Battery bank wiring help, inconsistent discharge

jimmyaz · January 2017

Hey guys,

I have 4x AGM 12volts 105 amps hour battery. I currently have it wire in series then parallels to make a 24volts 210ah bank....

But I am having problem when I drain the battery more than 20-25%.... Picture attached. When they are all charge up, the voltage seem to be within .30volts difference. But after discharging about 20-30ah.... Battery 1 and 4 voltage always drop significantly lower...

#1: 9.9v

Image: https://us.v-cdn.net/6024911/uploads/editor/jr/zp93kn49nitj.gif

#2: 11,9v
#3: 11.8v
#4: 9.9v

My actual wiring, instead of the negative going to battery 4 like the diagram, I have it connected to battery 2. But I can't imagine the distance of 6 inches would make that much of a difference?

DId I config it wrong, or is my battery dying? I am now thinking, maybe I should have parallels 2x together first, then series it? But everywhere I see online show series first than parallels.... But I also see Testa battery, they parallels all the cells first, then series?

thanks

jimmyaz · January 2017

Should I change to this config? would this be better?

Image: https://us.v-cdn.net/6024911/uploads/editor/1y/kuy79j48ecsj.jpg

706jim · January 2017

My batteries are set up similar to your first configuration. I think there may be a problem with the individual batteries; not the wiring.

jimmyaz · January 2017

Anyone else guys? Would I better off wiring it like this?

Image: https://us.v-cdn.net/6024911/uploads/editor/0e/g71k18nfkwd5.gif

dennis461 · January 2017

Put some yellow paint on battery #1.
Keep wiring the same as you have it now.
Move battery 1 to #3 position, and move battery 3 to #1 position.
Then check to see if the yellow battery still drops to 9.8 volts.

mcgivor · January 2017

jimmyaz said:

Hey guys,

I have 4x AGM 12volts 105 amps hour battery. I currently have it wire in series then parallels to make a 24volts 210ah bank....

But I am having problem when I drain the battery more than 20-25%.... Picture attached. When they are all charge up, the voltage seem to be within .30volts difference. But after discharging about 20-30ah.... Battery 1 and 4 voltage always drop significantly lower...

#1: 9.9v
#2: 11,9v
#3: 11.8v
#4: 9.9v

My actual wiring, instead of the negative going to battery 4 like the diagram, I have it connected to battery 2. But I can't imagine the distance of 6 inches would make that much of a difference?

DId I config it wrong, or is my battery dying? I am now thinking, maybe I should have parallels 2x together first, then series it? But everywhere I see online show series first than parallels.... But I also see Testa battery, they parallels all the cells first, then series?

thanks

The diagram would be the correct way to wire 4 × 12V series/parallel for 24V, you want to attach negative to battery 4 for even charging and discharging, but what jumps out in your post is the voltage readings, 9.9V would indicate a dead battery, 100% discharged.

Marc Kurth · January 2017

jimmyaz : Are you aware that 10.5v is a dead battery? That is the point where you have removed all of the rated capacity.
You can drain them lower - just like you can rev your engine way past its rated limit - but there are consequences.....

I know that you said that they are within 0.30v from each other when charged. But what are the voltages that you are seeing when fully charged?

What brand of battery do you have and what was their manufacture date?

Marc

mike95490 · January 2017

I think you have 2 bad batteries. One in each string. Shuffling positions around won't fix anything, Sorry

BB. · January 2017

And a 0.30 volt difference between batteries is, in theory, the difference between a battery at 100% state of charge and at 70% state of charge (at least for typical flooded cell batteries, not so sure if AGM are quite the same). (basically assuming ~12.7 volt rest voltage--AGMs can read higher on their rest voltages).

-Bill

jimmyaz · January 2017

Well, add another mistake to my list for buying slightly used battery... One of them die after 2 months, so I had the supplier replaced with a different one, this may be why the whole bank is now screwed.

I'm now quite disappointed with AGM... but oh well, can't and won't spend more money on battery. Leaning toward them salvage Tesla Model s module. A guy locally here is parting out, stating the car only has 1050miles and selling each 24v 5kw module for 1000.00 ea... Price is not bad right? Considering it a 5kw pack and you can safely use 3kw.

