Pack Troubleshooting - 4S4P

DeeCee

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May 19, 2020
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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 a12v 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?


image_rkjant.jpg
 

ajw22

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Nov 16, 2018
Messages
506
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.
 

DeeCee

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ajw22 said:
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.
 

DeeCee

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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.
 

daromer

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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?
 

DeeCee

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[ATTACH said:
16660[/ATTACH]
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 measurethe load while increasing the temperature.

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


image_tjpeht.jpg



Main pack terminal voltage

image_advjpv.jpg



Main pack voltage after BMS

image_vauwfc.jpg


Voltage output from buck converter

image_dnrvgs.jpg


Inverter input voltage

image_gzkhmg.jpg


Inverter input voltage under load

image_gzgomk.jpg


Maximum watts before fail

image_fctzsj.jpg


Maximum Amp before fail

image_lcvfxk.jpg


Buck output under load

image_ymojcr.jpg


Inverter outputting 70W on separate battery

image_ocdovh.jpg


I'm stumped! Anything other testing you can think of? Thanks again.
 

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daromer

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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 :) 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.
 

ajw22

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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
 

DeeCee

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ajw22 said:
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 thishttps://www.amazon.com/gp/product/B081YQQVHC/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=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 stilltesting, but I have been running a continuousload 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-ionpack or switch to different chemistry (LiFePo4)if the primary use is to provide power to an AC inverter.

Thank you both for your input!
 

jonyjoe505

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I seen those buck converters stress tested on youtube, none could do the rated amp output, and at lower amperage they got too hot. I also use those small buck converter but only to power small 12 volt fans/lights that are less then 1 amp.

Your idea of building a 3s pack is a good one, I been using a 3s16p 31 ah battery to power my 200 watt inverter for years, even when I drain the battery all the way down the bms will shutoff before the inverter does. My 3s li-ion will run everything my 4s lifepo4 does.
 

daromer

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25w is more than 1A. Thats like 25/14 = 1,78A on the output. Then add losses and you are way above 2A.
You can meassure current with your voltmetera as long as you stay below 10A (I think you have it on that multimeter?)
 

ajw22

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Whenever dealing with dirt cheap Chinese devices, best to assume:
"Max/peak" performance values -> pure fantasy
"Rated" values -> may overheat and burn out if used too long at that rate
30%~70% of "rated" values -> actually safely usable

It's not like US/Jap/German made brand name devices that often over-deliver on what they promise.
China _can_ and do produce quality devices too, but they're not that cheap.
 

DeeCee

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daromer said:
25w is more than 1A. Thats like 25/14 = 1,78W on the output. Then add losses and you are way above 2A.
You can meassure current with your voltmetera as long as you stay below 10A (I think you have it on that multimeter?)
Thank you!


jonyjoe505 said:
I seen those buck converters stress tested on youtube, none could do the rated amp output, and at lower amperage they got too hot. I also use those small buck converter but only to power small 12 volt fans/lights that are less then 1 amp.

Your idea of building a 3s pack is a good one, I been using a 3s16p 31 ah battery to power my 200 watt inverter for years, even when I drain the battery all the way down the bms will shutoff before the inverter does. My 3s li-ion will run everything my 4s lifepo4 does.

Thanks for the input.

I suppose it's all about what the intended use case is for the pack. This was my first pack using 18650's and I used the ******* caps instead of soldering so I can re-configure as needed.
If an AC inverter is the targeted use case, then selecting the cell chemistry to end up with a working voltage range that suits the inverter is key. I'm glad to hear you are having good success with your pack!
 
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