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Voltage drop very fast on load

Montanat

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Mar 15, 2022
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Hi. I bought a brand new 5000w inverter (Chinese Soer) + brand new 4x200a tubular Indian lead acid batteries. It's a 48v / 800amp setup. No solar panel, only grid for recharging.

I see 54v float when electricity is on and when it goes off voltage drop immediately to 53.2v without any load on the batteries at all.

In batteries mode (when electricity goes off) i tried 1300w load and the voltage of batteries dropped from 53.2v to 48.2v immediately.
I switched off that load after 10 seconds , the voltage went back to 50.5v.

Checking the batteries with a voltmeter there is one of them always 0.07v lower than the rest on both float and load and rest mode.
Example of Batteries voltage like this : 12.72v / 12.71v / 12.74v / 12.64v is that low one.

Inverter is installed 4 meters away from the electricity board with 12mm wires. Batteries under it connected together in series with 2x6mm wires.

I'm new to the inverter batteries thing, I need to know if dropping this much and fast under this load is normal and if that battery which is always lower than the rest by 0.07v is defected or not.

Thank you
 
When you get a fast voltage drop under load I would suspect either a bad connection or bad batteries.

If you have 4 200Ah batteries in series to make 48 V .... that would be 200Ah at 48V.

How old are the batteries? Do you have a picture of your setup? Can you test the batteries individually?
 
When you get a fast voltage drop under load I would suspect either a bad connection or bad batteries.

If you have 4 200Ah batteries in series to make 48 V .... that would be 200Ah at 48V.

How old are the batteries? Do you have a picture of your setup? Can you test the batteries individually?
Hi, the batteries are 3 weeks old only, yes I tested the batteries with a voltmeter the difference between them and the inverter is 0.2v only. If inverter shows 50.2v the batteries shows 50.4v
Attached is a pic of the setup. Thank you
 

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Hi, the batteries are 3 weeks old only, yes I tested the batteries with a voltmeter the difference between them and the inverter is 0.2v only. If inverter shows 50.2v the batteries shows 50.4v
Attached is a pic of the setup. Thank you
More pics
 

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I think what you need to do is measure between the battery and the inverter. Connect the voltmeter from the + terminal on the battery and also the + terminal on the inverter. (directly across the wire) Do this with a 1300 watt load. If you have a greater than 50 mv measurement then your wire size is to small and needs to be increased to get the voltage across the wire to be low. You can also use this approach to determine if you have a bad connection or not. Just measure each part of the system and see where you have a voltage drop with a high load. No voltage drop is suspect because when you flow that much current you will have a voltage drop.
 
Looking at your pictures, the wire size is not capable of handling the type of current the inverter is drawing. Use some copper bus bars and some very heavy wire. Looking at a wire chart it is showing 2/0 copper for 200 amp service. Make sure you use a fuse as well to protect your inverter.
 
Are you sure the wire is 6mm and not 6AWG? The wire looks pretty small to me ... Is it copper wire?

1300 W at 50V would only be about 26A .... so it doesn't seem like you should be dropping the voltage that much.
Do you have a way to disassemble and load test the batteries individually?
 
Yes copper attached is are photos. The ones coming in are 6mm the ones going out from inverter are 12mm.
Did the test (+ of the voltmeter on the + of the battery, - of the voltmeter on the + of the inverter ) the inverter shows 49v and I got 34v on 1st battery and 24v on the other 3.
 

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Last edited:
Are you sure the wire is 6mm and not 6AWG? The wire looks pretty small to me ... Is it copper wire?

1300 W at 50V would only be about 26A .... so it doesn't seem like you should be dropping the voltage that much.
Do you have a way to disassemble and load test the batteries individually?
Yes I can dissamble the batteries what shallni do to test them
 
I think what you need to do is measure between the battery and the inverter. Connect the voltmeter from the + terminal on the battery and also the + terminal on the inverter. (directly across the wire) Do this with a 1300 watt load. If you have a greater than 50 mv measurement then your wire size is to small and needs to be increased to get the voltage across the wire to be low. You can also use this approach to determine if you have a bad connection or not. Just measure each part of the system and see where you have a voltage drop with a high load. No voltage drop is suspect because when you flow that much current you will have a voltage drop.
In did this I got 34v 1st battery and 24v on the other 3. Inverter was 49 v
 
Are you sure the wire is 6mm and not 6AWG? The wire looks pretty small to me ... Is it copper wire?

