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Question about diy 4S5P battery (not working normally )
#1
Hello everyone, i m new as a writer on this forum, 
I m starting a small storage battery for outdoor lights powered by 2 solar panels ( 2x250W ) and with 5-7 small led projector (12V and about 7W each )

My goal is to have outdoor light when it is dark ( most in winter ) during 2 or 3 hours, i will add a présence detector ( detect movement ) after that.

So i decided to do 4S5P battery's because : first the 14.4 V is near the 12v i need for my lights and with a DC-DC adjustable regulator i can habe the good voltage.
it allow me to place them in small boxes protected from rain, dust etc, easily....


So my problem is : 

I ve done my first battery pack, i use 20 18650 tested cells between 1800 and 2100maH 

I ve done my pack, i try a charge of the battery with the imaxB6 mini, and i have my 16.4 v ( from memory ) when charged  ( i have a slow voltage loss when charge stop, but it stay to more than 16 if i don't touch the battery )

I try a discharge test ( to have the capacity of the battery )  and the imax stopped it at 2112mah.

Is that normal ? 
If i m right, i should have about 7000 - 10000 mah ( it depend capacity of battery ) no ???
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#2
Try an old fashioned discharge test, using a known load (a few lamps ?) and a timer - a little math will show what the capacity is.

An ammeter in series with the load will improve the accuracy.
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#3
5 * 1800 = 9000. So yeah, your math comes out right.

Did you double check all your connections?
A slow drop after charging is normal, but it should be low and then stabilize relatively shortly after disconnecting the charger. I think around 50mV drop per parallel section (so for 7s, that'd be about 350mV for the whole string). But shouldn't drop below that. If it does, then there's a self discharger amongst the packs somewhere.

For the iMax to stop, that seems more like a disconnect issue, or maybe it needs the balance wires hooked up(?)
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
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#4
i ve done a small test and a 10w light is on since more 4 hours... so if i m right 10w light at 14V drain 0.7Ah and for now this is a capacity superior at 0.7*x4 = 2.8Ah,

i will let the light on for testing the "real" capacity...

so i think the imax b6 discharge test maybe end by timeout or wrong connection between B6 clips and my battery wires.

------------

But it is not ended Smile as i am here, i will ask another question, i ve put this cheap BMS for under/over voltage protection and cell balancing.
https://www.ebay.fr/itm/4S-14-8V-30A-BMS...2709387736

My question is maybe stupid but, i assembled the battery and my cells was charged ( thinking about this, it is maybe a dangerous idea, shold discharge them before next time )


did the BMS will balance the cells at the next charge ? or will it balance the cell now, i need to wait ?
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#5
Remember, voltage drops as power is consumed. So if it started off at 15V, after 2hrs, you could be around 13V. You can get an "average" reading sure. But the battery might "appear" to have higher capacity than what it really does.
If you do the test this way, you need to take voltage measurements every 15 mins or so. The more measurements, the more accurate. Because you need to also find out when the battery does a rapid voltage drop. This is your cutoff point.
For example, if the battery drops .1V every 15mins, then after 3hrs it goes to .3V, that's a huge drop and the test is terminated. Then take the average voltage at that time and calculate the capacity.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
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#6
I ve good new about my new battery, i see this evening that the bms was hot, everytime, even when i don't use the battery, (maybe deffective/wrong quality , for 3€ i can't be hungy)

So i ve done a BMS replacement , i ve bought 3 of them because of the small price,
With the new BMS , i don't have this heating on the PCB.

I ve done again the discharge test with my B6 MINI and now it is during since 800minutes ( all the night )
i m at 5100mah, and voltage down to 14.55V for the moment. The test is not finished yet, so i believe i have about the half of the battery left at this voltage

it is very long because the discharger can only discharge at 5Wh but i will let it a complete day if needed, well it is really great to make his first battery working.

