Voltage Drop

[Carbage]

Hey all! I searched the forum, but didn't find anything that specifically answered my question (although I'm sure one of the 40,000+ posts on here would answer my question, I couldn't find it.)

I have a system comprised of a 48v 100ah 15 string LiFeP04 battery, a Renogy 3500w 48v inverter charger, and the primary use is to run an 1800w heater/AC unit. Connecting the battery to the inverter, is 3ft of 1/0 OFC wire with solid crimp connections (both positive and negative.)

The issue I am dealing with is that when I turn on the heater or AC, I drop from whatever current voltage is down 5-6 volts. I'm unsure if this is a common thing or not, or if there is a root cause. After the appliance is turned back off, voltage returns. I've checked all voltages on all points, and everything seems to add up. Is this amount of voltage drop normal?

The solution I've come up with (if you can call it that) is to turn my low voltage cutoff down from 44v, to 40v. I haven't actually done this yet, because my largest question is: will a voltage under load of 40v hurt the cells if it returns to a resting voltage over 44v? Sorry for the noob question and the rambling, I just wanted to try to cover all the bases. Thank you all for any help!

 

[Bob B]

Are you proficient with a DVM? You are going to need to take some voltage readings to determine where the voltage drop is taking place.
Start by taking a voltage reading at the battery terminals .... compare that voltage with the voltage at the DC input of the inverter.
If the drop is happening at the battery .... it is going to be necessary to troubleshoot the battery itself.

 

[Carbage]

Are you proficient with a DVM? You are going to need to take some voltage readings to determine where the voltage drop is taking place.
Start by taking a voltage reading at the battery terminals .... compare that voltage with the voltage at the DC input of the inverter.
If the drop is happening at the battery .... it is going to be necessary to troubleshoot the battery itself.

I will check tomorrow after work, and find out if the battery is actually experiencing that drop at the terminals. If so, is it actually a bad battery, or could this be normal because of the high current demand? Also, if it is the battery, could it be the internal BMS? Thank you for the quick response!

 

[Bob B]

I will check tomorrow after work, and find out if the battery is actually experiencing that drop at the terminals. If so, is it actually a bad battery, or could this be normal because of the high current demand? Also, if it is the battery, could it be the internal BMS? Thank you for the quick response!

Should be less than 40 Amps .... most 100A lifepo will handle that .... it could be the BMS or the some connections in the battery .... We will probably need some pics of it to get a better idea how to troubleshoot it if the drop is caused by the battery.

 

[Carbage]

Should be less than 40 Amps .... most 100A lifepo will handle that .... it could be the BMS or the some connections in the battery .... We will probably need some pics of it to get a better idea how to troubleshoot it if the drop is caused by the battery.

Okay, that was my thinking as well when I purchased it. I believe it's 32-35a of draw depending on factors. I'll see what I can come up with. For reference, it is a smilaxe battery. Amazon special, had good reviews though. If I can get it opened up (if it's "serviceable") I will get pics inside.

 

[GXMnow]

I agree with Bob B

That sounds like far too much drop. 1,800 watts at 48 volts should be under 40 amps. You should have less than 1 volt of drop at 40 amps.

Do you have a smaller load you can apply for testing? Something like 200 to 300 watts? Does your BMS report cell voltages? If not, do you have a meter you can measure all of the cells while the system is under load? Do they all measure a similar drop, or is it just one cell giving up?

My guess is that you have a bad connection somewhere. 1/0 cable should be fine, but inspect it closely. My friend used some 2/0 welding cable for a trunk mount battery in his car, and it would not crank worth a crap. Turned out the new cable he bought had a cut nearly 3/4 the way through in the middle of the cable. Probably got hit with the razor knife while opening the box the wire was in.

15S is kind of an oddball string for solar storage. Is this a DIY battery build? What cells? What BMS? Most LFP "48 volt" batteries are 16S as that is a much closer match to a string of 4 lead acid 12 volt batteries.

 

[Bob B]

Okay, that was my thinking as well when I purchased it. I believe it's 32-35a of draw depending on factors. I'll see what I can come up with. For reference, it is a smilaxe battery. Amazon special, had good reviews though. If I can get it opened up (if it's "serviceable") I will get pics inside.

Do you have the specs for it? I wouldn't open it up if there is any chance you can return it ..... maybe post a link to it.

One step at time though ... make sure whether the problem is in the battery or not .... Take your voltage readings on the battery terminals themselves .... not on the cable connection at the battery.

 

[Carbage]

I agree with Bob B

That sounds like far too much drop. 1,800 watts at 48 volts should be under 40 amps. You should have less than 1 volt of drop at 40 amps.

Do you have a smaller load you can apply for testing? Something like 200 to 300 watts? Does your BMS report cell voltages? If not, do you have a meter you can measure all of the cells while the system is under load? Do they all measure a similar drop, or is it just one cell giving up?

