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Overkill / jbd PCB limiting SOC

I

idontknowwhatimdoing

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Jul 6, 2020
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Hey all,
Been running this BMS 4s 12v for over a year. It runs like a dream. My battery is the 280AH. I just followed recommended settings at the time I believe.

I want to make this setup last as long as possible and I'm not sure if I'm taking this battery to 100% all the time. I want to limit it to 80 or 90% SOC so I get lots of cycles. Also, sometimes it sits full SOC for a week or so when I'm away and it's connected to mains power.

I have following settings:
Total battery Capacity 280000mv
Total Cycle Capacity 224000mv
cell over voltage trigger at 3600mv
Cell under voltage trigger at 3200mv

What should I change to ensure I'm max charging say 80 or 90 %.

Could I change the battery capacity to 224000mv, then do a discharge cycle and recharge. Then BMS will disconnect when the shunt counts to "100%" SOC?
Then when I do a big trip (vehicle mounted setup) and want to charge it up more, could I then change the capacity back up again?

I'm guessing the depletion SOC can only be controlled by cells undervolting?

Or is there another way or am I off track?

Thanks for reading!
 
I believe it is best to limit charging by reducing the voltage of the charging system. Likewise don't discharge to the bottom.

I don't think the BMS should be used as a charge controller. BMS primary function is to protect the battery.

I would keep your capacity correct so the phone application will display the charge level correctly.

Is this in an RV? Post the converter or inverter/charger model for some recommendations on limiting charging voltage and avoiding float charge at 100% while you are away.
 
Oh really, wow.
Yeah it's a 4 wheel drive, so same principle as RV.
It's an enerdrive DC2DC charger.

Bulk 14.5
Float 13.6

I'm surprised that it is as simple as changing these voltages! I guess to me it wasn't intuitive because .1 volt change is a huge SOC difference.

Would be interested to know what the recommended settings are :)

** Thinking about it, I have been pretty cruel to my battery with those figures ?
 
I would start with going to the minimum at 13.9 and 13.5 volts shown on the charging profile for LFP.
Monitor the app and see where the charge level floats out. Some of this might be trial and error and may need to be adjusted to match your usage.
Possibly move the L setting to the high set point to drop out charging sooner. I am not sure how the vehicle intermittent voltage supply level might affect the charge algorithm.

https://enerdrive.com.au/wp-content/uploads/2018/12/DC2DC-PLUS-User-Manual-WEB-1.0.pdf

Is there a different system charging when on mains power?
 
bulk 14.0
float 13.5

these are the settings i use for now on 4S frey/fortune 100Ah DIY pack with goal of long cycle life, solar charging. just sharing for reference. not to say it’s appropriate for your needs☮️
 
Unreal thanks crew.
So I'll see the battery not reach max SOC on the shunt? I'll try the lowest numbers and see where it goes up to.

I have the L setting at max, 8Amps.
Just discharging the battery now, at 20% so hoping the BMS catches it soon.

I actually have a couple of server power supplies on the solar input, in series. That way I can charge at max amps, tricking the charger into thinking they are solar panels. I have a transfer switch to an Input anderson that disconnects them so I don't back feed the power supply. This works GREAT. Benefits are that I only have to deal with one charger.
 
Keep in mind that there is a voltage setting on the BMS where it kicks in balancing. If your charger never gets to that point then balancing never happens.
 
So the BMS shut the battery off at %20 SOC (according to the shunt (which is programmed for 280AH)

Not sure if the cell undervoltage (3200mv) or pack undervoltage (12500mv) triggered the BMS to disconnect.

Anyway I left the charger to do its thing with bulk at 13.9 and float at 13.5. The pack is at 13.58 and the shunt is reading 100%SOC (so 224 AH have gone in to the battery over the charge)

So, I'm a little confused I kind of expected the SOC to be a bit lower.

My main concern is that the battery actually is at 100% SOC and I am reducing its life. Or is it that the battery capacity is just nice and healthy and it's capacity is a bit higher (300AH or so)

Thanks crew!
 
