Bottom Balancing? Top Balancing? Charging and BMS

[dnmun]

actually headway does not make the headway BMS. they have a source, i don't know who. the new 12S headway BMS is still the anlalog style with the shunt resistor divider but now the new 16S BMS is digital. they may still have the 16S in the analog version or if you can get the older 16S BMS, it is possible. i will look at mine. i only have two left, hate to give them up but the ping v1 signalab would work too since it has the 22 ohm shunt resistors, so about 170mA shunt current at max.

 

[999zip999]

When you parelle charge the cells can be put in smaller 8p for easier handling. Charge to 3.6v and see how they hold there charge over time. Still a chart sorts things out and a black marker.

 

[heathyoung]

if you are going to be using just 15S then the 16S BMS will not work since you will have a channel empty and that will set off the LVC control signal. i can hack the ping v1 and v2.5 signalab BMS to 15S but not the new 16S digital BMS from headway.

i think i can hack the older 16S BMS from headway but it depends on how much of the black goop they pour on the control lines. you have to get access to the transistor that turns on the LVC signal line and unsolder the resistor so it just depends on each of those old BMSs and if you can reach the resistor with the soldering iron.

the ping is easy, and you are gonna have to discharge outside the mosfets too if you are gonna pour this much power through them.

Maaaaaaannnnn, what a pain :? think i just best email Headway and tell them what i need see if what they can do for me? Surely they will be able to do that all for me.

Since you are in Australia, and you are buying from evpower, get some of their cell-top BMS units - they are very good and reliable, and get their battery management module as well.

http://ev-power.com.au/webstore/index.php/ev-power-bms/ev-power-lfp-battery-management-system/bms-cell-modules.html
http://ev-power.com.au/webstore/index.php/ev-power-bms/ev-power-lfp-battery-management-system/bms-master-units/bms-mcu-micro-08.html

Ah crap - they are out of the master units - oh well, just use the cell modules to run a relay then or if you get stuck, I can sort you out with some of the modules I am using for my Thundersags in the vectrix.

 

[999zip999]

Can one be used for a 2-4 parelle string ?

 

[Frank]

If someone were to poll those who actually use LiFePO4 w/o BMS you'd probably find they were quite happy. There is no known self-discharge mechanism for these cells and most of the troubles I've experienced personally and read about have been due to parasitic loads on the cells. NB: I'm talking about prismatic cells here (like you have) not small jobbers.

The key is to undercharge and under-discharge. ANYTHING you do to keep cell voltage within conservative limits will work. I have 24 100Ah T-Sky's in a motorcycle with a few thousand miles on it and generally speaking never use more than 60-65 Ah. Is the pack oversized? Possibly - but these cells will last forever. The only time they diverge is near end of charge (I aim for 3.5 VPC) and most of the time stick within .02V of each other. I've also saved quite a pocket of change by deciding not to use BMS.

Sure, Jack is arrogant, but I respect the hell out of what he's doing. My only specific advice: if you're going to shunt current I'd make sure you're standing there every single time you near end-of-charge.

 

[dnmun]

i have no respect for him. he is an idiot.

you should not mislead people into thinking they can build a pack and use it in operation without a BMS to balance and protect it. your pack will not last forever, it will die the first time the charge puts one of the cells over 5V.

you just end up hauling around a lot of excess baggage rather than pay the $75 it takes to install a BMS to balance it and protect it. none of you guys ever report it when you ruin a pack that way. lucky tommy lost three of the high power headways when he drained his dead.

there is nothing difficult about hacking the 16S BMS to 15S.

 

[Jeremy Harris]

If someone were to poll those who actually use LiFePO4 w/o BMS you'd probably find they were quite happy. There is no known self-discharge mechanism for these cells and most of the troubles I've experienced personally and read about have been due to parasitic loads on the cells. NB: I'm talking about prismatic cells here (like you have) not small jobbers.

The key is to undercharge and under-discharge. ANYTHING you do to keep cell voltage within conservative limits will work. I have 24 100Ah T-Sky's in a motorcycle with a few thousand miles on it and generally speaking never use more than 60-65 Ah. Is the pack oversized? Possibly - but these cells will last forever. The only time they diverge is near end of charge (I aim for 3.5 VPC) and most of the time stick within .02V of each other. I've also saved quite a pocket of change by deciding not to use BMS.

Sure, Jack is arrogant, but I respect the hell out of what he's doing. My only specific advice: if you're going to shunt current I'd make sure you're standing there every single time you near end-of-charge.

