Does the 91 Johnson 70 hp have an alternator?

[Captain Bart]

I was discussing putting another battery on my 170 with an on board charger when I was told that my 91 Johnson 70hp has the ability to charge my battery when it's running at a rate of 15 amps.  Can anyone advise me as to whether or not adding another battery is worth my efforts.

[Capt. Bob]

I was discussing putting another battery on my 170 with an on board charger when I was told that my 91 Johnson 70hp has the ability to charge my battery when it's running at a rate of 15 amps.  Can anyone advise me as to whether or not adding another battery is worth my efforts.

Well Capt. Bart you got a couple of questions here.

First a manual would help a lot (remember the prop nut torque question).
From what I found, you have a stator so you have the ability to provide a charge (as well as powering the ignition system) to a battery while underway.

Is it 15 amps????
I can't tell.
Is it regulated?
Not sure.

Extra battery worth the effort?
Well redundancy can be a good thing (you can never have enough of a good thing) when it comes to starting power so an extra battery is good but.....
170s aren't noted for their abundance of space so finding room and the extra costs (switch/wiring/cables) may steer you in the opposite direction.

That stated, IMHO if you can find the room, install it. Your motor can charge one at a time (when properly switched) then again, if you are in somewhat decent shape, you probably can pull start a 70 hp outboard and a pull rope doesn't take up as much space, weighs less and is much more cost effective.

Good luck.

PS... you could still install the on board charger for when it's sitting at home.

[kaptainkoz]

From my limited experience, it might be enough to top off the battery charge after light use but you will probably have to supplement from time to time with an external charger. Unless you are under motor with decent throttle for long periods of time the charge will have moderate effect on your battery.  That's usually 15 amps max at a higher rpm, not at idle. A second battery isn't a bad idea but now you are trying to charge 2 batterys off of a small charging system.