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dual charging setup question


ozzmonaut

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This isn't written in stone yet, so please don't blast me too bad for asking. When I bought my 48 Special Deluxe, the seller had a new 6v +ground alt with it. He was going to switch it over. I opted to have the original generator rebuilt instead. I also have access to a couple of Optima 6V batteries super cheap. I was thinking of using the alternator as a tensioner of sorts and running it to a separate battery. I could build a stainless battery box to house two optima batteries. Were this a negative ground system, this would work fine. But with positive ground, both the gen and alt will be sending voltage into the chassis. I have read about the common issues with dual charging systems. If I understand correctly, having two sources with a 6V output should be fine since neither flows to the negative side of the other battery, which would create a "run-in-series" situation that would add the voltages together. With this setup, it should be possible to run the batteries separate by having one negative terminal connected to the generator and one to the alternator. I am just not sure how the batteries would react with positive power flowing from 2 sources. Since the batteries would both be new, I could also just run the batteries in parallel, creating one large 6v battery with two charging sources. But this could lead to the "in-series" condition I mentioned above. As well,  I would prefer to run the batteries separate, to allow one to be used for starting if the other is compromised. I know there are threads about dual charging, but so far I have not found any that discuss running two charging sources, two batteries, and the added complication of a positive ground system.

 

The other issue that might be more obvious is that the voltage from the alternator could cause the voltage regulator of the generator to open and keep the gen from charging.Thanks in advance for your help, and maybe this thread will help enlighten other newbs like myself in the future as well.

Edited by ozzmonaut
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I'm not familiar with a dual system like you are describing (except in aviation and that is a different animal) so can't exercise judgment on it. However.....I'm wondering why it is needed. With a healthy conventional charging system, the proper batter cables and a starter that isn't clapped out you should have a reliable starts. My preference is the single-wire alternator (no external regulator to go bad) since you get full charging even at idle and the 6v 65a alternator on my P15 keeps the battery hot at all times. After installing new cables the car is a very reliable starter hot or cold.

 

battery-cables.jpg.170835f77eef60de30c32a996c568276.jpg

Edited by Sam Buchanan
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Dual batteries would have to be run in parallel. This keeps the voltage the same. In series, the voltage is doubled. It would be fine as long as the charging system never fails. I am most interested in having a backup battery that stays fully charged without having to place it on a charger or do some kind of maintenance other than just driving the car. Plus a 35A generator will have difficulty even maintaining one Optima battery to its full potential, much less two batteries.

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3 minutes ago, Sam Buchanan said:

I'm not familiar with a dual system like you are describing (except in aviation and that is a different animal) so can't exercise judgment on it. However.....I'm wondering why it is needed. With a healthy conventional charging system, the proper batter cables and a starter that isn't clapped out you should have a reliable starts. My preference is the single-wire alternator (no external regulator to go bad) since you get full charging even at idle and the 6v alternator on my P15 keeps the battery hot at all times. After installing new cables the car is a very reliable starter hot or cold.

 

battery-cables.jpg.170835f77eef60de30c32a996c568276.jpg

I am just investigating for now. I would like to be prepared for anything. My main hope is to be able to get around a dead battery or failed charging system without need for attempting to jump start from a 12V source.

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No need for dual charging systems. It sounds like more of a headache than its worth. Once you connect your 2 6v batteries in parallel they basically become 1 larger capacity 6v battery. Your one generator will easily keep them charged. The advantage with an alternator is higher output at lower RPM’s so its less likely to discharge at idle would the lights on, as it will on a generator. 

Also, don’t over complicate your thinking with a Pos. Ground system. Electric theory is all the same regarding current flow, its just that the Neg and Pos are reversed. It only really affects polarity sensitive components, such as electronics with diodes and transistors. Maybe a tube radio would fall into that as well. And when doing diagnostics you need to remember to connect your volt meter leads correctly or you’ll read negative voltage. 

Edited by Merle Coggins
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10 hours ago, Merle Coggins said:

No need for dual charging systems. It sounds like more of a headache than its worth. Once you connect your 2 6v batteries in parallel they basically become 1 larger capacity 6v battery. Your one generator will easily keep them charged. The advantage with an alternator is higher output at lower RPM’s so its less likely to discharge at idle would the lights on, as it will on a generator. 

Also, don’t over complicate your thinking with a Pos. Ground system. Electric theory is all the same regarding current flow, its just that the Neg and Pos are reversed. It only really affects polarity sensitive components, such as electronics with diodes and transistors. Maybe a tube radio would fall into that as well. And when doing diagnostics you need to remember to connect your volt meter leads correctly or you’ll read negative voltage. 

That would be awesome. My friend works at a parts house and they ordered 2 optima 6v batteries for somebody that changed their mind. 6V for cars don't really sell around here and they don't want the batteries to sit in stock for too long. Apparently they aren't allowed to sell them more than a certain number of months after the manufacture date. With his discount I can get them for $40 each. I might just run them parallel and throw the alternator in. I like the generator, and so far my lights don't dim. But my ammeter never gets above maybe 7A. It sits at 0-2 quite a bit, but never goes in the negative. Do you really think the generator will charge both batteries? My friend said that one optima is higher capacity than a normal 6V, and would require a higher output than the 35A the gen provides in order to maintain the battery efficiently. Would it be smarter to use an isolator to run dual batteries off the alternator, so that the 2nd battery becomes a backup battery?

