Can you put in too many amps when the volts never go up?

[Deleted member 9967]

Ok so I top balanced my cells. [Go to beginners section to learn how] months ago.
They all topped out at 3.650 volts each.

I let them rest a day or two and then started to use them. They went down to about 20% or 3.2 volts each cell or 25.60 volts total [for an 8 cell 24 volts system].

I charged them up to about 80% or 26.6 volts total. After a few hours rest the cells were within 0.001 volts of each other at 3.325.

I have done this a few more times.

I then decided to charge them up to 28.2 volts. I knew how many amps I had used and so knew about how long it would take to charge them up etc.

Well after a number of hours the coulomb meter said they were full. But the voltage was still at 27.14 volts. It was not moving.

I let it sit a bit longer knowing that it would not go too high on the voltage because it was preset to 28.2
So I felt safe.

However after three more hours of putting in 10 amps per hour when the amp meter said it was already full. I began to get worried.

I left it running a little longer thinking that maybe the coulomb meter was not working or something.
But even after four more hours. The voltage never went about 27.14 and the amps were still saying 10 amps were going into the cells even though all the meters said they were full.

After 7 hours of putting in 10 amps per hour when the amp meter said it was of amps, the full the voltage never changed from 27.14 volts.

So is this normal?
Is something wrong some place?
Can you put in too many amps even when the voltage reads all is ok?
Anyone have a clue as to what is happening here?

Thank you

 

[newbostonconst]

What size battery? In Amp Hours

10 amps is high for no response. Try flipping you amp meter, it should read negative 10 after flipping.

 

[Deleted member 9967]

Lifepo 280 amp cells.

Flipping it? Not sure how to do that. It is digital [the reader] and attached to a shunt.

 

[labeeman]

Did you tell the coulomb meter what size battery you have?

 

[mikefitz]

maybe the coulomb meter was not working or something.

What meter have are you using? Its possible the auto reset to 100% is occurring too early.

Any voltage at 3.4 volts per cell will almost fully charge given time. Because you are charging at a low current compared to cell capacity, the voltage rise and associated current fall, will not occur until you are very near 100% charge.

Are the voltage readings accurate? Even an error of 0.050v can make a significant difference.

Its possible if you have been short cycling the cells, extra power may be needed to overcome the memory effect.

To see the effect of charging at a low current I have taken screen shots of tests carried out by the 'off grid garage' guy. The blue line is 30 amps into a 109 Ah cell, the green 5 amps. You can see that with the lower current the voltage did not change much until the cell was near full charge. Over much of the range it was steady at 3.375 or less.


Mike

 

[Deleted member 9967]

Did you tell the coulomb meter what size battery you have?

Of course. Like I said I have been using it for a while now.

Edit for spelling.

 

[Deleted member 9967]

What meter have are you using? Its possible the auto reset to 100% is occurring too early.

Any voltage at 3.4 volts per cell will almost fully charge given time. Because you are charging at a low current compared to cell capacity, the voltage rise and associated current fall, will not occur until you are very near 100% charge.

Are the voltage readings accurate? Even an error of 0.050v can make a significant difference.

Its possible if you have been short cycling the cells, extra power may be needed to overcome the memory effect.

To see the effect of charging at a low current I have taken screen shots of tests carried out by the 'off grid garage' guy. The blue line is 30 amps into a 109 Ah cell, the green 5 amps. You can see that with the lower current the voltage did not change much until the cell was near full charge. Over much of the range it was steady at 3.375 or less.

View attachment 54034
Mike

Lithium cells do not have a memory. I think you are thinking of NiCad.

Yes, I double check with another multi meter for the voltages etc.

I am aware of the low curve on the charging. I just never seen this before is all.
I don't want to hurt the cells. I know cells can take more than their rated voltages [which of course hurts them].
But can they take more than their rated amperage?

 

[labeeman]

Cells like you have are usually .5c rate charge at most I suspect you have discharged the battery more than you have measured. As long as you do not go over the max voltage of 3.65 vdc per cell you can continue to charge them. Charging them at a 10amp rate would take 28 hours.