If my inverter shut off is fixed set to 19.00 volts, is this ok for use with LI-ION? 3.2volts X 6 = 19.2, 3.2volts cut off for Li-Ion sound about right?

mike95490 · January 2017

Doe the tesla battery management system come with the battery? (Voltage and temp monitor)

westbranch · January 2017

why is it being scrapped? How old is the car, when was it made? A friend with an S model is thinking about what to do if his battery shows weakness, probably in 2 years.. He has read that Tesla does NOT sell repair parts on the street, have to go to an authorized dealer!

jimmyaz · January 2017

The guy said its a wreck and battery is fine. Car was at 1050miles.

He said each module does have some type of circuit board but he said it require the main bms in order to work and probably need all the module to work.

mcgivor · January 2017

My personal opinion for what it's worth, is you are thinking beyond your current knowledge, dealing with sophisticated battery systems is the last thing you should be thinking of, these companies research and spend millions developing battery monitoring systems, more than likely they will not share their information and furthermore they would be unlikely to sell parts to complete anything you have in mind. Tesla builds cars, for now, they have other aspersions, but they will keep their cards close so as not to liberate technology, although they claim not to.

Marc Kurth · January 2017

I would venture a guess that you bought used batteries that were far more than "slightly" used. One failed in two months and now you feel that the rest have also failed. So now, you are "quite disappointed" with AGM batteries.

I see that you are contemplating buying a different type of used battery bank of untested condition. The type of battery is more specialized relative to the monitor/management system - but you will not likely get the entire control system with the batteries.

Seems like there might be a lesson in there somewhere.

Marc

jimmyaz · January 2017

Thank you for all the positive comment... lolz. This is simple a small test project and there's no reason to spend all my money on it. I am no idiot when it come to battery, I have been using Lithium Polymer in RC for 15 years now and have used a lot of batteries.

For the past couples day I undo the batteries, put them all in parallels then charged it all together. Hook it back but this time, I parallels first the two, then series the pack. I after testing, I believe this is a much better way to wire it.

After a few charged and discharge. I was able to sync the voltage much better...

They are both get to 13.10volts when fully charged. Both packs are within .05 volts from each other at rest now. But under 25amps load, there's a .10 volts difference.

Marc Kurth · January 2017

I didn't mean to be negative - just cautious. Best of luck!

mcgivor · January 2017

jimmyaz said:

Anyone else guys? Would I better off wiring it like this?

If this is the way you ended up wiring you batteries, it goes aginst conventional wisdom, the battery voltages will appear more ballanced as the better two will feed into the worst two. My suspicion is that there will be problems with this configuration, it would be better to seperate the bad 2, series them, charge them to full capacity, repeat with the other 2, then wire in a conventional configuration, series first, then parallel, with the charge and discharge connections diagonally opposed.

My opinions, for what they are worth.

jimmyaz · January 2017

mcgivor said:

jimmyaz said:

Anyone else guys? Would I better off wiring it like this?

If this is the way you ended up wiring you batteries, it goes aginst conventional wisdom, the battery voltages will appear more ballanced as the better two will feed into the worst two. My suspicion is that there will be problems with this configuration, it would be better to seperate the bad 2, series them, charge them to full capacity, repeat with the other 2, then wire in a conventional configuration, series first, then parallel, with the charge and discharge connections diagonally opposed.

My opinions, for what they are worth.

Interesting thought... that may be true now that them battery is all screwed up. But if with brand new matching battery, would it be against conventional wisdom then? Isn't that how all the battery in Tesla car are wired? Bunch of cells in parallels first, then series together?

mcgivor · January 2017

Even if they were new there would be imbalances during charge and discharge, so conventional wisdom would prevail. What Tesla is doing is completely different, they are using small cells to create a single large cell, then stringing these cells to create a battery, a lot more complex setup, but essentially it is still one battery, not multiple batteries fighting one onother.

Battery bank wiring help, inconsistent discharge

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