Battery Selector Switch

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carver 2557

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Someone had mentioned that I should not run my selector switch in the All position when underway...I have always done this, and thought that it would charge both batterys at the same time...
Should I run it on only one battery at a time?...
What problems could happen by running both at the same time?...
 
An alternator is not a charger...it's a "maintainer". The "all" (or both) position is to use both batteries when starting if one battery is weak. Running with the switch in "all" mode is maintaining the stronger of the two batteries and the slave battery, if weaker, is leeching the charge from the other one.

Use 1 for starting, for example, and 2 as a house position.
 
Mike,

Now you have me a bit confused (beyond my normal confusion, of course). You say that "an alternator is not a charger...it's a 'maintainer'." I have always been under the impression that an alternator recharges your battery as the engine runs. If I have a dead battery in my car, for instance, I can usually (if the battery is fairly new and in good condition) jump start the car and let it run for a while. That has always recharged the battery (from the alternator, I assume, since there is nothing else on the car that would recharge the battery). Is a marine alternator different?

If I run down my boat's house battery over a weekend on the hook, won't running the engines recharge the battery? I've done this (before buying a Honda EU2000i generator) and it has recharged my house battery, I believe. Once I've started my engines on my starting battery (Position 1 on my battery selector switches), I've always switched the battery selector switches to all and have never had a problem with the batteries. When I'm hanging on hook, I always have my battery selector switches on my house battery (Position 2 on the switches).

I'm just trying to understand why an alternator doesn't charge, only maintains, the batteries.
 
Most marine alternators only put out 55 amps at full speed. Most boats don't run at full speed all the time, so the alternator puts out less current. The boat engine, and accessory loads such as the VHF, lights, refrigerator, stereo etc, all are using power. I think what Mike means is that the alternator is keeping up with the running loads. There might be a little left over for some battery charging, depending on the speed of the engine.
 
My issue with the switch in "all" position has nothing to do with charging but the fact that sooner or later someone will forget and "all" batteries end up DEAD (embarassing).
 
i dont' agree with some of the above comments...

an alternator is not a MAINTAINER (what the heck is that???).... an alternator is designed to CHARGE the battery(ies)

Alternators do not have to run at full speed to put out full current, fast idling at 1200rpm is usually enough.

There are 2 reasons why you are better off not running on BOTH:
1)- if you forget to switch to either batteries when you get somewhere, you could drain both batteries and be DIW
2)- indeed, both batteries will not charge at the same rate and one battery may not be fully charged... but we're talking small differences here, not a huge deal
3)- if one battery fails, it will take down the other one, at least in part.

oops. that's 3 reasons... not 2 :-)
 
This is a subject that is hacked to death quite often.

If your engine is running, it is safe to put the battery selector in the both position. (Caveat 1: neither battery is DEFECTIVE.) (Caveat 2: Never switch through the off position while the engine is running.) Just be sure to switch to one or the other when the engine is off to prevent discharging both and getting stranded. Get a battery combiner - does the same thing automatically.

The Alternator will charge the batteries just fine - even at a just a fast idle. The weaker of the two will get most of the charge. There will be no leakage from the charged battery to the weaker if the engine is running. Even if the engine is off, the leakage between the two is small.
 
An alternator is not a full-on charging unit...even the ones in your car.

I'm quoting now from my ASE Certification manual:

"It's important to remember that alternators are not battery chargers, they are battery maintainers. If an alternator fails, then it is probable that the battery has been run down as well. If the battery is not checked and recharged, the excessive load placed on the new alternator as it tries to recharge the battery may cause it to fail quickly"

The alternator's job is to convert AC to DC current via a rectifier. A battery maintainer, Pas, is what you would typically use when storing batteries for long periods of time. Unlike rapid chargers, these "maintain" the battery by essentially, trickle charging...which is what the alternator does.

source: Nat'l Instit for Automotive Service Excellence (ASE)
 
I think you guys are arguing semantics and getting way off the original post.
 
quote:
Originally posted by stmbtwle

I think you guys are arguing semantics and getting way off the original post.

Click to expand...


I think you're right, Willie. But, it's what we do best - chop into little bits, swallow and then throw up.

