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Chuck Lott (Chuckmc8)

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Posted on Thursday, November 04, 2004 - 7:17 am:   

In studying the 24 volt charging situation on my MC8 conversion, I thought about this scenario:
24 volt house bank of 8 golf cart batts
2 group 31's engine/coach batts
The bus is equipped with proper switching equipment to "connect" the house bank to the coach charging system for charging the house batts from the coach alternator while underway.
OK- knowing all that- IF after boondocking for a weekend, the house batts are at, say 80% charge. Since the coach group 31's only duty when we leave this morning is to energize the bus starter, they are nearly (I suppose)either 100% or only very slightly discharged after the burst of power to start the engine ( which starts almost instantly).
The coach goes underway and the "switch" is flipped to connect the house batts into the coach charging circuit. Now there are 2 battery banks in the picture, a small one at 100% or so and a very large one at 80%. The large bank will require significant charging time and output from the coach alternator to bring them up.
What happens to the group 31's? Are they overcharged? Do they boil?
In this situation,Would the proper proceedure be to start the engine, "switch" in the house bank and then use the main disconnect to the coach starting batteries to take them from the loop and let the house batts provide power for the "coach" rather than the group 31's?
Am I way off base? Thanks Chuck, Douglasvile Ga
Marc Bourget

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Posted on Thursday, November 04, 2004 - 8:35 am:   

Is your batt switch "1" or "2" or "1, 2, Both"?
TWODOGS (Twodogs)

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Posted on Thursday, November 04, 2004 - 8:47 am:   

my theory might be wrong...here is how I look at it...the next morning...say you did not start the bus...just flipped that switch...what do you think would happen ? I think all batterys hooked together would seek a certain leval,and all would start chargeing when the voltage regulator made the decision (when started)I think it does the same thing when you fire up the bus like you are supposed to...they will seek a certain leval then recharge when needed
Gillig-Dan

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Posted on Thursday, November 04, 2004 - 9:06 am:   

I wonder if anyone has put a current limiting resistor into the line between the start battery and the house battery? If you could limit the current to about 24 amps max, the house batteries shouldn't boil, should they? The 1-2 ohm resistor could be made from ni-chrome (toaster heating element) wire housed in a heat resistant enclosure. The batteries would eventually equalize. Might take a long drive to get them to 100%.

Does anyone have any thoughts whether this might work or, has anyone used this type arrangement?

Gillig-Dan
Don/TX

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Posted on Thursday, November 04, 2004 - 9:20 am:   

Although several systems are in use by people with some success, one of the elements missing is that you SHOULD have a three stage charger on the house batteries set up for deep cycle charging, and this is somewhat incompatible with the start batteries. What I chose to do was change the engine alternator regulator to a top notch three stage one feeding the coach batteries. Now you are doing the right thing to your deep cycles whenever you have the coach engine running. For only a very few bucks ($20-$50), you can have any old Delco alternator changed over to a 24v one wire, hang it on the coach engine (most have a spare pulley) to maintain the Group 31's, and now you have the best of both worlds. In addition, with the battery switch, you have a redundant system if either fails. Sure worked great for me.
In fact, I also hung on a 12v one wire and added one 12v battery, now all my 12v needs are satisfied also!
Tim Hoskinson (Tdh37514151)

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Posted on Thursday, November 04, 2004 - 9:21 am:   

Dan is on the right track. A curremt limiting resistor or solid state switching device should be used. Repeated introduction of a fully charged bank of batteries to a lesser charged bank will cause reduction of battery life.
Richard Bowyer (Drivingmisslazy)

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Posted on Thursday, November 04, 2004 - 10:04 am:   

I have to respectfully disagree with Tim's statement.

In my opinion, when the fully charged bank is connected to a partially charged bank, the fully charged bank will act as a "battery charger" and bring the partially charged bank up to a level where the two banks are equalized in voltage. At that time the alternator will then bring both battery systems up to full charge. This partial discharge should in no way affect the life of the battery.
Richard
Gillig-Dan

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Posted on Thursday, November 04, 2004 - 10:38 am:   

But, without current limiting between the two banks, you're only limiting factor is the internal resistance of the batteries and cabling between them. Large CCA batteries have very low internal resistance. Even a two volt difference between the banks could cause a 200 amp flow between them. That current would be flowing regardless of the alternator's output so, it wouldn't matter what kind of alternator was doing the work.

