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Old 03-12-2009, 05:00 AM   #21
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An update:

I bought 3 new identical group 27 batteries, ~100AH each. Fully charged all 3 separately. Combined them into one 300AH battery. Installed 4 precision shunts (100A/100mv): One for each battery, and one for the combined battery. Start drawing a 15A load from the combined battery and watch the current going out:

Combined current = 15A
Battery #1 = 5.5A
#2 = 4.5A
#3 = 5.0A

So, even identical (same brand, same type, same capacity, same month of manufacturing) batteries do not behave identically. Same observation when the batteries are under charge.

Sam
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Old 03-12-2009, 07:57 AM   #22
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Quote:
Originally Posted by Sam View Post
An update:
Quote:
Originally Posted by Sam View Post
Quote:
Originally Posted by Sam View Post
I bought 3 new identical group 27 batteries, ~100AH each. Fully charged all 3 separately. Combined them into one 300AH battery. Installed 4 precision shunts (100A/100mv): One for each battery, and one for the combined battery. Start drawing a 15A load from the combined battery and watch the current going out:

Combined current = 15A
Battery #1 = 5.5A
#2 = 4.5A
#3 = 5.0A

So, even identical (same brand, same type, same capacity, same month of manufacturing) batteries do not behave identically. Same observation when the batteries are under charge.

Sam


Are you asking why this is?

My theory would be that the batteries are designed to operate within certain parameters. I would guess that the internal resistances are slightly different. Remember, a DC battery is essentially a capacitor. Ideally, the capacitor would have an infinite resistance. However, practically the capacitor is not a perfect inductor.

To expound on the automobile example, Imagine you have three trucks pulling three trailers down a 3 lane highway, Each going from point A to point B.

You set the cruise control of each truck at 70 mph. You wouldn't expect each truck to be going EXACTLY the same speed. You may have one truck going 70.00015, while another is going 69.99985 mph.


From an engineering perspective, the method for controlling the outputs of each source would be to add an appropriately sized resistor in series with each individual battery prior to your junction point. However, I wouldn't recommend that for your application.
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Old 03-12-2009, 08:58 AM   #23
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Quote:
Originally Posted by Sam View Post
An update:

I bought 3 new identical group 27 batteries, ~100AH each. Fully charged all 3 separately. Combined them into one 300AH battery. Installed 4 precision shunts (100A/100mv): One for each battery, and one for the combined battery. Start drawing a 15A load from the combined battery and watch the current going out:

Combined current = 15A
Battery #1 = 5.5A
#2 = 4.5A
#3 = 5.0A

So, even identical (same brand, same type, same capacity, same month of manufacturing) batteries do not behave identically. Same observation when the batteries are under charge.

Sam
Yes, there will always be some variation battery to battery, and this is what you will see in parallel. However, if they are very similar, I bet this evens out over a longer set of charge/discharge cycles. You added another variable by charging separate, then discharging in parallel.

As landonew mentions, trying to do something about it will cause more problems than it solves. You might just have to accept that 3 batteries will give you maybe 2.9x the amp hours rather than the theoretical 3x.

-ERD50
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Old 03-12-2009, 09:27 AM   #24
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You added another variable by charging separate, then discharging in parallel.
-ERD50
I only charged them separately once, after bringing them home from the store. I wanted to make sure they are all fully charged before combining them. Now, they are all charged together as one. I have gone through several charge/discharge cycles and the individual currents through each battery still differs.

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Old 03-12-2009, 10:30 AM   #25
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The small current differences you found are not significant. If you used different capacity or age batteries the differences would be much larger, especially after several hours of use.
Having 3 100AH batteries is an excellent choice. You are keeping the discharge rate around (C=capacity) C/20, which will provide good longevity. Providing they don't get deep discharged.
New batteries come up to full capacity after about 4 to 5 charge discharge cycles.
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Old 03-12-2009, 10:55 AM   #26
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The way I see it, when a load is applied, each battery will provide a current proportional to its capacity, 1/3 from #1 and 2/3 from #2. The reverse is true under charge. There will be no (or only negligible) current going from one battery to the other. So, I don't see any problem.

What am I missing?

