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BMS/Inverter turns off after pushing 10A

sonny93

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Joined
Dec 17, 2020
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119
4S, 12V 272aH
Individual cells 3.23V LifePo4
Daly BMS 4S 12V 100A
powertech 12VDc pure sine inverter

Set it all up last night finally and applied some load. I can charge my battery fine, but when I plug in my laptop charger, the multimeter climbs to roughly 10A and the power turns off at the inverter.

what is causing this? I also just checked the app and the soc is now reading 0% whereas before it was at 100%. Do I need to short the P- and B-?
 

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Welcome to the forum.

4S, 12V 272aH
Individual cells 3.23V LifePo4
Daly BMS 4S 12V 100A
powertech 12VDc pure sine inverter

Set it all up last night finally and applied some load. I can charge my battery fine, but when I plug in my laptop charger, the multimeter climbs to roughly 10A and the power turns off at the inverter.

what is causing this? I also just checked the app and the soc is now reading 0% whereas before it was at 100%. Do I need to short the P- and B-?

"The multimeter climbs to roughly 10A" - please elaborate on the specifics. Unit and how you're using it.

B- to P- resets the BMS if it's off.
 
It's a clamp meter. I have it around the positive lead just before it goes into the inverter.
 
It's a clamp meter. I have it around the positive lead just before it goes into the inverter.

Okay. Thank you. Folks have taken standard 10A multimeters and probed like a voltmeter creating a short. I thought that might be tripping short circuit protection.

What is the voltage AT the inverter terminals prior to shutdown?
 
Red lead of multimeter prong on red terminal of inverter and black at black terminal of inverter: At inverter power on, it starts off at around 13.5V then slowly goes down to

12.53V

Then inverter switches off.

EDIT: Probably nor important, but FYI laptop and charger load. (Input of laptop charger = 100-240V~1.5A 50-60Hz, OUTPUT = 20V 6.75A
 
Holy shit. That's a HUGE drop under what sounds like a light load. What is your wire gauge?

Double check that all your connections are properly torqued and any crimps are solid and of high quality. If you have a fuse/breaker between battery and inverter, measure voltage across it under load.

Compare battery terminal voltage to inverter voltage just prior to shut down.

All voltages to be taken with the multimeter.

Gut says you have a big voltage drop due to a poor connection/thin wire.
 
"Holy shit. That's a HUGE drop under what sounds like a light load. What is your wire gauge?""
Glad we found something suspect in that case. Wire gauge should be fine: positive terminal to fuse to inverter is probably 8AWG. Not sure what the negative gauge is, but Im connecting directly from the BMS P- Black wire to the inverter terminal.

"Double check that all your connections are properly torqued and any crimps are solid and of high quality. "
I haven't torqued my connections down and that could be the cause. I've just been doing them really tight by hand until I get a tool for it. I'm a little scared of tightening too much as I've heard of them getting damaged.


"If you have a fuse/breaker between battery and inverter, measure voltage across it under load."
Okay, plugge in a fan. I placed my positive prong of multimeter on the battery terminal where one side of the inline fuse connects and the other black prong of multimeter at the positive terminal of the inverter- reads about 8VDC with fan turned on

"Compare battery terminal voltage to inverter voltage just prior to shut down."

Measurement between Battery terminal positive and negative terminal also drops to about 12.5V just before turn off


---
The only place where there could likely be a poor connection is with the nuts not being torqued and also the crimps I have applied on the inline fuse connecting to the positive battery terminal and the inverter.
 
I noticed with the laptop charger load plugged in, just before shutdown, I measure the voltage jumping from 12 VDC to 20 to 30 back again really rapidly from the positive battery terminal and the inverter terminal
 
"Holy shit. That's a HUGE drop under what sounds like a light load. What is your wire gauge?""
Glad we found something suspect in that case. Wire gauge should be fine: positive terminal to fuse to inverter is probably 8AWG. Not sure what the negative gauge is, but Im connecting directly from the BMS P- Black wire to the inverter terminal.

I would look carefully at the wire insulation and try to determine. 8awg is good for 10A.

"Double check that all your connections are properly torqued and any crimps are solid and of high quality. "
I haven't torqued my connections down and that could be the cause. I've just been doing them really tight by hand until I get a tool for it. I'm a little scared of tightening too much as I've heard of them getting damaged.

