Do I need a Shunt? Battery level drops instantly when current is drawn :|

[coalfield]

I have recently installed a inverter / charger, which displays the SOC on the controller panel.

What I have noticed, is when current is drawn from the inverter (in particular those with higher loads), the voltage at the battery drops significantly so my SOC can drop from 100% to 50% in a matter of minutes. After turning the load of this slowly recovers.

Is this normal, or could I be drawing too much power for my batteries. My batteries are 2x 12V 100Ah AGM batteries in series, I could not find on the spec sheet anything about a max power draw, but my inverter can handle up to 3KW which would be over 100Amps.

I was thinking to get a Shunt to at least allow me to more accurately read the SOC, wanted to check if the above is normal and if indeed that would be the best thing to do and almost ignore the controller readout?

 

[DThames]

You are most likely putting too much load on your batteries and they voltage is dropping. Too small of a cable from battery to inverter would act similar but as soon as you stop the load, the voltage should jump back without the delay you described. 3kw inverter could easily pull 250 amps and while your battery might deliver that, its volts would drop in the process. Voltage coming back up a little after the load is off, is normal.

 

[MisterSandals]

wanted to check if the above is normal and if indeed that would be the best thing to do and almost ignore the controller readout?

You are definitely pulling the voltage of the batteries down with a 3000W load. I would put the money into more batteries before a shunt.

 

[coalfield]

@DThames @MisterSandals thanks for the replies.

I have purchased 2 more batteries arriving tomorrow, so will have 4x 12V 100Ah AGMs... will run in 2P 2S so am hoping this will help.

If I have the right number of batteries, should the voltage remain relatively stable?

 

[DThames]

@DThames @MisterSandals thanks for the replies.

I have purchased 2 more batteries arriving tomorrow, so will have 4x 12V 100Ah AGMs... will run in 2P 2S so am hoping this will help.

If I have the right number of batteries, should the voltage remain relatively stable?

If you have an infinite number of batteries, the voltage will be rock solid.

 

[coalfield]

If you have an infinite number of batteries, the voltage will be rock solid.

fair point

 

[DThames]

@DThames @MisterSandals thanks for the replies.

I have purchased 2 more batteries arriving tomorrow, so will have 4x 12V 100Ah AGMs... will run in 2P 2S so am hoping this will help.

If I have the right number of batteries, should the voltage remain relatively stable?

Power supply theory suggests your supply be 10 times what the load is for the supply to be "stiff" and not see much of a voltage sag. This is not practical in most battery power systems so you do the best you can....afford.

 

[coalfield]

Power supply theory suggests your supply be 10 times what the load is for the supply to be "stiff" and not see much of a voltage sag. This is not practical in most battery power systems so you do the best you can....afford.

As I am running in in 2S I am guessing that does not count either.

So based on a 3000W output, I would need 3000 / 24 = 125A draw... so a 125A *10 = 1250Ah

1250Ah in 2S is 2500Ah's worth of 12V batteries, i.e. 25?


I guess on a separate note, with less batteries, even if I temporarily drop below 10.5V will I damage the batteries?
(note I an not sure if the volt reading when the temp drops on the controller is the same as measured at the batteries, but I presume it would be)

 

[DThames]

As I am running in in 2S I am guessing that does not count either.

So based on a 3000W output, I would need 3000 / 24 = 125A draw... so a 125A *10 = 1250Ah

1250Ah in 2S is 2500Ah's worth of 12V batteries, i.e. 25?


I guess on a separate note, with less batteries, even if I temporarily drop below 10.5V will I damage the batteries?
(note I an not sure if the volt reading when the temp drops on the controller is the same as measured at the batteries, but I presume it would be)

"125A *10 = 1250Ah" 125A * 10 is 1250 amps. "Amp hours" is a different unit of measure. A 100ah battery discharged at 1C would be 100 amps, 2c would be 200 amps. I am not sure what lead acid discharge "C" rating is, but if it is 1C, I can see what your are thinking.

In general a battery cell is a chemical voltaic cell with a series resistance as part of its physical nature. Under load the voltage at the terminal is the point in a voltage divider that between the load and the cell's internal resistance. So if the internal resistance is a measurable percent of the load, then a measurable drop will be seen across this internal resistance. So if that voltage happens to be 1 volts (drop across the internal resistance, the battery cell voltage is not as low as it would seem when measured under load. As soon as the load is removed, the cell should be measured if you want the best idea of what the voltage is. What I am saying is that under load, the voltage might be lower than it really is. But after the load is removed, if the voltage is still below your ideal, I would say you need to change something. Most batteries will rebound somewhat after a few minutes but I would not count that as okay value to read. If that makes any sense. That is my understanding anyway, and might be flawed somehow. But for sure, the internal resistance is part of the voltage drop under load. If you double up on your batteries, I would expect for the same load you will see a marked improvement.

 

[HRTKD]

When you put the system under load, check your connections (lugs, ring terminals, terminations, etc) to ensure they aren't heating up. Check each one with a bare finger. If a connection is hotter than the others then you likely have found a bad connection. You may find nothing, but it doesn't cost anything to check the connections.

 

[Zil]

Basically you now have 100 amp hour battery at 24 volts. That can never run the inverter. You upgrade to 200 amp hour at 24 volts will not do it for very long. And! do what HRTKD said. Although it would be better to use a volt meter to check the connections and wire for voltage drop.
I have successfully run a 2000W inverter from 690ah GC2 battery at 12 volts. I have friends that run 2000W with 460ah GC2 at 12 volts. If you can figure the math conversions maybe you can make a guess at what you need for 3000W at 24 volts, other than heavy cables.

