Experiences or opinions on the Chargery BMS?
- Thread starter Dzl
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Hmm...I just realized I made an incorrect assumption.
I incorrectly assumed that using Top Balancing also implies frequent re-balancing (almost daily, automated by the BMS).
But that's not necessarily true.
You could do manual Top Balancing once and just leave it alone for a year...just like people do with Bottom Balancing.
Stay out of the knees (limit usage to 2.90 - 3.40 V) to minimize risk of over-discharge / over-charge and it should be fine for a while...
But this would work if the following conditions are met:
1. The battery capacity is big enough relative to the typical charge/discharge currents so that you can stay away from the knees and still benefit from an acceptable amount of capacity. If you charge with 100 A into a 200 Ah battery, you must let it get into the upper knee, or you will not have access to significant capacity.
2. There are no asymmetrical current draws on the cells to ruin the balance.
Charge with 0.001 C to 3.38 V until current drops to nearly zero. That WILL take a week or so...
I just measured the AH sucked in by the battery starting from 2.75 V. It added up to the label capacity + 3% losses.
I incorrectly assumed that using Top Balancing also implies frequent re-balancing (almost daily, automated by the BMS).
But that's not necessarily true.
You could do manual Top Balancing once and just leave it alone for a year...just like people do with Bottom Balancing.
Stay out of the knees (limit usage to 2.90 - 3.40 V) to minimize risk of over-discharge / over-charge and it should be fine for a while...
But this would work if the following conditions are met:
1. The battery capacity is big enough relative to the typical charge/discharge currents so that you can stay away from the knees and still benefit from an acceptable amount of capacity. If you charge with 100 A into a 200 Ah battery, you must let it get into the upper knee, or you will not have access to significant capacity.
2. There are no asymmetrical current draws on the cells to ruin the balance.
Yes, that seems correct.
Charge with 0.001 C to 3.38 V until current drops to nearly zero. That WILL take a week or so...
I just measured the AH sucked in by the battery starting from 2.75 V. It added up to the label capacity + 3% losses.
I initially thought of external power to the BMS as a safety measure against accidental over-discharge of the battery.
You are out on a two-week summer vacation. Your solar charger breaks down. Your refrigerator drains your battery down to the point of Inverter Low-Voltage Shutdown at 3.00 V / cell.
But the BMS still draws around 50 mA from the battery and the relays also pull around 50 mA (maybe even more).
100 mA * 24 hours * 5 days = 12 Ah ... a significant amount for a typical 200 Ah battery.
After 5 days, the cells would reach 2.75 V, the BMS turns off the relays, power draw drops to 50 mA (the BMS self consumption).
After another 5 days, the BMS self consumption adds up to another 6 Ah...so we have drained nearly 10 % of capacity in 10 days...danger of over-discharge.
To work around this problem, we could use a BMS with external power input, powered from the battery through the discharge relay.
So when the BMS does a low-cell voltage shutdown, it turns off EVERYTHING, including its own self consumption.
To re-boot the system, you could have a momentary switch (push button) that would temporarily bypass the discharge relay and power the BMS directly. Keep the button pressed for about 5 seconds, until the BMS boots, initializes and energizes the Discharge Relay. Now you can release the "boot-up" button because the BMS will be powered through the Discharge Relay.
The momentary "boot-up" button could be a double-pole button.
- One pole would provide power to the BMS, bypassing the Discharge Relay.
- The other pole could pre-charge the inverter capacitors via a 20 Ohm resistor. So when the BMS finishes initialization and energizes the Discharge Relay, the capacitors are already near full battery voltage.
I actually had this exact set up on my work bench and I triggered a full emergency low-voltage disconnect as a test. After the shutdown, I started poking with the multimeter to make sure there is no current drain.
That is how I discovered that in OFF state, Chargery drains an asymmetrical 5.50 mA on (8+) / 0.71 mA on (1-) cell leads.
You're right, I checked and it's 3.5 V
So replace 3.65 by 3.5 when reading my previous post.So at 3.5 V you can't overcharge it and it's plenty high to do some top balancing
That's what I tried to explain, what you already do with bottom balancing you can also do with top balancing but then can use pretty much any BMS as they also do top balancing
You're right, I checked and it's 3.5 V
So at 3.5 V you can't overcharge it and it's plenty high to do some top balancing
That's what I tried to explain, what you already do with bottom balancing you can also do with top balancing but then can use pretty much any BMS as they also do top balancing
Good thread with lots of info for folks.
