Getting a Lithium battery charging / balancing circuit right - Page 1
Forum section: electronics / beginners

author: raphaelcasimir
author_rank: Contributor
timestamp: June 15, 2017, 02:21:24 pm
content: Hello,I am designing a 3 to 6 cell 18650 battery charger and balancer for a 3D printer UPS application. Some printer uses 12V and others 24V, so on some boards the cells will be used in 2 packs of 3S in parallel and on some others 6S.I thought to always charge them in the 2x3S configuration so I only need one charging circuitry for both cases, and then use mosfets to switch them to series mode when they need to be used in a 24V printer. I also thought to charge them all in parallel with one charger and then use mosfets to make the number of series cell that I want, to get rid of the cell balancer.The problems that I face are:- They seems to be no all-in-one ICs that does charging and balancing, and I don't know if I will be combining the two chips correctly, where can I find reference designs?- I am not sure if my charging ideas above are going to work of make the packs explode, which is a slight problem, please give some feedback- I found a charger for 6 cells that has a boost converter included,http://www.linear.com/product/LTC4020, but that still makes 2 chips, is there really no all-in-one solution? Do you know the simplest chips for this application since I am still a newbie?Thanks in advance for your help.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 15, 2017, 02:28:17 pm
content: You could put them in parallel and use a boost converter with a switch to produce two different voltages. Or all in series and step down with a buck converter.

author: raphaelcasimir
author_rank: Contributor
timestamp: June 16, 2017, 09:44:11 am
content: Thanks for your answer but I don't think you understood my problem, I need to use the voltages of the batteries directly, and this is no problem. I just would like a feedback on how to charge them.

author: Zinglish
author_rank: Newbie
timestamp: June 19, 2017, 09:27:52 am
content: There are ICs that do charging and balancing at the same time,TI offer multiple chips that do thisso doesMaxim. As for a single chip solution for boosting, charging and balancing, for your needs especially the high voltage and likely high current draw for the motors and heating element there may not be a specific single IC solution for your needs.Also: try not to go down the Linear Technology route, I found that out the hard way by finding the perfect chip from them only to find out it was roughly 20 Euro for a one off purchase where as TI offered a better solution for my needs at 4 Euro of a one off.

author: tatus1969
author_rank: Super Contributor
timestamp: June 19, 2017, 10:23:04 am
content: have you seen this?https://www.eevblog.com/forum/projects/introducing-my-project-'kicksurfer'/msg1203499/#msg1203499

author: raphaelcasimir
author_rank: Contributor
timestamp: June 20, 2017, 07:54:10 am
content: Quote from: tatus1969 on June 19, 2017, 10:23:04 amVery interesting for the BMS indeed, I have seen the schematics and the boards, they are pretty neat. I have slightly different needs so I will have to come with different solution but the design will be mainly the same! That will probably save me from some mistakes. Thanks!Quote from: Zinglish on June 19, 2017, 09:27:52 amThey are not all-in-one ICs, they only do charging or dc/dc + charge. I think that's normal since in most applications you want the BMS to be directly on the battery.Thanks for the price advice.

author: raphaelcasimir
author_rank: Contributor
timestamp: June 20, 2017, 11:45:51 am
content: Also, if I use my battery in series (22.2V) can I recharge it in the 3S x2 configuration (2x 11.1) ? Will it be any unbalancing risk even with a BMS like this onehttps://www.eevblog.com/forum/projects/introducing-my-project-'kicksurfer'/msg1203499/#msg1203499attached to the battery ? That is the big question. Will there be any issue when I put the cells back in parallel ?

author: tatus1969
author_rank: Super Contributor
timestamp: June 20, 2017, 03:06:24 pm
content: once you have paralleled the two halves, you can charge them as a normal 3S2P pack. But it could be difficulty to get there, because all cells will have slightly different voltages after discharge, maybe 200mV. You may have significant equalization currents, and the max charging current may easily be exceeded.

author: raphaelcasimir
author_rank: Contributor
timestamp: June 27, 2017, 12:58:17 pm
content: Thanks. Yes that was my fear. I will design two different systems for the different voltages.

author: raphaelcasimir
author_rank: Contributor
timestamp: June 27, 2017, 02:32:06 pm
content: I will use an LTC4020 charger and a ISL94202 BMS. I will just have to change some resistors and maybe capacitors to switch from 12V supply with 3 cells to 19V with 5 cells I guess (yes I will use 5 not 6 finally).I ran the numbers and these are pretty well-priced compared to alternatives, especially if we have high volumes.

author: tatus1969
author_rank: Super Contributor
timestamp: June 28, 2017, 06:03:51 pm
content: Quote from: raphaelcasimir on June 27, 2017, 02:32:06 pmThis is the feedback from Intersil's review, could be useful for you if you go with this chip.The number of cells is configurable for this chip, maybe that helps to reduce the mux effort.

author: raphaelcasimir
author_rank: Contributor
timestamp: June 29, 2017, 11:38:47 am
content: Thanks! But I don't understand the 5, did they add the resistor on the schematics themselves?

author: tatus1969
author_rank: Super Contributor
timestamp: July 01, 2017, 05:49:28 pm
content: Quote from: raphaelcasimir on June 29, 2017, 11:38:47 amThe reference design has them, but I omitted because power GND and VC0 potential are one big plane. He recommends to still add them to symmetrize all the cell voltage inputs. This way all bias currents create the same bias voltages. But I have measured, omitting them does not show any measurable difference. Also the noise figures are identical. But keep in mind that you can only do this when you do not have a separate cell 1 minus lead as in my case.