Charger vs BMS  - Page 1
Forum section: electronics / beginners

author: MathCa
author_rank: Contributor
timestamp: March 03, 2023, 11:31:30 pm
content: If I understood correctly, the charger should be the one that measures the voltage and controls the charging, and the BMS, during the charging process, would only be the last protection in case the charger malfunctions?I charged my batteries twice (lithium-ion 4S2P) using a power supply that provided a constant voltage of 16.8 V and limited the current to 4 A, and the BMS seems to have carried out the charge correctly, balancing and stopping the process.But it was not a good idea I think?The charger should have lower limits than the BMS so that the BMS only serves to protect the batteries if the charger limits are exceeded?So is it also the job of the charger to do the balance at the end?Thank you!

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2023, 11:52:48 pm
content: Yes, the BMS is supposed to be primarily a protection device, sometimes they are meant to handle balancing. The charger should always be a proper charger for the chemistry though.

author: shapirus
author_rank: Super Contributor
timestamp: March 04, 2023, 12:07:20 am
content: 16.8V / 4 = 4.2V per cell (assuming that the balancer does its job). This is a normal CV stage voltage where the BMS is not yet supposed to break the circuit. Overcharge protection will normally kick in a little later.BMS functions, in general, are not limited to protection (which can be over/undervoltage and the same for current). They can also include balancing and monitoring, which in turn can get quite intricate.

author: MathCa
author_rank: Contributor
timestamp: March 04, 2023, 12:17:53 am
content: Слава Україні!!!@shapirus Good morning hero. I'm so happy to see your beautiful flag, you are the best in everything!Все буде Україна

author: MathCa
author_rank: Contributor
timestamp: March 04, 2023, 03:21:14 pm
content: OMG I was sleeping I think. I finally understood, we should never allow the charger to reach the limits of the BMS!Now it's so simple to understand why a separate charger is required with similar functionalities!

author: Siwastaja
author_rank: Super Contributor
timestamp: March 05, 2023, 06:34:38 pm
content: If multi-cell pack is going to be CV charged until full, then the BMS has to provide per-cell voltage measurementsand communicate that to the charger, so that charger can regulate the CV voltageof the highest cell, not the full pack voltage. That's the proper way of doing it.In practice, that communication part is difficult and BMS just separately tries to balance the cells by removing charge from them, and in case of some cell overvolting beyond the BMS protection limit, communicates that to the charger in a simpler on/off signal - or if even that is too challenging, sometimes every BMS cell has their own disconnection switch and they just break the whole chain. The disadvantage is N times repetition of these disconnection switches, each of which must deal with full pack voltage and current; but it's workable up to maybe 6s.

author: shapirus
author_rank: Super Contributor
timestamp: March 05, 2023, 06:49:22 pm
content: Quote from: Siwastaja on March 05, 2023, 06:34:38 pmTrue, but it requires a charger specifically developer or modified for the purpose. It shouldn't be difficult, but it's not required, because off the shelf parts work very well as they are.Quote from: Siwastaja on March 05, 2023, 06:34:38 pmThe balancer is often a separate device which is not a part of BMS, because the balancers that typical BMS have are low-power passive ones which don't do the job well, except when charging small batteries at low currents. Once you reach charging currents of several amps, you are likely to need balancing currents that require using (external) active balancers, which also can balance battery packs at any SoC level.Quote from: Siwastaja on March 05, 2023, 06:34:38 pmIn practice, it's not an issue. Never really happens. If the balancer is chosen adequately for the battery capacity and charging current and if the cells aren't mismatched too badly, then by the time the battery voltage gets closer to end of charge and the current drops, the cells are already well balanced and BMS doesn't have a chance to break the circuit for protection.

author: MathCa
author_rank: Contributor
timestamp: March 05, 2023, 07:37:23 pm
content: So I can make my own charger with a stable 16.8v limited at 4A (for a 4S2P pack of 4000 mAh lithium ion batteries), a current sensor and 4 voltage dividers connected to an Arduino board?The Arduino could cut the connection to/from the BMS in case of problems.Sorry for the diagram, I still have a lot to learn but I guess it's enough to understand?EDIT BY MODERATOR: Please keep your replies in English and on-topic (no political topics here please).

author: Siwastaja
author_rank: Super Contributor
timestamp: March 05, 2023, 07:45:00 pm
content: Quote from: MathCa on March 05, 2023, 07:37:23 pmThe voltage dividers cause different leakage currents to each cell, making it an un-balancer. Maybe the easiest way around this is to choose some li-ion balancer/monitor IC which provides cell voltage measurement and integrated level shifting; these are designed for minimal loading of the cells (microamps). For example, with I2C interface, which you can then connect to your Arduino.(With 4s, you can maybe just barely do it with properly chosen resistor values, but the accuracy when used with the AVR's integrated ADC will suck.)

