Why have a thruster battery?

[serpa4]

I was looking at the C30 wiring diagram. I have a C30 with a generator.
Why do we have a separate thruster/generator battery?
I get that it is for starting the generator and running the thrusters.
However, if you re-wire for the house bank, you'd gain 100Ah of capacity or 25% more house capacity.
My logic, tell me if I'm wrong.
The generator can easily start off the 4 (same G31) batteries as the single (G31) thruster/generator battery.
If the engine battery goes dead, you start with the house batteries using the combining switch already there in the boat.
If the house batteries go dead, then you cannot start the generator. However, you can start the engine and it will charge all batteries from the 115amp alternator. Even if you only need to run the engine long enough to charge the house battery enough to start the generator. That is IF you kill the house enough not to start the generator.
If you are using the thrusters, then the engine is highly likely to be running and providing charging to the house batteries running the thruster. And there is 500Ah of house capacity with 5 group 31 batteries. This should never be an issue. Maybe you don't want to kill the house batteries with the thruster, but how would you use entire 500Ah of capacity with the thrusters? Not even remotely possible (especially with the engine running anyways).
Why not just move the thruster battery to the house and also ditch a battery combiner switch while you are at it?


On a separate note. Someday I'll jump to lithium.
I could do 2 100Ah battleborn batteries which are rated to 100% DoD. This is the same as 4x 100Ah AGM G31 drained to 50% DoD.
Then, I can find Lithium start batteries rated at 70Ah and 1200CCA for the Volvo D6. This single battery is about 20lbs.

3x Lithium house: 3x30lbs=90lbs and 300Ah usable (2x house +1 more for thruster if it is even needed).
1x Lithium start: 1x20lbs=70Ah for engine at 1200CCA
Lithium weight: 90+20= 110 lbs
AGM:
4 house at 50Ah usable = 200Ah
6 x Group 31=6x80lbs=480lbs

Weight savings: 370 lbs or the weight of a dinghy and engine and davits and still be ahead by 110 lbs.

 

[Levitation]

A separate thruster / winch / generator battery is not necessary if you are marina kept. If you do significant anchoring out or camping it will save you from sitting with the engine running for an hour before you can get under way.

As far as spending thousands of dollars on fancy batteries, it's not for me. Standard batteries properly maintained cost a fraction of lithium and can be replaced at the nearest Autozone store when you are DOA in some tidal creek far from the nearest supply of lithiums.

 

[serpa4]

I don't understand your reasoning for running an engine for hours before getting under way. Please explain.
PS. Lithiums are cheaper in a long run for the same capacity. It may take 10 years to realize that though.

 

[BB marine]

Your thinking about the thruster/house combination is spot on. Why have a battery bank sitting back there that has limited use? Combining the thruster/generator bank to the house bank will add AH to the house bank or in a situation where the thruster/generator bank gets discharged from heavy use of the thrusters (wind and current when docking). The ability to crossover to the house bank would be a plus.

Disadvantage of having them combined all the time is probably minimal. We all have opinions and mine is I want to have a way of keeping them seperate. The electronics are sensitive to voltage drops, quick voltage drops. This could happen if the thruster and house bank were connected all the time. With a bank that large you would have to be laying on the thrusters to cause an issue. But in the event that either bank had a bad battery then you may end up with an issue in both systems instead of one.

A good solution is to install a cross over battery switch between the Thruster/generator bank and the house bank. This gives you the best of both worlds.

I installed a 4 position battery switch replacing the single position inverter battery switch. I use this as my cross over switch. My thought was I will need the thruster bank combined when at anchor and when using the inverter. The 4 position inverter switch will combine the house and thruster bank when the selector is positioned on both. It will use just house bank when the selector switch is on battery 1 and will use the thruster bank when on battery 2 ( I have never used this option but it can be used if I choose) This gives me more options. Inverter on with battery (1) house bank while running the engine ( all banks are combined because the ACR's are closed unless there is a large voltage drop for more than 10 seconds). Inverter on both when at anchor combining the house and thruster banks. Using the inverter battery switch for dual purpose eliminates the need of an additional switch.

Lithium batteries is a thought. The reason I would like to go with them is the weight ratio ( lbs per AH ) My experience with battery weight in the aft section of my stern heavy Cutwater is additional battery weight effects the performance. I went from 44lbs maintenance free batteries to 67lbs NorthStar AGM's. 23lbs per battery x 4 =92 lbs increase. Same equipment installed just additional weight from the Northstar batteries (loss of 1kt) In a boat that makes 19kts WOT the loss of a KT is a lot. Enough that I regretted spending the additional 600.00 for the AGM's over the maintenance free. Boats that do not truly plane from design but run at planing speeds from HP. Additional weight is a large contributor to loss of performance. I never realized how much until I experienced it with my C26.