I like Don's method of two alternators. Unless you're operating air conditioners off the batteries, the second alternator could be relatively small, amperage-wise. I know it's in these messages somewhere but, how does a 2-stage alternator work? Is it by current flow regulation?

I am currently (no pun intended) looking into my own charging situation and trying to make some decisions. Unfortunately, it's going to take some serious rigging to get an extra alternator and the $10 (a new York 210 I stole off E-b*y yesterday) air conditioning compressor to work off the current belt/pulley system. But, that challenge is the main point to this bus hobby I jumped into this year. The two XKE's will just have to wait another decade in the shed.

Gillig-Dan
Gary Stadler (Boogiethecat)

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Posted on Thursday, November 04, 2004 - 11:40 am:   

But Dan a bank discharged to 80% isn't going to be anywhere near 2 volts different from a 100% charged one. It will be more like 1/2 volt difference, one volt at most, and this alone will limit current to safe and reasonable values. Keep in mind that to charge batteries your charger has to be up in the 14-16 volt range, while a fully charged bank will be more like 13-13.5 volts at most when disconnected from the charger. Definitely not high enough to cause massive current to flow into an 80% discharged bank.

I do this all the time with my bank of four T-6's and my starting battery, a solenoid and my sole bus alternator. In three years, no matter how low the house bank or main battery is, never more than about 30 amps flows in either direction, and no problems ever. Happy batteries to date...

Here's a good dissertation on charging if you're interested
http://www.homepower.com/files/battvoltandsoc.pdf

Cheers
Gillig-Dan

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Posted on Thursday, November 04, 2004 - 12:07 pm:   

Thanks Gary,

That article clears up quite a bit of my confusion on the subject. I guess my theory fell apart when I figured an 80% charge on a battery meant 80% voltage. Need to get back to school on some of this stuff. Computers and throw away electronics have made my old solder-melting skills kinda obsolete. Rather than "doing it my way" I think I might "do it your way". Anyway, 'keep it simple' seems like it may work here. It's going to be tricky enough installing that air conditioning system Chuck sold me.

Dan
Richard Bowyer (Drivingmisslazy)

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Posted on Thursday, November 04, 2004 - 12:42 pm:   

Good post Gary. Even if the battery was discharged 50% the inrush current would not be sufficient to cause any damage to either battery. The voltage on the charged bank would immediately drop and the voltage on the discharged battery would immediately rise, so the maximum differential would probably not be more than 1 volt for a few seconds.

I operated my system like this for many many years and often the house battery was much lower than 50%, but I never experienced any problems.
Richard
Stan

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Posted on Thursday, November 04, 2004 - 1:08 pm:   

If a system is designed correctly the batteries are not tied together until the alternator is charging.

The output voltage of the alternator is higher than a fully charged battery so both batteries are receiving a charge and there is no current flow out of either battery.

Each battery will only accept the charge needed - that is the charge current is determined by the difference between the alternator output (which is fixed) and the battery voltage.

A three stage charger (properly set up) is good for either type of battery but not a mixture of different types.
Richard Bowyer (Drivingmisslazy)

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Posted on Thursday, November 04, 2004 - 2:15 pm:   

My belief is that at the instant the two batteries are tied together (regardless of whether the alternator is charging or not), and that current will flow from the higher DC battery voltage to the lower DC battery voltage. Current will continue to flow until those two voltages are equalized.

If an alternator is connected to the junction of the batteries, then the voltage regulator will see this common point voltage and set the field voltage of the alternator regulator to supply a certain voltage output of the alternator which will be above the common voltage sensed at the common junction. Typically about 13.6 volts as long as the amperage output at this voltage does not exceed the capacity of the alternator.

The output current of the alternator will divide between the two batteries. The majority (or all) of the current will flow into the battery with the lowest voltage due to its lower internal impedance. At the same time, current may be flowing from the higher voltage battery to the other battery. This equalization only takes a few seconds and never lasts long enough to create overheating or cause internal battery damage.

As soon as the two battery banks equalize with each other, then the output current of the alternator will divide proportionally between the two loads, depending on their internal impedance.

Regardless of the initial state of charge of either bank, the DC voltage will rapidly rise within a very short period of time and start limiting the amount of current the alternator is supplying as charging current. Since the alternator DC voltage is limited by the regulator, then the amperage will rapidly decrease to a safe and reasonable level.

