Biggest battery for 40amp alternator ?

V

Vronsky

Well-Known Member
Joined
8 May 2014
Messages
72
Visit site
Dear All,
My outboard has an alternator of 12V - 40amps >
what would be the biggest battery that this alternator could charge properly ?

I'm experiencing voltage drops when cranking that malfunctions some electronics:
perhaps a bigger battery (single one on my boat) might be the simplest solution.

THANKS,
V.
 
I don’t think there is any size limit on the battery you can have (within reason). Your alternator will keep pushing it out until full charge is reached. What we don’t know is what voltage your electronics cuts out or the cranking voltage drop for different sizes of battery. I’d be tempted to buy a second battery same as the one you’ve got and have cranking on one and electronics on the other with some kind of switch so both are charged together. There may me some kind of clever electronics that do this automatically
 
Thanks.
Yes, adding a second battery is also possible but will be a bit more substantial.
My engine has a seperate NMEA2000 network, which shows no problems, just the CAN-bus datacommunication of the bowthruster. Maybe CAN-bus is more sensitive to voltage drops.
 
Agree with the above by DavidJ ^^^^^^

We need to know more, what size battery, do you also have a shore powered charger?
If you are relying on the OB only charging the battery then it may never get to 100% SOC unless you are doing many hours of boating. If using a charger then rule of thump would be around 10% charge rate to battery capacity but saying that a 5amp charger will look after a 400 ah bank if connected 24/7.

By all means add an extra battery but if you are only "topping up" via the engine alt to ~75% capacity because of short trips you may still have some problems with volt drop.
 
CAN-bus datacommunication of the bowthruster. ??????

Why are you using the bowthuster whilst starting the engine? A little confused here
 
It's a proportional thruster with joystick control >
https://www.vetus.com/nl/about-the-bow-pro

I put the thruster on standby when I crank the engine.
This cranking causes a voltage drop, which creates a malfunction of thruster's CANbus network.

Adding a second battery to an older, different existing battery is not recommended; I also have a 50W MPPT PV to top off the battery, and if this PV works well with 2 combined batteries is uncertain.
So just a bigger battery appears more attractive, if this suffices of course.
 
How old is the battery ?

Voltage drop on cranking is normal but it should not go so low as to do what you say if it is in decent Nick.

The main consideration with boats and batteries is usually the size of the battery bay which usually won’t allow anything bigger than the factory fit battery.
 
I'd also be making sure that all connections are spotlessly clean with no corrosion and properly connected. It could be a poor or undersized connection to the bowthruster system which suffers just a little too much when the battery voltage gets pulled down by the starter.
 
Battery is a 100Ah semi traction unit: I understand these types are not the best for starting.
Cables/connections are fine.
Strange to me is that NMEA2000 -on same circuit- has no probs. Any voltage drop cannot be very much.
 
Some things are more sensitive to voltage drop than others. Simple solution would be to start the engine before switching anything on.
 
I also suggest starting the engine before switching on equipment.
A zener diode solution was suggested to me but I havent tried it yet.
 
I would check the CCA of the battery because a 40 A/H battery means nothing other that you can use 2 amps for 20 hours and nothing more.
 
You must log in or register to reply here.

Share this page

Top