I'm running a jbd and enerdrive in the back of my ute too.

Toying with a similar idea too but 10%-90%.

I think your settings are very conservative to achieve what you want.

Are you just trying to dial in the SOC now to reflect your new settings?
 
3.4V per cell will achieve a full charge, it will just do it slower than 3.65V, and might require an absorb stage (holding 3.4V for a while before switching to the lower float voltage.)

So if you want to not achieve a full charge, you should set the bulk voltage slightly below 3.4V per cell. Experiment to find what works for you. It will not be accurate, but it works (I do it also) One day it might charge to 80%, and another 90%, but it will always be a fairly high state of charge, and never get to 100%

It is important to note 2 things. One is balancing, that I think most people realize. The second is that the SOC reading in the BMS relies on achieving 100% once in a while to calibrate and reset. After a week of not reaching 100%, the reading might be off by as much as 10%. So, you need to either live with not having an accurate SOC indication, or occasionally charge to 100%. I charge to 100% about every 2 weeks, just to reset the SOC, and to check balance.
 
wholybee said:
3.4V per cell will achieve a full charge, it will just do it slower than 3.65V, and might require an absorb stage (holding 3.4V for a while before switching to the lower float voltage.)

So if you want to not achieve a full charge, you should set the bulk voltage slightly below 3.4V per cell. Experiment to find what works for you. It will not be accurate, but it works (I do it also) One day it might charge to 80%, and another 90%, but it will always be a fairly high state of charge, and never get to 100%

It is important to note 2 things. One is balancing, that I think most people realize. The second is that the SOC reading in the BMS relies on achieving 100% once in a while to calibrate and reset. After a week of not reaching 100%, the reading might be off by as much as 10%. So, you need to either live with not having an accurate SOC indication, or occasionally charge to 100%. I charge to 100% about every 2 weeks, just to reset the SOC, and to check balance.
Click to expand...
at 3.4v will take ages to reach 90%, and will NOT be fully charged.
that's 3.65.
faster charge can be done by using 4.2v
lifepo4 can ne safely overcharged to 4.2v
-warning!! NOT keep at 4.2v, it CAN reach up to 4.2 without damage, and charging should stop directly.
if you place a 4.2v charged cell aside, a few hours later it's 3.9, and half a day 3.6v.

At 3.4v the charge capacity is really low above 3.3v, and stopping charge after several hours will give 70-80% SOC
If you have enough solar to recharge your sells to 70% before noon, sure, this will give you (a lot) more cycles.
If you havent hit the 3.36v before noon, you won't reach the 80% if the charge speed stay the same.
basically it will take 4 to 5 hours at 3.4 to get from 60 to 90%
I assume there is enough solar power available for at least C0.25 charge (from empty to full in 4 hours)
you can (temporary) set the maximal voltage to 4.2 per cell to get maximal charging capacity from the solar panels.
going from +3.5 to 3.65 at 3.65v charge already takes several hours,
at +3.5 you have about 60-85% SOC (voltage hangs, till its enough saturated and then "quickly" rises to 3.65 where usually the BMS stops the charge cycle, burns off a few milivolts, and starts the cycle again, till it reach 3.65 again.

if you ever have watched this, you will notice that the first few times it takes up to 10-15 minutes to reach 3.65 again.
after a few bumps this time frame is a lot less. or in other words, more saturated / more charged.

the last few percentage (10-15%) is safer not to use, less risk for overcharge and more total cycles.
it doesn't really hurt to use, except you don't get the bonus.
knowing your SOC will be less accurate.
from time to time top balance (all the cells reaching several times 3.65) will make it accurate again.
stopping when the first cell hits 3.65 and the others are "close" isn't a top balance and doesn't "reset" the SOC counter.
 
Interesting stuff, thanks for the replies.
I'm trying to work out the simplest way to run the setup for the best battery life in the long run.
I often park my personal car at work and leave it plugged in for 7 days. Then I head out bush on days off exploring, but when I head off exploring I want to start with 100%, to save getting out the solar panels.