 

[Deleted member 9967]

Cells like you have are usually .5c rate charge at most I suspect you have discharged the battery more than you have measured. As long as you do not go over the max voltage of 3.65 vdc per cell you can continue to charge them.

Thank you. Ok so I have the voltage on the power supply set to 28.2 volts. And you say it is 100% safe and will never put in more amps than the 280 amps it can hold.
Is this correct?

 

[labeeman]

Thank you. Ok so I have the voltage on the power supply set to 28.2 volts. And you say it is 100% safe and will never put in more amps than the 280 amps it can hold.
Is this correct?

With a BMS yes because some cells can reach saturation before the other cells and cause overcharging for that cell which will cause it to swell. The BMS will cut off the charging if any cell gets too high.

 

[Deleted member 9967]

Cells like you have are usually .5c rate charge at most I suspect you have discharged the battery more than you have measured. As long as you do not go over the max voltage of 3.65 vdc per cell you can continue to charge them. Charging them at a 10amp rate would take 28 hours.

Also my cells can handle a 1C charge. Thing is though at 10 amps it is more like one third of 10 % LOL. or one third of 0.1c
So it will go slow but it works.

They are legit Eve cells at 280 Ah each. I have 8 of them for a total of 24 v 280Ah cells.
yes it is overkill but I never have to worry about running out and can ride for 140 miles if I didn't sleep or go to the bathroom LOL

 

[newbostonconst]

If you disconnect your battery does the BVM/shunt still read 10 amps? Maybe it just needs to be zero'd.

 

[Deleted member 9967]

With a BMS yes because some cells can reach saturation before the other cells and cause overcharging for that cell which will cause it to swell. The BMS will cut off the charging if any cell gets too high.

Ok but the BMS only monitors voltages NOT amperage.
Which is what my question is about.

 

[Deleted member 9967]

If you disconnect your battery does the BVM/shunt still read 10 amps? Maybe it just needs to be zero'd.

Actually. The voltage went u to 28.24 which is what the psu was set for and then the amps stopped.

So I think you were correct in that the coulomb meter was not recording the proper amount of amps going out. THAT is disturbing to me.

Dang it.

Ok thanks for all of your help here.
I really appreciate it.
Now I need to find a reliable coulomb meter. I thought I had one but obviously not.

For reference I have the juntek. Which we now know is garbage.

Thanks again for helping out.

 

[labeeman]

Ok but the BMS only monitors voltages NOT amperage.
Which is what my question is about.

I know of NO smart BMS that does not monitor amperage.

 

[Deleted member 9967]

Well the Daly I have does not tell me how many amps I have used at all. It might say the amount of amps in use. But I have no clue how many amps I have used or have left.
So now you KNOW of one that does not.
Wait, I know of others too. But whatever.
Might be easier if you can tell me of the BMS's that record amount of amps used over time please and thank you.

 

[labeeman]

Well the Daly I have does not tell me how many amps I have used at all. It might say the amount of amps in use. But I have no clue how many amps I have used or have left.
So now you KNOW of one that does not.
Wait, I know of others too. But whatever.
Might be easier if you can tell me of the BMS's that record amount of amps used over time please and thank you.

Yes, there are a few BMS that do not have a coulomb meter but they ALL monitor amps so that they can disconnect in case of overload.
The ones that do have coulomb meters are not very accurate and have to be read with a computer or smartphone.

 

[Deleted member 9967]

Yes, there are a few BMS that do not have a coulomb meter but they ALL monitor amps so that they can disconnect in case of overload.

Monitor as in how much is CURRENTLY in use. Ok sure.
But it does not say how much has been used NOR how much you have left.
THAT is what a coulomb meter is for and why I tried to use one.
Until today when i found out the one I have ifs garbage that is.

So no, not ALL BMS's "record" amperage. They just monitor it.

 

[Deleted member 9967]

Anyhow. I stilla thank you for helping me out.

 

[JeepDaddy]

Don't confuse Amps and Amp-hours. Take this in the kindest way, but a lot of the way you are using some your terms - amps, coulomb meter, amp meter, etc. suggest a learning opportunity.

BMS AH readings are not very accurate.

What meter are you using? Use a quality shunt meter, like a Victron Smartshunt if you want an accurate measurement.