1300 W at 50V would only be about 26A .... so it doesn't seem like you should be dropping the voltage that much.
Do you have a way to disassemble and load test the batteries individually?
Please check the log here is a pic
 

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In did this I got 34v 1st battery and 24v on the other 3. Inverter was 49 v
I really do not follow what your measurements were.

1-Please measure each each individual battery with the inverter off. (4 voltages around 12v)

2-Turn on the inverter with a 1300 watt load. Measure each battery separate with the inverter loading the batteries (4 voltages with battery under load)

3-Please measure the voltage at the terminals of the inverter under load. (1 voltage around 48v)

Please label the batteries as 1,2,3,4 and post the results.

You should come up with something like this

Battery1 Battery2 Battery3 Battery4
1 12.4v 12.3v 12.5v 12.3v
2 11.9v 12.1v 12.1v 11.7v
3 48.5v


If you do this then I may be able to suggest what the issue is.

Thanks
 
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Hi. I bought a brand new 5000w inverter (Chinese Soer) + brand new 4x200a tubular Indian lead acid batteries. It's a 48v / 800amp setup. No solar panel, only grid for recharging.

I see 54v float when electricity is on and when it goes off voltage drop immediately to 53.2v without any load on the batteries at all.

In batteries mode (when electricity goes off) i tried 1300w load and the voltage of batteries dropped from 53.2v to 48.2v immediately.
I switched off that load after 10 seconds , the voltage went back to 50.5v.

Checking the batteries with a voltmeter there is one of them always 0.07v lower than the rest on both float and load and rest mode.
Example of Batteries voltage like this : 12.72v / 12.71v / 12.74v / 12.64v is that low one.

Inverter is installed 4 meters away from the electricity board with 12mm wires. Batteries under it connected together in series with 2x6mm wires.

I'm new to the inverter batteries thing, I need to know if dropping this much and fast under this load is normal and if that battery which is always lower than the rest by 0.07v is defected or not.

Thank you
It is not a 800 amp setup sense the batteries are in series. It is a 48v 200 amp setup. You should expect around 8kw of useable power maybe less due to lead acid technology
 
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I really do not follow what your measurements were.

1-Please measure each each individual battery with the inverter off. (4 voltages around 12v)

2-Turn on the inverter with a 1300 watt load. Measure each battery separate with the inverter loading the batteries (4 voltages with battery under load)

3-Please measure the voltage at the terminals of the inverter under load. (1 voltage around 48v)

Please label the batteries as 1,2,3,4 and post the results.

You should come up with something like this

Battery1 Battery2 Battery3 Battery4
1 12.4v 12.3v 12.5v 12.3v
2 11.9v 12.1v 12.1v 11.7v
3 48.5v


If you do this then I may be able to suggest what the issue is.

Thanks
Hi thank you for your time.

Inverter off:
13.37v 13.31v. 13.36v. 13.38v
53.4v

Inveter load 1300:
12.06v 11.99v 12.06v 12.08v
48v

Removed 1000w load kept 300.
50.5v

Note: the wires from breakers to AC in & AC out are 4mm each (4 of them 2L & 2N) (around 5 meters).
The wires from POS & NEG of the inverter to the batteries are 6mm (around 2 meters). The wires used to join the batteries in series are 4mm x 2 wires.

As you told me yesterday, under 1300 load I put + of the multimeter on + of the battery (the cable that goes up to the inverter POS) & negatif of voltmeter on + of inveter (POS), I got 151mv and on 300 load I got 41mv. Still don't know if I'm doing it right coz when inverter was not on load and main power was on I got 1.1mv only.

Hope it helps
 
Hi thank you for your time.

Inverter off:
13.37v 13.31v. 13.36v. 13.38v
53.4v

Inveter load 1300:
12.06v 11.99v 12.06v 12.08v
48v

Removed 1000w load kept 300.
50.5v

Note: the wires from breakers to AC in & AC out are 4mm each (4 of them 2L & 2N) (around 5 meters).
The wires from POS & NEG of the inverter to the batteries are 6mm (around 2 meters). The wires used to join the batteries in series are 4mm x 2 wires.

