# Cell Cycle Capacity Checker: Arduino based charger / discharger for LiIons



## Nabla_Operator (Aug 5, 2011)

By the time my LiFePo4 cells arrive I want to have a simple measurement device to measure the capacity of each cell. 
After a few years, when the BMS reports a defect cell, I can check and conclude which cell has to be replaced.
One cell measurement may take 24hrs; a total pack takes 3 months, which is the time I expect to need for the batteryboxes. 
This device doesn't contain accurate parts, because 'charge' and 'discharge' are measured with the same components.



















Your comment is appreciated. Can I find such a device or the algorithms somewhere on the internet?
I'd like to measure the internal resistance too. But how?


( refer to: discussion on fast charging  and *Determine which LiFePO4 cells are "bad" in pack* )
-paul-


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## Siwastaja (Aug 1, 2012)

A few comments;

Log coulombic efficiency (Ah efficiency) and energy efficiency (Wh) separately. The first should be _very_ near to 100%, but I would like to see it checked if you have such an automated device.

If at all possible, try to make a constant current discharger. In any case, add a support for two different discharge currents (maybe with two relays, two loads), for example, 0.5C and 2C. This allows you to measure "dynamic" internal resistance, (U2C-U0.5C) / (I2C-I0.5C). This may be a bit more relevant than (Uopen-Uload)/Iload.


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## major (Apr 4, 2008)

Nabla_Operator said:


> By the time my LiFePo4 cells arrive I want to have a simple measurement device to measure the capacity of each cell.
> After a few years, when the BMS reports a defect cell, I can check and conclude which cell has to be replaced.
> One cell measurement may take 24hrs; a total pack takes 3 months, which is the time I expect to need for the batteryboxes.
> This device doesn't contain accurate parts, because 'charge' and 'discharge' are measured with the same components.
> ...


Hi nab,

Ref: http://www.diyelectriccar.com/forums/showthread.php/quantified-lot-a123-20ah-cells-phev-79846.html 
Like I was telling Iam, capacity is measured in Ah, not Wh. Check out the link in post #29. http://www.cse.anl.gov/us-china-wor...ePO4 battery performances testing for BMS.pdf 

Of course you're interested in cell energy. But to get a valid comparison to manufacturer's spec and against themselves as they they age, you should use charge.

Re: Internal resistance. http://www.diyelectriccar.com/forums/showpost.php?p=328900&postcount=18 

Good luck,

major


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## Siwastaja (Aug 1, 2012)

Yes, or to rephrase:

Internal resistance and Wh efficiency are practically the same thing, just redundant information. 

This is because Ri models the inefficiency of the cell; there is practically no Peukert effect involved, but all the losses you have are in the form of voltage drop. You may end up with some Ah "loss" too at high currents if you stop discharging/charging earlier due to hitting the voltage limits, but you can always finish discharging/charging at lower amps to end up with full capacity.

So, if you measure dynamic Ri from real DC load tests, that single value can be used to calculate the Wh efficiency if you ever need that number. Note that losses are I^2R, hence, the efficiency goes down when current goes up even when the Ri is a constant. Furthermore, Ri varies as a function of temperature and SoC, and so varies the efficiency.

Due to the I^2 nature of the losses, Ri is a very important number on high discharge (low range but high performance) applications, and insignificant in high range moderate performance applications.


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## DIYguy (Sep 18, 2008)

Nabla_Operator said:


> By the time my LiFePo4 cells arrive I want to have a simple measurement device to measure the capacity of each cell.
> After a few years, when the BMS reports a defect cell, I can check and conclude which cell has to be replaced.


Or u can just buy a Powerlab 6 (or 8). . . and save some time and work + get the ability to do a whole bunch more stuff. . .


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## Nabla_Operator (Aug 5, 2011)

the Powerlab-6 seems to be an excellent advise for me, DIYguy. Have you ever programmed the charger in multi-cycle mode as they claim? :


... this kind of performance with the ability to autonomously cycle batteries, graphing all aspects of charge and discharge at the cell level (using the free Charge Control Software) and over multiple cycles provides you with key information about any chemistry battery you might have. As well it will tell you the quality of your battery by monitoring and graphing internal resistance at the cell level; more accurately and faster than ever


And it drives 40 Amps back and foreward, wow.

-paul-


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## DIYguy (Sep 18, 2008)

Nabla_Operator said:


> the Powerlab-6 seems to be an excellent advise for me, DIYguy. Have you ever programmed the charger in multi-cycle mode as they claim? :


Yes I have. I have cycle tested over 200 A123 cells (32157's and 32113's) with it. I've also used it to test and bottom balance CALB SE180 prismatics. I've charged my camera battery after loosing the charger and the other day I took my daughters key fob remote for her VW that no longer worked, took out the flat lithium watch battery and charged it. The FOB has worked fine ever since. lol
I've powered it with a battery and a power supply.
The software is easy, intuitive and completely wide open to modify and create your own charge profiles. The regen is awesome. IR measurements is a must. My biggest beef is the 40 amp rating. The unit will do it... but the wires are too light to sustain it IMHO. If u want to do 40 amps, I would upgrade banana jack wires. Get the USB board for the pc software and make up a set of balance wire clips.

I have two PL8's.


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