 

[coalfield]

Thanks for all the advice. My new batteries have arrives so 2S 2P will be 24v / 200Ah and hoping this will help.

However am coming to terms with the fact that I wont really be able to hammer the inverter much!!

My batteries are:
https://www.ultramax.co.uk/media/catalog/product/pdf//s/l/slaumxnp100-12-tech_1.pdf

So based on this with 4 batteries, I am looking at 120W draw to give me the maximum 100Ah capacity, lasting 40 hours (but am guessing that's with full discharge which I understand I should not look to go below 50%).

Also looks like a 408W draw (more realistic) would give me up to 10 hours (5 Hours to 50%), so for short period will try to keep below 500W.

I can see why people go Lithium, expensive but much more practical on the space. Unfortunately my EPEVER UP3000 does not support them

 

[Texas-Mark]

the voltage at the battery drops significantly so my SOC can drop from 100% to 50% in a matter of minutes.

How are you determining the SOC now without a shunt?

 

[Short_Shot]

You can't use voltage to determine state of charge while under load, fyi.

Voltage drop under load is normal but doesn't mean the battery is suddenly discharged to whatever % capacity your chart or meter indicates.

 

[coalfield]

How are you determining the SOC now without a shunt?

You can't use voltage to determine state of charge while under load, fyi.

Voltage drop under load is normal but doesn't mean the battery is suddenly discharged to whatever % capacity your chart or meter indicates.

Well there lies in the initial question to be honest as I imagine its the only reliable way to actually know. I have purchased a Renogy 500A Shunt and awaiting for this to arrive so will hopefully be able to take out the guess work!

 

[Short_Shot]

If you want to find soc of a lead acid by voltage you have to take the load off and let it rest for a while lol

 

[Texas-Mark]

Were you looking at a voltage meter, or just one of those bar graph displays? If you have a voltmeter, you should measure the voltage at the battery terminals while it's under the inverter load, and also at the inverter battery input terminals. You may also have a voltage drop due to cables and/or bad connections.

 

[DThames]

Thanks for all the advice. My new batteries have arrives so 2S 2P will be 24v / 200Ah and hoping this will help.

However am coming to terms with the fact that I wont really be able to hammer the inverter much!!

My batteries are:
https://www.ultramax.co.uk/media/catalog/product/pdf//s/l/slaumxnp100-12-tech_1.pdf

So based on this with 4 batteries, I am looking at 120W draw to give me the maximum 100Ah capacity, lasting 40 hours (but am guessing that's with full discharge which I understand I should not look to go below 50%).

Also looks like a 408W draw (more realistic) would give me up to 10 hours (5 Hours to 50%), so for short period will try to keep below 500W.

I can see why people go Lithium, expensive but much more practical on the space. Unfortunately my EPEVER UP3000 does not support them

If this is your UP3000 unit, it looks like lithium could be handled under the USER battery type. That is normal (user) for a lot of systems, so you can tweak it however you desire.

 

[coalfield]

Were you looking at a voltage meter, or just one of those bar graph displays? If you have a voltmeter, you should measure the voltage at the battery terminals while it's under the inverter load, and also at the inverter battery input terminals. You may also have a voltage drop due to cables and/or bad connections.

Yeah I was looking at the display, but have checked with a multimeter and (from memory only) think I saw a significant drop. I am pretty sure my terminals are good, the cabling is 35mm copper (should be good for over 100A), and connections are sound, not heating up and all copper crimped. Will give it another test to check when the 4 batteries are setup.

If this is your UP3000 unit, it looks like lithium could be handled under the USER battery type. That is normal (user) for a lot of systems, so you can tweak it however you desire.

Thanks for this I did wonder... when I contacted EPEVER they said it was not compatible with Lithium and another seller said they don't recommend as it often conflicts with the BMS. I guess with careful setup of the USER setting and BMS it would be possible.

Are the charging phases for a Lithium and AGM battery the same? i.e. just just the voltages they change bulk, absorb, float just need to be tweaked?

 

[DThames]

Yeah I was looking at the display, but have checked with a multimeter and (from memory only) think I saw a significant drop. I am pretty sure my terminals are good, the cabling is 35mm copper (should be good for over 100A), and connections are sound, not heating up and all copper crimped. Will give it another test to check when the 4 batteries are setup.



Thanks for this I did wonder... when I contacted EPEVER they said it was not compatible with Lithium and another seller said they don't recommend as it often conflicts with the BMS. I guess with careful setup of the USER setting and BMS it would be possible.

Are the charging phases for a Lithium and AGM battery the same? i.e. just just the voltages they change bulk, absorb, float just need to be tweaked?

Boost voltage is what you want to use for "charge to" and then boost duration is set to give time to fully charge. That is hard to determine charging with a variable current like from solar. If boost duration is too short or current during that time is too low (cloudy day) then you might not get fully charged. But you can set Boost Reconnect at a point that under load if it is not fully charged, it will go back to boost mode again. Float, might be set a little above what lithium settling voltage is, then maybe boost reconnect a bit lower than that. It depends on what the "user" wants, which is nice. I think I have mine at boost=27.2v and float at 26.7v. It will go into Boost every morning and top off the battery. Mines is for standby power and sometimes I just run it to take some load off of my house because I have those panels just sitting there. We are fully grid tied with a big system so I don't need to work my battery system to save money. Just to play with and wait for when it might really be needed.