DIY charging two Li-Ion cells in series? Any chips/boards known? - Page 1
Forum section: electronics / projects

author: Wisnaeme
author_rank: Unknown
timestamp: May 05, 2016, 02:17:34 pm
content: Hi all,I'm looking for a simple circuit to charge two cells in series. There's a couple of IC's out there but they appear to be awkward to solder etc. Ideally I'd want a ready made module or something SOIC so I can solder it easily.Digging around Ebay and the likes doesn't seem to find me a module to suit my needs.Any thoughts?Ta.Robert

author: stmdude
author_rank: Frequent Contributor
timestamp: May 05, 2016, 03:04:16 pm
content: BQ24005. 20-lead TSSOP.

author: mark03
author_rank: Frequent Contributor
timestamp: May 05, 2016, 04:00:47 pm
content: I think these packages were 0.5mm pitch, not SOIC, but perhaps useful as a reference. This was hand-soldered.http://www.hau.nz/posts/oly.htmlNote that if you are going to charge series Li-ion without a balancing tap, the battery pack should be designed for that--either some internal balancing network, or really well matched cells. (I'm not sure which approach the Olympus batteries use, but there is no tap brought out to the connector.)

author: Buriedcode
author_rank: Super Contributor
timestamp: May 05, 2016, 04:20:23 pm
content: A quick google I found this:https://www.vishay.com/docs/70658/si9730.pdfSOIC8, dual cell, and monitors the centre terminal. Not sure about availability though.It wouldn't be that difficult to roll your own, unless you were dealing with high charging currents and/or high capacity cells. A CC/CV supply with tight voltage regulation, set for 8.4V and a few comparators would work. There are some cell balancing circuits that are put across each cell floating about on the web, often based on a comparator and 3-pin voltage reference. Whilst CC/CV supplies can be really rather complex, for charging lithium batteries the most important thing is the voltage limit, plus, making it a linear supply doesn't help with efficiency but negates the hassle of a switch-mode solution.I'll admit an of-the-shelf dedicated IC would be great for this purpose, but it does seem that dedicated IC's are either relatively simple, single cell linear chargers, or complicated multi-cell chargers with lots of bells and whistles and SMBus capability.The above solution is perhaps something I would not trust to be absolutely safe, but much easier to treat the battery as a 'single cell' at 8.4V, with protection/shunt resistors across each cell.

author: Habropoda
author_rank: Regular Contributor
timestamp: May 05, 2016, 05:54:35 pm
content: You are very vague about your needs, but for charging known-good cells the lm2596 cc/cv modules on ebay work fine:https://sites.google.com/site/3dprinterlist/electronics/chinese-electronics/lm2596-buck-converter-charger-cc-cvIt is lacking in some charging features like balance charging, low voltage detection, shutoff after charge and battery temperature monitor, so if your needs include any of those look elsewhere.

author: Wisnaeme
author_rank: Unknown
timestamp: May 06, 2016, 09:02:15 am
content: Hi all,Thanks for the replies.I have thought of using the LM2596 modules previously (I own a stack of the non CC versions) but didn't feel good with this.I guess setting up a suitable current limited feed at a fixed output voltage of 8.4v would work fine but it feels over simplified given the nature of the true charger circuits out there. They tend to do all end of charge transitions which I understand from the sheets.I'm not worried about balancing the charge as the balance will be easy to monitor in the Arduino ADC. If thing get too out of balance I can stop the charge.I will investigate the other IC's mentioned too.

author: Buriedcode
author_rank: Super Contributor
timestamp: May 07, 2016, 10:10:07 am
content: If you break it up into two main parts: the power supply, with set limits on voltage (obviously) and current (low side current sensing, with an opamp) and the charge controller - that can be a small uC or a comparator setup, then it can seem rather straight forward.All lithium polymer chargers I have seen, have the charge termination when the battery draws <10% of the charge current. That figure varies, often as chip options, from 7.5% to 20%. As you'll need current sensing anyway, and the charge termination doesn't have to be *too* accurate, a simple comparator hooked to the current sense amp, with hysteresis, controlling the enable pin of your power supply would do nicely.Most of the cheapo chinese LMxxxx modules on ebay that claim to be CC/CV use one half of an LM358. Whilst using opamps as comparators isn't ideal, you could use the second opamp for this. The use of two LED's, or a tricolour LED for inidication would give the usual 'red, yeollow, green'. I would just add an LM393 for this, with the red LED on for an output current of >10% of charge current, and the green for Vbatt voltage > 8.3V. That way red is CC, yellow is CV and green is 'done'.I might knock up my own actually - not jumping on the bandwagon here.. as I've been charging a 2000mAh cell with a 300mA charger for a breadboard PSU. That and I have no work right now, so I'm bored out of my tits..