Sam
Sam, another issue might be when you go to recharge them. If they are still connected in parallel and you use a common recharger, one of them will get topped off before the other one. The charger would still be running current into the larger battery, though, which might cause boilover in the smaller battery.
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Old 03-12-2009, 11:22 AM   #27
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I only charged them separately once, after bringing them home from the store. I wanted to make sure they are all fully charged before combining them. Now, they are all charged together as one. I have gone through several charge/discharge cycles and the individual currents through each battery still differs.

Sam
As I said before, the internal resistance characteristics of each battery differs slightly. Charging them together and/or running them through numerous discharge cycles will not change this factor and thus will not affect their individual power outputs. In other words, the battery will reach its maximum voltage relitively quickly in the charging cycle. Voltage only begins to fall when the battery has almost completely dissipated its potential capacity.


Is this really a problem for you? The battery with the highest output will limit your total capacity, so just adjust your charge/recharge cycle using your highest output battery as your baseline. You will have to recharge slightly more often, but not significantly.

In other words, you want to charge your batteries before the first battery dips below 50%. The battery with the 5.5A output will only discharge 10% faster than the ideal 5A battery. This is not significant in the grande scheme of things is it?

If this 10% loss makes the total runtime insufficient, you could always add an additional battery.

There is no way to change the internal resistence characteristics of the battery. As i said before, the only real solution is to add a series resistor to the high output battery. This carries its own issues (extraneous heat, wasted energy, voltage drop).

Enjoy your RV, as well as ER.
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Old 03-12-2009, 01:20 PM   #28
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Sam, another issue might be when you go to recharge them. If they are still connected in parallel and you use a common recharger, one of them will get topped off before the other one. The charger would still be running current into the larger battery, though, which might cause boilover in the smaller battery.
I'm aware of that. However, modern chargers only pump in high amperage up to about 85-90% charged. Then the charging voltage will drop slightly to limit the incoming current. During this latter stage, the individual batteries "should" equalized somewhat, so it won't be a problem, in theory.



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As I said before, the internal resistance characteristics of each battery differs slightly.
Yep, that's why I bought 3 "identical" one, hoping that they have the same characteristic.

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Is this really a problem for you?
No, it's not. It's just a lot less than I expected.

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In other words, you want to charge your batteries before the first battery dips below 50%. The battery with the 5.5A output will only discharge 10% faster than the ideal 5A battery. This is not significant in the grande scheme of things is it?
But 20% faster than 4.5A battery. Of course it's significant. A 3% drop in the market is headline news, and it does not happen that often (barring the present time. Here we have 10-20% all the time.

I only watched the test for about 10 mins. But I'm relatively sure that given sufficient time, thing will get better. As the high output battery's voltage will drop faster, its current should decrease. The reverse should be true for the slower batteries. So, overall loss will be minimized, unless all the loads are removed. W/O load, the batteries will try to equalize, transferring energy among themselve, this is where the real loss is. But that should never be case in an actively used motorhome, there will always be some small load - about 1A, for the fridge, the propane/CO alarm.

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Old 03-12-2009, 01:37 PM   #29
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Sam.....

Where did you buy the shunts? How much did they cost?

I'm assuming you'll remove them after testing is complete. Or do you feel the voltage drop across them is of no consequence?

How have you been measuring the percent discharge?

Regarding charging them while they're connected in parallel.... The voltage across each battery connected in parallel will be equal to one another. But, chemically, one may reach a fully charged state before the others. Without independent battery voltages to serve as a charge indicator, you'd have to measure specific gravity to be sure of whats going on, yes/no? With similar batteries, this is probably of little concern.

I'm a possible future RV'er (pending stock market recovery!) and have been doing some reading regarding charging systems, just enough to be dangerous with my limited knowledge. Is your system set up so that your batteries charge from the vehicle electrical system while underway and from the DC converter while plugged into campground AC? Are both systems free of overcharge issues? Is it all automatic or is some manual switching involved?
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Old 03-12-2009, 01:47 PM   #30
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Having 3 100AH batteries is an excellent choice. You are keeping the discharge rate around (C=capacity) C/20, which will provide good longevity. Providing they don't get deep discharged.
New batteries come up to full capacity after about 4 to 5 charge discharge cycles.
So far, my discharge rate averages about C/30 at night and less than C/50 during the day.

