Balancing question

[duckwize]

part purchased.

[Cobb]

That sucks.

Interesting enough if you drain the stick to .9 volts or even zero you can in turn reverse polarity charge them and they will even function in that manner.

Kind of adds a new idea to the old trick of rotating someones monitor screen 90 degrees. Whats better than reversing the polarity of their rechargeable batteries?

[kennybobby]

i don't know your chemistry or cell arrangement or the rated capacity, but if you measure capacity then you could compare that to the rated spec capacity to determine how worn out they are. Over time they will wear out. Aren't the Leaf batteries supposed to retain ~70-80% capacity over 10 years?

There is no spec on internal resistance--You don't know the internal resistance when they were new--and what can that measurement tell you now, there is nothing to compare against? Plus that value can change depending upon how heavy you load during testing--i don't see any use for it, whereas capacity is a known quantity that you can easily measure and compare and use to determine remaining life, etc.

Lithium cells get permanently damaged if they go into reversal, any talk of reverse charging is voodoo science--probably want to stay away from voodoo

[IamIan]

Good enough is a bit subjective... it doesn't have to be 100% new to be usable.

But Here's my 2 bits.

You want the cells / sticks to be as close to each other (balanced) as possible ... close in all aspects : Ah or Wh of discharge capacity ... Resistance ... Self Discharge Rate ... etc.

In order top 4 sorting criteria :

1. Balance of all 20 sticks as close to each other as possible , is better
2. less resistance (Ohms) each stick is better.
3. more capacity (Ah or Wh) each stick is better... whole pack only as good as the weakest cell or stick.
4. less self discharge is better

- - - - - -

Test result values will vary with conditions ... different testing methods and conditions can give different results ... be it Ah or Wh numbers or Ohms of resistance , etc... ( so be as consistent in all testing as you can be ).

to determine resistance ... V=IR --> dV / dI = R
dV = the change in voltage from zero amps to under a load.
dI = the change in current from zero amps to the amps of the load.
R = Ohms of resistance.

A simple method is to use a volt meter before and less than 10 second after the amps change to a steady value ... you can use the superbrain (or any controlled load) to act and the load ( charge amps or discharge amps )... and a basic multimeter to see the change in voltage (dV) before and after that amps change (dI)... more than 10 seconds you are getting a bit too much into the natural voltage change as the battery charges or discharges.

- - - - - - - - - -

If you can verify the following minimums you know you'll be fine ... but plenty of people don't weed out as many as the bellow would.

Originally Posted by retepsnikrep

Cells/sticks with less than say <4500ma capacity at 10A should probably be binned.

Cells/sticks that can't sustain a 70-80A load for at least 15 seconds without dropping below 0.9/0.8V should probably be binned.

Cells/sticks that heat up by more than say 10C during the above high current test should probably be binned.

Cells/sticks that self discharge more than 25% over one week should probably be binned.

- - - - - -

I would say if you can't do the 70+ Amp high load test described above ... do the resistance testing as high of Amps as your setup can do... Brand new 6 cell NiMH sticks (depending on temperature and SoC) can have as low as 18mOhms (0.018Ohms) of internal resistance ... the lower your sticks / cells are the better ... but I would personally give an absolute maximum high of about ~43 mOhms per stick ... more than that , and you can use it for something else ... but I wouldn't put it back into a HEV.

[duckwize]

Before I destroyed my super brain, I was working on the internal resistance. It would seem all my sub packs were in decent shape. one stick has a capacity of 5400 (which was the lowest) and one stick couldn't hold a charge like the others, but only dropped down to 7.3 volts, the others sat around 8.

From the sounds of it I might be able to have the car back on the road in no time. I ended up getting one stick on ebay in hopes of replacing the one stick that drops to 7.3 volts.

The stick at 5400 might be able to be revived with another cycle or 2.


As far as my measuring of the IR I will try again soon. It seemed my super brain was able to stabilize in 8 seconds at a 6 amp load. and was taking a measurement at 20 seconds. The next time I run this test I will up the discharge to 10 amps and get the voltage measurement within a few seconds after stabilization.

