Voltage Drop

[GXMnow]

I think there is a misunderstanding. I only have a single 100ah 48 battery.

Sorry, that was me, I got it mixed up with another thread I was helping on.

Even with one battery, I would hope it's output drops less than 4 volts at 100 amps. That would be 40 milliohms of total series resistance. And we calculated your 1,800 watt load should be less than 40 amps on a 48 volt system.

The ad does claim it has bluetooth monitoring. Is there a QR code anywhere in the instructions or on the battery? My BMS units had a QR code that brought me to the App for my iPhone. If you can't find a Smilaxe app I would guess they are using the same BMS as the Chins battery, or even the Ography one shown on the same Amazon page you linked. I would try the other apps and see if they find the BMS. Those 3 batteries all look like the same case, just different colors and stickers.

 

[Carbage]

The culprit is likely to be a bad connection somewhere ..... That bad connection can be inside the battery.

How long does it run before it drops out on low voltage? The suggestion GXMnow made earlier about trying to test it with less load .... is a good suggestion ..... enough load to get voltage drop but not drop out the inverter in order to extend the time for troubleshooting ...... sure would be nice to get that App working. It can be used to troubleshoot the battery.

If there is a bad connection, there is likely to be a heat build up where the bad connection is.

It will run fine for a few cycles after the battery is charged, it's just when the battery gets down to around 50 volts. Around that voltage, it will start shutting itself off the second the load kicks in. Keep in mind, yesterday was my first time ever using the system, so I've only had one full charge on it ever. After I get off work I will be charging everything up, changing my parameters to 16s, and watching the voltage while it is working for the first time. Because I didn't even watch the voltage until the problem occurred, so I'm not even sure if the voltage is dropping that much when it's charged. I will find out in a few hours and get back to you guys. The entire problem may be occurring because it was only charged to 54v (during charging, dropped closer to 53.2 off the charger) and in fact it's supposed to be charged up closer to 58v if it's a 16 string battery. Since we've figured out it's 16s, at 49v resting that means each cell is at 3v. Which is dead. So it very well could be short run time do to not being fully charged like I thought, and huge droop because it is out of juice. Once I change my settings, charge it up fully, and start everything back up, I will put a voltmeter on the actual battery and on my connections at the inverter and see if I'm experiencing any real drop static or under load. I will then see how much droop I am experiencing under load, and I will get back to you guys with all this information. Thank you again, I will be back on here in maybe 5 hours with some answers hopefully

 

[Carbage]

Sorry, that was me, I got it mixed up with another thread I was helping on.

Even with one battery, I would hope it's output drops less than 4 volts at 100 amps. That would be 40 milliohms of total series resistance. And we calculated your 1,800 watt load should be less than 40 amps on a 48 volt system.

The ad does claim it has bluetooth monitoring. Is there a QR code anywhere in the instructions or on the battery? My BMS units had a QR code that brought me to the App for my iPhone. If you can't find a Smilaxe app I would guess they are using the same BMS as the Chins battery, or even the Ography one shown on the same Amazon page you linked. I would try the other apps and see if they find the BMS. Those 3 batteries all look like the same case, just different colors and stickers.

I will check on that as well. Hopefully I can find some sort of monitoring, and get a bunch of answers for us

 

[Carbage]

Sorry, that was me, I got it mixed up with another thread I was helping on.

Even with one battery, I would hope it's output drops less than 4 volts at 100 amps. That would be 40 milliohms of total series resistance. And we calculated your 1,800 watt load should be less than 40 amps on a 48 volt system.

The ad does claim it has bluetooth monitoring. Is there a QR code anywhere in the instructions or on the battery? My BMS units had a QR code that brought me to the App for my iPhone. If you can't find a Smilaxe app I would guess they are using the same BMS as the Chins battery, or even the Ography one shown on the same Amazon page you linked. I would try the other apps and see if they find the BMS. Those 3 batteries all look like the same case, just different colors and stickers.

Everything is charging. The battery was most definitely dead, but it is indeed 16s. It was down to 48v. Set constant charging to 57.6v (recommended by the inverter book for 16s). The BMS app and inverter screen agree about the battery voltage, but my handheld voltmeter actually claims it is about 0.5v higher at the battery posts. I'll let you guys know when it's up to full charge, how it handles the load.