author: MathCa
author_rank: Contributor
timestamp: March 05, 2023, 07:59:48 pm
content: I would love to find a good charging module that could communicate easily with an Arduino/ESP32/etc.What I saw that was not a BMS was only able to go up to 4A, and I guess it's not the best idea to draw 4A (0.5C) continuously if it's the limit. I could also charge those batteries at 6A (0.75C) when a faster charge would be required so without using a fan, I would ideally need a charger able to draw at least 8A, ideally 12A if it's something not super trustable from china?Maybe it should be a project for my first PCB 🥴

author: shapirus
author_rank: Super Contributor
timestamp: March 05, 2023, 08:45:20 pm
content: Quote from: MathCa on March 05, 2023, 07:37:23 pmWhat's your final goal? Or, rather, are you ok with an off-the-shelf solution? I mean, you could make your own, but it gets rather tricky as you dive deeper into it and try to implement everything properly. Ready-made solutions are also very likely to end up much cheaper.If you ask me, I'd say that designing the load switching part (if I understand it correctly that you want to make something like a UPS) is in itself a project significant enough to be able to satisfy the DIY itch and it's where putting your effort will make the most sense.The charging+balancing+protection part is what should rather be built from ready-made parts. Not really worth the hassle of building them yourself. Any decent adequately rated (plus chinese-derated) CC+CV dc/dc converter for charging, acapacitive active balancerfor balancing and whatever has a lot of good reviews and is rated for the necessary current (+chinese derating) on AE for BMS.There are also inductive active balancers, which are cheaper, but I've never tried them, so can't tell how good or bad they are. The capacitive one that I linked above is very good and it balances the cells to a sub-millivolt difference. It also has an on/off jumper that allows to control the balancer using a relay or an optocoupler.

author: MathCa
author_rank: Contributor
timestamp: March 05, 2023, 10:15:47 pm
content: Дуже дякую, ви добре знаєте свій предмет.Quote from: shapirus on March 05, 2023, 08:45:20 pmI agree 100%. I already have the power supply, buck converter with CC+CV and a good enough BMS I think. That BMS(https://www.aliexpress.com/item/4000527872921.html) can do the balancing at the end, so if I understand well, it's also a charger when used with a CC+CV source? Else I don't understand when it could "decide" to do the balancing if it cannot communicate with a "regular" charger? We must surely pass a limit that trigger the balancing, like the equalizing voltage of 4.18v? In that case, I could only have to mesure the voltage at the output of the BMS and when it reach 16.8v, disconnect the power supply from it?So the process would be like:1. Charging start at 4A and the voltage increase slowly.2. Once it come close to the end, the BMS draw less and less current.3. At ~16.2v, the BMS would start the balancing.4. At 16.8v, I could stop the charging & balancing process.5. If there is a problem and the system doesn't stop at 16.8v, the BMS would stop it for me at ~17v (~4.25v x 4).If I'm right, I think it's easy enough for me 👌Quote from: shapirus on March 05, 2023, 08:45:20 pmExactly, but it's fine for me if the power cut a fraction of a second. I will surely use relays first and change them later for MOSFETs. I want to learn more about them first.Thank you very much! 🤝

author: shapirus
author_rank: Super Contributor
timestamp: March 05, 2023, 10:48:36 pm
content: Quote from: MathCa on March 05, 2023, 10:15:47 pmThese two comprise the charger.Quote from: MathCa on March 05, 2023, 10:15:47 pmYes, it's supposed to do the balancing, but the question of how well it does it remains open. They claim 60mA balancing current, and it's probably okay with a 4A charging current, if the cells are not too unbalanced and are well matched in terms of internal resistance and capacity.It can always be verified experimentally. Try to unbalance the cells and then charge them via this BMS, starting with lower current, like 500-1000 mA and monitor each cell voltages (simply with a DMM) as the most charged one gets closer to 4.20 V. See if it goes over this value, how much, if it does, see if the other cells catch up or if they don't before the BMS breaks the circuit.Maybe you won't need an external balancer for this combination of battery/charger/BMS, maybe you will.Quote from: MathCa on March 05, 2023, 10:15:47 pmNo, it's just a monitoring circuit with a (mosfet-based) circuit breaker activated when a certain condition is detected. These conditions are usually over/undervoltage on any of the cells and current rising over the limit. Some BMS also have temperature monitoring.Quote from: MathCa on March 05, 2023, 10:15:47 pmThat I unfortunately don't know. I've been too lazy to try and understand how the passive resistor-based balancer circuit actually works. But it should be simple enough, search for it.Quote from: MathCa on March 05, 2023, 10:15:47 pmIt's a design choice of cyclic charge vs float charge. Float charge is easier (no need to disconnect the charger), but it's believed that in this case it's better to set the CV output to a lower value, e.g. 4.1V per cell.Quote from: MathCa on March 05, 2023, 10:15:47 pmBMS doesn't draw current (save for the negligible amount to power its own circuitry). It simply passes current between the battery and load/charger until a fault condition is detected.Quote from: MathCa on March 05, 2023, 10:15:47 pmRemains to be determined by understanding how the passive balancer works.Quote from: MathCa on March 05, 2023, 10:15:47 pmOr keep it at 16.4V indefinitely and don't disconnect the charger.Quote from: MathCa on March 05, 2023, 10:15:47 pmAs long as the BMS is functioning correctly and as expected, it will break the circuit (turn off the pass MOSFETs) ifanyof the cells reaches 4.25V. It may or may not be at 17V total depending if the balancer was able to fully equalize the cell voltages.

author: MathCa
author_rank: Contributor
timestamp: March 05, 2023, 11:25:56 pm
content: Quote from: shapirus on March 05, 2023, 10:48:36 pmExcellent advice again 🙏Quote from: shapirus on March 05, 2023, 10:48:36 pmI didn't know that, very interesting, I will read more about this.Quote from: shapirus on March 05, 2023, 10:48:36 pmTrue! I have to keep that in mind.You have been very helpful, I'll made another donation for drones 🤝 I can't wait to visit your country, I felt in love with it because almost all my favorite photographers were from there, and I planned a trip just before the date you know, but this summer the party organised by Saint Javelin will be great on the Ukrainian beach of Sevastopol 💙💛 I hope to see Patron too!