been very busy with boat construction. Looking at the specs more closely - you are 100% correct. The HE whilst having its applications is not up to the high discharge pattern likely on 15kw elec motors. I think this would be a great house battery as you use on your motor home. Looking at the Smart range - the clear winner is the 12,8/200-a. High energy density with good discharge & life cycle. I believe this battery is new and has not been released as yet. Any idea when it will be available and price ?

Regards Rob

kai avatar image
kai answered ·

Re: #4 - as far as the BMS is concerned, regen looks like a charger. All it sees is current going in/coming out of the battery pack. I don't think the BMS differentiates between loads and chargers on a channel basis - at least this is what the image on page 6 of the manual implies. All loads/chargers are funneled through the main safety contactor at the middle of the BMS unit.


#2,#3- Have a think about how you want to assign bi-directional devices (load + source such as motor and any combined charger/inverter). Could also use additional contactors and make a 3rd bi-directional power bus. Depends on how you want to (safely) shed excess load/supply, under all normal and abnormal conditions.


#7 - combined charger/inverter is a plus and minus. If you separate the charger and inverter you have more flexibility, and improved resilience in event of single failure.

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John Rushworth avatar image
John Rushworth answered ·

PS regen on my boat due to LWL, monohull, hull speed of 6 knots is but around 80W @ 5.5 knots. You will get much more with your Cat for obvious reasons. Regen simply goes straight back into the battery and Cyrix relays control voltage cut off. To regen I simply click the throttle into the first detent tick over position and off it goes in regen via my controller. Nothing to do other than that than accept the increased drag and slight drop in speed. The main thing is to have a way not to regen at a higher than rated battery charge voltage. I use a fixed 3 blade, slightly higher pitch than equivalent diesel. Personally I’d prefer to use a Flexofold two blade prop but no room in my aperture. But as my hull speed and therefore regen is so small I usually freewheel my fixed prop anyhow.

John

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