>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>On Behalf Of Rolf
>Sent: 17 January 2007 02:50
>To: Microcontroller discussion list - Public.
>Subject: Re: [EE] Li-ion charging questions
>
>>   
>Hi Harold, others.
>
>I have looked in to some of the available chips. The I2C and 
>1wire "fuel 
>gauges" seem overly complex to interface to... I will have to 
>re-arrange 
>a bunch of stuff on the main board. Additionally all the registers and 
>other control values seem like too much effort to get to grips with.
>

There is a reason for the complexity!

>The Coulomb counters though seem like a grand idea. The simplicity of 
>the LTC4150 device which gives a pulse per unit charge is great. I am 
>also developing a bias for Linear since their datasheets 
>explain things 
>in a way I can understand .... ;-)
>

The coulomb counter is the heart of the smart battery IC's, the complexity is to take into account the numerous other factors that affect state of charge.

>The problem as I see it, is that the charging process will charge at a 
>much higher current (up to 1.5A), but the device will drain 
>that current 
>at about 40mA on average. Now, I understand that the coulomb counter 
>just measures current (charge). The amp is a SI unit, and the 
>coulomb is 
>a derived unit defined as the amount of charge delivered by 1 
>Amp for 1 
>second. By counting all the "coulombs" that are used to charge 
>a cell, I 
>can calculate it's "charged" capacity because, if 1 amp for 1 
>second is 
>1 coulomb, then 1 amp for 1 hour is 3600 coulombs, or 3600coulombs is 
>1000mAh. see http://en.wikipedia.org/wiki/Coulomb
>
>The problem is that the capacity of a cell changes depending on the 
>drain current. The greater the drain current, the lower the capacity. 
>This is quantified by Peukert in Peukets Law: 
>http://en.wikipedia.org/wiki/Peukert%27s_law

Not only discharge current, but cell temperature and aging can greatly influence cell capacity.

>
>Peukert's law assumes that each type of battery (or is that each 
>individual battery) has some constant that can be used to 
>determine it's 
>capacity at a given discharge rate. It seems the law is somewhat 
>incomplete, and does not reflect reality all that well. Still, given 
>that the recharge curve for the cell is not going to change much, and 
>the discharge curve will be stable as well, I believe I can 
>calculate an 
>Peukert constant for my system with a little bit of training. 
>Install a 
>discharged cell, charge it and track it. Count the coulomb units 
>required for a charge (Cc). Then, discharge it at the regular 
>rate, and 
>count the discharge coulombs (Cd). Cc/Cd gives me a factor I 
>can use to 
>measure the capacity available in a cell given that each discharge 
>coulomb will require (Cc/Cd) charging coulombs to replace. This should 
>work out to be accurate enough to be within 10% or so, and 
>that would be 
>fine.
>

You need to ensure you zero your coulomb counter when the cell reaches it cut-off voltage, and also that you store the maximum coulomb reached during charging, otherwise the gauge will become increasingly inaccurate as the cell ages.

Regards

Mike

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