 

[Bob B]

Everything is charging. The battery was most definitely dead, but it is indeed 16s. It was down to 48v. Set constant charging to 57.6v (recommended by the inverter book for 16s). The BMS app and inverter screen agree about the battery voltage, but my handheld voltmeter actually claims it is about 0.5v higher at the battery posts. I'll let you guys know when it's up to full charge, how it handles the load.

Glad you got the App working .... That will help a lot.

 

[Carbage]

Glad you got the App working .... That will help a lot.

So the app is stating that after charging for the last 2 hours or so, that I've actually gone down slightly in voltage, but the capacity is up to 45ah remaining. Before it was stating 7ah capacity remaining. How has the voltage actually decreased while it's been charging? Interesting.

 

[Tomthumb62]

So the app is stating that after charging for the last 2 hours or so, that I've actually gone down slightly in voltage, but the capacity is up to 45ah remaining. Before it was stating 7ah capacity remaining. How has the voltage actually decreased while it's been charging? Interesting.

That’s 100% normal. Once it reaches the target voltage (or if charging is interrupted), it will settle down to its resting voltage.

 

[Bob B]

So the app is stating that after charging for the last 2 hours or so, that I've actually gone down slightly in voltage, but the capacity is up to 45ah remaining. Before it was stating 7ah capacity remaining. How has the voltage actually decreased while it's been charging? Interesting.

That's the reason voltage alone is not a good indicator of SOC til it reaches the knee.

 

[Carbage]

That's the reason voltage alone is not a good indicator of SOC til it reaches the knee.

Awesome news. I will keep this post updated, app claims 4 hours of charging remaining (can only charge at 25a with my little Honda generator)

 

[GXMnow]

Another reason for the voltage dip could be the temperature. The cells were at just 9.3C in the first screen shot and it warmed up to about 20C in the second screen shot. Cell internal resistance will drop with the rising temp. And yes, 57.6 volts is plenty high to get you well into the upper knee. I would not be too surprised in you get a single cell over volt before the whole battery get's that high. If it can make it up there, then the cell balance is excellent.

 

[Carbage]

Another reason for the voltage dip could be the temperature. The cells were at just 9.3C in the first screen shot and it warmed up to about 20C in the second screen shot. Cell internal resistance will drop with the rising temp. And yes, 57.6 volts is plenty high to get you well into the upper knee. I would not be too surprised in you get a single cell over volt before the whole battery get's that high. If it can make it up there, then the cell balance is excellent.

Well it decided it's charged. Claiming it has 135ah of capacity, if so that would be very impressive from a 100ah rated Chinese battery. But the low and high string worry me

 

[Carbage]

That's the reason voltage alone is not a good indicator of SOC til it reaches the knee.

And now that it's rested for a few minutes, the voltages have landed here. Seems very low to me..

 

[Bob B]

And now that it's rested for a few minutes, the voltages have landed here. Seems very low to me..

Good news is that all the cells were pretty close while charging ..... probably not a bad connection on the cells. I wouldn't worry about the 90 mv difference.

Now that fully charged, what is the voltage drop with that 1800 W load?

 

[Carbage]

Good news is that all the cells were pretty close while charging ..... probably not a bad connection on the cells. I wouldn't worry about the 90 mv difference.

Now that fully charged, what is the voltage drop with that 1800 W load?

The voltage settled down a bit more, I waited for it to finish settling before I turned on a load so I could get an accurate reading. Under load all the cells seem to be matching voltage. Droop is only 0.3v@350w, 0.9v@1300w, and and my draw is sitting at 1650w right now and we're seeing 1.05v of droop or so. Safe to say the battery was indeed just dead! A good full charge and it's happy as can be!

 

[webyoo]

I have a similar question about my 24V LifePO4 system. The voltage drop when powering a 1200W kettle is 1.6V. The battery voltage is 26.6 volts, with no load falling to 25 volts under load. The total voltage drop from the battery to the inverter is 1.9 volts. Is this typical or is it too high a voltage drop?

 

[robbob2112]

Nobody has mentioned it yet, but loads like you are using typically have a very high starting current. In some cases 2 to 3 times the rated wattage. Now the kettle should be a pure resistive load depending on how they work.

From my reading most renogy inverters are the high frequency type which is basically a switching power supply and they don't handle inductive loads very well and the higher wattages the list are only supported for a few seconds or less.