So I can sit at 100% SOC for a week. I'm trying to figure out the best way to maintain 80% for those days. I am often unable to get back to the car to turn it off and let it drain a bit. Etc. My work is a bit unpredictable.

I'm happy to do the odd top balance. From what I have read that's good to do anyway.

The other tricky thing with the enerdrive is that those charge parameters at the bottom end and somehow the shunt shows 100% after 12hours charging. So I'm not sure how I can charge at lower voltages if it wont let me...

It's nice to have the SOC on the shunt pretty accurate, but I don't really push the limits on the capacity too much anyway. The lowest I have had it to while out bush is about 30%. However, I will be adding some higher loads in the coming weeks, so want to be close-ish. But the main priority is I don't kill the battery, which the BMS should do its job anyway ?. My undervoltage settings are pretty conservative at 3200mv/cell I think .

The other thing, the SOC reading in the BMS seems pretty arbitrary/irrelevant anyway isn't it? Especially if I have the separate shunt... That should be dead accurate yeah? I kind of wish I had one of those fancy relay shunts that shut off the battery charge/discharge at different SOC parameters you set.
 
Beg to differ. A fully charged (resting voltage) LFP is 3.4V. Key word, is resting. You charge to 3.65V, and it is fully charged, but you let it rest, and the voltage will settle to 3.4V. It is still fully charged, even at that lower voltage.

If you charge to 3.4V and immediately stop charging, the voltage will settle at lower then 3.4V, and the battery will _not_ be fully charged. However, if you *hold* 3.4V until current drops to nearly 0, then when you disconnect, the voltage will not settle to a lower voltage, and it will be fully charged. Yes, it will take a long time, but it will fully charge the LFP eventually. If you hold 3.4V indefinitely, you can even slightly overcharge at 3.4V. This is why it is so important that float voltage be set to 13.5V or less. 13.6V (3.4V per cell) would slowly overcharge the battery. But if the battery has settled to 3.4V per cell the float charge of 13.5 (3.375V per cell) is sufficiently less that charging will stop.

I bring this up, because the original post was asking about how to charge to less than 100% by adjusting the bulk voltage. He set bulk at 13.9V which is 3.475V per cell, and was surprised that it still charged to 100%. My statement that he needs to charge to something less than 3.4V holds true. 3.38V or so would be a good starting point for him to experiment with. Depending on his charger, if it has a timed absob stage, etc, that could get him close to 90%

The SOC in the BMS is not arbitrary, but it does take a few cycles to calibrate. Anytime you adjust something in the BMS, it needs to recalibrate for another few cycles. So eventually it will settle in. A good shunt like a Victron will be better _if_ you set up up correctly, but could be worse if you don't. Neither will ever be dead on accurate. That is why you need to charge to 100% once in a while for them to resync.
 
@wholybee
"Long time" and solar usually don't match... Here in Thailand the real charge is between 09.30 and 15.30.
Before and after is just about enough to have the daily usage.

@ 3.4v you will get eventually things saturated but it takes days for solar, without really using the system, to get it saturated enough that it won't drop below 3.3 when you start using the cells.
(+/- 70-80% SOC)

Higher up north there is longer light, and longer low charge capacity.
That might just tip the balance.
Not something that would work in Florida :) but Canada, probably ...

Safe to say we are both right, and depending on how long a person have time to charge without use, 3.4v will get you eventually really close to 95-99% SOC.

@idontknowwhatimdoing ,
Yes, external shunt CAN give you accurate readings.
There are high accuracy shunts, those are pricey.
Quality EV should be equipped with one.
A standard China AMmeter won't be able to use it....

Speaking of those, expect 0.25 to 2% offset per cycle.
A more heavy shunt of 400A doesn't register accurate 10A draw.
where a 200A shunt won't be accurate if you draw 400A via it.
(Last one would be pushing it, probably the back paint smell funny, but won't melt like a fuse and does register, less accurate)

Ideally you have steady loads / charges and matching shunt to measure.