At least that is my belief and is based on many years of practical experience.

Richard
TWODOGS (Twodogs)

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Posted on Thursday, November 04, 2004 - 2:18 pm:   

yeah....what he said
davidb

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Posted on Thursday, November 04, 2004 - 3:16 pm:   

I may be wrong but what about a dual blocking diode. Mounting one on each charging wire with the anode of each one attached to the altenator then one cathode to the starting battery and the other cathode to the house batteries. The diodes would not allow any current draining from one battery to the other but would allow the charging of both battery sets at the same time. You would have to reset the voltage regulator to compensate for the voltage drop of the diode. Try semikron.com part number SKKD260 (260 amp rating).
TWODOGS (Twodogs)

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Posted on Thursday, November 04, 2004 - 3:26 pm:   

how would the regulator know when to charge
davidb

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Posted on Thursday, November 04, 2004 - 3:56 pm:   

Good questions bring up others. The requlator would be enabled by the ignition switch but the altenator requires power on the windings to generate power.
What about a relay connected accross the starting diode. This would entergize the alternator after startup using oil pressure to activate the relay. If the coil of the relay was wired thru the N.O. contacts of the relay it would only momentarly short out the diode only after oil pressure was up.
TWODOGS (Twodogs)

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Posted on Thursday, November 04, 2004 - 4:30 pm:   

I think you are danceing around the question..yes,you can enable with the ignition switch...but...how would the regulator know when the batteries were low wired up like you say..
Stan

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Posted on Thursday, November 04, 2004 - 8:32 pm:   

Richard: I can't argue with your theory except for the several seconds of current flow. A solid state regulator will set the output voltage in millliseconds above the higher voltage battery and there will be charge current to both batteries. As I said in my previous post, this is dependent on the alternator charging one battery before the other battery is swiched into the circuit. This is the only correct way to do it and the switch should be completed by the R terminal on the alternator and not by an engine oil pressure switch.
Jayrjay

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Posted on Thursday, November 04, 2004 - 9:29 pm:   

I would be very careful of the dual diode setup. I don't know the PIV of the suggested diode, but if you loose one and the current inrush against the remaining one exceeds the PIV, it may well be setting up an alternator failure. Inrush current could exceed 3 to 5 times the normal current rating for as long as 40 milliseconds,(too long for the alternator) and since Leece-Neville 260 amp alternators are about $1600.00 exchange,I would consult someone with more experience than I about it. I express uncertainty, since there are so many variables in the final circuit design. The solenoid route is very stable, inexpensive, and nearly bullet-proof. Too many switches and interconnects to make me feel comfortable with that type of setup. Hmmmm, why not a good pair of knife switches? Ha! ...JJ
niles

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Posted on Thursday, November 04, 2004 - 10:21 pm:   

Too bad Gene Rochester isn't here - he's the guru on this stuff

Niles
Gary Stadler (Boogiethecat)

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Posted on Thursday, November 04, 2004 - 10:26 pm:   

I doubt you'd have any PIV (peak inverse voltage) problems with dual diodes- loosing a diode in a situation like this wouldn't really do much at all to the remaining diode. And it's totally easy to simply purchase diodes with 100 or more volt ratings if you have any doubts. And if you're worried about inrush currents just get a massive diode. Typical 40 amp diodes can withstand thousands of amps for a short time. Diodes rated at hundreds of amps are quite common and easily avaliable.
That said, I personally don't think inrush current and PIV are even things to worry about, because they are both terms that describe aspects of diode operation that really have little to do with this discussion if the diodes you use are properly sized and heatsunk. If you are designing high frequency switching power supplies, then inrush and PIV along with a bunch of other terms are things you must pay attention to. But not here....

The biggest problem with using diodes in series with batteries for charging is the inherent .7 volt drop across the diode. This means that if you put a diode in series with the path from your alternator to your battery, the alternator will think everything's groovy and the battery is fully charged when actually the battery is 7/10 of a volt lower than the alternator thinks it is, so it will never fully charge.
Systems that use diode isolators that are "properly" set up involve tweaking the alternator's regulator to increase it's output by .7 volts, to compensate for the .7 volts lost to the diode, something that is not easily doable by "users" like us busnuts. In truth, unless you want to squeeze every bit of charge out of your system, even this .7 volt problem is not much of a problem. There are millions of diode isolators out there with untweaked alternators that are giving their owners plenty fine service.