Yes, I hope their capacity will better. My informal 10 hour test told me I have a lot less than 300AH.

BTW, what voltage do you use to "guesstimate" the 50% state of charge? Batteries are at rest of course. I've seen number ranging from conservative 12.25V (Batteries -- and Other Electric Stuff by phred) to 12.40V (some solar panel selling co in AZ or CA). The difference between them is the equivalent of ~50AH for my battery bank!

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Old 03-12-2009, 01:56 PM   #31
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Sam.....

Where did you buy th shunts? How much did they cost?

I'm assuming you'll remove them after testing is complete.
I was about to buy them locally at $25 each, then I stumbled onto a good deal on EBay. $72 for 12 of them (he would not sell them individually). So I bought them all. Mines are Empro HA 100/100 (100A/100mv). They are big (5"x1"x1" LHW) and heavy (I guess 1/4 lb). They appear to be very accurate, at least for my use.

No, I don't plan to remove them at all. I will leave them there so I can check on them periodically.

Sam
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Old 03-12-2009, 02:00 PM   #32
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Thanks Sam.. BTW, I added some questions to my post with an edit. Perhaps you could go back and pick up those as well?
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Old 03-12-2009, 02:12 PM   #33
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Sam.....

Or do you feel the voltage drop across them is of no consequence?

How have you been measuring the percent discharge?

Regarding charging them while they're connected in parallel.... The voltage across each battery connected in parallel will be equal to one another. But, chemically, one may reach a fully charged state before the others. Without independent battery voltages to serve as a charge indicator, you'd have to measure specific gravity to be sure of whats going on, yes/no? With similar batteries, this is probably of little concern.

I'm a possible future RV'er (pending stock market recovery!) and have been doing some reading regarding charging systems, just enough to be dangerous with my limited knowledge. Is your system set up so that your batteries charge from the vehicle electrical system while underway and from the DC converter while plugged into campground AC? Are both systems free of overcharge issues? Is it all automatic or is some manual switching involved?
Hey, you edited while I was responding to your first message! ;-)

The voltage drop is of no significance. At 100A draw, the drop is 100MV. Most of the time, my draw is less than 10A, so 0.01V drop.

Percent discharged (or charged) is a mistery in my mind. No one seems to agree on what the voltage should be, even at rest! I will use 12.35V as the mid point (50% discharged) when under small load (1 to 5A, up to C/60 for my battery bank), and 12.40V at rest. I guess that my guess is just as good as anyone's? What's your opinion?

I don't have to worry about being overcharged while underway. My alternator is only 63amp, and it has to go through a humongous diode (battery isolator) before it gets to my house battery.

At home, I will be using a intelligent 25Amp battery charger. So I'm safe there. I have discarded the converter that came with the motorhome. It's old, weak, and takes day to charge the battery. So, no converter for me to worry.

On the road, I have a small DC generator capable of pumping 90A (according to specs) into the batteries. I have not seen anything higher than 30A so far. But then my batteries are more than 60% charged when I tried it.

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Old 03-12-2009, 02:20 PM   #34
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Thanks Sam. And also for the reference to the article in your post #30 above.

Quote:
I have discarded the converter that came with the motorhome. It's old, weak, and takes day to charge the battery. So, no converter for me to worry.
So parked at a commercial campground, you'd be running your dc appliances off of batteries? And perhaps recharging the batteries simultaneously with your smart charger?

I have no clue on the percent discharge measurement. But this is teaching me that the various indicators I have available, such as the led readout on my trolling motor, are all probably just a shot in the dark!
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Old 03-12-2009, 02:45 PM   #35
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BTW, what voltage do you use to "guesstimate" the 50% state of charge? Batteries are at rest of course. I've seen number ranging from conservative 12.25V (Batteries -- and Other Electric Stuff by phred) to 12.40V (some solar panel selling co in AZ or CA). The difference between them is the equivalent of ~50AH for my battery bank!

Sam
12.4 is a good conservative voltage for 50 charge, 12.3 would be closer to the actual. Discharged to 11.94 is a dead battery. All these numbers are unloaded i.e. open circuit 12 HR no load conditions for marine type quasi deep cycle batteries.,

Typical dead battery readings on per cell basis for flooded lead acid:
auto 1.75, traction 1.70, stationary 1.85 volts.
By the way none of these numbers are cast in stone, good bit of variations claimed by various mfg.

