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Bow thruster - battery type; and charging from shore power


Hawksbill

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I have a problem with the bow thruster in my NB and am seeking some battery and charging related advice please so I can decide best way to move forward.

 

I purchased it a few months ago - first boat I’ve owned and loving it, although I’ve rented boats for holidays over many years - and it came fitted with a 24v / 8kw bow thruster. [Aside: I know some folks opinions on BTs , I know they’re not essential but at 68’ long I’ve found it quite useful as a manoeuvring aid at very limited times when reverse mooring etc, so appreciate to keep advice away from ‘you don’t need one’ 🤣].

 

Current immediate problem is it will provide initially good thrust, but for only 2 or 3 seconds before stopping completely. I believe the reason is, because initial thrust is good before expiring the current batteries are shot - so once I confirm this I will need to replace them. It currently comes fitted with 2 x 12V 180aH dual purpose (which I understand means suitable for both general purpose or starting purposes) leisure batteries, sited very close to the BT. So I have 2 questions:

 

1. Someone has told me that although the current batteries are ‘dual purpose’ leisure batteries so theoretically should be fit for purpose, they would recommend replacing with dedicated starting/cranking batteries. These would however be a lot more expensive, so does anyone have experience/knowledge of the trade-offs of dual purpose vs dedicated starter type?

 

2. Once I select and fit the replacement batteries, another dilemma: currently the BT battery is only charged by the engine - not shoreline. But looking at the scenario of using a BT, it’s typically used for a short time with very high power - and often when coming to the home mooring, just before switching off the engine and switching to shoreline power. I’ve heard that unless you can ensure the battery is kept fully charged for most of the time, a new battery will quickly be destroyed. I’m not a live-aboard and can’t  typically run the engine for an hour after coming home, so in folks experience should this be a real concern? If so I’ll then look at options for adding another dedicated 24V charger which could permanently trickle charge from shoreline power (main concern being to add more cost/complexity)

 

Any constructive advice gratefully appreciated. 

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If the domestic bank is 24 volts I suppose you could use a VSR to parallel the BT and domestic  battery only when the charger is on. With careful selection of sense voltage, you could probably get it to drop out when the charger goes into float. Doing that might also charge the engine battery, but without more details of the engine circuit, I can't say.

 

Truck start batteries may well be cheaper, than the so-called leisure batteries that have been described on here as start batteries with handles.

 

The BT may well have a thermal cutout so if it's overheating because of seizure, a fouled prop, or lack of voltage it might be the overheat cutout and then after it cools down it resets itself ready for next time.

 

 

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BT takes  a heavy current for a short time, so high cranking current is more suitable.

 

Leaving batteries partially discharged is what kills them. A Voltage Sensitive Relay between BT and cabin batteries will allow the same shore powered charger to be used.

 

Tony always beats me to it!

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I would stress that VSR's seem to come with different drop out (cut out) voltages, so I would choose a high one so it drops out when the charger goes into float. Otherwise, it is possible that under certain circumstances a  dodgy bow thruster battery just might discharge the domestic bank.

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1 hour ago, Tony Brooks said:

If the domestic bank is 24 volts I suppose you could use a VSR to parallel the BT and domestic  battery only when the charger is on. With careful selection of sense voltage, you could probably get it to drop out when the charger goes into float. Doing that might also charge the engine battery, but without more details of the engine circuit, I can't say.

 

Truck start batteries may well be cheaper, than the so-called leisure batteries that have been described on here as start batteries with handles.

 

The BT may well have a thermal cutout so if it's overheating because of seizure, a fouled prop, or lack of voltage it might be the overheat cutout and then after it cools down it resets itself ready for next time.

 

 

Thanks Tony and Tracy for the really helpful advice. I’ll look into truck start batteries, any ideas on sources for these? 

 

I can confirm the domestic batteries are 24V, which are charged by a Mastervolt Mass Combi 24/2500/60 inverter/charger, so your suggestion to use a VSR to charge both domestic and BT batteries in parallel sounds a good way forward. 

 

You raise a very good point on the BT thermal cutout which it does have - so can’t rule out cutting out due to seizure / fouled prop, I thought it was unlikely though due to the initial ample 2-3 second ample thrust before cutout, I thought if it was due to a seizure you wouldn’t even get that hence my immediate thought being a shot battery. I haven’t worked out (yet) how to get into the BT tunnel to check if there could be a foiled prop, I’m assuming this could only be done with the boat out of water - as there’s no weedhatch equivalent like on the main prop , sorry if I’m missing something obvious as it’s all new to me. 

