Lithium Ion balancing circuits - Page 1
Forum section: electronics / beginners

author: jim_griff
author_rank: Regular Contributor
timestamp: April 17, 2015, 02:34:44 pm
content: Hi there,I've been scouring the Interwebs for the past few days trying to find simple Li Ion balancing circuits, but have come across some extremely complicated and difficult to build circuits that don't even seem to have anything to do with Li Ion balancing...*takes deep breath*So, I am left with the only option: Ask on EEVBlog in the Beginner's section... Here it goes!Is there not an extremely simple way of keeping series Li Ion batteries balanced while charging them in series? i.e. a 4.1V Zener diode, Darlington transistor or perhaps an op amp circuit that will "bleed" off any current once the cell reaches 4.15V? I'd love to be able to charge two batteries in series without having to resort to a specialised charger.P.S. Something like this would be nice to make:http://www.modmaker.co.uk/YiHi-9V-Balancing-Charger-BoardIt appears much simpler than the 20-30 component circuits I've seen on Google Images which look way too complicated to accomplish something that should be a fairly simple job.P.P.S. Or am op amp circuit like this:http://www.google.com/patents/EP1603212A2?cl=enWould this even work?

author: max666
author_rank: Frequent Contributor
timestamp: April 17, 2015, 04:33:59 pm
content: Quote from: jim_griff on April 17, 2015, 02:34:44 pmI was wondering the same thing, I guess the problem with zener diodes is that they aren't steep enough at that voltage range?

author: mikerj
author_rank: Super Contributor
timestamp: April 17, 2015, 04:46:08 pm
content: A zener is useless for this job. They have a soft 'knee' voltage, wide tolerances and limited power dissipation in the common package types.The YiHi module you linked to is not a balancing circuit as such. It's basiacly two separate isolated charging circuits which is the best way to charge cells and keep them balanced, but rapidly gets costly and complex if you have many cells.A balancing circuit has to be able to shunt the entire charging current in order to be able to keep a cell below it's maximum permissible voltage (~4.2v typically). This is why many designs use an array of surface mount resistors in series and parallel to get sufficient power rating. It also requires an accurate voltage reference that won't drift significantly with temperature or supply voltage.There are numerous designs onRcgroups, mostly using the TL431 reference which is one of the cheapest references which is good enough for the job.

author: mtdoc
author_rank: Super Contributor
timestamp: April 17, 2015, 11:59:14 pm
content: I'd recommend having a look on theEndless Sphere forum. Lots of experienced lithium BMS builders there.A quick search there foundthis threadwith this:I'm sure there's others there as well.

author: Psi
author_rank: Super Contributor
timestamp: April 18, 2015, 12:09:32 am
content: What sort of charge current are we talking about.It would be quite easy to do it at like 100mA and rather complicated at 10A

author: CatalinaWOW
author_rank: Super Contributor
timestamp: April 18, 2015, 12:21:15 am
content: Most of the lower cost commercial units solve a number of problems by splitting the charging and balancing problem.The general approach is to charge for a short interval (a few hundred to few thousand milliseconds), then stop charging current and measure cell voltages. If cell voltages are close enough to each other, another charge interval is applied, if not a small load (50 to 100 mA) is applied to the high cells for a short interval. The algorithm repeats until the battery is charged, or a time limit is reached. The high cell voltage usually controls the overall charge. The whole thing is done with a cheap microcontroller, a few isolators, FETs, and resistors in addition to the basic charging source.This works if the cells are not too far out of balance and are relatively well matched.

author: amyk
author_rank: Super Contributor
timestamp: April 18, 2015, 01:40:49 am
content: Theoretically, a 4.2V shunt regulator across each cell, which can handle the full charge current, should work.

author: hli
author_rank: Frequent Contributor
timestamp: April 18, 2015, 02:14:33 am
content: A while ago I wrote about this for a design contest. There is one post aboutcharge balancersas such and one with thefull schematicexplained. But I gues you wont't get a solution for fewer than 10 components per cell. If its just two cells, there are somewhat simpler solutions, and even complete chips available.

author: Psi
author_rank: Super Contributor
timestamp: April 18, 2015, 02:46:56 am
content: An important consideration is the standby current draw of the balance circuit when the cells are not being used.

author: jim_griff
author_rank: Regular Contributor
timestamp: April 18, 2015, 06:45:43 am
content: Wow, thanks for all the responses.Quote from: Psi on April 18, 2015, 12:09:32 amI was thinking only in the small current range (100-400mA ish) in order to extend the life of the batteries. I don't really need high current charging for the cells.Quote from: hli on April 18, 2015, 02:14:33 amThanks for the links. I'll check them out.I did find some TI and LT ICs for "charging" double Li-Ion cells (for 8.4V) but they didn't seem to include balancing in the reference circuits on the datasheets. What I have found are some "18650 protection circuits" that can be added to each cell. I'm not sure if this will allow series charging, though. Here's an example of what I'm talking about:https://www.fasttech.com/product/1451307-charge-discharge-protective-circuit-board-forI guess this could be worth buying instead of me worrying about designing something, as I'm not really a proper EE.

author: Psi
author_rank: Super Contributor
timestamp: April 18, 2015, 12:56:41 pm
content: if you only need 100-400mA charge you can just burn off the energy in a transistor once the voltage reaches 4.2100mA @ 4.2V is only 0.5W which a TO220 package can handle without a heatstink.You'd want a reference, compactor and transistor per cell

author: lutkeveld
author_rank: Regular Contributor
timestamp: April 18, 2015, 02:57:15 pm
content: Interesting document:http://components-asiapac.arrow.com/file_system/intranet/NEWS/TIME/id_109/FCS_oct2013_en.pdf

author: jim_griff
author_rank: Regular Contributor
timestamp: April 19, 2015, 07:32:55 am
content: Quote from: Psi on April 18, 2015, 12:56:41 pmAh yeah, I didn't even think of that possibility. I was thinking too much in terms of efficiency, but efficiency doesn't matter as much with charging. I'll look into that now.Quote from: lutkeveld on April 18, 2015, 02:57:15 pmThanks. Looks interesting!

author: jim_griff
author_rank: Regular Contributor
timestamp: April 19, 2015, 07:56:37 am
content: Here's the circuit I am designing (or had designed for me by the datasheet circuit examples) that needs the balance charger. Unfortunately, I can't find an IC that has better performance that uses a boost configuration, hence why I need serial Lion batteries, otherwise I'd have put a couple of 4.2V cells in parallel.Thedatasheetis for the LM2675 in the circuit. Should be an interesting build for a 1 Amp nominal output for charging/using devices. I think it's quite amazing that it can provide 90%+ efficiency of energy transfer. I've seen some as low as 50-60%.P.S. Just noticed a mistake in the resistor divider. It should be 2.0V for the D+ and 2.7V for D- in order to limit iPhone charging to 900mA, else it goes into 2,000mA charge mode. Eek!