Stock Alternator for 2 batteries?

Does anyone know if a stock alternator on an 18 foot I/O be enough to keep 2 batteries charged? I was thinking about adding a second battery to the mix and wasnt sure if the stock Alternator will work

An alternator is not a battery charger. What kind of acc. do you have on your boat? total amp draw? how big is yoru stock alt. Withthat being said...every boat on the water should haev at least 2 batteries IMO. When you haev equipment that is drawing alot of power from yoru alt...you shoudl stick to a good chraging regiment adn keep your batteries fully charged all the time to keep strain off yoru alt.

Thanks Adam, what do you suggest for a regiment?
As far as accessories, I have my stereo with 4 speakers, 1000 watt amp, and 4 tower speakers. I plan on adding a light bar this week with 4 lights and some neon/LED around the inside of the boat. I am not sure if I would need a second battery for this, but would like to play it safe and not be stuck out on the water.

The alternator size has nothing to do with the number or even size of the batteries. If your current alternator is working out okay with one battery then adding a second battery won't change that.

There are two primary issues to consider when thinking of an alternator upgrade:

1) Typical current useage
2) Desired battery recharge time.

It is really, really nice if the alternator will provide at least enough output to conver your maximum usage. The engine itself will take ~ 5 amps. The blower can take another 8 amps. If you have ballast pumps they can draw 5 to 10 amps each. The stereo is a little harder to figure out as the amps will have a peak current draw that is VERY high but you are only concerned about the average draw, which is much lower.

A good indicator of enough alternator output is to measure the voltage when you have everything turn on that you might want to run at the same time. Run the engine at 2000 RPM, blower on, ballast tanks filling (actually pumping water, not dry running), crank the volume up on the stereo. Now measure the voltage at the battery. If you have at least 13.2 volts then the alternator is able to provide what you consume.

The next question is how fast do you want to recharge a low battery. The primary purpose of the second battery is that you can play your stereo with the engine off without fear of not being able to start up again. After you have run this way for a while the stereo battery will be significantly discharged. In order to have it ready to go again then next day/weekend you would like for it to charge back up as you cruise back to your trailer at the end of the day.

How long of a cruise is that?

The answer may be that you can't get an alternator big enough for the job, and even if you could you don't want to charge a battery that fast.

A typical marine deep cycle battery is about 100 amp-hours. That would provide one amp for 100 hours. It would also take 100 hours to recharge at a 1 amp rate. Or 20 hours to recharge at a 5 amp rate. Unfortunately, the faster the charge the less efficient it gets, so it will take a lot longer than 5 hours at a 20 amp rate. If you attempt to charge at a 100 amp rate you will seriously degrade the life of the battery.

The bottom line is that if you significantly draw down the stereo battery during the day you should plan on hooking it up to a battery charger overnight.

To repeat what I said above, if your stock alternator was fine before, it will still be fine after you add the second battery.

Rod

Got it! Thanks Rod