Ian, you have me thinking that I may be doing the capacity ratings wrong. All I did was record the discharge values and charge values taken from the super brain. (excel spreadsheet on the OP) Am I missing something? I have seen others record their time it took to do a discharge and charge as well, but I didn't think those mater. I feel I might be wrong now.

[IamIan]

Originally Posted by duckwize

Ian, you have me thinking that I may be doing the capacity ratings wrong. All I did was record the discharge values and charge values taken from the super brain. (excel spreadsheet on the OP) Am I missing something? I have seen others record their time it took to do a discharge and charge as well, but I didn't think those mater. I feel I might be wrong now.

Not wrong ... there is always more depth you can go into ... at a certain point you have to set what you consider to be "good enough" to you ... and if you end up not being satisfied with that ... set the bar higher ... how high you want to set it is entirely up to you... better is always better ... but you might not need it that much "better" ... at a certain point it becomes a question of how much of your time and or money do you want to invest in getting ever higher amounts of "Better".

- - - - - - - - -

There are 2 main aspects of capacity.

#1> How much capacity the stick has.

#2> How much capacity the stick will be able to use in the HEV context.

- - - - - - - - -

Either 1 or 2 above can be recorded in Ah or Wh.

Ultimately energy is energy ... but the recording of the time to transfer that amount of energy can be used to get an idea of the Peukert effect of transferring that quickly.

For example if stick A gives you 5.0 Ah (~36Wh) of discharge capacity from a 10A discharge load ... and stick B gives you the same 5.0Ah (~36Wh) from a 1 Amp discharge load ... if both were equally charged SoC and discharged DoD ( at roughly the same temperature) ... Stick A that can give the same energy to you at a higher rate (amps of current or watts of power) is the better stick.

Because parts of the charge and discharge are not always constant current (Amps) nor is it always constant power (Watts) some people use an overall time method to get a gross ball park idea of how quickly they were able to get that ___ Ah or ___ Wh from the cell.

This additional time data for a gross ballpark of the Peukert effect can be useful , but is not absolutely necessary.

Just like doing Charge / Discharge Cycle efficiency tests for Ah or Wh can be nice ... some people do them ... others don't bother ... etc ... etc ... you can spend days testing or years testing.

- - - - - - - -

The basics just fall back to the top 4:

1. Balance of all 20 sticks as close to each other as possible for each of the 3 bellow.
2. Less resistance (Ohms) each stick is better.
3. More capacity (Ah or Wh) each stick is better... whole pack only as good as the weakest cell or stick.
4. Less self discharge is better

From there the basic guideline is:

Originally Posted by retepsnikrep

Cells/sticks with less than say <4500ma capacity at 10A should probably be binned.

Cells/sticks that can't sustain a 70-80A load for at least 15 seconds without dropping below 0.9/0.8V should probably be binned.

Cells/sticks that heat up by more than say 10C during the above high current test should probably be binned.

Cells/sticks that self discharge more than 25% over one week should probably be binned.

If you can't do the 70+ Amp high rate testing ... do the IR testing ... any stick over ~43mOhms should probably be binned (for some other use , not HEV)

- - - - - - - -

Lastly ... when you are done selecting your sticks you will use... and are ready to put it all together back into the car.

Make sure all 20 sticks have a balanced SoC.

There are 3 main methods (in order of my preference):

#1> Use a low (trickle) current (less than 300mA) high voltage grid charger to top off the whole assembled pack ... When you start the car let it sit and idle in neutral until it stops charging on it's own... If it charges on its own at all.

#2> Discharge all 20 sticks before assembling ... let them 'rest' until each stick is within +/- 0.01v of each other ... ... When you start the car let it sit and idle in neutral until it stops charging on it's own... If it charges on its own at all.

#3> Charge all 20 sticks before assembling ... let them 'rest' until each stick is within +/- 0.01v of each other ... ... When you start the car let it sit and idle in neutral until it stops charging on it's own... If it charges on its own at all.