As you told me yesterday, under 1300 load I put + of the multimeter on + of the battery (the cable that goes up to the inverter POS) & negatif of voltmeter on + of inveter (POS), I got 151mv and on 300 load I got 41mv. Still don't know if I'm doing it right coz when inverter was not on load and main power was on I got 1.1mv only.

Hope it helps
It looks like your batteries are defective.

They should not drop down to 12v from 13.3 volts with a 20 amp load.

You may purchase a battery shunt meter and put that in series to measure the current to make sure your inverter is not defective.

For a 1000 watt load it should be around 20 amp current draw at 50 volts.

Possibly charging the batteries to full charge and repeat the test.

Or you could disassemble your pack, charge each battery up and use a 12 v automotive headlight to determine capacity. (Measure the current draw of the light) An example would be if the light draws 10 amps then your battery should last around 20 hours or less.

State of charge for lead acid for around a 20 amp load
12.7 V 100%
12.4 V 75%
12.2 V 50%
12.0 V 25%

As you can see for your batteries to drop to 12 Volts with a 20 amp load that says that they are at a 25% charge. If in fact your batteries are fully charged then they are defective. You just need to verify that you have a 20 amp load. The automotive headlight test would take the inverter out of the equation.
 
It looks like your batteries are defective.

They should not drop down to 12v from 13.3 volts with a 20 amp load.

You may purchase a battery shunt meter and put that in series to measure the current to make sure your inverter is not defective.

For a 1000 watt load it should be around 20 amp current draw at 50 volts.

Possibly charging the batteries to full charge and repeat the test.

Or you could disassemble your pack, charge each battery up and use a 12 v automotive headlight to determine capacity. (Measure the current draw of the light) An example would be if the light draws 10 amps then your battery should last around 20 hours or less.

State of charge for lead acid for around a 20 amp load
12.7 V 100%
12.4 V 75%
12.2 V 50%
12.0 V 25%

As you can see for your batteries to drop to 12 Volts with a 20 amp load that says that they are at a 25% charge. If in fact your batteries are fully charged then they are defective. You just need to verify that you have a 20 amp load. The automotive headlight test would take the inverter out of the equation.
I understood. I checked the specs of the batteries and it says a 48v system should float at 54.4v . The inverter setup for flooded battery is set to 54v float. If I raise the float to 54.4v manulay will this change something?
Attached is the data sheet / specs of the batteries.
 

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It says in your spec that absorb voltage is 57.6 volts. Maximum time 4 hours. You could try that and see if it helps. Do not exceed the 4 hour. Float voltage is 54.4

This is my suggestion. Set your voltage to 57.6 volts for charge. Monitor your batteries. When the battery voltage reaches 57.6 volts then continue for 4 hours. Then repeat the inverter test. Your batteries should be fully charged at this point.
 
6mm² wire to the inverter from the battery is way too small. A 5000W 48V setup can pull up to 5000W / 48V / 85% efficiency = 125A. For that you need at least 2AWG or 1AWG or 35mm². The 6mm² is only good for about 35A which means no more than about 1400W from the inverter before you have big issues.

Replace all of the battery wires and the wires to the inverter (from the battery) with at least 35mm² and things should work a lot better.
 
It says in your spec that absorb voltage is 57.6 volts. Maximum time 4 hours. You could try that and see if it helps. Do not exceed the 4 hour. Float voltage is 54.4

This is my suggestion. Set your voltage to 57.6 volts for charge. Monitor your batteries. When the battery voltage reaches 57.6 volts then continue for 4 hours. Then repeat the inverter test. Your batteries should be fully charged at this point.
How can I continue for 4 hours when they reach 57.6 the inveter will stop charging
 
6mm² wire to the inverter from the battery is way too small. A 5000W 48V setup can pull up to 5000W / 48V / 85% efficiency = 125A. For that you need at least 2AWG or 1AWG or 35mm². The 6mm² is only good for about 35A which means no more than about 1400W from the inverter before you have big issues.

Replace all of the battery wires and the wires to the inverter (from the battery) with at least 35mm² and things should work a lot better.
Will do that for sure and see. But I'm not reaching 1400w load yet max is 1300. I'll see how it goes anyways with the wires upgrade
 
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