Depending on the BMS, reading can be accurate.
I've heard positive response about electrodacus installations, and I'd I recall correctly he does provide a shunt.

Bigger names like Batrium BMS will have accurate shunts available, calibrated with their equipment.
 
I just have the Aili shunt, it is reading the exact same SOC as the BMS.

I just noticed there is a custom setting for the enerdrive charger.

Pg. 11

Would that be appropriate to drop it to 13.8 absorption and float 13.4. any chance that will stop it charging over a week?

I'm surprised there isn't a simpler way of doing this to maintain loads while maintaining and healthy SOC for longevity. Changing the charge profiles all the time is a bit of a hassle ?
 
You are correct, it is a hassle.

Some companies choose standard lower voltages for longer life, and sell at lower Ah rating. (Use 280Ah and sell as 250Ah)
The cells can be really out of balance and no one would know.
(Battleborn?)

Otherwise you need to switch from time to time to top Balance.
Only when you have fully top balanced, you know they are at max, and can be used as reference.
(Or all at 2.5v)

Always full isn't a bad thing.
It doesn't reduce the cycle life, you just don't get the bonus.
Many people are happy with the +2000 cycles.

For SOC, how accurate do you need? For electric vehicle.. it's really important to know more exact. For day to day life..
Indication is enough.
Calibrate once every 3 to 6 months is enough to know if you can or better can not use the electric oven that day from the battery.
For a car, 5 or 50 km left on 500km charge is a huge difference.

Diybms is Arduino based and open source.

To my knowledge there aren't BMS available yet that you can program to cycle between 85-90% SOC for 14 days (or 1/2/3/6 months) and go to 100% SOC for a week or whatever, depending on how you like.
Or better, go to the 85-90% profile when all the cells have reached 3.6v twice.

The ESP32 WROOM is 4 to 16Mb,(part of diybms that gives Wifi and BT) more then enough space to program this " fancy" feature.

I think most BMS companies don't want to burn their hands on this, as they probably get tonnes of complaints "why does my battery never get fully charged??"

Heathy condition is without a problem the 100% SOC.
It doesn't overcharge.
And 2000 cycles is a lot
If you don't use daily basis down to 20-40%, just a few %, it's hardly countable as a cycle.
And will last 8-10 years.
If you want / need to hit the 15 years (up to 5000 cycles)...

Yes, for now it's a hassle.
You don't need to.
+2000 already is a lot.

When my system is up and running (when ever that will be, I hope this year!!) I probably will try to study enough Arduino to make this script/ profile option.

As things look now, I'm already happy if the setup reached 1500 cycles!!
800Ah@48v Lead acid died within a year.. too hot.
Now the cells are in airconditioning room, that does struggle with the heat we are facing now.
25c setting gets it down to 30, with outside peaks of 44c (yesterday 13.00)
For those who say it's too light..
Yes, on purpose, as it's the most energy efficient for year round.
Pre-cooling (water vapour) the outside units does help.
 
There is a lot of evidence that holding the cells continually in a high state of charge increases the ageing effect.
Thus if you have an application, like a RV, that spends some/much of the time inactive, having a charge regime for the inactive period that results in the battery being in a lower state of charge, should be beneficial. There may be small loads on the battery, for example security systems, so even in the inactive period some charging may be needed from AC or solar.

Tests show that charging a cell at 0.2C to 3.4 volts results in the cell reaching about 98% capacity, and with some absorption time, almost 100%.

The comments made by wholybee are correct, if you want the battery to charge to less than full, the charge voltages need to be lower than 13.4 volts for a 12v battery.

Since fractions of a volt can have a huge effect, ( equipment calibration), the actual settings will need to be tested experimentally.
In my RV installation the settings on a Victron charger for partial state of charge are 13.20 volts charge, 13.15 float, 13.10 re boost. This results in a state of charge around 60% capacity.
Its known that 'mini partial charge' cycles can introduce a memory effect, so a full charge is needed from time to time. The battery is not under heavy charge and discharge conditions during the inactive period so cell balancing should not be an issue.

Mike
 
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