Almost all outboard motors built in the last 20 years with an electric start have an alternator or stator that charges the start battery and provides some surplus current to run accessories and charge a house battery.
Why have a separate House Battery from the start Battery?
This concept has been around for ages and the answer is simple. When you are out and about, and you stop the motor all of the accessories would run off the start battery A single battery system would continue to drain the start battery and with sounders, stereos, fridges, and other accessories that can be a problem. It is very easy to forget about the amount of time and what you have running and pretty soon you have a depleted start battery. This can be a dangerous situation and at least very inconvenient. You might be stuck somewhere remote or have to wait for some assistance to get you going again.
A best practice is to run all your loads off a 2nd House Battery and keep your start battery separate so you can be sure it will always fire up your motor when you want to get underway again.
The House battery is typically charged from a DC-DC charger whilst underway and possibly with an AC power from shore power when you back at dock or home. When your motor is running you will get a 20-40A typically charge coming from the start system. This power is provided by the engine, and this is the point of our discussion today.
What is the difference between an Alternator and a Stator?
A stator is simple system that is inside the outboard and located on the top of the flywheel typically. A stator is a series of permanent magnets fixed in place that as the flywheel rotates create current.
An alternator is usually a fully self-contained externally mounted unit connected to a motor usually with a belt that uses electromagnets. One difference is that if you wanted to change to a larger alternator there may be some options whereas a stator is an integral part of the motor construction and is more difficult to change if you wanted higher capacity.
Essentially, they do the same thing. They produce an output current. Stators are usually on smaller motors and Alternators are usually on larger motors. They all have a nominated Charge Output Current which is clearly described on the motor datasheet.
What size DC-DC charger should I use?
As we mentioned above this really depends on the specifications of the motor you are using. Find the specification on the datasheet called CHARGE CURRENT. This may be say 40A.
The rule of thumb is to take at say 10A (usually less but to be conservative) off the top. So now we have 30A to use for external charging. A good rule of thumb is not to go beyond 80% of the surplus available so that would be 24A. In this example, I would use a 20A charger and no more.
In the current Yamaha outboards:
90HP for example has a 35A Output.
225HP has a 70A output
What happens if you choose an oversize DC-DC Charger?
Any electrical system should be used within the manufacturer’s specifications. If you choose a 50A DC-DC charger for a system that only outputs 30-40A you can expect problems. The charger might be smart enough to downscale its charge but at the switch-over point, there is commonly electrical spikes that have to go somewhere and can cause equipment to take a hit and fail. Also, the alternator charging at 100% workload is likely to fail early and not designed to used that way.
What is the take away on this?
1. Check your engine specifications.
2. Take off 10A and then take 80% to calculate the maximum charge current
3. If in doubt round down, not up.
4. Use a reputable DC-DC charger and make sure it is set up correctly
5. Use the right size cable to connect your DC-DC charger