Question regarding coach batteries tied in series/parallel

[John From Detroit]

Roamer.  We agreed on more than just the "2 six in serries are 1 12"  Your entire post was basically a re-statement of my post on Paralleling unequal batteries,,, Right down to your statement of socialism at work (I used a different phrase but with the same meansing, Each according to his need/abilities)

Wow, One of us must be sick!

Oh well, I'm about 10 minutes from where I have to take a test (Amateur Extra Class) in 87 minutes as I type this

 

[DonTom]

We agreed on more than just the "2 six in serries are 1 12"  Y

Who wouldn't agree to that?  Even a single six volt battery is really 3 two volt cells in series. You're just doubling the number cells by adding another battery.

                                                                        -Don-

 

[DonTom]

I have to disagree with your advice about wiring batteries in parallel, Don. When in parallel, each battery acts pretty much independently, supplying power according to its capability and drawing charge amps according to its need and internal resistance. Parallel wiring is true socialism at work.

Do you fuse each battery, so one shorted battery won't discharge the others?  I still would  prefer to buy a single battery at twice the capacity, if possible.
                                                          -Don-

 

[DonTom]

Oh well, I'm about 10 minutes from where I have to take a test (Amateur Extra Class) in 87 minutes as I type this

Good luck!

                                  73, Don, AA6GA

 

[Karl]

Don,

Do you fuse each battery, so one shorted battery won't discharge the others?

A fuse would do nothing but defeat the whole purpose of paralleling batteries. How would the fuse know whether it's a valid load, or a shorted battery? Even if it could, how would you determine the size to use? Is 10 amps too much? Is 100 amps too much? A better way would be to use diodes on the output of each battery. This technique is used extensively to prevent damage to a circuit if the battery voltage is inadvertently connected backwards. Same thing. The shorted battery would be more negative than the good one and current would not flow. Notice I said "better"; not good. You would still have to contend with the inherent .6V drop of silicon diodes or the .2V drop of germanium diodes. :'(

 

[John From Detroit]

We agreed on more than just the "2 six in serries are 1 12"  Y

Who wouldn't agree to that?  Even a single six volt battery is really 3 two volt cells in series. You're just doubling the number cells by adding another battery.

                                                                        -Don-

Thanks for the good luck by the way Don, as it turns out (I only missed 2 questions, you can miss 13 and pass) I did not need it )

The difference between a pair of sixes and one 12 is that you can not replace 1/2 of a battery, so when I say I treat a pair of sixes as one 12 I mean replace one, replace the other (normally, there is one exception which I won't go into now)  Where as if you have 12 volts in parallel (even if some of them are pairs of sixes) they will work out their differencs.  A pair in series have to match or bad things happen  Details of those things were in the earlier post

 

[DonTom]

Please see:

http://www.allaboutcircuits.com/vol_1/chpt_11/5.html


"With a parallel battery bank, one fuse is adequate for protecting the wiring against load overcurrent (between the parallel-connected batteries and the load), but we have other concerns to protect against as well. Batteries have been known to internally short-circuit, due to electrode separator failure, causing a problem not unlike that where batteries of unequal voltage are connected in parallel: the good batteries will overpower the failed (lower voltage) battery, causing relatively large currents within the batteries' connecting wires. To guard against this eventuality, we should protect each and every battery against overcurrent with individual battery fuses, in addition to the load fuse: "


I noticed several other sites that mentioned the same thing about lead-acid & other batteries used in parallel.


http://sub.allaboutcircuits.com/images/00276.png


                                                        -Don-

 

[DonTom]

Thanks for the good luck by the way Don, as it turns out (I only missed 2 questions, you can miss 13 and pass) I did not need it

Were the two you questions you missed about batteries?  (just kidding!!!) ;D

I had to take all my FCC tests at the FCC office in San Francisco. That was all back in the mid 1960's and early 1970's. Back in those days, it was required to have the General or higher for two years before I could even take the Extra. In fact, the Extra Class was new and the Advanced was restarted after not being available for many years.  And we were tested for sending and receiving 20 WPM. It has changed a lot since those days. Back then, they would never tell us if we missed any on any test, but if you failed sending or receiving CW, you were told right away that you flunked. For the written, pass or fail  would come a month or two later in the mail. It was always good news for me as I passed them all  the first time (3rd,  2nd and First Class with Ship Radar as well as all the ham licenses] but how many  missed was kept secret back in those days.

"The difference between a pair of sixes and one 12 is that you can not replace 1/2 of a battery, so when I say I treat a pair of sixes as one 12"

I too use two six volt gulf cart batteries in series in my RV.  My main reason is because each  are a lot easier to remove than a single larger 12 Volt. However, If one failed, I would replace both. Both are always the same age.  I replace all my lead acid batteries  in all my MANY vehicles every four years. That's just because I prefer to replace them at my convenience instead of when they decide to fail during a trip or whenever. I replaced several good batteries (four years old) just last summer. I don't try to get the last few hours out of them!

