Re: Dual Batteries good idea or overkill?
Not true... The characteristics of an alternator are that the output is self limiting, by the resistance of the stator windings. If an alternator fried it was because of an internal fault, not an overload. All that should happen if the current load is more than the alternator can provide is that the battery/s start to discharge. Once the load is removed the alternator can then start to recharge the battery/s.
Chris.........
P.S. I have two
HUGE(N70ZZ) batteries and a dual battery isolation switch in my boat. I can't get out and push.
Granted it is not known whether the alternator had an internal failure as it was never tested only replaced. Still, based on the following I have to disagree.
http://www.boatus.com/boattech/12volt.htm
Battery Charging
If you don't replace what you use, your batteries will eventually go dead, no matter how much battery capacity you have on your boat.
The rate at which you can recharge your batteries depends on a number of factors, including how much you discharged them, the temperature, the alternator's power (in amps) and its output (in volts).
Most boats charge their batteries with an engine-driven alternator. If your engine running time is minimal, you want to charge as quickly as possible, without damaging the battery. Battery damage begins when the internal temperature becomes too high, causing it to gas and heat up. If it feels warm to the touch, it's getting too hot. Use a voltage regulator to taper off the charge to prevent this.
Alternators are rated in amps; the rating refers to the maximum output in one hour at a certain temperature and rotation speed. Typical automotive alternators can't run continuously at full load in the warm temperatures found around boat engines, or in warm climates. A suitable marine alternator will be rated at full continuous load at temperatures up to 200?F.
You will need about 120% of the energy you used to restore it. And to prolong the life of your alternator, plan on running it at less than full output. Take into account any other power-draining loads you might be adding to the system as you are recharging, such as refrigeration. If you install an oversized alternator, you can recharge efficiently while at anchor, with the engine at idle and the alternator operating below its rated speed and output.
In general, charging capability should be approximately one third of battery capacity, plus any additional loads as mentioned.
The speed of your alternator's rotation is a function of engine rpms and pulley size. Once you have determined the maximum alternator output you require, add 25% so you won't have to operate it at full bore to achieve the required results. Now check how many alternator rpms it takes to reach that output. Then figure the minimum engine rpms at which you will be charging. You need a pulley ratio that gives you maximum required output at your minimum engine speed. If you need 110 amps, and it takes an alternator rpm of 4,000 to generate it, and your engine is running at 1,000 rpms, then you need a 4:1 pulley. Make sure that if you punch your engine up to 3,000 rpms, thereby increasing your alternator speed to 12,000, you are not exceeding maximum safe alternator speed.