[duckwize]

I'm glad I'm going in the right direction.

You said to make sure I have a balanced SoC, I want to build a grid charger that Mike designed. I've just started to do research on it, have my parts list and I'm about to get the supplies, but the small hurdle is the civic fan control. Sounds like it's not as easy as the insight. If you or anyone has input on a grid charger and fan control, or useful links that would be great!

I don't think I can even come close to charging all the sticks within .01v.

But like I said I haven't looked too deeply into my grid charger issue yet, but just looking for input.

Thanks again!!

[IamIan]

Originally Posted by duckwize

You said to make sure I have a balanced SoC,

The reason fall back to the 'usable capacity' thing... and the 'weakest link'.

For example:
If all 20 Sticks were identical resistance , capacity , etc.
But if Stick #1 were at 10% SoC ... the rest of the sticks are at 20% SoC ... the whole pack looses the bottom 10% ... because Stick #1 will bottom out first ... even if the other 19 have charge to give ... it wouldn't be usable to you.

The reverse also happens ... if Stick #1 were at 20% SoC while the other 19 were at 10% SoC ... than the whole pack looses the top 10% as Stick #1 would top out first before the others ... and even if the others still had room to charge ... it wouldn't be usable to you.

Attached is an example of the SoC variation that can creep into a pack over time ... The SoC variation (shown there) was effectively removing about ~2Ah of usable capacity from that pack.

Eventually if a pack get far enough out of balance you can get IMA error lights ... even if all the cells and sticks in the pack are otherwise still usable for resistance, capacity, self discharge , etc.

Originally Posted by duckwize

I want to build a grid charger that Mike designed. I've just started to do research on it, have my parts list and I'm about to get the supplies, but the small hurdle is the civic fan control. Sounds like it's not as easy as the insight. If you or anyone has input on a grid charger and fan control, or useful links that would be great!

That kind of high voltage whole pack low amp trickle charger (I think) is the over all easiest / best method of balancing the NiMH pack ... it is also will be useful afterward 2 or 3 times a year as maintenance to correct any small imbalances that might develop.

The fan is needed to dissipate heat ... you can work with hacking the OEM fan (you are correct the Civic version is not as easy as the Gen1 Insight fan) ... you can install your own fan to move the air ... or you want do a low enough amp rate that the heat generated isn't as much of a concern (but it would take longer).

There are a variety out there ... Mike's is a great system (but AFAIK he isn't building new ones anymore) ... Peter in the UK also has a good simple cycler (but AFAIK not a pre-built product) ... EVpowers and Bubblebatteries both have offered balancing services in the past (I don't know if they sell the equipment)... etc... etc.

Originally Posted by duckwize

I don't think I can even come close to charging all the sticks within .01v.

The +/- 0.01v is a level that you can be certain you will not have any issues.

Like other things ... you will have to decide how good is 'good enough' for you... some people are happy with +/- 0.1V.

The goal is to have all the cells and sticks you have tested and selected all be pretty close to the same SoC.

The difficulty of getting that SoC balance without using a whole pack low amp trickle charger is the self discharge rate of the cells ... See attached Graph I did from previous testing of the 40 sticks I have (other sticks might vary)... the time gap from when you finish the first stick to when you finish the 20th stick there is the potential for a SoC balance to be created ... due to the Self Discharge Rate... in a week or two it won't be a large difference ... but any difference is just effectively removing that % of your usable capacity.

Originally Posted by duckwize

But like I said I haven't looked too deeply into my grid charger issue yet, but just looking for input.

Thanks again!!

You can always just 'do the best you can' at the stick level now ... and then later use the low amp trickle charger to correct any imbalance that might be there... in the mean time you're just down that % of usable capacity.

[Cobb]

What about hooking all sticks in parallel to the hobby charger for a week or so?

[duckwize]

Originally Posted by Cobb

What about hooking all sticks in parallel to the hobby charger for a week or so?

I don't see anything wrong with that idea. a lot of wiring would need be done, but as far as I can see that shouldn't be too difficult with the battery pack sitting on my stand. hmmmm