------

About cell's voltage, on my 4 cell string, i have :

3.60v 3.65v 3.64v 3.65v

Is that correct for you ? or it is too unbalanced ?
In my mind that is a poor difference and it's ok for a small outdoor garden's light storage system ( not need to be 0.001V of precision for the balance ), but i prefer to have advice of experimented people
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#7
(01-18-2019, 08:35 AM)vivi0512 Wrote: About cell's voltage, on my 4 cell string, i have :

3.60v 3.65v 3.64v 3.65v  

Is that correct for you ? or it is too unbalanced ?
In my mind that is a poor difference and it's ok for a small outdoor garden's light storage system ( not need to be 0.001V of precision for the balance ), but i prefer to have advice of experimented people

Its unbalanced, have you accurately measured the cell voltages immediately after completing a balance charge with the iMax (without having the BMS connected) ?

You might want to confirm the iMax charge termination voltage isn't lower than the point at which the BMS will start balancing - if you dont, the battery will likely never be fully balanced, and it will possibly never be fully charged.
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#8
(01-18-2019, 11:12 AM)Sean Wrote:
(01-18-2019, 08:35 AM)vivi0512 Wrote: About cell's voltage, on my 4 cell string, i have :

3.60v 3.65v 3.64v 3.65v  

Is that correct for you ? or it is too unbalanced ?
In my mind that is a poor difference and it's ok for a small outdoor garden's light storage system ( not need to be 0.001V of precision for the balance ), but i prefer to have advice of experimented people

Its unbalanced, have you accurately measured the cell voltages immediately after completing a balance charge with the iMax (without having the BMS connected) ?

You might want to confirm the iMax charge termination voltage isn't lower than the point at which the BMS will start balancing - if you dont, the battery will likely never be fully balanced, and it will possibly never be fully charged.

I would suggest it is only slightly unbalanced, 50mV difference max to min is not much in the scheme of things..... not enough to worry about.

You are right, the BMS should not be hot unless it is balancing cells, eg you have charger on and some cells are high.
Maybe the voltage settings in the 1st BMS (hot one) were wrong?
What voltage does the BMS say is the balance voltage? eg 4.2V?
Running off solar, DIY & electronics fan :-)
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#9
i only have these specs from my chinese BMS :

Working current: 30A
Balanced current: 60mA
Over current: 60A
Temperature range: -30 - +80℃
Over charge detection voltage: 4.28+ 0.05 V
Over charging voltage: 4.095-4.195V
Over discharge voltage: 2.55±0.08
Over charge detection delay time: 0.1s
Short circuit detection delay time: 100ms
Over current detection delay time: 500 ms

so i m afraid to not be able to know the balance start voltage
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#10
(01-18-2019, 08:35 AM)vivi0512 Wrote: [...]

About cell's voltage, on my 4 cell string, i have :

3.60v 3.65v 3.64v 3.65v  

Is that correct for you ? or it is too unbalanced ?

[...]

If you bottom balanced them, then they are EXTREMELY well balanced.  About average for a top balanced system. 

My gut feeling is that as you are using a cheap BMS, then this is probably what keeps battery in balance too, which means it will be top balancing.  this means (amongst other things) that they will likely only be 'perfectly' balanced when they get to ~ 4.2V / cell.

NB they will only ever be 'perfectly' balanced when you balance them.  After that, it depends almost entirely on how well matched the capacity of each of the four groups of cells you are using.

With such a TINY battery, I would say that what you see is very normal, and consider it a success.

I have several 72p based batteries, and they all stay within about 10mV ... as they are very well matched packs, and much bigger than the loads need, so IR doesn't really matter.  occassionally I take them offline to rebalance them at 3.92V, but mostly they just chug along.

FWIW, your setup does however strike me as a bit 'out of balance' with the rating of your PV and the capacity of the battery -- (500W seems like a heck of a lot of solar for a battery of only 20 cells) -- but thats another rabbit hole for another time perhaps..
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