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If the batteries are suspect, then I could easily be low voltage. Electric motors are funny things in that the slower they run, the more current they draw. So seizure or a fouled prop would cause it to run slowly, but so would low voltage. That seems counter to Ohms law, but in involves inductance and that is different.

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If the boat is on a shore line then you could have a second mains battery charger at the bow. This would keep the 24V wires between the charger and battery short and save having to run very thick 24V wires from stern to bow to minimise voltage loss. If too much voltage is lost, then the bow batteries never get recharged properly, shortening their life. This could also recharge the bow thruster batteries while cruising. The alternator would charge the house batteries, near the stern. The inverter would convert some of this to 240V, which the bow charger would use to charge the bow thruster batteries.

Jen

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5 minutes ago, Jen-in-Wellies said:

If the boat is on a shore line then you could have a second mains battery charger at the bow. This would keep the 24V wires between the charger and battery short and save having to run very thick 24V wires from stern to bow to minimise voltage loss. If too much voltage is lost, then the bow batteries never get recharged properly, shortening their life. This could also recharge the bow thruster batteries while cruising. The alternator would charge the house batteries, near the stern. The inverter would convert some of this to 240V, which the bow charger would use to charge the bow thruster batteries.

Jen

 

True. But he should already have large enough 24 volt cables running from the back of the boat that could be used. Not "should", i would not bank on t.

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40 minutes ago, Tony Brooks said:

 

True. But he should already have large enough 24 volt cables running from the back of the boat that could be used. Not "should", i would not bank on t.

Thanks Jen, also an excellent alternative suggestion. Given the engine 24V alternator at stern is currently charging the BT battery, then I’m assuming as Tony says there must (or should) already be thick 24V cables running from stern to bow, however a second independent charger for the BT potentially sounds a more simplified solution. 

 

Also sounds like we’re agreed it’s really advised to have that BT battery permanently topped up rather than rely on purely engine charging which is good feedback thanks. 

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My VSR switches charge between start battery and. BT batteries and it does this whether charging from the alternator or battery charger. The battery charger itself is connected to the domestics & start battery (multiple output terminals) and the VSR does the rest. 

 

It's not this model but I have something similar from BEP

 

https://shop.chastheboat.co.uk/products/bep-dvsr-digital-voltage-sensing-relay-12-24v-aq8-90060?utm_campaign=gs-2018-10-25&utm_content=sag_organic&utm_medium=smart_campaign&utm_source=google&variant=31789406158906

 

Switching between start and BT batteries is a much more sensible option than between domestics and BT, because your start battery is generally fully charged anyway, but I'm assuming like me you have twin alternators?

 

I bought some Hancook sealed lead acid batteries for the BT (in 16 years they've been replaced once). The rest of my batteries are open lead acid and ideally you'd want the same type of battery being charged by your charger because open wet lead acid can be charged at 14.8v max and sealed at around 14.4v. Many multi-output chargers will only allow one battery type setting. However, I looked into this and found that the Hankook batteries I'd bought can go up to 14.6v and with the voltage drop down the cable that's all they get anyway. 

 

I do live aboard but even if I didn't my charger would still stay switched on if I had shore power. I judge the risk of bilge pumps running out of power and the boat flooding is probably greater than the risk of a fire from faulty charger. They are designed to be left switched on after all. Plus the benefits are that your batteries stay in great condition.

Edited by blackrose
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4 hours ago, boat911 said:

When replacing batteries, you need to think about LiFePO4. You will find all the information about it here.

 

Curious. I clicked the link and there was naff-all information about designing LiFePO4 battery installations.

 

it just took me to an on-line shop selling some, but not all of the kit you might need. My advice is LiFePO4 battery system design is a minefield and this site is no help at all. 

 

 

 

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22 minutes ago, MtB said:

 

Curious. I clicked the link and there was naff-all information about designing LiFePO4 battery installations.

 

it just took me to an on-line shop selling some, but not all of the kit you might need. My advice is LiFePO4 battery system design is a minefield and this site is no help at all.

 

I would suggest that the OP would get far more information and knowledge by searching the forum for LiFePO4

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11 minutes ago, Tony Brooks said:

 

I would suggest that the OP would get far more information and knowledge by searching the forum for LiFePO4

 

I think the complexity and expense of a LiFePO4 battery installation makes LA a better choice for a bow thruster.

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7 minutes ago, Tony Brooks said:

Tends to be my feeling as well for the next few years.

 

Even for a main domestic battery bank the products needed to get the best from them are simply not widely available. Witness the discussions searching for a driver for bistable relays, and the other subject little recognised that they need to be cycled up and down rather than kept at full or even high SoC to get max life from them. There are NO control products on the market that do this that I've noticed. 

