BMS - Where to start? - Page 1
Forum section: electronics / beginners

author: Muffins
author_rank: Regular Contributor
timestamp: February 07, 2018, 09:10:52 am
content: Hi guys.I'm developing a device to run around 6v DC, lithium ion cells would be great for the application as portability is desired.I would like to run a 2s2p configuration with 18650 cells. I see no-name Chinese solutions are available for these applications but I don't want to use these in my designs, I want to be able to put together my own battery management solutions. My goal is to eventually be able to design systems containing any number of cells.So my question is, where do I start down this path?I've been going through the websites of Texas Instruments, Microchip, Linear Technology and doing some general browsing and I'm confused by the multitude of solutions available.I would like to charge the cells in the device, control the balance, and protect them from over-discharge.I'm going to see what I can pick up to start playing with this stuff soon.

author: paulca
author_rank: Super Contributor
timestamp: February 07, 2018, 02:22:08 pm
content: You and me both. I've been looking for months to solve this problem easily without much joy.BMS boards are 99% for single cell battery packs. The rare few that exist for multi-cell batteries either don't balance or don't balance very well.To be honest it seems as though the normal way to deal with this today is inefficiently using a 4P pack and boosting up to your intended voltage.Even large powerbanks that produce 19V for laptop power do it this way.The other option is to use some more expensive Lithium BMS ICs and a whole bunch of mosfets and passives to produce what effectively becomes a full balancing battery charger. However without careful modification that cannot be used to load the battery in use, or to run charge + load simultaneously.I'm sure there are other solutions, but I haven't found them yet.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 07, 2018, 03:26:15 pm
content: I make and sell a TP5100 based module which until the version i am currently selling is intentionally set to 1SxP mode. The module has an onboard booster too... I am now waiting the prototype pcbs and parts for the next version which besides minor or major changes in the input or output sides has the 2S ability of the TP5100 unlocked.At some point, designing the "next" version i had the crazy idea to go to an under-board 2 cell battery holder which with the help of a "switch" would swap the batteries from 1s to 2s configuration .. this way, (and that is the simpliest and perhaps cheapest balancing method i know) you could charge and discharge your cells in 2S but once in a while go to 1S and charge them in parallel to balance them out. I scrapped the idea because of the switch itself... a toggle switch would be bulky, expensive (i needed a 3PDT switch like the one in the photo ($4+ at Mouser!) and the idea of 3 much cheaper pcb mounted switches was potentially hazardous. Adding an onboard BMS on the other hand would either lead me to a 2S only version or, could be wasted for 1S use. There are simple and cost effective BMS designs you can use and embed to such a project but for many reasons i would prefer something readily available attached to the batteries.For a 6V boost you DO NOT need to complicate things... depending on the current draw requirements, most common boost ic's will not complaint taking as low as 2.8-3V and up to 4.2V and turn it to 6V... i am sure the MT3608 i use will happily deliver 1,5A under these conditions without any issues... and its as cheap as you can go.Also, there is nothing wrong with the chinese offerings in this area.Some parts are so succesfull (like TP4056) that the danger using them is the risk of getting a fake or B grade part so you have to "know" your sources.

author: paulca
author_rank: Super Contributor
timestamp: February 07, 2018, 04:51:33 pm
content: The cheapest balancing method would be to charge them on the balance port and cycle the charger through the cells, but critically you can't connect the main power lead anywhere.The most common method is to pull the high cell(s) down through a load mosfet/resistor and lower the charge current to maintain sensible voltage across the cells.This is slow. The issue with the BMS boards I have seen demonstrated by Adam Welch etc. is that they do not limit the charge current in this phase. So they are pulling the high cell through a 100mA load while pushing 2 or 3A through the pack. They thus will only work if they pack remains "nearly" balanced anyway. I suppose in a lot of devices they will remain "nearly" balanced, if the cells are nicely matched. It also highly inefficient. There are techniques you can do if you are in a hurry to charge a badly low cell individually and it works out much faster than letting the balancer do it.I have been pondering trying to create a LiPo charger which will switch between the normal mode above and individual cell management with the power lead disconnected by a mosfet. It can then apply voltage across individual cells at up to 1A through the balance lead until the cells are balanced, then resume "Power" mode.The trouble comes when you look at the voltages. You can apply 0V - 4.2V across any individual cell. You cannot apply 0V-4.2V across more than one cell (with the main series power lead disconnected) while they are in series.EDIT: To clarify. You can put 0V-4.2V across Cell 2 in the pack fine. You can't put 0-4.2V across Cell 1 and then put 0-4.2V across cell 3 at the same time. You will reverse polarity on Cell 2. If you try and put 4.2V across cell 1 and cell 2 you will short your 0 to 4.2V lines.Cross cell charging is something i have heard claimed some LiPo chargers will do, but if you consider the voltage differentials it... well it make my head hurt. If the power lead is attached the cells are in series. If you want to charge cell 1 from cell 3 you have to drop 8.4-12.6V down to 0-3.7V relative. I gave up at that thought. Although with the cells out of series completely there is no reason why you can't charge a low cell from a high cell trough a current limiting mosfet config. Just disconnecting the power lead will not work in this case though.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 07, 2018, 05:01:51 pm
content: Any manufacturer would assume the cells in a pack are "identical" coming from the same supplier, same plant, same batch, hell they would be sequential serial numbered if needed...... The do not make a BMS trying to balance odd bits of recycled 18650's and if that is what we all try to do then we at least need to 1. make as best a choice of cells in a pack as we can and b. keep our "balancing" demands in perspective with what we put the bms do! Putting cells from laptop batteries into a pack with capacity and age and resistance etc different as night and day and calling it a Battery Pack expecting the 1-2$ chinese (not bad to be chinese really) bms to do miracles is just not going to happe.. then we can blame the chinese for their el cheapo bms not worth the solder they used to put them together!