There is an alternate type of diode, shottky, that has a much lower forward drop, like .1-to-.3 volts. These would help the problem some, but then shottkys have their own problems- they are not avaliable in voltages much over 40 volts due to the physics of their construction, and any little spikes above their rated voltage tend to poof them quite easily. In an automotive environment, normal old .7 volt silicon diodes are much more robust, difficult to kill, and forgiving of abuse.
H3 (Ace)

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Posted on Thursday, November 04, 2004 - 11:30 pm:   

Where is ole' Gene anyway? I used to have his contact number but have lost it. I think it went with my Eagle when I sold it!

Anyone know how to reach him?

Ace
Niles

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Posted on Friday, November 05, 2004 - 12:24 am:   

Ace - he's in wallhalla South Carolina - can't remember the name of his shop - don't know if he's in the phone book - my buddy Dale Cyr has his # if you need it -

Niles
Tom Caffrey (Pvcces)

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Posted on Friday, November 05, 2004 - 12:51 am:   

A battery combiner that uses a heavy enough constant duty relay is a much better solution than a diode isolator. Pathmaker is one brand, Mastervolt another. These units look for approximately 13.2 volts at either battery and then interconnect them.

Since you will not have 13.2 volts in a flooded lead acid battery of 12 volts unless it is charging, it will stay disconnected until one battery or the other is charging.

As long as you are using compatible batteries, these are a nice, simple solution. They are made for 24 volt batteries, as well. Their main drawback is a tiny current from each battery to monitor the voltages.

For what it's worth.

Tom Caffrey PD4106-2576
Suncatcher
Richard Bowyer (Drivingmisslazy)

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Posted on Friday, November 05, 2004 - 9:07 am:   

Tom, I have not seen the devices you mention, but they sound like a great idea. I will see if I can find some info on it. I know all the Mastervolt folks and have visited their facility in Amsterdam several times.
Richard

I do not think anyone ever directly answered Chuck's original question, but with the two battery banks parallelled, you will not boil one bank while the other bank is coming up to charge.
At least thats my opinion, and I'm sticking to it. LOL.
Richard
Marc Bourget

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Posted on Friday, November 05, 2004 - 9:27 am:   

I have this recollection in the back of my mind that the regulator sensing the lesser capacity start battery(ies) will stop the charge when they get "full" leaving the (generally greater capacity) deep cycle house batts at less than full charge, which, I understand, it a recipe for accelerated battery death.

Can anybody explain what I missed above or comment on this point?
Jock Fugitt (Busjock)

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Posted on Friday, November 05, 2004 - 12:48 pm:   

Hi everyone, This post has been pretty much covered, but I would like to add a few notes and data I have from several years of running such a system.

1. I have paralelled the fresh 8-D's with a 1/2 charge house bank, engine off.. About 50 amperes initial inrush, then tapers off to about 10-20 anmperes as the batteries try to equalize.. Remember there is no real over voltage to force more current in this situation

2. Paralelling in with the bus on normal idle, The inrush is about 50 amperes only, the alternator does not put much out when at idle.

2. Our system is set up to automatically parelell when the alternator is charging, we use the terminal that indicates that blowers can be turned on. We have a 250 ampere fuse in our cross tie circuit. We have never blown it..

When paralelling with a low house battery, and full high idle, the inrush is about 150 amperes at the beginning, It drops fairly quickly as the bank voltage comes up. We see normally about 50 to 75 amperes charge on a 1/2 or 3/4 charged bank while going down the road.

If you decide to use a heavy load on the alternator, like running roof airs off the inverter, the voltage drops enough that you will get very little charging on the house bank.

It is true, that unless you are going on quite a long drive, the bus cross tie will not fully charge the house bank. If we pull out with a 1/2 charged house bank and drive most of the day, We are fairly up, but we may still need to use the genset and trace to fully charge,, Of course if plugging in, then it is all done with the trace while sleeping..

The old type of open construction for the regulator would be very easy to add one relay and a resistor, and it would be a multi stage unit, Just a switch up front to make it go to bulk voltage.. I have been to lazy to do it.. I believe then you might have to be more careful with over charging the 8-D's but I dont think that would be serious. Remember a fully charged battery will float at about 28 volts without really much input current. 29 volts for a few hours would problbly not boil much water.