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10 hours ago, Tony Brooks said:

 

True. But he should already have large enough 24 volt cables running from the back of the boat that could be used. Not "should", i would not bank on t.

Good point. Worth @Hawksbill checking the cross sectional area of the wires and calculating what the volt drop is. If it is inadequate, then the new batteries will have a short life. If new wires need to be run, then it may well be cheaper to install a mains charge, as well as much less hassle and disruption to the boat interior. Twin core 25mm^2 cable, for example, is coming in at £12.81 a metre, so over run of say 14 metres, a mains  to 24V charger would be similar in material.costs with minimal disruption.

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6 hours ago, boat911 said:

When replacing batteries, you need to think about LiFePO4. You will find all the information about it here. Or call them and get advice

Lithium batteries are the wrong choice for a bow thruster because they are not normally rated for the high current demand unless you install a large bank

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5 minutes ago, PeterF said:

Lithium batteries are the wrong choice for a bow thruster because they are not normally rated for the high current demand unless you install a large bank

 

I'd agree with this. Although LiFePO4 batteries are capable of truly massive discharge currents, this leads to a very short cell life and surprisingly, manus generally recommend a max discharge rate of 0.3C. So using LiFePO4 as a bow thruster battery demands a pretty big battery to avoid pushing that limit.

 

What current does a typical BT draw anyway? 300A or so I would guess. 

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3 hours ago, blackrose said:

My VSR switches charge between start battery and. BT batteries and it does this whether charging from the alternator or battery charger. The battery charger itself is connected to the domestics & start battery (multiple output terminals) and the VSR does the rest. 

 

It's not this model but I have something similar from BEP

 

https://shop.chastheboat.co.uk/products/bep-dvsr-digital-voltage-sensing-relay-12-24v-aq8-90060?utm_campaign=gs-2018-10-25&utm_content=sag_organic&utm_medium=smart_campaign&utm_source=google&variant=31789406158906

 

Switching between start and BT batteries is a much more sensible option than between domestics and BT, because your start battery is generally fully charged anyway, but I'm assuming like me you have twin alternators?

 

I bought some Hancook sealed lead acid batteries for the BT (in 16 years they've been replaced once). The rest of my batteries are open lead acid and ideally you'd want the same type of battery being charged by your charger because open wet lead acid can be charged at 14.8v max and sealed at around 14.4v. Many multi-output chargers will only allow one battery type setting. However, I looked into this and found that the Hankook batteries I'd bought can go up to 14.6v and with the voltage drop down the cable that's all they get anyway. 

 

I do live aboard but even if I didn't my charger would still stay switched on if I had shore power. I judge the risk of bilge pumps running out of power and the boat flooding is probably greater than the risk of a fire from faulty charger. They are designed to be left switched on after all. Plus the benefits are that your batteries stay in great condition.

Thanks for the suggestion on the Hancock batteries. Re:”Switching between start and BT batteries is a much more sensible option than between domestics and BT, because your start battery is generally fully charged anyway, but I'm assuming like me you have twin alternators?”, yes I have twin 24v/12v alternators feeding domestic and starter battery respectively when engine is running. 

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6 hours ago, boat911 said:

When replacing batteries, you need to think about LiFePO4. You will find all the information about it here. Or call them and get advice

 

LiFePO4 bow thruster batteries?

 

I think I understand the benefits for domestic battery banks for liveaboards, but LiFePO4 bow thruster batteries for a leisure boater seems a bit over the top and unnecessarily expensive to me. It could end up costing a few thousand pounds for the system, whereas all the OP really needs to spend is about 500 quid on a 24v charger, a VSR and a couple of new BT batteries.

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58 minutes ago, MtB said:

 

I'd agree with this. Although LiFePO4 batteries are capable of truly massive discharge currents, this leads to a very short cell life and surprisingly, manus generally recommend a max discharge rate of 0.3C. So using LiFePO4 as a bow thruster battery demands a pretty big battery to avoid pushing that limit.

 

What current does a typical BT draw anyway? 300A or so I would guess. 

I think discharging at 1C is their normally rated discharge current, so you’d need 3 or 400Ah. No doubt they would last longer if the discharge was kept below 0.5C but then again with the life being so long, does it matter?

 

The main impediment is the need to have some protection circuitry with some means to electronically isolate the battery in the event of over or under charge. It is having that circuitry capable of passing 300A or so which I think is the limiting factor making LiFePO4 not really advisable for a bowthruster, plus the complexities of charging it correctly. I’m a big fan of Li for domestic batteries but I agree it would be best to stick with LA for a bowthruster.

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