My guess is that you have a bad connection somewhere. 1/0 cable should be fine, but inspect it closely. My friend used some 2/0 welding cable for a trunk mount battery in his car, and it would not crank worth a crap. Turned out the new cable he bought had a cut nearly 3/4 the way through in the middle of the cable. Probably got hit with the razor knife while opening the box the wire was in.

15S is kind of an oddball string for solar storage. Is this a DIY battery build? What cells? What BMS? Most LFP "48 volt" batteries are 16S as that is a much closer match to a string of 4 lead acid 12 volt batteries.

I guess I should have said in the original post; it's a Smilaxe battery, purchased on Amazon. I could not find any info on string count, so I called renogy and they claimed to know it was a 15 string. My initial instinct is it was 16 string as well.

I will see if it opens up tomorrow, because the BMS is internal to the battery. I will gather more info and see if I can get pictures in the morning.

 

[GXMnow]

I am looking around Amazon, I am not fining that battery.

 

[Carbage]

Do you have the specs for it? I wouldn't open it up if there is any chance you can return it ..... maybe post a link to it.

One step at time though ... make sure whether the problem is in the battery or not .... Take your voltage readings on the battery terminals themselves .... not on the cable connection at the battery.

The battery itself is no longer sold anywhere, although there are a few suspiciously similar (same dimensions, case style, built in rope handles etc) batteries under different names for sale on Amazon. It came with a book that has very vague information, but I took Renogy's word for it being 15s even though they have no affiliation with the battery at hand. When I called to find out if there was an easy way to tell how many strings, the tech on the phone told me it was 15s. I asked how he got the info, and he claimed to find the answer on Google (I could not find any TDS on the battery) so I should be suspect of that.

 

[Carbage]

I am looking around Amazon, I am not fining that battery.

https://www.amazon.com/gp/aw/d/B0C5MFJP1K/ref=yo_ii_img?ie=UTF8&psc=1

 

[Bob B]

https://www.amazon.com/gp/aw/d/B0C5MFJP1K/ref=yo_ii_img?ie=UTF8&psc=1

Specs for that show it should handle 100A. Note the voltage before and after adding the load. If it has Bluetooth, maybe post a pic of individual cell voltages and watch those on the app when applying the load .... also watch the battery total voltage in the app and compare to your readings.

Don't do a tear down if there is any way you can do a return.

 

[Carbage]

Specs for that show it should handle 100A. Note the voltage before and after adding the load. If it has Bluetooth, maybe post a pic of individual cell voltages and watch those on the app when applying the load .... also watch the battery total voltage in the app and compare to your readings.

Don't do a tear down if there is any way you can do a return.

Noted. I will look into the app, although I think I need a separate module it did not come with for that. I'll find out. I will also check voltage at the battery terminals, and see if it matches what the inverter is claiming. I will get back to you tomorrow after work, thank you so much!

 

[Mattb4]

Battery voltage droop under load as opposed to voltage drop due to resistance between battery and load can confuse people. The capacity of a battery to maintain voltage when loaded is a function of its chemistry and internal construction. Example are LA starter batteries that are designed to deliver heavy current while maintaining sufficient minimum voltage during cranking a engine.

Your 5-6 volt droop is high but for you 15S battery might be about what it will do if it does not really have the 100ah claimed capacity.

 

[Carbage]

Battery voltage droop under load as opposed to voltage drop due to resistance between battery and load can confuse people. The capacity of a battery to maintain voltage when loaded is a function of its chemistry and internal construction. Example are LA starter batteries that are designed to deliver heavy current while maintaining sufficient minimum voltage during cranking a engine.

Your 5-6 volt droop is high but for you 15S battery might be about what it will do if it does not really have the 100ah claimed capacity.

Okay, so if that's the case, then my original question; will the battery get damaged if it goes under the minimum recommended voltage while under load, even if it comes back up to an accepted resting voltage?

 

[Mattb4]

Okay, so if that's the case, then my original question; will the battery get damaged if it goes under the minimum recommended voltage while under load, even if it comes back up to an accepted resting voltage?

No the battery will not be damaged however you might end up getting a low voltage disconnect depending on how far the voltage droops. This catches folks all the time with batteries at about 20-30% capacity and a large load turns on . They see their unit trip (or transfer to grid) but by the time they look at battery voltage it is in a decent range.

 

[Carbage]

No the battery will not be damaged however you might end up getting a low voltage disconnect depending on how far the voltage droops. This catches folks all the time with batteries at about 20-30% capacity and a large load turns on . They see their unit trip (or transfer to grid) but by the time they look at battery voltage it is in a decent range.