Literally finger tight by hand? No tools? Is it possible the bolts have bottomed out in the terminals?

That definitely won't work. If every connection is by hand, then I would be concerned you have significant losses at every connection.

I recommend you get a wrench and hold it with your thumb, index and middle fingers as though they were a tripod and make sure it's snug. With your fingers like this, you have very little mechanical advantage to permit over-torque:

1617156829160.png

"If you have a fuse/breaker between battery and inverter, measure voltage across it under load."
Okay, plugge in a fan. I placed my positive prong of multimeter on the battery terminal where one side of the inline fuse connects and the other black prong of multimeter at the positive terminal of the inverter- reads about 8VDC with fan turned on

That sounds atrocious, but not possible.

With the fan running, what is the voltage at the (+) battery terminal to the (+) inverter terminal - yes, you're measuring voltage at the opposite ends of the same wire.

"Compare battery terminal voltage to inverter voltage just prior to shut down."

Measurement between Battery terminal positive and negative terminal also drops to about 12.5V just before turn off

Strongly supports crazy voltage drop due to poor connections throughout.

I noticed with the laptop charger load plugged in, just before shutdown, I measure the voltage jumping from 12 VDC to 20 to 30 back again really rapidly from the positive battery terminal and the inverter terminal

I don't follow this.

Snug things up and see if the problem goes away.

Bad connections can introduce HUGE resistance.
 
I've tightened the nuts far more than they were before using a mini wrench with your instructions. I am using grub screws I think they are called with a nut on the top .

I am still very anxious to try tighten them any further as I have no measure of how tight they can go before I damage them. I'm still encountering the same issue so I suspect the fuse cable coming from the positive of the battery to the inverter is where the problem lies. The crimps may be the problem. Just not sure which hypothesis is accurate so I am very averse to keep tightening.
That sounds atrocious, but not possible.

With the fan running, what is the voltage at the (+) battery terminal to the (+) inverter terminal - yes, you're measuring voltage at the opposite ends of the same wire.

I'm pretty sure this is what I measured so I tried to check again.
No load, positive battery terminal to positive inverter terminal is of course 0VDC
Fan on first setting positive battery terminal to positive inverter terminal = 7.2VDC
Fan on second setting positive battery terminal to positive inverter terminal = 8.7VDC

What should I try next?
 
Are you sure those aren't millivolts (mV), and you're misreading the meter?

With fan on second setting, measure:

Voltage at battery terminals
Voltage at inverter terminals
 
battery terms = 13.3V
inverter terms = 13.28V
battery positive to inverter positive 8.9V WOOPS, See post below
 
WOOPS YOURE ABSOLUTEY RIGHT
its 8.8mV on the battery positive to inverter positive

I am super sorry, haven't gotten used to this clamp meter compared to my usual one
 
Whew. Rest assured, I can identify these mistakes because I've made them at some point... :)

0.02V drop is negligible.

I'd attempt 10A and see what happens.
 
Whew. Rest assured, I can identify these mistakes because I've made them at some point... :)

0.02V drop is negligible.

I'd attempt 10A and see what happens.
Oh I tried it in between measuring a few posts ago and yeah still not working. Powers down with the laptop charger plugged in.

ill try it again now that its morning but since the value has not changed Im uncertain anything would be different.
 
No load
Battery positive and negative measurement = 13.35V
Battery positive to inverter negative measurement = 13.35V

Laptop load
Battery positive and negative measurement = drops to as low as 12.94V, turns off after about 10-15 seconds

turned on inverter again for next measurement
Battery positive to negative inverter measurement = drops to 12.94V then wavers between 12.94-13.05V (but does not turn off?!? wtf. This might be because my laptop battery is reasonably charged at 93%)

Ill let the laptop battery run down and try again, but I expect these measurements to be similar
 
0.4V drop

10A

V = I * R

R = V / I = 0.4 / 10 = 40mΩ

This seems very high.

These Lishen cells tend to have about 0.15mΩ, i.e., 0.6mΩ total resistance through all 4 cells. Everything else contributes to the resistance... wires, connections, terminals, BMS, etc.

Your cells and BMS should be able to handle 100A with no problem. A 0.04Ω resistance would see a 4V drop... that's outrageous.
 
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