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Pack Troubleshooting - 4S4P
#1
Any troubleshooting/ideas are appreciated. 

Pack details:
4S4P 
Panasonic NCR18650B cells
fully charged and capacity tested to ~95%
low IR 
All 20 cells are the same voltage
4S 30A BMS

Testing:
Using buck converter to reduce voltage to 12.8 so I can use a 12v 150W inverter, rated for operating voltage of 11-13V
Using Kill-a-Watt on the AC side of the inverter. 
Load equals a 60W variable temp soldering iron.

Problem:
Pack will power the inverter, but the inverter shuts down when the load exceeds 20-25W/2A. The pack voltage remains in the acceptable range under load. 
It seems that this pack should have no problem powering this load at full (60W). 

What am I missing?

 
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#2
Look at the voltages going into the BMS and coming out of the BMS. Just before and after the inverter trips. That should tell you if the BMS is tripping.
Also the voltage after the buck converter. They often have a overheat protection, which may be tripping.
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
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#3
(05-19-2020, 11:55 PM)ajw22 Wrote: Look at the voltages going into the BMS and coming out of the BMS.  Just before and after the inverter trips.  That should tell you if the BMS is tripping.
Also the voltage after the buck converter.  They often have a overheat protection, which may be tripping.
Thank you. 

I tried bypassing the BMS completely and going straight to the pos & neg of the pack with the same results. 
I'll also measure the voltage out of the buck converter while under load to see what I find.
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#4
Tested again and increase the voltage coming OUT of the buck converter under load to 14V. The inverter was running better, but still cut-out with a load of only 22w and .25A.
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#5
Starting to look like a bad inverter. Can you try it out on a car battery?
Does it perhaps have a broken cooling fan?
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
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#6
Please answer the first question. What is the voltage going into the Inverter? You need to meassure the voltage during the load and when it trips. Also note that Cheap inverters cant start any high current load. Example is Laptop charger that can use 5x surge during start due to rather large caps inside. This often trips those cheap inverters.

What voltage do you have during and when it happens?
What are you trying to power?
What inverter is it?
Image of the contraption?
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
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#7
[diy Wrote:srbqpq[/diy] daromer pid='62673' dateline='1589956998']Please answer the first question. What is the voltage going into the Inverter?  You need to meassure the voltage during the load and when it trips. Also note that Cheap inverters cant start any high current load. Example is Laptop charger that can use 5x surge during start due to rather large caps inside. This often trips those cheap inverters.

What voltage do you have during and when it happens?
What are you trying to power?
What inverter is it?
Image of the contraption?

Greetings! 

Ajw - I tested the inverter with a 12v 20Ah battery pack and it was functioning correctly, so we can eliminate the inverter. 

Daromer - In regard to the "first question" you refer to - the pack voltage under 20W load is 15.42 before the BMS and 15.40 after the BMS. once the load is increased 2-3W, the pack voltage is ~15.20 and the inverter stops.

For testing purposes, I'm using a 60W variable temperature soldering iron so I can measure the load while increasing the temperature. 

Im using a cheap Bestek inverter - but have tested it up to 100W on a separate 12v 20Ah pack. 




Main pack terminal voltage



Main pack voltage after BMS


Voltage output from buck converter


Inverter input voltage


Inverter input voltage under load


Maximum watts before fail


Maximum Amp before fail


Buck output under load


Inverter outputting 70W on separate battery


I'm stumped! Anything other testing you can think of? Thanks again.
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#8
Cool! that was some serious explanation and testing! I know that i may sound grumpy some time but some people expect us to answer and guessing Smile You have done your homework and this helps alot.

Does the inverter have an indication why it stops? Like low voltage or overload?

Since you tested the inverter with another battery and it works fine we can state some things
* The inverter do take the load.

You said you did test the system bypassing the BMS and same outcome?If thats the case we can rule out the BMS. In that case we have 2 factors left and thats either

1. Battery is not able to deliver
2. The Buck converter fails and disconnects or drops to much

What type of buck converter is it? I have hard time to think it can be the battery currently and i would aim for the Buck converter with the information you have provided.
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#9
100% the buck converter. I believe it's a 3Amp LM2596 chip based buck converter, and has a chip integrated thermal shutdown. You could probably get more power by removing the heat shrink and adding a heat sink on the chip... but the proper solution would be to use a higher rated buck converter. Something like this would be better for your use case:

https://www.aliexpress.com/item/32805882664.html
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
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#10
(05-20-2020, 10:36 PM)ajw22 Wrote: 100% the buck converter.  I believe it's a 3Amp LM2596 chip based buck converter, and has a chip integrated thermal shutdown.  You could probably get more power by removing the heat shrink and adding a heat sink on the chip... but the proper solution would be to use a higher rated buck converter.  Something like this would be better for your use case:

https://www.aliexpress.com/item/32805882664.html

The original buck converter was indeed the LM2596, which has a 3A current limit. I had this https://www.amazon.com/gp/product/B081YQ...UTF8&psc=1 in the parts bin, so I gave it a go. Similar results. It would also cut-out while only outputting ~25W and less than 1A. 

So... I eliminated the buck converter from the equation. I attached a DC load to the pack to reduce the voltage to just under 15V. My cheap inverter has an input of 11-15V, so I was able to go directly from the pack to the inverter. VIOLA. I am still testing, but I have been running a continuous load of 75W from the pack without any problems. 

It is still puzzling as to why I couldn't pull more than 25W through the buck converter, as it is still below the 4A max current. 

Lesson learned - build a 3S Li-ion pack or switch to different chemistry (LiFePo4) if the primary use is to provide power to an AC inverter. 

Thank you both for your input!
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