Here is my takeaway for my use case:
NEVER top or bottom balance as there is some amount of risk with each. The only way to definitely remove that risk is to not do it.
The cost of that protection is to be happy only working your cells between 2.9 - 3.4v. A cost I am willing to pay. Just live between the knees! ?. My new motto!
In addition I added a BMS just for the cell level monitoring but I do let it balance during charging.
ALL of these things are tradeoffs that each individual has to weigh and use in making their own decision but for me top balancing is a risk that can be avoided at minimal cost.
That's completely incorrect.You're right, I checked and it's 3.5 V
So at 3.5 V you can't overcharge it and it's plenty high to do some top balancing
The resting voltage of a full LiFePo4 cell is a little less than 3.40 V.
3.50 V can be attained only during charging, in several ways:
1. Charging with a high current. When you reach 3.50 V, you are still below 100% SoC, but you see the effect of what you called "Ohm's law" causing an early rise in cell voltage.
2. Charging with a low current. In this case, "Ohm's law" has very little effect, the voltage does not raise significantly during charging. When you reach 3.50 V, you have overcooked your battery.
Here's another article confirming that you do not even need to exceed 3.40 V to get very, very close to 100%.
Charge voltage experiments with lithium iron phosphate batteries showing how capacity varies with charge voltage and higher cycle live with lower charge voltage
Engineering resources for designing equipment using lithium iron phosphate batteries from PowerStream
www.powerstream.com
I got even closer to 100% than him because:
- he charged with 1.6 Amps into a 2.2 Ah cell = 0.72 C-rate
- he terminated the charge when the current dropped below 30 mA on a 2.2 Ah cell = 0.013 C-rate.
- I charged with 20 Amps into a 200 Ah cell = 0.1 C-rate
- I waited much longer, several days, and terminated the charge when the current dropped below 20 mA on a 200 Ah cell = 0.0001 C-rate
That's what I tried to explain, what you already do with bottom balancing you can also do with top balancing but then can use pretty much any BMS as they also do top balancing
You need a good BMS that does not have asymmetrical current draw, not even 1 mA, because that would just ruin your balancing within a year.
And 95% of the people using Top Balancing and a BMS have their system set up to charge up to the upper knee very often, just so the BMS can do the balancing, when in fact no balancing would be needed.
Charging frequently into the upper knee with an unpredictable current source such as solar, will occasionally overcharge the battery, shortening its life.
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Ok, I'll stop arguing on the 100 % SoC part, we're not going anywhere.
No because if you can then use a BMS who does some balancing (even if not much because you want to stay at 3.4 V or below) you don't have this problem and as a side benefit you also keep your pack balanced without the need to do it manually every year or so.
That's why you don't connect panels directly to the battery but have a SCC (and a BMS as a backup). But if you prefer to do all that manually and have to babysit your battery it's fine too, just not my cup of tea...
No because if you can then use a BMS who does some balancing (even if not much because you want to stay at 3.4 V or below) you don't have this problem and as a side benefit you also keep your pack balanced without the need to do it manually every year or so.
That's why you don't connect panels directly to the battery but have a SCC (and a BMS as a backup). But if you prefer to do all that manually and have to babysit your battery it's fine too, just not my cup of tea...
If my SCC breaks down I'm screwed no matter what lol thats why I have 3.
I can see reasons to run without a BMS however I still think your chances of ruining the battery without a BMS are far far greater than having one.
Lastly I would like someone to reproduce your 5.50mA draw in the off state I think there is something else going on there but I dont have the capability of testing that. To me off is off the screen is off and the boxes all stay cold so I dont know what it would be doing with the energy you think it is using.
You need a BMS to clean up the mess that the BMS creates...not my cup of tea.
Maybe we are exaggerating with this need to balance, balance, balance, day after day, and the cells would stay in balance by themselves if left alone and not abused. Then the BMS is just another weak link, something that can break and ruin your vacation...
I totally agree that some cell-level voltage protection would be nice to have, but I want one meeting these requirements:
- does not cause imbalance
- can be set up to self-disconnect in the event of under-voltage
The main risk I see is how well the cells will stay balanced.