                                          -Don-

 

[DonTom]

How would the fuse know whether it's a valid load, or a shorted battery? Even if it could, how would you determine the size to use? Is 10 amps too much? Is 100 amps too much?

Some simple math should figure out that one. The fuses should be just large enough to never blow when each battery is doing it's normal job on a full load. Of course they will be of large amperage. A shorted battery is a BIG dead short and that's all you're trying to protect from.  A hundred amps on each might be too low!

I would think it could even be dangerous to have no fuse on each. The shorted  battery could blow up!

I certainly would go with large fuses before even thinking about diodes!

                                                                -Don-

 

[Shayne]

Interesting thread but got to give it up? ?these old eyes along with the Yellow glasses don't make it easy for reading all the colors and then the changes of colors.? To me it's like reading caps.  BTW don  Thank you.

 

[Tom]

I'm not far behind you Shayne. There's no real reason for anyone to post their messages using anything other than black font. If you need to quote text from a prior message, just use the Quote button above and edit out the parts you don't want to quote. Alternatively use the quote /quote tags (use square brackets around those tags and put your quoted text between them). Either way, the result will look like:

these old eyes along with the Yellow glasses don't make it easy for reading all the colors and then the changes of colors.

.

 

[DonTom]

"these old eyes along with the Yellow glasses don't make it easy for reading all the colors and then the changes of colors.  To me it's like reading caps."

For some reason I find the colors to be easier to read than this pure black. Have you tried taking off your yellow glasses?? ;D

                                                                    -Don-

 

[Karl]

Don,

Some simple math should figure out that one. The fuses should be just large enough to never blow when each battery is doing it's normal job on a full load.

Sorry, but 'simple math' doesn't get the job done in the real world, and requires an understanding of Ohm's Law, Kirschoff's Law, planar diode theory, and many other things too numerous to mention. Your reliance on "allaboutcircuits" is both misleading and erroneous. Almost all battery "short" failures are not entire shorts at all, but a cell that fails to output it's normal 2 volts (approx.), normally due to a shedding of plate material which causes a low output. As per my previous post, I would challenge anyone to determine what the fuse rating should be.

 

[Shayne]

sorry  I'll skip the colors and keep the glasses.  I have others and it's just as bad.

 

[DonTom]

" Your reliance on "allaboutcircuits" is both misleading and erroneous."

There were several sites all saying the same things, not only that one. Do your own web search on "batteries in parallel" an you will see what I mean.

But it is not important to me, because I don't put batteries in parallel anyway. At least not permanently.

                                                            -Don-

 

[Gary RV_Wizard]

Don's red type is a bit tough on these eyes too. A little bit of color for emphasis is OK, but all red is a bit much.  Got that, Don?

I think the notion of fusing individual batteries in a series pair is all but academic. The likelihood of a cell shorting is the same whether the cells are two groups of three (a pair of 6V) or one group of six (a single 12V). And the article quoted is discussing paralleled  batteries, not series pairs. Yeah, a fuse could still be placed on the series wiring between batteries, but it doesn't do much since all current for both batteries flows through one fuse anyway. That's why Karl suggests a diode to prevent reverse flow if the good battery is downstream from the shorted one.

And shorts are not always complete battery shorts - in fact they rarely are. Usually one cell shorts to another, dropping the battery voltage by 2.1VDC, e.g. making a 6V battery into a 4V. That drags down the series pair, but is not explosive.

I do agree with Don that a larger 12V is a more reliable solution than paired small batteries for moderate needs, but for larger requirements a 400-500 amp-hour 12V is a huge and  expensive battery that requires a fork lift to move.  Even an 8D size, which some of us have, is a real handful to deal with and that's only 250 A-H. An 8D weighs about 165-175 lbs and often costs $400-$500.  A pair of 6V golf cart batteries provides nearly the same power in two smaller packages (similar total weight) and should cost no more than about $200-$250

 

[Tom]

Don already switched to black font in his messages yesterday (thanks Don!) I guess Shayne is a very persuasive guy.

 

[Shayne]

Let's don't go that far.  But thanks  for  the compliment anyway.  To and for me those come far and few between.  Guess it's the stubbornness and unsophisticated way I state things.

 

[Karl]

The last pair of T-125's I bought was from, of all places, Gander Mountain. Seems they sell a lot of them for both golf carts and trolling motors. $109 apiece; roughly $10 cheaper than T-105's anywhere else I could find. Not all that much difference in power, but an ampere hour is an ampere hour

 

[Karl]

Shayne,

Guess it's the stubbornness and unsophisticated way I state things.

What's that old saw about calling a spade a spade? Nothing wrong with that. Some of us, myself included, sometimes use a hundred words when just a few would get the point across!