Depends on how you use the pack, how quickly one or two cells drift off from temperature variations (self discharge and charge efficiency are both cell temperature dependent) and how able you are to detect potential problems. Doing as that bloke suggests is not wise, but then I doubt he ever puts hundreds of cycles on any of his projects; he probably just moves on to the next interesting thing once one is finished. I can cope with arrogance, but dumb arrogance is a bit wearing, and potentially costs people who, unlike him, may not be best placed to afford it. It's clear from what he says on the couple of videos I've seen that his technical knowledge is very severely limited, dangerously so in my opinion.

I'm not an advocate of permanently wired BMS units, either, as I've seen too many fail and kill battery packs. I'd rather always charge a pack to a known state of charge and not discharge it to lower than maybe 20%, to be sure that no cell will be over discharged or over charged. I do believe firmly in periodically charging so every cell in the pack is at the same state of charge. Those two measures combined give me peace of mind, others may well have different views.

 

[arkmundi]

For the higher amp applications, I tend to agree that a "human bms" can be the way to go. Get two cellog 8's, and you can have a readout of each cells voltage. Then a Cycleanalyst in the higher power version will make a nice display of full pack voltage, and all the other wattmeter functions.

Thanks. My next project. I already have the Celllogs in hand, wire, andersons for a harness to the Agniusm kit - he put on a nice cell-by-cell set of leads and connector. That way getting a reading, additional data and cell alarm.

 

[arkmundi]

If someone were to poll those who actually use LiFePO4 w/o BMS you'd probably find they were quite happy. There is no known self-discharge mechanism for these cells and most of the troubles I've experienced personally and read about have been due to parasitic loads on the cells. NB: I'm talking about prismatic cells here (like you have) not small jobbers.

Update on my saga, having recently done a test, is that the two cells I replaced failed because of a "parasitic load." Like the term because it says it. I left the Watts Up connected to the controller left on, for 10 days in the bike shop. That was enough to trickle discharge the pack. Why the two middle cells discharged fully and left the other 10 cells within a safe range may have something to do with those cells or their position in the pack.

 

[Frank]

I use 3 cell-logs (8P) to monitor my pack on the cell level and a CA for overall info. I route pin 1 (power) on all 3 units through a 3PST switch as leaving them connected all the time will lead to imbalance over time (not just pack drain.) This will be the 3rd winter for this pack and I plan to unplug the cell-logs. I didn't do this last year and they slowly drained the pack a bit more than planned, albeit evenly, even when powered "off." Before that I tried to modify some 8S's for even power drain per instructions floating around; although this seemed to work they eventually failed. I did a manual balance on the pack after this experience to get things back into line.

Please advise where someone can buy a BMS for $75! Maybe we have a semantics issue: I don't consider keeping an eye on voltage via cell-logs or similar units a BMS.

I have a 90AH T-Sky cell in the basement that I bought 3+ years ago to get an idea of what it looked like, etc. before buying larger quantities for this pack. I did some very quick testing but basically left it alone. 3 months ago I bought a PowerLab 6 and decided to test that cell. It took about 50 AH to charge it up after sitting. Rickard had a bunch of cells sitting unopened for 2-1/2 years and he had the same experience. There just doesn't seem to be any self-discharge mechanism with large format cells.

I don't want to sound like I'm defending Rickard, but his vehicles do get driven. I think he has kids, his wife as well as himself who use his EV's.

 

[heathyoung]

Yes, but he comes across as an epic douche sometimes.

In the words of Shakespere - more matter, less art.

 

[dogman dan]

Backing up to Dunmun's comment. Prices of bms's vary greatly by the output amps, and how many cells in series. I do believe there are lots of 20-30 amp continous lifepo4 bms's out there intended for 20 ah or less, for 50-100 bucks depending on the vendor in china. Delivered to your door might get a lot more costly though. A bigger bms is of course going to cost more.

For sure, I'm not advocating going without a bms to charge, except for perhaps the on the road charge that has no chance mathematically of ever completely recharging the battery. That is to say, you plug in for one hour, your charger puts out 200w, and your wattmeter says you need 400wh to fill the battery. No problem to put in 200wh with no bms in that case.

More and more, I really like the RC chargers I use on my RC lico packs. You have either a balancing bms, or a solid dependable top of charge cutoff, depending on the setting you choose. They aren't all just 50w chargers anymore at all, so the large ones can charge a big pack fairly quick. Pricy yes, but once you invest in some good RC chargers you don't need a charge bms anymore, it's in the charger.