Awesome. Then I will do some quick minimum checks, because I do suspect something is up (a bad cell causing a low string, or a poor connection inside, or a crappy BMS) and if I find nothing, then I will set my shutoff voltage down to 40 or so for testing. A second battery may be necessary, not for capacity, but simply because it should help absorb this droop if what you're saying is correct (it does make sense)

 

[GXMnow]

I see now why I couldn't find it. It's "not currently available".
From the link you sent, the voltages suggest it is 16S not 15S.

Charging voltage range is listed as 56.8-58.46 volts. At 15S that 58.46 would be 3.9 volts per cell, which is way too high for LFP cells. But divide it by 16 and we get 3.654 per cell which is really close to the recommended 3.65 volts per cell. Then the "Full Charging Voltage Range" goes up to 55.2 volts. That is 3.45 volts which is the resting full float voltage as well.

With 2 of these in parallel, you should have a solid 200 amps of current available. And with typical voltage drop, you should see only about a 1 to 3 volt drop at 100 amps on the pair. They don't quote an internal resistance value, but the cells and BMS should all add up to less than 30 milliohms, or 0.03 ohms per battery. With 2 in parallel, it should even drops that in half. My battery bank is a bit stiffer as it is a total of 720 amp hours, you only have 200 total. I found a spot where my load went from 18 amps to 52 amps, a 34 amp increase. The voltage at the inverter input dropped just 0.3 volts 54.6 volts to 54.3 volts. Since you have just 27% of the battery capacity, I would still expect the drop to be 0.3 volts / 0.27 = 1.11 volts. With your thinner cables, maybe weaker BMS units, and any other items like a breaker or disconnect switch, I would still hope for less than a 3 volt drop.

Set you bulk charge voltage up to at least 55 volts for 16S. Off Grid Garage uses 55.2 to get his cells right to the float voltage. Then set the float voltage to 55 volts even so the system keeps the batteries up there. Part of your problem may be that the batteries just are not really charged. Even at 50 volts, 3.125 per cell with 16S, the cells are nearly depleted and that may be falling into the lower knee. From 54 volts, even a 4 volt drop is still 50 volts and should not show any read flags. Once you get charged up to 55 volts, then measure the voltage at the terminals of each battery with no load, a 200 or 300 watt load, and then with your 1,800 watt load and see what you get.

 

[Carbage]

I see now why I couldn't find it. It's "not currently available".
From the link you sent, the voltages suggest it is 16S not 15S.

Charging voltage range is listed as 56.8-58.46 volts. At 15S that 58.46 would be 3.9 volts per cell, which is way too high for LFP cells. But divide it by 16 and we get 3.654 per cell which is really close to the recommended 3.65 volts per cell. Then the "Full Charging Voltage Range" goes up to 55.2 volts. That is 3.45 volts which is the resting full float voltage as well.

With 2 of these in parallel, you should have a solid 200 amps of current available. And with typical voltage drop, you should see only about a 1 to 3 volt drop at 100 amps on the pair. They don't quote an internal resistance value, but the cells and BMS should all add up to less than 30 milliohms, or 0.03 ohms per battery. With 2 in parallel, it should even drops that in half. My battery bank is a bit stiffer as it is a total of 720 amp hours, you only have 200 total. I found a spot where my load went from 18 amps to 52 amps, a 34 amp increase. The voltage at the inverter input dropped just 0.3 volts 54.6 volts to 54.3 volts. Since you have just 27% of the battery capacity, I would still expect the drop to be 0.3 volts / 0.27 = 1.11 volts. With your thinner cables, maybe weaker BMS units, and any other items like a breaker or disconnect switch, I would still hope for less than a 3 volt drop.

Set you bulk charge voltage up to at least 55 volts for 16S. Off Grid Garage uses 55.2 to get his cells right to the float voltage. Then set the float voltage to 55 volts even so the system keeps the batteries up there. Part of your problem may be that the batteries just are not really charged. Even at 50 volts, 3.125 per cell with 16S, the cells are nearly depleted and that may be falling into the lower knee. From 54 volts, even a 4 volt drop is still 50 volts and should not show any read flags. Once you get charged up to 55 volts, then measure the voltage at the terminals of each battery with no load, a 200 or 300 watt load, and then with your 1,800 watt load and see what you get.

I think there is a misunderstanding. I only have a single 100ah 48 battery. But I will put charging up to 16s, as you recommended, and see how the voltages behave when it's charged fully. I seriously appreciate your help! Also with 1/0 cable I should be seeing no issues, I have run the same size cable for a 3000w draw at 12 volts in other applications and seen only 0.1v drop under load between the battery and appliance, so I think it is a combo of under-charging, and then it may be somewhat "normal" with only 100ah of capacity. Let me know your thoughts on this, thank you again for the help

 

[Bob B]

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.