author: paulca
author_rank: Super Contributor
timestamp: February 08, 2018, 08:04:32 am
content: Maybe I'm tainted by using high power LiPos but I can tell you that guaranteed matched Ohm Lipo cells in a pack when put under stress WILL unbalance. I've seen 200mV or more in a single discharge. I have seen inbalance of over 0.5V but only when over discharged and that won't matter due to the steep edge on the charge curve. On a large LiPo charged at 5A a 100mA balancer is not enough unless you reduce the charge current to 0.5A or less.I can also tell you that a proper Lithium charger will handle completely unbalanced cells. You can put a 2.8V cell into a pack with a 4.2V cell and it will handle it and not overcharge them. It will just take it's sweet time to charge the pack. If this case you would be better jury rigging the flat cell and charging it as a single cell via the balance port.Under light load my little pack I made here, LiIon 2200mAh per cell in 3S2P has got itself to 3.54, 3.53, 3.48 and it's only been producing 0.5-0.9A. I don't really want to do the maths, but on a 2A BMS with a 100mA balancer will it be able to balance before it's fully charged or will it overcharge the 3.54V cell while trying?Also, if you check the datasheet on these cheap BMS boards, the stated cell cut off voltages are horrendous. I have seen 2.6V under discharge and 4.3V over charge protection. That in itself is bad. Then you have to factor in their voltage measuring tolerances are abismal and you can at very least be damaging your pack and left unattended you could be facing much worse. Some of course go the other way and barely charge to 4.1V.I also believe that should a cell reach 4.3V when charging it will just stop the charge and disconnect the battery completely, leaving the other cells where they ended up, no matter how low. Over time the pack balance will just get worse and worse and worse, until just like 99% of laptop batteries they die young. Not because the pack is broken, but because it has become too unbalanced for the BMS to handle and the BMS decided the pack is faulty.Even hobby grade chargers have buckled to safety concerns regarding their voltage accuracy. I have two, once 8 years old and one bought this year. Both the same brand and model. Both came from factory over reading by 100mV. The older one had voltage calibration, so experienced users could correct the safety margin. The new one doesn't.I don't mind as I currently want my packs slightly undercharged to increase cycle life.

author: paulca
author_rank: Super Contributor
timestamp: February 12, 2018, 08:16:43 pm
content: So I got a sample 3S BMS.This one:https://www.banggood.com/3S-20A-Li-ion-Lithium-Battery-18650-Charger-PCB-BMS-Protection-Board-12_6V-Cell-p-1118043.html?rmmds=detail-top-buytogether&cur_warehouse=CNFor a start, it charges and discharges via the balance leads.It has no voltage regulation. I don't believe it will balance cells, though I did notice voltage anomalies while charging on cell 1.On it's first discharge test it allowed the pack to drop to 7.64V before disconnecting it. Roughly 2.54V per cell. Too low. It was under 1.3A of load and when I tested the cells at rest they were 3.11, 3.12, 3.24. So you could argue that's fine. However if it had of been under a much lighter load, 2.54V per cell is definitely too low.Testing it with a charge voltage of 12.60V it did not disconnect even when charge current fell to 10mA. This is expected. When I lifted the voltage to 12.63 voltages it allowed all 3 cells to come up to 4.21V, when I gave it 12.66V it allowed the cells to come up to 4.22V. I do not want to try further. The datasheet says it's over charge protection can go as high as 4.30V per cell. WAY too high.So, you need high current balance leads on each cell to use it for higher outputs. Normal balance leads where warm at 1.4A. They are typically rated for 1A, 24 or 26AWG wire.It over discharges the cells too low. It allows overcharging too high.The only functional purpose this board has to me is as an emergency cut off if I want to leave a pack discharging while I am not present.The audible LiPo alarm warnings are probably better as they are at least accurate.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 12, 2018, 08:34:27 pm
content: Can you check the codes on the three SOT23-6 ics between 8.4V & 12.6V ?