Discharging, of course you can't read a bank of cellogs at 50mph. But a good wattmeter and voltmeter will tell you when to start paying attention to them. A conservatively set lvc on the controller can be a good enough drop dead stopping point, preventing ruination of all your cells.

If one cell got that much lower than all the others that the lvc doesn't save it, it was a junk cell anyway right?

 

[agniusm]

My view on this would differ. I think that there is no sense in using RC charger unless you are having fun fiddling with web of wires and charging you pack at home. I like my charger on the vehicle, balancer to protect my cells and chemistry that won't set my house on fire. At the moment i have built in 5amp charger, box for my chain and charger lead. I pull it out where i find a socket and plug it in, be it top up or full charge in 4 hours. Job done, nothing to think about, do about, just to enjoy the ride.

 

[999zip999]

To go bms free you have to marry the pack and know all your little ducks to keep them in a roll. As the cells are a little different and we can only try and match cells over time ? How much different ? I do miss those little blinky leds on a signalab love those things.

 

[dogman dan]

Well, if a bms that costs under $100 will do the job, then indeed why not use one? It's part of why I keep telling folks to just get a ping for a 20 amp controller.

But if you are running something larger, when a bms costs a bundle, then moving the charging part bms into the charger starts to make some sense to me. I've got no objection to a bms per se, but I've learned the hard way that the exact same kind of monitoring you do riding without a bms is wise to be doing with a bms.

So there you are anyway, riding along keeping an eye on your pack voltage. Even with a bms, it really pays to know that charging actually happened.

 

[agniusm]

Well, if a bms that costs under $100 will do the job, then indeed why not use one? It's part of why I keep telling folks to just get a ping for a 20 amp controller.

But if you are running something larger, when a bms costs a bundle, then moving the charging part bms into the charger starts to make some sense to me. I've got no objection to a bms per se, but I've learned the hard way that the exact same kind of monitoring you do riding without a bms is wise to be doing with a bms.

So there you are anyway, riding along keeping an eye on your pack voltage. Even with a bms, it really pays to know that charging actually happened.

Agree, same as riding a car and time to time watching your fuel gauge even if the light comes on and you get a buzz. Voltmeter on bike=fuel gauge. You can use small BMS for charging big batts for HVS, balance and wire it as LVS controller disconnect as we discuses on other thread. Of course you have your voltmeter to check your fuel.

 

[dogman dan]

There are so many ways to monitor a pack, charge it , balance it.

Most of em, except for crap like bottom balancing, are fine. It's just a personal choice which you choose. Just don't choose to blindly trust a bms is my advice.

I pedaled a long way home, uphill all the way btw, several times because I didn't have a voltmeter on the bike to tell me that my battery never charged while at work. Flaky plugs on chargers, arrgghh.

I just wanted to put it out there that the RC charges have pretty good charging bms's built in, and work great for single cell charging for quicker balancing when it is needed. Handy to have at home even if your pack has a charging bms. Even if you seldom use it, an RC charger is a great tool to have in the house for any EV enthusiast.

 

[scoot]

Yeah, I recall back in the days of the Rickard thread. I was following it with some interest... enough so I guess that I decided to try out the bottom balancing concept with my newly assembled 8s Headway pack (first one BTW) ... mostly because I thought would be an interesting learning excercise. To begin with, it was quite a challenge to get all the cells at around the 2.7 - 2.8 volt level (steep part of the discharge curve). Posted a charge graph of all 8 cells in that thread. It was quite obvious to me that this was not the way to go, LOL. Been top balancing ever since, and never looked back :wink:

Edit: Spelt his name wrong... corrected now I hope Freudian slip I suppose. :?

 

[arkmundi]

There are so many ways to monitor a pack, charge it , balance it... just a personal choice which you choose. Just don't choose to blindly trust a bms is my advice... the RC charges have pretty good charging bms's built in, and work great...

My sentiments exactly. I have the batteryspace smart charger for 12S bulk charging, so includes digital charge control. I've never gotten an overcharge using it - it charges the pack great. Should a cell get out of balance, I use one the niffty single cell chargers to bring it back in line. By "out of balance" I mean any cell that is 0.2 volts lower than the average, so a cell that's 3.2V when the average is 3.4V, for instance.

 

[dogman dan]

I consider myself quite a balancing slacker, but my tolerance is only .1v off. Sometimes I'll run a balancing charge on my packs just for grins when it's less. Just because it's been quite a few months since last time.

I really really get a laugh when folks post about a pack that is giving them fits, because they can't get it to balance to .005v. Fusssy!