I apologize Wayne...carry on!
 
it's true that an alternator may be damaged if you attempt to recharge a battery that's completly dead... but, in a normal situation of a normally low/discahrged battery the alternator will charge that, not just maintain it like a trickle charger.

50, 75, 100+ is hardly a trickle charge.
 
Pascal...it's not worthy of an argument. An alternator is what it is, whether you like the terminology or not. I don't make up the definitions...just pass them along. Take it or leave it.

Let's get back to Wayne's question.
 
is there something you dont' understand in... "discussion forum" ?
 
Come on guys........I think your both correct to some extent depending on the circumstances. Most conventional alternators will burn up if you attemp to use them at capacity for long periods of time. They aren't designed to deal with the amount of heat generated, but I don't think this is the kind of use carver 2557 is talking about. On a side note, Balmar makes alternators specifically designed to handle loads like these. They can deliver upwards of 300+ amps! These alternators can charge, ALOT!

To the original question: I typically do what you are referring to and haven't had any problems with it. I have seperate house and starting batteries. If I spend the day on the hook and run down my house batteries I will leave the switch in the both position for the return trip home to replenish the house bank. Just remember that the typical alternator is rated at somewhere from 50-80 amps but only puts out a fraction of that in the real world. Heat, rpm, condition, etc. all play a part in determining how many amps they actually produce. If you happen to have a Balmar or equivelant than that is a different animal.
 
To answer the OPs question I would get a combiner and use it to connect the two batteries and run with your battery switch set on the starting battery. Both batteries will get charged when the engine is running and you leave the switch on the starting battery unless you need more power to start the engine if for some reason your starting battery does not have the power to start the engine.

Here is a link to one type of combiner:

http://www.yandina.com/c100Info.htm

As far as the alternator "maintaining" the battery as opposed to charging it it depends on the details. Battery size and state of charge, alternator output and type of regulator and the electrical load while spinning the alternator. While is SOME truth to your ASE book Mike it only applies to the real world in a somewhat limited sense. Less so to the marine world. In the automotive world alternators are usually sized to handle the the typical electrical load with a little extra buffer. In the marine world alternators usually have a considerably larger buffer and upgrades are not uncommon as boaters add to the electrical load with more stuff.

Generally the lower output small frame alternators (40-80 amps) will give your a larger percentage of their rated output at lower RPMs then higher output (80-120 amp) small frame alternators. Of course the devil is in the details noted above. There is a world of difference between a small outboard boat with a single battery and little electrical load and a long range cruiser with a much larger electrical load, massive battery bank and a large frame Balmar alternator and 3 stage regulator to charge that massive battery bank.
 
If you had an amp meter on your car or boat you would see that an alt can charge the battery at a rate in excess of 10 amps...depending on the state of discharge. The alt senses the state of charge in the batt and charges accordingly. If you have two batteries charging at the same time(without a batt combiner) you'll never get BOTH batt to absolute full charge.

I seldom hang on the hook overnight with a bunch of elec running...I run BOTH all the time but I'm either trolling or running to or coming back from a fishing trip.
 
Let me chime in an add to the confusion. An alternator is more of a maintainer than a charger. Can it recharge? Sure. Is it engineered to be a charger? No. An alternator is designed to replenish the discharge of the battery after starting. Their output is lowered as the batery comes up to proper voltage, hence a maintainer. As electrical loads increase they will put out amperage to run the load.
Back to the post. There is no single best way to use the battery switch. It depends on many factors. I would not use the battery switch on both while running. If you're on the hook using the house battery, it will be discharged more than the starting battery. It is not a good idea to try and charge both. One may under charge while the other overcharges. If you're house battery is still strong enough to start the engine, use it. After starting, the alternator will recharge it more effectively since it does not see another fully charged battery. It will recover the house battery although not as good as a battery charger. You still have a fully charged starting battery if ever needed. Should the house battery fail to start the engine then start it on starting battery. Let it run for for awhile and charge it back up. Then turn the switch to the house battery so it can try and recoup.
 
Charging two batteries at the same time is typically a non-issue. As a battery discharges the internal resistance decreases so having one well charged battery connected to a partially discharged battery for charging means since the partially discharged battery has a lower internal resistance then it receives more charge than the charged battery. Basically they self regulate the charge. While there can be issues with over/undercharging in my experience they are rare and even then don't show up until many hours running.
 
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