author: paulca
author_rank: Super Contributor
timestamp: February 12, 2018, 09:15:06 pm
content: Codes:From battery terminals forward.3x IC 6 pin "DW01KA"6x IC 3 pin "2L"3x IC 3 pin "G1"4x Large IC 3 pin (1 cut) + tab "D472" "BAxxxx" varied.EDIT: Apologies, this was the item I bought:http://www.ebay.co.uk/itm/263099688192

author: soubitos
author_rank: Frequent Contributor
timestamp: February 12, 2018, 09:44:12 pm
content: Quote from: paulca on February 12, 2018, 09:15:06 pmListing says was removed!As for the parts, 2L G1 and D472 are all transistors.. npn and mosfets.Nothing to write home about.DW01KA though is the culpritFrom DW01K datasheet the numbers need attention are (min nom max)? VOCP -- 4.25 4.30 4.35 V (over charge voltage)? VOCR -- 4.05 4.10 4.15 V (over charge voltage release)? VODP -- 2.30 2.40 2.50 V (over discharge voltage)? VODR -- 2.90 3.00 3.10 V (over discharge voltage release)You can improve these somehow by replacing these DW chips with DW01C-G which is a little better, or as good as DW chips goVOCP - 4.25 +/- 0.05VOCR - 4.05 +/- 0.05VODP - 2.38 +/- 0.1VODR - 2.98 +/- 0.1Or with FS312F-G which is much betterVOCP - 4.25 +/- 0.025VOCR - 4.145 +/- 0.025VODP - 2.90 +/- 0.08VODR - 3.00 +/- 0.08I use FS312F-G and can send you some if you likeAlso note that the board is not a charger... its function is to save the batteries from a faulty charger!It is the chargers job to detect a 4,2V 1/10C steady current draw and terminate the charge cycle...This is why the overvoltage protection they offer is always a little higher than 4.2VOn the other hand though, the over-discharge protection they offer (DW01x chips) is a bit problematic, as its always in the 2.4V rangeLast but not least, a better quality of chinese BMS would most of the times be based on Hycon chips like HY2213-BB3A balancing chip (VOCP 4.200±0.025V VOCR 4.190±0.035V) but they match them with also Hycon chips similar to DW01x ....

author: paulca
author_rank: Super Contributor
timestamp: February 12, 2018, 09:58:21 pm
content: The purpose I originally started on this hunt for was to create a cycle UPS for my dash cam. ie. It would run off the car and charge while the car was in use. It would run off the battery while the car was parked.3 Cell Lithium seemed an option, but being able to manage the 3S properly, safely and reliably seems very difficult to do.The best option I have is to use a LiPo charger which does not require pushing a button and about 4 relays to full disconnect the charger when the car supply goes to 0V. Messy.Sealed Lead acid is bulkier but only requires a small voltage drop off the 14.4V car and a current limiter. It will fill my glovebox though.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 12, 2018, 10:00:37 pm
content: Quote from: paulca on February 12, 2018, 09:58:21 pmWhat kind of current draw are we talking about?

author: paulca
author_rank: Super Contributor
timestamp: February 13, 2018, 07:57:14 am
content: Dash cam has "Maximum 1W", but I doubt it actually pulls that very often, probably more like 0.5W. Runs on 10V-15V. So way less than an amp.Charge current would need to be fairly high to get the pack charged during the week when the car is only used for 2 hours a day. However, not too high, the car socket can only provide 10A. An SLA might only like about 4A max.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 13, 2018, 09:41:40 am
content: I am no expert in car batteries and avoid doing work in or around cars anyway but....I doubt such a small load can affect the car batteryI would worry mostly about the automotive grade power supply requirements of any kind of charger you might decide to use, either its a 3S or 1S+booster choice.How would you feel if when you power the "charger" and it starts charging, it counts 3hrs and then stops?

