7.4V Dual cell Li-ion Battery management - Page 1
Forum section: electronics / beginners

author: brenesrob
author_rank: Newbie
timestamp: August 07, 2012, 09:37:22 am
content: Hey there,I was wondering if I would be able to successfully charge a 7.4V dual cell Li-ion battery by using an 8.2V Li-ion charge management control chip, specifically the MCP73862 from Microchip. If this wasn't the case, then what chip could I be using for this feature?Thanks in advance,Roberto

author: brenesrob
author_rank: Newbie
timestamp: August 07, 2012, 10:24:23 am
content: If anyone's interested, here is the link to the datasheet:http://ww1.microchip.com/downloads/en/DeviceDoc/21893E.pdf

author: digsys
author_rank: Unknown
timestamp: August 07, 2012, 12:13:50 pm
content: Absolutely NOT. The CC/CV point, Max Voltage and other parameters are all critical !! There are, however, "Universal" Lithiumcharge management ICs out there, where you can preset these points. To do it properly, you really need the factory graphs,usually find them on-line. Would you consider making your own with a CPU? There's not a lot in it and you have control.

author: brenesrob
author_rank: Newbie
timestamp: August 07, 2012, 06:23:11 pm
content: I'm guessing then that a chip like this onehttp://www.digikey.com/product-detail/en/LT3652EMSE%23PBF/LT3652EMSE%23PBF-ND/2225686is much more suitable for this application since it is programmable to different voltages and it is specifically for solar charging, which is what I'm using.

author: M. András
author_rank: Super Contributor
timestamp: August 07, 2012, 08:03:49 pm
content: you can charge to less voltage a cell it will improve its lifetime but not overcharge them 0.2 volts less charge will likely give a good 2-30% more life

author: brenesrob
author_rank: Newbie
timestamp: August 07, 2012, 08:48:46 pm
content: Quote from: M. András on August 07, 2012, 08:03:49 pmGreat advice! So it would be much better to configure the charger to deliver 7.2V. I'm using a 9V solar array, but I think I will wire it in series with another one in order to produce 18V and have enough power to charge the Li-ion battery.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: August 07, 2012, 09:10:22 pm
content: Quote from: M. András on August 07, 2012, 08:03:49 pmWhich you'll need, since the reduced voltage also reduced the battery capacity by 16% - 40%;http://www.helifreak.com/showthread.php?t=333661Few - if any - manufacturers will provide accurate data for these uasge scenarios,so unless you'll do the necessary testing, these recomendations and values are just broscience.

author: amyk
author_rank: Super Contributor
timestamp: August 08, 2012, 10:16:30 am
content: It depends what is exactly meant by "7.4V" here, because this means the cells are going to be 3.7V each. Lions have a nominal and max voltage. For example Cobalt types (most common) are usually 3.7V nominal and 4.2V max, Phosphates are 3.3V nominal and 3.6V max. The max is what they're charged to when full but nominal is the average voltage they'll stay at throughout the relatively flat discharge curve. It sounds like your cells are Cobalt type (check the datasheet to be sure) so that IC you mentioned may be applicable. The flat discharge curve is also why it's difficult to determine the state of charge from voltage alone.

author: brenesrob
author_rank: Newbie
timestamp: August 09, 2012, 02:31:11 am
content: Quote from: amyk on August 08, 2012, 10:16:30 amI'm guessing that is the case, but I'd better find a Li-ion with a good datasheet, because this one has almost no information on it. It is my first time using Li-ions and initially I was meaning to use other batteries because these ones are more intricate when it comes to charging, but I watched EEVBlog's video con Li-ions and decided to give it a try. I intend to use solar panels to charge a battery and then have the battery connected to a USB-A jack in order to charge phones and stuff. I already have the circuit for the USB done, I only needed to devise the circuit for the battery charger.