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You are still way over-thinking this. One battery with an alternator is all your car needs. Also, don't underestimate the value of installing proper battery and starter cables....they will be worth more than the second battery......   :)

Edited by Sam Buchanan
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10 hours ago, Sam Buchanan said:

If you want to be prepared for anything........get an AAA membership with free jump starting and/or towing.......   :)

I would love to have AAA, but by the time they get to me I might be late for work. We are extremely strict on tardies. Like, well beyond any normal level of strictness. I have never been late , but in 5 years I have been absent once and it still comes up. LOL

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Just now, Sam Buchanan said:

You are still way over-thinking this. One battery with an alternator is all your car needs.  :)

I know. I worry a lot. I just like the idea of being able to get going quickly if a battery or gen/alt fails.

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One, or two in parallel will work fine, the generator will charge one or both, the alternator will do a better job charging either combo.

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1 minute ago, ozzmonaut said:

Also, I'm that guy that carries a spare coil, points, cap, rotor, plugs, wires, carb, etc.   My vw bus has a parts dept under the z-bed

 

Can you get all that stuff replaced in time to get to work?   ?

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Not all at once. I did swap a coil in a couple of minutes, 5 minutes to swap carbs, changed a tire in 3 minutes (use antiseize on lugbolts to make this possible)  LOL  I leave 10 minutes early for work just in case.

Edited by ozzmonaut
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8 hours ago, ozzmonaut said:

 My friend said that one optima is higher capacity than a normal 6V, and would require a higher output than the 35A the gen provides in order to maintain the battery efficiently. Would it be smarter to use an isolator to run dual batteries off the alternator, so that the 2nd battery becomes a backup battery?

 

While it's true that your Optima's will have a higher amp rating than a replacement lead acid that doesn't mean they'll be discharging all of that amperage. Your alternator, or generator, only needs to output enough current to displace what your car is using. The largest current consumption is at cranking to start the engine. After that the ignition system draws a small amount. If you have lights on there is more current draw from the battery, and etc. So, the charging system just needs to replace the amperage that was used during the starting process and power consuming devices while running. Normally you may see your ammeter move higher to the Positive side right after startup, to replace that amperage draw, then it'll settle down and show a slight positive charge during normal driving. The generator, or alternator, will only output the amount of current (amperage) needed to maintain the battery state of charge.

Since you aren't adding any large current consumers to the system there is no need to increase the charging capacity. Stick with the single alternator, or generator, and you'll be fine.

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2 hours ago, Merle Coggins said:

 

While it's true that your Optima's will have a higher amp rating than a replacement lead acid that doesn't mean they'll be discharging all of that amperage. Your alternator, or generator, only needs to output enough current to displace what your car is using. The largest current consumption is at cranking to start the engine. After that the ignition system draws a small amount. If you have lights on there is more current draw from the battery, and etc. So, the charging system just needs to replace the amperage that was used during the starting process and power consuming devices while running. Normally you may see your ammeter move higher to the Positive side right after startup, to replace that amperage draw, then it'll settle down and show a slight positive charge during normal driving. The generator, or alternator, will only output the amount of current (amperage) needed to maintain the battery state of charge.

Since you aren't adding any large current consumers to the system there is no need to increase the charging capacity. Stick with the single alternator, or generator, and you'll be fine.

times 2!  Charging system need is determined by car's current draw, not the battery's storage capacity.

 

As far as redundancy/reliability is concerned, IMO, it would be best to install one battery and alternator. Simple is always best. Then buy a good lithium jump starter for your trunk kit.  Doesn't take up much space, always ready to go and can be used to help others if needed.

Edited by kencombs
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On 1/7/2020 at 11:00 AM, kencombs said:

times 2!  Charging system need is determined by car's current draw, not the battery's storage capacity.

 

As far as redundancy/reliability is concerned, IMO, it would be best to install one battery and alternator. Simple is always best. Then buy a good lithium jump starter for your trunk kit.  Doesn't take up much space, always ready to go and can be used to help others if needed.

I haven't found any 6v jump starters. Do you have a source handy?

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1 hour ago, ozzmonaut said:

I haven't found any 6v jump starters. Do you have a source handy?

No don't know a source.  But, a 12v will work fine, just be sure the lights are all off and jump direct to starter.  actually it is a benefit as it will provide a hotter spark during cranking, just remember to remove quickly.    been done many times.  And, it is useful for all the 12v 'others' you may encounter, or the rest of your fleet.

Edited by kencombs
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  • 4 weeks later...

Well. I decided to leave my generator in and go with the two optima batteries, since I could get a good deal on them. Everything was fine until I was hooking up electric wipers and noticed the battery voltage was staying low. So I checked the regulator and it seemed to be working fine. I decided to put the 6v alternator in, and the next day my batteries were dead. I hooked them up to charge and the trouble light comes on once the voltage gets to 4.64. Then battery voltage keeps dropping. I pulled both and put the old battery back in. Now everything is good but two brand new  optima batteries are bad. 

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Glad your one battery, alternator system is working well. It took you awhile to get here.....but now you should be good to go.  The endearing quality of our old cars is their simplicity......  :)

Edited by Sam Buchanan
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I figured 2 optima batteries would be the way to go. Are they really such poor quality now? My current battery is very old. Everyone seems to be going with the dual optima setup with an alternator, but apparently they don't last more than a couple of months.

Edited by ozzmonaut
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5 hours ago, ozzmonaut said:

I figured 2 optima batteries would be the way to go. Are they really such poor quality now? My current battery is very old. Everyone seems to be going with the dual optima setup with an alternator, but apparently they don't last more than a couple of months.

 

Seek out your local farm supply store that sells batteries for tractors (Tractor Supply in my neck of the woods), they will have 6v wet-cell batteries. You should get several years out of one if you use an alternator that keeps it charged all the time. The old-school batteries worked pretty well when our cars were youths....and they still do (with proper cables).   :)

Edited by Sam Buchanan
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