Example: I had a 26' Searay 5.7 EFI Bravo II drive. WOT 4800rpm 42MPH. If 1 passenger or 4 passengers where sitting in the cockpit the performance at WOT or planing speeds did not change. The performance did change at displacement speeds and did change while the boat was working its way up to put the boat on the pad or plane. ( it took more power to get it on plane and more trim tab) Once it was up and planing I could back the throttle down and we were cruising the same 1 person or 4 persons and additional equipment.

The Cutwater at least my C26 never really planes. It is trying to get up on the pad but by design there is not a pad to get up on. It is a semi displacement hull. It is always trying to plane but gets harder to do with additional weight in the stern. If I have 4 passengers on board I know they are there at anything above hull speed but see no difference at hull speed. Just the opposite of the planing hull.

The reduction of battery weight is a plus, The lithium technology is a plus, The cost at this point is a minus. When the installation and technology for adapting lithium to the engine charging systems is less expensive and the batteries become less expensive I will consider them. Until then I will stick with old school. For those that have installed them Cudo's to you and I hope to see threads on the battery performance and overall opinions of the decision to use the lithium.

 

[Nwdiver]

I don't understand your reasoning for running an engine for hours before getting under way. Please explain.
PS. Lithiums are cheaper in a long run for the same capacity. It may take 10 years to realize that though.

I believe his reference to running the engine refers how long it can take before the house batteries are charged before safely getting underway. Draining the house batteries is very easy to do, leave a few lights on, an air conditioner. If you have your thruster on the house batteries and they are drained it will provide a good opportunity to exercise your boating skills to get out of a tight anchorage 😀. Also, getting underway with no navigation or radio is a bit unsafe

 

[GaylesFaerie]

My 2012 R27 was modified by the original owner to combine the Thruster with the House battery bank for the reasons you mention. You can view his project description here: viewtopic.php?f=5&t=3665&p=27090

GF

 

[Chimo]

I liked the idea of being able to combine thruster with house when on the hook but I wanted a disconnect. There's a photo of the switch layout in our album.

As an aside, I'm really glad to have separate charger and inverter. The stories here about the combined unit dropping into inverter mode in case of power failure don't sound good to me. If I lose AC I know I lose battery charging but the loads are minimal in the winter. I don't want my house batteries trying to run the dehumidifier and the heater. Have I misunderstood this issue?

 

[Scuba Too]

In my case having the separate thruster battery saved me from the ignominy of being towed back to port. :lol:

My house and engine battery both "fell off the cliff" with respect to holding a charge. On my last trip out to Catalina I was having to run the generator daily to top off the house battery. I was on the island for 4 days. On the last day I went to start the engine and not enough juice to start! I hadn't used the engine and had disabled it at the helm so I didn't think it'd be a problem (hadn't really thought of it actually since engine starts hadn't been an issue). I then crossed the engine to the house battery bank - but no joy. Not enough juice between the five batteries. Fortunately I did have the thruster battery and was able to start the generator. After an hour of recharging of the gen I was good to go. Now perhaps the remaining juice on the house/engine batteries might have been enough to crank over the gen since its a much smaller engine - but I do feel better having the backup. I did swap out the batteries and replaced the engine battery with an Optima so everything is good now.

 

[knotflying]

If the house batteries go dead, then you cannot start the generator. However, you can start the engine and it will charge all batteries from the 115amp alternator.

This is not necessarily correct. If the house batteries go below 12.1 volts the relays will not allow charging. You would have to start the engine and then switch the parallel switch to on for the batteries to charge.

 

[SJI Sailor]

@Chimo -- with the Kisae inverter (and presumably many others) there is a setting "In0" where the inverter will not supply AC power unless shore power is coming in. We set that for peace of mind. Another option -- if I understand correctly -- but easier to forget is to turn off the inverter "power" button.

I love the idea of a parallel switch for house + thruster only (not engine). Will look into that sometime on our (long) project list!

 

[Chimo]

@Chimo -- with the Kisae inverter (and presumably many others) there is a setting "In0" where the inverter will not supply AC power unless shore power is coming in. We set that for peace of mind. Another option -- if I understand correctly -- but easier to forget is to turn off the inverter "power" button.

I love the idea of a parallel switch for house + thruster only (not engine). Will look into that sometime on our (long) project list!

Thanks for the clarification, much appreciated! The absence of the setting you describe would have been concerning if I had one of those units.