Charging 3 batteries

[Guest]

I have just fitted a bow thruster and will be placing its 105 ah battery alongside the motor to minimise the high current cable run. I also have a 240ah (2X120)domestic battery bank and a 90ah dedicated starter battery as normal with the starter battery protected by a diode. The current battery supply switch is the normal OFF/1/BOTH/2 arrangement. Everything is 12 volt The alternator is 1987 and is rated at 80ah I think.

Can anyone advise me how I might suitably control the charging of the now 3 battery bank so that no damage can be done and also that they all do get charged? Is there a reference book on this subject? Has anyone bought any special kit for this purpose?

 

[ccscott49]

You need a three battery bank, smart charger, this will solve all your problems. Adverc, Sterling etc. Is easy and simple to fit, just follow the instructions. Or if you dont feel you can do it, it wont be an expensive job for a boat leccy!

 

[charles_reed]

Fit a 3-way splitting diode, with leads to each of the batteries.

That avoids any flat battery discharging the others.

I would recommend having a separate rotary switch on the thruster battery and would also consider making it part of the total battery system.

You would be well advised to get a smart regulator such as the Adverc, that's in parallel between alternator and splitting diode input.

If you do so I would recommend beefing up the belt drive (get a serrated heavy duty one) and also increasing the section of wiring from alternator to splitting diode.

You may find your 80 amp alternator somewhat underpowered for the task you're expecting it to discharge, so a 25 amp pulse charger for connection to shorepower would be sensible.

Do you really need to reserve a 90 amp hour battery purely for starting the motor?

Speak to Brian or Trevor at Adverc on 01902 380494, they have all the gear and are not your usual marine rip-off merchants.

 

[Guest]

Nigel Calders "Boatowners Mechanical and Electrical Manual" (?) will give you what you need.

 

[chippie]

A local magazine here in NZ (Boating New Zealand December 2001) is currently running a series of articles by Nigel Calder and his current thinking is that if at all possible two alternators are the preferred option. One with a smart charging setup for the house and ancilliaries and the other with an automotive type setup for the starting bank.There is a linking switch arrangement that is only for emergency charging/starting to the other banks.
It seems like good thinking to me.

 

[pvb]

Now, this may be controversial..

...but I'm not sure that installing a dedicated battery next to the bowthruster is necessarily the best solution. To start with, you'll need to isolate it in some way for charging purposes (others have suggested a 3-way blocking diode) and this will incur expense. Secondly, a simple blocking diode won't stop high current draw along the wire from diode to thruster battery in circumstances where you use the thruster when the engine's running (ie everytime you use it!) so you'll either need to install some clever switchgear or use quite hefty cable. Again, expense. Then there's the consideration of the dedicated battery adding even more unwanted weight at the bows.

So my proposal would be to add an additional battery to your domestic bank, and then simply run heavy cable through to the bowthruster. Cable really isn't that expensive (even if you have to use 50 or 70 sq mm stuff). You'll gain additional domestic battery bank capacity, which means that in normal use your batteries will be discharged less heavily (so they'll last longer). And, you won't have to make any changes to your existing charging arrangements.

In recommending this, I'm assuming that you've bought a fairly small thruster (because if you only have 240Ah of domestic battery capacity you presumably have a moderately-sized boat).

You say you have diode isolation for the engine starting battery. If you don't already have one, you must get a smart regulator (Adverc, etc) in order to ensure that your batteries are charged as quickly as possible. And I'd suggest that increasing your domestic bank to maybe 500 or 600Ah would be another benefit. If your alternator is 80A, it will rarely be able to pump anything like this current into a 240Ah domestic bank (most of the time it's more likely to be charging at around 25A) whereas with a bigger battery bank you'll be able to accept a bigger charging rate and therefore replace the used Ah more quickly.

As Chippie has pointed out, the optimum solution is to use 2 alternators, enabling you to dispense altogether with diodes, but that's not necessarily feasible for every engine installation.

Incidentally, the Vetus catalogue has a useful chart showing recommended cable sizes for various combinations of bowthrusters and cable lengths.

 

[HaraldS]

Re: Now, this may be controversial..

I think that's indeed controversal and always a compromise. For one thing the extra batterie puts extra weight into the bows, where you normally would like to avoid it. Second I would be concerned about not fully charging the bow battery because of the voltage drop on the long wire forward. Your regulators will likely have their sense line connected to one of the closer batteries and will take those as the reference. It's a bit like mixing different batteries, not ideal.
Also as others pointed out, an otherwise unsused battery is a lot of dead weight.