 

[agniusm]

I consider myself quite a balancing slacker, but my tolerance is only .1v off. Sometimes I'll run a balancing charge on my packs just for grins when it's less. Just because it's been quite a few months since last time.

I really really get a laugh when folks post about a pack that is giving them fits, because they can't get it to balance to .005v. Fusssy!

Yea, i firs thought that my BMS does not balance well, within .03V, then started thinking that i'm being paranoid :lol:

 

[Njay]

Not considering arrogance from some people, I really don't see what's that wrong about bottom balance for large prismatic LiFePO4 cells. What I think is, and I've seen it quite often, people are not context-synchronized. The BMS/no-BMS wars alone show the biggest out-of-phase of them all, and in my opinion it is because BMS may mean so many different things (not to say there so many BMS models) and people just say "BMS". I'll have a BMS on my motorcycle - actually, let me rephrase it: I'll have a battery pure-monitoring system in my large-prismatic-LiFePO4-cells based motorcycle. I'll bottom balancing with an out-of-the-vehicle balancer device from time to time if needed (my BMonitoringS will tell me if needed) and just constantly monitor the pack at a cell level.
In fact, just saying "bottom balancing" isn't descriptive enough. It's bottom balancing of large prismatic LiFePO4 cells while charging each cell to 3,6-3,65V max (or the point where they haven't yet diverged significantly) and avoiding deep discharges.

 

[Jeremy Harris]

Not considering arrogance from some people, I really don't see what's that wrong about bottom balance for large prismatic LiFePO4 cells. What I think is, and I've seen it quite often, people are not context-synchronized. The BMS/no-BMS wars alone show the biggest out-of-phase of them all, and in my opinion it is because BMS may mean so many different things (not to say there so many BMS models) and people just say "BMS". I'll have a BMS on my motorcycle - actually, let me rephrase it: I'll have a battery pure-monitoring system in my large-prismatic-LiFePO4-cells based motorcycle. I'll bottom balancing with an out-of-the-vehicle balancer device from time to time if needed (my BMonitoringS will tell me if needed) and just constantly monitor the pack at a cell level.
In fact, just saying "bottom balancing" isn't descriptive enough. It's bottom balancing of large prismatic LiFePO4 cells while charging each cell to 3,6-3,65V max (or the point where they haven't yet diverged significantly) and avoiding deep discharges.

If you're charging each cell (LiFePO4) bto 3.6 to 3.65 then you are definitely not "bottom balancing". The process that Rickard describes is to discharge all the cells to a set minimum voltage then bulk charge them with a set capacity during charge. AFAIK, he doesn't use cell level charge cutoff to determine full charge, if he did I wouldn't have taken issue with his supposed advice.

 

[Njay]

If you're charging each cell (LiFePO4) bto 3.6 to 3.65 then you are definitely not "bottom balancing". The process that Rickard describes is to discharge all the cells to a set minimum voltage then bulk charge them with a set capacity during charge.

Yes, that's what I meant to say, bulk charge calculating the total voltage such that each cell (theoretically) gets to 3.6V - 3.65V (I don't remember the exact value he uses but I understand the theory).

AFAIK, he doesn't use cell level charge cutoff to determine full charge, if he did I wouldn't have taken issue with his supposed advice.

Yeap, he doesn't. As far as I remember, the "trick" is to bottom balance and then charge to a point before cell voltages start to diverge; since they're balanced below, the capacity differences will show up at the top, but if you charge a bit "below the top", you won't see them diverge much in voltage (LiFePO4 has a very flat voltage curve for most of the SoC state sinking or going up in voltage only at the extremes). According to him and some other people, (at least this tech in particular, TS/Winston/GBS/Sinopoly/... all the same) hardly get out of balance. I will have cell monitoring always on, then I'll see what happens.

 

[dumbass]

Njay, I'm way to tied of reading anymore about "to BMS or not to BMS" and the values and/or disadvantages of bottom balancing. And I know you said you will have constant cell monitoring. But I truely hope you keep a watchful eye on your monitoring and don't assume all is well. I tried the so called bottom balancing one on a small 24v 20ah pack of Thunder Sky prismatic cells. I manually ran each cell down to 2.5v then tried to bulk charge them with a 24v 6a smart charger. I guess it would depend on your idea of what is a balanced pack. I can tell you that my lowes cell was still in the 3.5v range when the highest cells were pushing over 4v when I pulled the plug. Remember this was on a small pack of only 24v 20ah. I did not feel this was in anyway "balancing" my pack and there was no way I could trust it.

Hope you have better luck and please keep us advised of your results.

Bob