Alternator size vs Amp Hours

Man my head is spinning after all of the stuff I have read about alterntors and charging systems on this board.

Here is my question, what is the recommened Alternator size for a 455 Ah bank of batteries. I have seen recommendations of 20% of the bank which is a 90 amp alternator. 30% is a 135 amp alternator and after that I would run a dual alternator set up.

My thought is if 20% is what I should run then I could use a Balmar Max Charge Regulator and the Duo Charge off of my 90 Amp Alt to charge all of my batteries. The question is would a larger Alterntor be able to charge it faster or is that going to hurt the batteries if I go up to a 135? Or am I better off running 90 amp alternators and getting rid of the duo charge (same cost overall is my guess so cost is eliminated). Thanks for the help

Coach:

You have to keep in mind that the 20% number is what the batteries want to see, which is not really the size of the alternator. The recommendations are based on the assumption that the batteries were drained over a long period of time (like sitting at anchor all weekend) and that the amount of current going to just operating the boat is negligable.

For the case of a typical wakeboard boat that is not true. If you are running ballast pumps, the heater, mabye left the blower on and cranking 1000 watts of tunes then the boat can easily be consuming 75 to 100 amps. If you expect to recharge that battery bank in a reasonable amount of time while you continue using the stereo then you will need the larger alternator.

On the other hand, if the large battery bank is so you can sit with the engine off at your and play the stereo at your favoriate beach and then you go back home or to a dock where you can plug the charger in all night then I wouldn't worry about recharging and just stay with the smaller alternator.

Rod,
Thanks for the info. Here is how my battery matainance works (not the best way, but conveince is key). I show up late and pull my boat from storage. I put a full charge on my batteries overnight. Launch the boat and the boat stays in the water for 2-3 days. I use the battery bank for the stereo only which will be close to 2200 RMS. I run the stereo all the time but it will run most of the middle of the day beached with the engine off. I think you answered my question, but see if this should work. I am going to leave the stock alt and electical system alone.I am going to mount a second Alternator and hook up a Balmar Regulator to the 2nd alt and have it hooked up to my battery bank for the stereo (4 6V Trojan AGM). Put a perko between the two banks but always leave it on 1 (this is just incase my starting battery doesn't work then I can switch over to the battery bank to start). What size Alternor will work the best? Can I get a 90 amp alt or am I better off getting a 125+ amp alt? My thought is I can use just a second 90 amp on because the only thing it does is charge the stereo bank and doesn't control anyother electoronics in the boat. Thanks for your help.

Coach you keep on using the word "charge" with respect to alt. your alt is not a battery charger, but more of a maintainer. if your trying to routinely recharge your batteries (from low voltage) with just your alt, they won't last long. What your trying to do is not a great idea. I'm not saying it won't work, but expect to go thru alternators. Carry a spare. Your alternator should be matched up to your stereo's current draw (if your running a separate one to this battery bank). Inevitably what will happen is you'll be out running your stereo all day batteries run low and you fire your boat up to go home, start cranking the radio and smoke yoru alt....yes I've done it. After thsi is when I was told by my local guy that if you try and use alternators as chargers you will burn them up.

My advice to you is stick with a 90-105 amp alt (most are very solidly built and are cheap), keep your fingers crossed that your 4- 6volt batt's will last 3-4 days. don't expect yoru battereis to be charged over that time, especially while playin the stereo loud. all the alt is going to do is slow down the rate of discharge.

coach:

You said your stereo is 2200 RMS, which I assume means that that it is capable of cranking out 2200 watts of real power. At that setting it would draw about 170 amps from your system.

Okay, nothing short of an air raid siren would crank out a continuous tone at full power. But if you assume that you might average 50% of your systems power you would still be averaging 85 amps. At that rate your 90 amp alternator would only have 5 amps left to charge the batteries with, assuming that the alternator was running at full power.

Like Adam said, the alternator will basically slow the rate at which your batteries go dead.

I guess the better question is to ask how it has been working for you so far? I would expect that by the end of the third day those batteries have got to be near dead.

You also said that you get the boat from storage, then put it on the charger overnight. What kind of charger are you using?

I would expect that it would take more than one night on a typical charger to fully recharge a 445 amp-hour battery bank. It would also be better for the batteries if you recharged them before you put the boat away so they didn't sit for a week in a discharged state.