The low frequency inverters typically are rated and perform at 3 x the spec power for a 30 seconds or more depending on the size of the current spike.

If you have a clamp meter that can capture instantaneous spikes you can prove or disprove this pretty easy.

Now that battery dropping voltage and turning off make be suspect the guts bms/cells are not up to performing what they are rated for. That ornthe super high starting current is tripping one BMS off and the other battery is being dragged down by it.

 

[GXMnow]

I have a similar question about my 24V LifePO4 system. The voltage drop when powering a 1200W kettle is 1.6V. The battery voltage is 26.6 volts, with no load falling to 25 volts under load. The total voltage drop from the battery to the inverter is 1.9 volts. Is this typical or is it too high a voltage drop?

1,200 watts into the kettle could be even a bit more from the battery tp the inverter. Let's use 90% efficient so 1,200 / 0.9 = 1,333.33 watts from the battery at 25 volts = 53.333 amps. A 1.6 volt drop at this current is 1.6 / 53.33 = 0.03 ohms, or 30 milliohms. That's not too bad. But to be sure, you should look up what the internal resistance should be for your cells.

Each cell has some internal resistance, and you have 8 of them in series, and 7 bus bars between them, then the wiring, fuses, BMS breakers, and maybe a disconnect switch. Add up all of the resistance, and see how close it comes to 30 milliohms. While it is a little more voltage drop than I would like, I don't think it is a serious issue in a 24 volt system pushing that much power. The inverter in this estimate, is losing 133.33 watts to heat. This 30 milliohm of series resistance is losing 1.6 volts x 53.33 amps = 85.33 watts. That's not horrible. It's just under 6.5% power loss to battery heating.

To make sure you don't have a problem, can you measure the voltage between the battery terminals on the same bus bar? The idea here is to measure from battery post to battery post. If the connections are good, and the bus bars are heavy enough. the voltage between the battery terminals when running your 1,200 watt kettle should be under 0.1 volts, and they should all be pretty close to equal. This test checks the bus bar and both battery to bus bar connections. In a perfect world, the voltage drop would be zero, but we don't have superconductor bus bars yet.

 

[webyoo]

Nobody has mentioned it yet, but loads like you are using typically have a very high starting current. In some cases 2 to 3 times the rated wattage. Now the kettle should be a pure resistive load depending on how they work.

From my reading most renogy inverters are the high frequency type which is basically a switching power supply and they don't handle inductive loads very well and the higher wattages the list are only supported for a few seconds or less.

The low frequency inverters typically are rated and perform at 3 x the spec power for a 30 seconds or more depending on the size of the current spike.

If you have a clamp meter that can capture instantaneous spikes you can prove or disprove this pretty easy.

Now that battery dropping voltage and turning off make be suspect the guts bms/cells are not up to performing what they are rated for. That ornthe super high starting current is tripping one BMS off and the other battery is being dragged down by it.

Thanks I will do more research great feedback.

 

[webyoo]

1,200 watts into the kettle could be even a bit more from the battery tp the inverter. Let's use 90% efficient so 1,200 / 0.9 = 1,333.33 watts from the battery at 25 volts = 53.333 amps. A 1.6 volt drop at this current is 1.6 / 53.33 = 0.03 ohms, or 30 milliohms. That's not too bad. But to be sure, you should look up what the internal resistance should be for your cells.

Each cell has some internal resistance, and you have 8 of them in series, and 7 bus bars between them, then the wiring, fuses, BMS breakers, and maybe a disconnect switch. Add up all of the resistance, and see how close it comes to 30 milliohms. While it is a little more voltage drop than I would like, I don't think it is a serious issue in a 24 volt system pushing that much power. The inverter in this estimate, is losing 133.33 watts to heat. This 30 milliohm of series resistance is losing 1.6 volts x 53.33 amps = 85.33 watts. That's not horrible. It's just under 6.5% power loss to battery heating.

To make sure you don't have a problem, can you measure the voltage between the battery terminals on the same bus bar? The idea here is to measure from battery post to battery post. If the connections are good, and the bus bars are heavy enough. the voltage between the battery terminals when running your 1,200 watt kettle should be under 0.1 volts, and they should all be pretty close to equal. This test checks the bus bar and both battery to bus bar connections. In a perfect world, the voltage drop would be zero, but we don't have superconductor bus bars yet.

Many thanks for the math feedback. I will use this for more research.