Parallel batteries

[FinallyRetired]

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.

 

[landonew]

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.

 

[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.

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.

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.

Is this really a problem for you?

No, it's not. It's just a lot less than I expected.

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.

Sam

 

[youbet]

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?

 

[Sam]

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!

Sam

 

[Sam]

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

 

[youbet]

Thanks Sam.. BTW, I added some questions to my post with an edit. Perhaps you could go back and pick up those as well?

 

[Sam]

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.

Sam

 

[youbet]

Thanks Sam. And also for the reference to the article in your post #30 above.

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!

 

[ls99]

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.

 

[ERD50]

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.

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....

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

 

[Sam]

I think that you guys are over-thinking this.

Isn't that where the fun lies? ;-)

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?

Sam

 

[Sam]

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.

Sam

 

[FinallyRetired]

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.

 

[Sam]

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

 

[landonew]

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.

Sam

Not to nitpick, but this is a little misleading.

20% is the differential between the slowest discharging battery (4.5A) and the quickest (5.5A). In an ideal scenario, all three batteries would discharge at 5A, and thus satisfy your load requirement of 15A. Consequently, the net difference in the ideal/non-ideal scenario is .5A (5.5 -5), or only about 10%.

You would be correct about the 3% drop in the market, and I was not attempting to marginalize your problem. I apologize if it appeared that way, I was just trying to explain the effect. Regardless, it is not my place to say what is sufficient for you and what is not.

If it is your power-bill that concerns you, then you should sleep soundly in knowing that you are not wasting anymore power than you would have where the battery's to have the same output. The slower discharging battery offsets the quicker discharging battery.

Anyhow, Enjoy your ER. We should all be so lucky.


BTW, I am curious. What made you break out the multimeter anyways? Is nice to know that people still test the products they buy.

 

[youbet]

.

it is not my place to say what is sufficient for you and what is not.

There it is! The understatement of the day!

 

[youbet]

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.

Was this based on your own testing? Or did you somehow average the far flung numbers available in the literature and claims of various manufacturers?

 

[ls99]

Was this based on your own testing? Or did you somehow average the far flung numbers available in the literature and claims of various manufacturers?

Roughly averaged a bunch of mfg numbers I had collected over the years.

 

[ERD50]

Roughly averaged a bunch of mfg numbers I had collected over the years.

It would be worthwhile for the OP to contact the mfg. I'm sure they would tell him what the voltage values are for state of charge for his particular mfg/model of battery.

I just know batteries in general, I'm no expert on real specifics like that. But I would think that little details in the design and components could change those voltages a bit. I'm also pretty sure the mfg would *have* to know them, or they couldn't rate their batteries.

-ERD50

 

[Sam]

BTW, I am curious. What made you break out the multimeter anyways? Is nice to know that people still test the products they buy.

I was curious too. Almost everyone in-the-know strongly recommends the use of same age/type/capacity batteries when used as one. I was already convinced that different batteries are not efficient, but still wanted to know how efficient "identical" batteries are.

I'm satisfied with the performance of the battery bank, although I still wish for a better distributed system.

However, I am confident that with proper test equiqments, I would have found the same discrepancy amongst the 2V cells of one single 12V battery. Oh well...

Sam