If they stay balanced, then battery voltage is good enough to control charging and discharging. So I just started this multi-year experiment, planning to do one deep-discharge test every 6 months, to learn more about how well they stay in balance. I am willing to share the results here, if anyone is interested.
I started with Bottom Balance because I needed a 0% SoC reference for my charging tests.
I think Top Balance would probably work equally well in practice, just not "frequent automated Top Balance", due to risk of over-charge.
I am an experienced electrical engineer.
Could there have been something wrong with my setup? Very, very unlikely...
Could there have been something wrong with my BMS? Yes, I accept this possibility, maybe I had a broken one. I bought it from a chinese seller on e-bay (icgogogo).
apctjb
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Informative thread; thanks to all contributing. I am new to LiFePO4 batteries and have been pondering the questions of should I top balance or not, and is a BMS an added safety device I really need or added point of failure? It would be great to have definitive answers but suspect it all depends. As the cost of LiFePO4 batteries continues to come down the cost of adding capacity to keep "between the knees" does not seem that high.
So just to clarify. I had a BMS 8T.
- the Power Supply slide switch was in the EXTERNAL position
- I unplugged the jack providing external power
I clearly remember that when I unplugged the connector of the 2 black thermistors, the current draw on (8+) dropped by a lot, possibly around 1 mA...unfortunately I did not write this value down in my notebook.
mrdavvv
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Hello @MondeoMan
I too realized that sometimes its difficult to achieve higher voltages with solar charging as the current its not always constant. In charging at 28.2V @60A (280AH bank)... and in the charging process the voltage starts increasing as expected... but a cloud passes and then current goes minimal... starting the cycle again. I think this could repeat a number of times before the voltage gets high enough for the SCC to stop the charge process, with the risk of overcharging the battery.
When i charge with AC the voltages goes steadily up and nice, with the SCC terminating at the propper time.
So with this in mind, what voltage do you recommend for charging and float?.
--
In my particular case, i also avoided top balancing, but after some cycles i discovered that my BMS was cutting before the LVD fromt he inverter (23.5V). Teorically im outside the knees and should be safer, but in practice one of the cells was very unbalanced and needed additional charging. Sometimes top balancing its needed even in new cells
You don't get it: you want a BMS to manage your battery instead of doing it manually (and as a human you'll make a mistake on day or another and ruin your battery, as I would do or anyone else).
The more the battery is cycled the more the weak cells gets weaker so the differences get bigger with time. I would not trust a battery without balancing on the long term.
Then don't use fuses either, they are weak links too... ?
Very nice, results would be much appreciated
Please quote one member who had this problem here; I never saw one even if I don't really trust chinese BMS either.
Would you risk an expensive battery because you didn't check a voltage in time? or just wasn't here when something happened?
That's very interesting. Maybe they just use a divider network for the temp measurement and the resistor values are very low?
I'm sorry to disappoint you, but there is no standard answer. It all depends on your typical charge currents, battery capacity, personal preferences.
Best I can tell you is to watch your coulomb counter and adjust your setup based on what you measure.
I cannot give you a voltage guideline, because voltages by themselves are not harmful, not even up to the mythical 3.65V.
But I can give you a SoC guideline: between 10% and 90% SoC.
Just keep in mind that allowing the cells to high voltages above 3.40 V makes it easy to overcharge if you have low charging currents.
Now the questions becomes: What is 0% SOC? What is 100% SOC? You need to establish one of them as your reference and measure from there.
As I said before, the manufacturers typically define like this:
- to get to 0% SOC, discharge with a constant current of 0.5 C and cutoff at 2.50 V
- to get to 100% SOC, charge with 0.5 C up to 3.65 V and maintain 3.65 V until the current drops below 0.05 C.
- If you use a lower current to charge, you will have to stop below 3.65 V.
- If you use a lower current to discharge, you will have to stop above 2.50 V.
I have discharged with around 0.2 C down to 2.50 V and I noticed that the voltage re-bounded up to 2.80V.
So I have chosen 0% SOC to be at around 2.75 V resting voltage and I balanced all my cells to this voltage.
One more thing I want to clarify.
Coulomb counting starting from a known reference is good only for a limited number of cycles, let's say 5 cycles.
After more cycles than that, errors begin to accumulate and the readings will be wildly inaccurate.
