Outboard Battery Charging

I am looking to buy two electric start 10hp engines to put on a small power catamaran. One of the things I am looking at is the battery charging capability of the various outboards. I have a 55 amp hour agm battery that will be adequate for my needs and would not like to go bigger for weight reasons. The battery literature says "limit initial current to 16.5 amps" with no mention of further charge current. I was thinking the more the better as far as charging current from the outboard goes until I looked at the new Honda 9.9 which puts out 12 amps so with two that would be 24 amps. I recognize that would be at WOT and I wouldn't be running that way very often if ever. Maybe I could dump excess amps into a heater but would like a system that was mostly automatic and could't accidentally damage the battery. Of course I could buy another outboard, all the others are rated at 6 amps.

 

More is not always better. A battery needs time to charge, not more amps.

I would use one battery dedicated for starting both motors, charging it off one motor. All the other stuff that you want to power, charge that battery with the second motor.

That way you will have a fresh starting battery every time, and a peripheral battery that may be used as a jump start should the first battery give problems.

Post a picture of your boat, will you ?

 

I would not use 2 motors(charging systems) on 1 battery. The charging systems on the outboards have internal regulators to prevent overcharging of batteries by constantly sampling battery voltage. With 2 charging systems they would be constantly switching on and off, building up heat, and failing quickly. Best advice, like the above post 2 engines charging should be going to separate batteries.
Dan

 

:!: What they said...

You mention not reaching full output 'till WOT;-

On a modern alternator, they are at full output from about 1200 - 1500 rpm and the regulator will be cutting in at that point.

Think about the positioning of the battery - you don't want long runs of heavy duty cable - the battery is best as close as PRACTICAL to the OB.

You also may have little choice about configuring the charging - most modern OB's use the same heavy duty battery cables both to draw current for starting and to deliver it for charging

So either add a second smaller battery for motor 2 start or dump the 55 amp in favour of 2 smaller batteries.

The potential savings in weight from a single Battery setup would be mostly lost to the weight of the extra cabling - as you increase the distance you have to increase the gauge of cable used!

 

I suspect you are fighting a loosing battle here. The internal regulators are going to defeat you. You should contact the motor mfg tech services for support on this one. What they will probably tell you is that you need separate batteries if you have two motors with alternators in separate hulls. I would just order one with and one without if you only need one battery. There may be some way to circumvent this, but it will involve using battery isolators to isolate the motors instead of batteries, and I don't know how that will work out. And not cheap either. If you are really serious about weight, you could use a pair of motorcycle batteries. Again, not cheap long term. See also Odyssey PC925 for example.

 

Thanks everyone for the replies. It looks like I will have to rethink this. I like the simplicity of one battery and there will never be a problem starting engines even with a low battery. The engines are only 10hp and can easily be started manually. I will investigate further but it looks like I will have to go with two batteries. I am familiar with battery combiners that combine banks when charging and isolate them when there is no charging source. I guess I could use the opposite to separate the batteries when charging and combine them otherwise for house use. I wouldn't trust my memory to use a manual switch to separate the batteries when running with both engines.

 

I thought I might explain a little more with photos. I built the boat last winter and installed a single 20hp outboard. I have tried many adjustments, cowlings, fairings, dolefins etc but still have trouble with ventilation in rough weather. I think the problem can be solved but I have decided to go with twin 10hp engines mounted on each hull. You don't have to go far here to be in remote waters with no nearby help. The motors will have independent fuel tanks/filters.

 

I was just thinking that I could use a regular normally on continuous duty solenoid connected to one outboard so that when that outboard is running the solenoid would be activated and the batteries separated. If just the other outboard were running there would be no conflict it would charge both batteries as one bank. As soon as the other engine was started the batteries would be isolated.

 

Have the charging system disconnected on one motor then connect both motors to one battery.

 

Just disconnecting one engines charging system is a very good idea but it would be nice to be able to do something with all those amps available.
In researching 10hp outboards the charging specs for the Honda stood out at 12 amps all the other brands are 6 amps. I could have 24 amps to do something with and I thought of using a 12 volt heater. There are good quality small 12 volt heaters in various sizes available. The boat has a very small cabin and a little heat would be useful here in Alaska year round. A 10-12 amp heater would leave 12 amps for battery charging. The heater would mostly be always on.

 

I have been pondering something like this for a while to put on an outboard that doesn't have a transom that regulates the water in front of the motor.

It can be made of carbon fiber or aluminum, it just have to be stiff enough not to bend from the oncoming water.

The thought is the plate forces the water down and forces it to the right height over the prop...

What do you think ?

 

The boat looks good ! Displacement is the hull shape of the future because it reduces costs and you need small lightweight motors, even on fairly large hulls. It's just not as fast.

Do you have a picture of the bows as well ?

I see you have nice steps where you can sit and fish or you can sit and drink

 

I see no problem with the regulators. The engine producing the higher voltage will charge the battery. Twin engines wired directly to the batteries have been used for decades without anything bad happening.

 

They're not exactly free amps - you're burning fuel to make them!

On cars, the roadside check for a working alternator used to be to disconnect and re-connect the field coil wire - motor would rev up and down about 250rpm at 'idle' due to the extra load on the engine from the alternator... (if it didn't = faulty alty).

You get a similar drop in revs when you switch on the A/C and the compressor engages...

 

The heater thing can definitely work. Look up shunt type battery charge controllers.

Also, one of my concerns earlier was the distance between the hulls and the length of cabling needed - especially if you needed to run separate starter and charging leads to each motor. Your hulls are so close together that cabling isn't a big expense. On a different boat, the cost of the additional wire could have been as much as the price of a battery.