Help: Balance 5 Li-ion Cells in Series Without Draining Battery in Standby? - Page 1
Forum section: electronics / projects

author: SuzyC
author_rank: Frequent Contributor
timestamp: January 17, 2023, 05:48:12 pm
content: Attached is the circuit I have designed to balance my battery pack. It works fine but discharges the battery pack through the voltage-dividers that connect to the A/D inputs of the battery balancing MCU.I plan to install five 21700 cells(in series) to power my handvac. These cells will replace the old NiMH battery that has worn out.I will try to develop a two MCU BMS where a single MCU does almost all of the BMS work and the primary MCU manages and communicates by I2C or RS-232 with the MBS chip and the master MCU is responsible for motor control, display, etc while also managing the BMS chip.I found the perfect battery to provide the needed cells at a very good price at a local supermarket. It is a "Parkside" brand snap-in battery rated as a 20V 4-Ah battery. It snaps into a charger module and into a variety of hand tools of the same brand that are sold by this supermarket chain.What is amazing about this Li-Ion battery is that is rated at 6-amp max in operation, uses the new 21700 high-capacity Li-Ion cells and even has bluetooth charge monitoring and cell balancing. For its intended use, it is even guaranteed for 3-years.I plan to re-purpose this battery for use in my old handvac by removing the cells from the asm and parts of the plastic cell holder that holds the cells together so that I can get it all the cells to form a new battery pac that fits inside my handvac.The battery interfaces to the charger power module and to all the tools that it can power with a 4-pin connector.Two of the three pins on the battery pack connector go directly to the + and - of the battery. The other two measure voltages close to the positive battery-connector pin.The third pin I guess must be used to detect/identify the battery when charging and also identify the battery when plugged-in into any of the tools it powers.The charger power module this battery can be plugged into is only a SMPS with what I guess is a fixed output voltage of around 22-24V and is able to supply perhaps up to 3-amps of charging current.Looking at the PCB circuit board mounted on the battery cells assembly I can see that it has at least 8 tiny TSOP power MOSFETs to balance the battery while charging.The charging PCB uses a single custom part number CPU that manages the whole charging and bluetooth operation. The MCU has 28-pins.What I am trying to figure out is how this battery balancing charger can balance the battery while charging without also discharging the battery while the battery is in standby, that is, disconnected from the charger or not inserted into a tool.I would like to get some idea how a battery balancing circuit can monitor the voltage of 5-cells and switch in/out balancing resistors without the use of relays or optoisolators to prevent the balancing circuit from drawing any power/discharging when disconnected from the charging station.I have designed a constant current(.5 to 1A) charging circuit that works well to monitor the voltage of each battery while charging and switch in a shunt resistor(22-ohms) across each cell if a cell needs to be balanced. The balancing uses a FET switch that bypasses some charge current of any cell that is charging and has reached significant (~50mV) higher charge voltage than others in the daisy-chain.What I haven't figured out is how to disconnect each of the cells voltage divider to the A/D inputs of my MCU chip without using an opto isolator or relays and also remove any powering of the balancing MCU when the battery is removed from the charger.I can take and post a picture of the charging PCB embedded into the battery assembly if anyone thinks that can help!

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 14, 2023, 10:51:46 am
content: There are awesome solutions for measuring/balancing/protecting, like the BQ76942:https://www.ti.com/lit/ds/symlink/bq76942.pdfhttps://www.ti.com/lit/ug/sluuby1b/sluuby1b.pdfIt does all you’ve asked. It can even do it without an MCU, or you can roll your algorithm.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 15, 2023, 02:41:44 am
content: The BMS chips are designed to have a very low quiescent current.

author: SuzyC
author_rank: Frequent Contributor
timestamp: February 15, 2023, 04:52:16 am
content: Fascinating, many thanks to all repliers to aware me of this TI BMS chip. The linked apnote is particularly helpful to me to develop my own BMS strategy and protection code.I've read the datasheet, which is quite a long document that describes a very complex device.What remains a mystery is thatthis particular battery BMS is doing all this using a 28-pin MSOP chip and the TI chip has 44 pins, so they cannot be the same. So what is this identity of this BMS chip?Interesting, the date code on the battery asm shows it was built/assembled in 2021 and yet offered for sale in 2023,My conclusions about the suitability of using the TI chip is not to use it because of it's complexity. price and doubt about future availability.While I would pay a price penalty in my design that is using at least five optoisolators to disconnect each of the voltage sensing resistor dividers, my developed design would be better IMHO because I can build a BMS that uses readily available off-the-shelf components and might even be repairable if it fails and battery replacement without BMS circuit replacement is possible.I am also interested in hearing about how practical it is to use cell balancing since I built a simpler circuit that uses a LI-ion battery pack composed of 5 series Li-Cobalt cells in a handvac and was smart CC recharging daily without any balancing BMS. I limited max cell V to 4.1V and max Icharge to .5Amp (C/4), and although It is been only used for a few minutes daily at Iop=C*1.5, the vac managed to take 5-yrs before having such a short use time(~1-min) to encourage me to now replace the entire string of cells.A postmortem of the cells showed one was still charged to 4.2V, one had close to zero voltage and shorted? and the others <3V.

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: February 16, 2023, 05:07:14 pm
content: As dobsonr741 pointed out, TI's BQ76942 BMS ICs are prefect for this application (they were made for the very purpose!).I have worked on an electric bicycle BMS before with a BQ76940. These chips are quite rugged so as long as you follow the example application circuit diagram. There isn't much that can go wrong (until it does). Pretty much every major brand has their version of these chips, NXP, ST etc makes them.Cell balancing is not really something that you need to be concerned with when the battery pack is in use regularly, and the cells are in good health. Doubly so when you are the one assembling the battery pack yourself (assuming this is not a commercial project).If you look at the life cycle of a BMS, the only time where the cell balancing matters is either when the cells are straight out of the factory and at an uneven state of charge or when they're old and the differences in their chemistries is easily noticeable. In the prior case, you can manually balance them and the pack would work perfectly fine. In the latter case, welp, its time to change out the cells anyway.Usually the cells should never go "out" of balance in normal operation, where all cells are healthy.Lastly, don't worry about balancing the cells too closely. For a small battery pack even if the cells are slightly out of balance, they shall balance themselves eventually and with low current draws, nothing should ever go wrong

author: SuzyC
author_rank: Frequent Contributor
timestamp: February 16, 2023, 07:15:34 pm
content: The TI BMS chip requires me to learn how to optimize 100 or more initialization settings than required for example, for initializing an Intel I5 processor for hobby use in a smart kitchen timer!The cells I just extracted from the 5-cell battery pack match in O.C. voltage (they are brand new) to <50mV.I wouldn't think or mixing non-identically matched cells, only make pacs with new identical and already V matched cells.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: February 16, 2023, 08:47:03 pm
content: As far as code complexity LibreSolar have a open license for their source for personal use, covers an entire BMS system:https://github.com/LibreSolar/bms-firmware/tree/main/app/src/bq769x2