But it still is the best method to get an idea of where you're heading with LiFePo4. You're at 3.40 V...did you push 170 Ah or 200 Ah into a 200 Ah battery? Voltage by itself will not tell you the answer, because the voltage also depends on the charging current. So this is where the Ah counter helps.
After the initial system tune-up, the Coulomb counter should be programmed to reset to a known reference point on every cycle. Typically this is the 100% SOC point.
For example, in the Victron BMV-712 battery monitor, I can program the "Charged voltage" and the "Tail Current".
When the battery voltage is above the Charged Voltage and the current is below the Tail Current, for a specified amount of time, then the counter resets to 100% SOC.
During initial testing, I had Charged voltage set to 29 V, an unrealistically high value, so that the counter would just track the battery Ah.
I charge with low current (0.1 C-rate) and I tested that starting from 22V resting voltage, I can push 200Ah + losses into the battery before reaching 27.20 V. I have Absorption set at 27.10 V. So I set my Charged Voltage to 27 V and Tail Current to 0.05 C.
The battery monitor now resets to 100% on every charge cycle, some time during Absorption when the current falls below 0.05 C-rate.
Coulomb counting starting from a known reference is good only for a limited number of cycles, let's say 5 cycles.
After more cycles than that, errors begin to accumulate and the readings will be wildly inaccurate.
But it still is the best method to get an idea of where you're heading with LiFePo4. You're at 3.40 V...did you push 170 Ah or 200 Ah into a 200 Ah battery? Voltage by itself will not tell you the answer, because the voltage also depends on the charging current. So this is where the Ah counter helps.
After the initial system tune-up, the Coulomb counter should be programmed to reset to a known reference point on every cycle. Typically this is the 100% SOC point.
For example, in the Victron BMV-712 battery monitor, I can program the "Charged voltage" and the "Tail Current".
When the battery voltage is above the Charged Voltage and the current is below the Tail Current, for a specified amount of time, then the counter resets to 100% SOC.
During initial testing, I had Charged voltage set to 29 V, an unrealistically high value, so that the counter would just track the battery Ah.
I charge with low current (0.1 C-rate) and I tested that starting from 22V resting voltage, I can push 200Ah + losses into the battery before reaching 27.20 V. I have Absorption set at 27.10 V. So I set my Charged Voltage to 27 V and Tail Current to 0.05 C.
The battery monitor now resets to 100% on every charge cycle, some time during Absorption when the current falls below 0.05 C-rate.
apctjb
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Thanks for post on Tiny BMS; I looked at the specs but somewhat confused on the current ratings. So could I use a TinyBMS with a 48V bank, 6kW PV array and 6kw inverter (Yes it the rating max charge 750A but clearly no if sustained charge rating is 30A). Can anyone explain?
I believe the two ratings are due to the fact that the TinyBMS can function as a FET based BMS (in which case the lower rating applies), or use external relays like the chargery BMS in which case the higher current rating applies.
mrdavvv
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Good answer!, just want to clarify, there is any orientation about what could be considered "low charging current"?, maybe /1C?, .01C?
Not sure if i get this... your guideline about "real" voltage its with the battery specifically underload and its relation with resting voltage at the same SOC?
About the coulumb counters... really interesting information.
While i was installing mine, i was thinking that after some time they need to start loosing accuracy... and now you confirmed it!, bet didnt expect to be as low as 5 cycles.... so after 20 - 30 cyles i cannot lunger trust the calculated capacity?
Problem mine doesn't have any algorithm to detect charge termination and reset itself (right @Gazoo ??).
I think for this kinds of battery monitors we are talking +200USD?
I guess I should have been following this thread too. Lots of good info. here. I thought after going through a few cycles the capacity would be accurate. I only use mine once a month. I will take a closer look at it later.
However I don't think the capacity reading on the meter has any effect on the termination settings. In other words as I recall, if you set the voltage termination to 12 volts using a relay to disconnect, the unit will disconnect at 12 volts no matter what the capacity reading is. Same with the other settings. Also the coulomb readings will remain accurate. It's just the capacity reading that's a problem with these. But yes I get your point and I don't know of any way to program the meter to reset itself like the Victron. The one I had with the hall effect sensor did the same thing and I had to reset it often. But I still like these meters.
Other than the capacity thing, how do you like the meter? Did you have any trouble connecting the DC cables? I am still planning to order the one you did. If you don't want the second unit you received I would be happy to purchase it from you.
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