Thanks for the replies. This is for a future boat. Right now I have 2 Trojan T-105's with a 400 RMS system. I have let the batteries get too low on water too often and they can play hard for about 3 hours before I need to go out and run the boat. My hope was the Balmar regulator would help charge the batteries back up. I should say that I don't normally ever go more than 3-4 hours before running the boat for a while when hanging out. I am not sure if that would help or not. I still am going to put an on board charger in so I can charge the batteries. My storage unit I can't plug in a charger (and if I do I could lose the space). Many times my T-105's take 2 days to bring back up to full charge. My guess I need to pull the boat every night and charge up the batteries if I want to be DJ (or just run a 150ft extension chord to my boat over the water each night) Any other ideas or suggestions are much appreciated.

Okay, you were referring to a future boat. This clears up some of my confusion. I really didn't understand how you could operate three days as described.

Your 400 watt system would draw ~33 amps peak, and probably average about 16 amps. A pair of T-105s would operate that load for 13 hours. Even a stock 65 amp alternator would have enough capacity to aways stay ahead of demand and provide 20 amps or so of charge, so if you ran a couple of hours a day you would probably stay reasonably charged.

If you increase to your 2200 watt goal you may experience a lot of difficulties. Why are you trying to run so much power? Hosting a small concert?

Alternator size vs battery bank? Humm.

I’m running 480 hrs of AGM batteries. I’m running a 210 Amp Balmar alternator. IMO that’s the right size. IMO you can never have enough power. I have built a bunch of charging systems for exactly what you are doing. IMO its better to have power to spare then to be trying to tax your charging system to what some book or rule of thumb says. I say take advice from people and systems that work and copy them instead of trying to re design something people have already worked out. Adam had some great advice about carrying an extra alternator LOL. Trying to get 2 alternators correctly lined up an nd a belt system that doesn’t smoke belts is a pain in the but. Good luck with engine noise coming form 2 different alternators as well. Sorry but I see nothing but problems from 2 alternators, IMO 1 is tough enough why add a second unless you have a boat with 2 motors. Balmar has excellent customer support. They know way more about HO charging systems then most of us. Take advantage of it. I’m sure they can help you select what will work best for you. I don’t make anything for promoting their product I just like it because it works hassle free. Like I said I have built many charging systems each one very different and each offers unique problems and draw back's. Yes I spent a bunch of money doing my system but you don’t need to spend that much you can still do something nice and solid its just not going to be over the top. IMO the electronic voltage regulators that work hand in hand with the Balmar Alternators are the key. They keep your alternator from destroying itself in a melt down over charging situation and regulate voltage via battery temp, time, and battery status,IE.voltage,

I love having a system that I can pound all day and night and know its all being taken care of electronically, its all out of my hands the system does its job all by it self there is enough to worry about when you are out on the water what add another worry to your list

Bling wrote:
"I’m running a 210 Amp Balmar alternator. IMO that’s the right size. IMO you can never have enough power."

If I understood your post correctly you are suggesting a 210 amp alternator with a single belt. Then you also recommend taking the advice from the manufacturers. Interesting, considering that you are going against the recommendations of Balmar (quote: "Any alternator in excess of 110-amp output will require dual belts") and battery manufacturers (quote from Trojan battery: "When selecting a charger, the charge rate should be between 10% and 13% of the battery's 20-hour AH capacity")

Rod: Yes I guess you mis understood my post and Im guessing you havent seen my set up. I have a dual belt Balmar 94-210 alternator. It uses dual 1/2 belt's for the 8 hp at max draw alternator. I had to have custom pully's made to run this set up. The pullys I had made have 2" 1/2inc groves for the alternator and one 3/4 groove for the raw water pump. Im also using Balmars Max Charge voltage regulator with a optional alternator temp sensor for the alternator and another temp sensor for the battery's here are some pic's



800-900 rpm's 170 amps at 14 volts



Thats pushing some juice. When I took these reading's with my amp clamp the alternator was luke warm. I could hold my hand directley on it.
try that with your alternator.

Max Charge and Duo Charge
Temp sensor
Alternator:

This was my old charge wire the shiny smaller one 2awg. My new charge wire is larger 0-awg directley from the back of the alternator to the battery bank