We keep the bus Voltage regulator set to just keep the coach batteries fully charged, without very much water being added.. about 28.0 volts

Jock
Stan

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Posted on Friday, November 05, 2004 - 12:57 pm:   

A normal automotive system is a constant voltage and not a constant current system. The voltage will rise to whatever voltage the regulator is set at. If it is set at 13.6 volts any battery connected to it will charge until it reaches 13.6 volts. If the battery is almost dead it will charge at a very high current for a short period of time and then the current will go down as the voltage rises.

If you use one of the more exotic regulators it will monitor temperature and maintain the current at a high rate until the battery is close to full charge or the temperature exceeds the preset limit. In order to do this, it has to raise the output voltage of the alternator to overcome the rising voltage and internal resistance of the battery. As with any resistor, if you put too much current through it you get excessive heat.

Hope this is the answer you were looking for.
Richard Bowyer (Drivingmisslazy)

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Posted on Friday, November 05, 2004 - 12:57 pm:   

To my knowledge, no voltage regulator senses battery capacity. They only sense battery voltage and possibly total amperage.

With 13.8 volts (for example) applied to two different capacity batteries, then the individual batteries will charge to a maximum of 13.8 volts. Depending on the individual battery state of charge, the output amps will divide as required to bring each battery up to the 13.8 volt level of the charger (alternator). It is conceviable that each battery could be receiving a significantally different number of amps, but again the voltage will never exceed 13.8 amps.
Richard
H3 (Ace)

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Posted on Friday, November 05, 2004 - 2:04 pm:   

Niles, yes if you can get me his number I would appreciate it!

He told me once that he plain got tired of all the know it all bs on here hence why he stays away!

Gene was the first guy I talked to about a bus back when he converted his Black Eagle. From that day forward I was hooked!
Look at me now! A 1967 4107 GMC, A 1965 01 Eagle, and now a 1992 H3-Prevost :-)

Ace
Niles

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Posted on Friday, November 05, 2004 - 3:51 pm:   

Ace emailed you the # today - Niles
H3 (Ace)

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Posted on Friday, November 05, 2004 - 4:25 pm:   

Thanks

Ace
H3 (Ace)

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Posted on Friday, November 05, 2004 - 4:49 pm:   

I just talked to Gene and he is doing fine! Said he might come to Florida sometime in the near future but not for Arcadia. Knees need work but health for him and better half is good for the most part.
He's building Mack Trucks in SC.

It sure was nice to talk to him!
Thanks agin Niles!

Ace
H3 (Ace)

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Posted on Friday, November 05, 2004 - 4:50 pm:   

I just talked to Gene and he is doing fine! Said he might come to Florida sometime in the near future but not for Arcadia. Knees need work but health for him and better half is good for the most part.
He's building Mack Trucks in SC.

It sure was nice to talk to him!

Thanks agian Niles!

Ace
Stephen Fessenden (Sffess)

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Posted on Saturday, November 06, 2004 - 5:02 pm:   

A second single wire 24 nolt alternator is cheaper than a battery isolator. If you want an isolator, Camping World sells them.

I have two separate 24 volt alternators for engine and house batteries. I have suggested the value of redundancy as Don suggests. Now I have a story:

On this trip, my house alternator failed in Windsor, Ontario. I have to go all the way back to Florida. I have been running with the batteries interconnected by a solenoid that was already there. I just flipped a switch and kept driving. The initial house battery voltage (four 8Ds) was 18 when I discovered the problem. I believe that put quite a strain on my engine batteries (two group 31s)when I first interconnected them. I can also charge the house batteries with the gen set at a cost of $1.40 per hour if I have no other need for it.

If I had not had two 24 volt alternators I would have been stopping to fix the problem where ever I could find help. With the second alternator, I will fix it at home in a week. I believe I paid $70 for that alternator.
Niles

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Posted on Sunday, November 07, 2004 - 1:05 am:   

Ace - I got a chance to see and ride in his first mack a couple weeks ago - first in branson - then we stopped in walhalla - he's workin on #2 - 1st one is for sale - he's workin at the shop - seems to have too much business - he has lost interest in buses though - Niles
H3 (Ace)

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Posted on Sunday, November 07, 2004 - 10:29 pm:   

Yes so he told me but due to a lot of un-needed BS the bussin hobby has lost a great asset! Truly a remarkable person that is willing to help anyone with almost any problem using tried and trued facts instead of all the "theory" that is mostly dished out here!

Ace

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