Happy camping.
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Old 03-12-2009, 03:32 PM   #36
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Sam, another issue might be when you go to recharge them. If they are still connected in parallel and you use a common recharger, one of them will get topped off before the other one. The charger would still be running current into the larger battery, though, which might cause boilover in the smaller battery.
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I'm aware of that. However, modern chargers only pump in high amperage up to about 85-90% charged. Then the charging voltage will drop slightly to limit the incoming current. During this latter stage, the individual batteries "should" equalized somewhat, so it won't be a problem, in theory.
I think that you guys are over-thinking this. The charger goes by voltage to know what to do next. Let's just say that 13.7V is a trigger for something. Well, any of those three batteries charged alone would cause a trigger to occur when they reached 13.7V. Tie them together in parallel, and the trigger still occurs at 13.7V on each battery.

Now, the lower state battery would hog more of its share of the current as it comes to equilibrium with the others (just like they deliver more/less current to a load). This might have a minor effect - if your charger is rated to max out a 300 A/H battery - that one hogging might gets more than its share, and that could over-heat it etc. I doubt you are close enough to the line to make these minor differences significant, and if you were, you'd need to worry about temperature, humidity, barometric pressure, phase of the moon....


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Yep, that's why I bought 3 "identical" one, hoping that they have the same characteristic
.

+/- 3dB. No you didn't, you bought three batteries of the same model, you didn't pay the manufacturer to give you a precision measured "matched set". It costs a fortune to get a manufacturer to "bin-out" parts to high accuracy. And they certainly won't do it w/o a volume purchase.

OK, now I see you really did mean "shunt", I thought you just meant straps to connect the batteries. so 5 Amps is 5mV - you are getting down to fractions of a millivolt, and ~ 5% deltas. Plus, throw in some inaccuracy for the shunts (I see these are accurate, 0.25%), but you connections would factor in also.

I'd worry more about the general charge/discharge cycles than this minor difference.

-ERD50
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Old 03-12-2009, 05:13 PM   #37
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I think that you guys are over-thinking this.
Isn't that where the fun lies? ;-)

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Now, the lower state battery would hog more of its share of the current as it comes to equilibrium with the others (just like they deliver more/less current to a load). This might have a minor effect - if your charger is rated to max out a 300 A/H battery - that one hogging might gets more than its share, and that could over-heat it etc. I doubt you are close enough to the line to make these minor differences significant, and if you were, you'd need to worry about temperature, humidity, barometric pressure, phase of the moon....
The charger has no idea how big the battery is. I think it makes decisions based on voltage and current being accepted by the battery.

What's "+/-3dB"? Sound level has something to do with battery performance?

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Old 03-12-2009, 05:16 PM   #38
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So parked at a commercial campground, you'd be running your dc appliances off of batteries? And perhaps recharging the batteries simultaneously with your smart charger?
Yes, but rarely I hope. I haven't spent a lot of time in commercial campgrounds and don't plan to either.

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Old 03-12-2009, 05:31 PM   #39
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The charger has no idea how big the battery is. I think it makes decisions based on voltage and current being accepted by the battery.
OK, let's overthink this some more. The charger has no idea of your battery configurations, or how many batteries you have in parallel. It just sees a resistance out there that is the combined resistance of both batteries. So it puts out a current at the set voltage.

As the load (both batteries) get charged, their resistance changes, and the charger adjusts its current to a lower level. The problem is, it's seeing the resistance from both batteries in parallel according to the formula 1/((1/R1)+(1/R2)).

The current that flows, however, goes to both batteries, and when one of them is charged to nearly full, most of the current then goes to the less charged battery. However, the fully charged battery is seeing the full current, causing it to overheat. So, you can charge them both, but will lower the life of the lower capacity battery.
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Old 03-12-2009, 06:07 PM   #40
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What you said is true, FinallyRetired. But in real application, it has not cause much problems. Keep in mind that even one single battery is actually six 2V batteries connected in series. So each sub-battery is also subjected to the same potential overcharge problem. And they do, as evidenced by different amount of electrolyte left in them after sometime.

Sam
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