Lithium-ion charging schemes, or modules?

Lithium-Ion is charged with scheme known as CCCV(i.e Constant Current Constant Voltage).Since the impedance of the battery is very low always it is required to control the current else it will cause gasing and result can be dangerous.

For your application there are many standard IC's available from TI,National,Maxim,Intersil etc.

http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?&familyAliasId=1000411&familyId=411&uiTemplateId=NODE_STRY_PGE_T&techFamId=null&rootFamilyId=null&sectionId=null&tabId=null&appId=null&viewDeviceCallingPage=null&totalCount=262&showAdditionalParameters=no&lc=3000435&lc=3000430&lc=3000431&lc=3000432&lc=2001192&compare=yes&download=yes&sort=yes&customize=yes&paramResults=yes&paramCriteria=yes&familyTree=yes&military=no&baSystem=yes&paramTable=no&sortOption=PA_PARAMETER_2001636&sortMode=DESC&searchPaths=1000394&pageId=undefined&templateId=0&navigationId=0&family=analog&paramTable=no&military=no&compare=yes&download=yes&sort=yes&customize=yes&paramResults=yes&paramCriteria=yes&familyTree=yes&military=no&baSystem=yes&paramTable=no&sortOption=PA_PARAMETER_2001636&sortMode=DESC&searchPaths=1000394&pageId=undefined&uiTemplateId=NODE_STRY_PGE_T&techFamId=null&rootFamilyId=null&sectionId=null&tabId=null&appId=null&viewDeviceCallingPage=null

http://para.maxim-ic.com/en/search.mvp?fam=batt_chrg&tree=powersupplies

http://www.maxim-ic.com/appnotes10.cfm/ac_pk/5

Choosing a Li-ION battery charger is based on the efficieny requirement of your system.Thee are two types Linear and Switched.First one is less efficient but component count is less(also cost) and the 2nd one would be highly efficient(and cost ).

Also it is important to add a hysterysis at the switch which controls the supply switch over.

Regards,

Arun

Arun,

Welcome to Element 14. Thanks for the reply. It is in depth.

On the hysterysis point, any thoughts on how to implement a solution? I was thinking of adding a supercapacitor bank to rapidly store energy, and form there charge the batteries for longer term storage.

E

I had a similar project (adding storage to an existing system, requiring bumpless transfer between external and internal power sources) and selected NiMH. Your Li-ion situation will have significant differences and things may work just fine, but perhaps it's prudent to consider the possibility that it will be a lot harder than you expect!

I had two significant issues that will apply to your situation. The first is reliable end-of-charge detection. Lead acid batteries are a luxury in this department - the coarse voltage changes are easy to detect. For Li-ion and NiMH however, detection of full charge is much more subtle. In particular you will need a very good measurement of cell voltage (in the order of 10mV resolution) and most likely a very stable measure of cell temperature. These are not difficult per se, but if you combine the noise and duration of a switched mode charging circuit operating for hours on end, it's not hard to fall foul to the occasional measurement glitch, environmental drift or battery cell variation. In other words, keep your voltage and temperature measurement circuits neat and tidy!

The other problem was switching between power sources. Again, seems simple on the surface, but since the cell voltage of your batteries change over their charge cycle, you could be switching from and to a variety of different voltages. Additionally, the charging power supply might in one instance only be supplying the batteries and in another, suppling the batteries and the load. Consider early in the design whether it would be worthwhile putting a regulated DC-DC converter between your power supplies and the load. It may turn out that two DC-DC converters are required. Then of course you need to consider max current draw from the load and carefully evaluate the hold-up capabilities of the power supply variations.

I used the BQ2005 chip from TI for battery charging (and the BQ2010 for state-of-charge display). It was far from idiot-proof, but did the job in the end. There was still a consider amount of external circuitry to design, mostly to do with the switched mode power supply and signalling between the two chips.

Hysteresis was handled using a p-channel MOSFET plus a couple of schottky diodes to create an OR of the power sources. The MOSFET's gate was driven using the external power supply via a resistive divider. The resistive divider was tuned to ensure that switch over from internal to external power source and vice-versa was done before the load voltage dropped too far. This negated the need for significant capacitance to hold up the load voltage - a relatively standard amount of capacitance on the output of the DC-DC converter was enough.

Eves,

I hope post by heath gives you a good understanding of the system and also about implementing the Hysterysis part.You can implement it using a widow(or a hysterysis) comparator or discrete circuitry based on transistors.But the discrete will not be a good solution since the switching between the supplies may not be ideal.

I agree using the super capacitor bank will yield a better charge capacity and extends the longetivity of the battery.I feel using a supercapacitor bank shall be quite costly because of the protection required to be provided(like inrush current limit).There are many standard products(Li-Ion chargers and battery gauges) which can make your system look less costly.

I am not sure if i understood your question correctly.Let me know.

Regards,

Arun

Hi E,

You might want to try a part like the LTC3101EUF#PBFLTC3101EUF#PBF.  This is designed to regulate several supplies, but has two inputs to the converter.  A charger/USB input and a battery input.  This is great because now the supplies powering your board will have minimal transients and the charger circuit does not have to talk to the regualtor at all and the hysteresis is handled for you.  Let me know what you think.

If you are thinking of using supercapacitors, a part like the LTC3127 was made to regulate the input current to program the caps, and it is programmable.

Let me know what you think.

-Mike

Mike,

Thanks for the parts.

The LTC3101EUFLTC3101EUF might be just what I need.

And the LTC3127LTC3127 I will have to investigate further. I may be able to deliver much higher amperage to a bank of super caps at a low voltage, so I may have to increase the chip's specs.

E

Hi,

Please check the application note below.

http://www.intersil.com/data/an/an1589.pdf

Thanks,

Chethan R.S.