author: paulca
author_rank: Super Contributor
timestamp: February 13, 2018, 09:46:57 am
content: Quote from: soubitos on February 13, 2018, 09:41:40 amIt's an option, but in automotive stuff there is a rule. Don't run stuff off your starter battery. They are not designed for it. Save the alarm and door sensors etc. To run stuff off a lead acid it needs to be a deep cycle battery.The manufacturer (of the dashcam) solution is a direct fuse wire with a low voltage cut off sensor.Either that or a Lithium pack with built in charger, but they want 200 Euro for it and claim it only runs the cam for 4 hours.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 13, 2018, 09:48:44 am
content: Give me30 minsome time and i will introduce you to a solution for your problem...The original idea i had fails to work as i wanted... i will try a couple alternatives and do some tests

author: paulca
author_rank: Super Contributor
timestamp: February 13, 2018, 10:05:00 am
content: If I got with 18650s in parallel and use a buck converter to charge them via TP4056's and a boost converted to get the 12V output it might be doable off the shelf. Still need a current limiter circuit though. Unless I use a current limiting buck converter IC.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 13, 2018, 10:56:40 am
content: why you need that for? 4056 will never draw more than the charge current setting

author: paulca
author_rank: Super Contributor
timestamp: February 13, 2018, 11:31:49 am
content: Quote from: soubitos on February 13, 2018, 10:56:40 amOh, I forgot it had a current limiter in it.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 13, 2018, 02:54:08 pm
content: What you want to do, the way you want to do it is PITAMy problem in all this is the following...You have a car battery and a 12V-1W load to run on it 24/7..The car will not operate to recharge the battery for 3-4 days in a row.Instead, you want to add a UPS so to speak between the car battery and the dashcam.Which means, you need a charger (possibly a voltage regulator too) and a booster which all consume power then the power needed to recharge the battery so in the end, you will still be consuming power from the car battery....A real solution to this really is ... two lithium packs big enough to keep the dashcam going for a week (1s14p is adequate i think) and a simple charger to charge them "offline" from the grid.. and once a week you swap the batteries fully charged without affecting the car battery at all....Or, you simply get power from the car battery !

author: paulca
author_rank: Super Contributor
timestamp: February 13, 2018, 03:23:06 pm
content: This is why I haven't bothered yet.I need a battery that can charge at 5A while the car is running and discharge at 100mA when it's not. 1s5p and 5 TP4056's in parallel could be amusing. But then when the car is off they need to through a boost converter to get 9-12V.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 13, 2018, 03:43:13 pm
content: Quote from: paulca on February 13, 2018, 03:23:06 pmOh, you want a FAST charger too! LOL.... you know a charger to go over 2A is serious business and anything but cheap.... a 1S5P setting would take 10-12hrs to charge at 1A... 5-6hrs at 2A...

author: paulca
author_rank: Super Contributor
timestamp: February 13, 2018, 04:37:22 pm
content: I can get a 3S 6A charger for £15. Single cell (ornP) chargers are hard to find that current as they aren't popular with RC folks.I worked out I need something like 7Ah and a peak charge current of 5A to cover most of the time, it would still run flat (another concern) over the weekend if the car wasn't used. At least then it would be in my home parking space.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 13, 2018, 05:10:40 pm
content: Got a link of said 3S 6A charger?

author: paulca
author_rank: Super Contributor
timestamp: February 13, 2018, 07:02:19 pm
content: Oops, it's only 4A. This is of course for a charger with no user interface. Plug the battery in, power it and it goes.https://hobbyking.com/en_us/turnigy-b4-compact-35w-4a-automatic-balance-charger-2-4s-lipoly.htmlIf you want 6A+ the one I use is the Accucel 6 80W:https://hobbyking.com/en_us/turnigy-accucel-6-80w-10a-balancer-charger-lihv-capable.html

author: soubitos
author_rank: Frequent Contributor
timestamp: February 13, 2018, 07:17:19 pm
content: Can you charge while load is on with these?

author: paulca
author_rank: Super Contributor
timestamp: February 13, 2018, 07:23:21 pm
content: Probably not. Hence why I remarked that I would need 5 relays. To completely disconnect the 4 balance leads from the charger and connect the battery to the dash cam.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 13, 2018, 07:33:59 pm
content: Have you considered something like this?http://bit.ly/2EoCLbYThis plus a TP4056FlexAdv and 2-3 fully charged 18650s to begin with and 3-5hrs of sun a day and i think you are more than ok

author: paulca
author_rank: Super Contributor
timestamp: February 13, 2018, 09:15:33 pm
content: Yea, I have considered a beefy power bank, especially one of the ones that Jullian Ilett plays with that will do up to 19V on the output. I can double up the 18650s to 8P or more to provide decent Ah. Would still need a 5V USB power thing to charge it.4-5 hours sun light? I see you are in Greece. I'm in Northern Ireland. We get as much rain and overcast days as you get sunny days. Around about 220 days of rain a year. (That comparison is actually between the UK and Spain).It's something that will knock around on ice until someone dings my car again, then that will spur me into action to get the dash cam running in parking mode.Thanks for the help though.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 13, 2018, 09:41:50 pm
content: Last call for something... sane... get a second lead acid battery...