24v Lithium resting voltage

FrozenToast

FrozenToast

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Howzit Everyone

My two Blue Nova batteries in series are reporting 26.4v on the inverter. The inverter is set to 28.8v bulk charge and no float as oddly enough my off grid Growatt provides PV power for load instead of the battery when using the USE2 mode. Ive read USE is normally chosen to have this work.

So the batteries last the full 2 or 4 hours load shedding but its just weird that its spikes up to 28.8v on charge then after an hour or two the inverter stops charging showing the batteries are full and the batteries show 26.4v.

I know its not wise to use the voltage to determine SOC on an axpert type inverter. So I just want to know if its something I should investigate or just leave it if it works type of thing.

Thank you for your time
 
@Stokstert has made a somewhat similar discovery ...

2 Sets of batteries with different behaviour.

I have 2 different sets of 2 x12V (24V) Lipo batteries in series on 2 different inverters. After being fully charged with 28,8V and rested at float at 27,4V, the mains as well as the inverters were shut off. The voltage of the two battery strings were then monitored for 11 hours. The voltage...
mybroadband.co.za
 
FrozenToast said:
Howzit Everyone

My two Blue Nova batteries in series are reporting 26.4v on the inverter. The inverter is set to 28.8v bulk charge and no float as oddly enough my off grid Growatt provides PV power for load instead of the battery when using the USE2 mode. Ive read USE is normally chosen to have this work.

So the batteries last the full 2 or 4 hours load shedding but its just weird that its spikes up to 28.8v on charge then after an hour or two the inverter stops charging showing the batteries are full and the batteries show 26.4v.

I know its not wise to use the voltage to determine SOC on an axpert type inverter. So I just want to know if its something I should investigate or just leave it if it works type of thing.

Thank you for your time
Click to expand...
Hi have a 15s battery so 48v

Charge to 53v
And after charging stops it comes to settle at 50.5v
This takes about 1.5hrs
The bms reports no current flowing out and the SOC at 100% at the resting voltage


With lifep04 you have to judge if it's fully charged

Dependant on if it is charging or not

So while charging the voltage lifts as the inverter pushes up the voltage to push charge into the battery

So if it is at 3.35v per cell it may be there because of this voltage push and not because it is full , if full it will come rest at this 3.35v per cell
But since the curve is so flat it could be a miniscule amount less and be 70% SOC

hence why it is best to charge to above 3.45v per cell to be sure it is full

the rule of thumb is if it is above 3.45v per cell it is full while charging

Edit :the closer you go to 3.65v a cell the more sure you can be , you have overcome voltage losses and amp push

ie if charging at 60amps
The voltage lift is more than on 20a


And 3.35v if resting but this resting voltage in the flat curve can fool you

ie my dc cables are a bit longer unfortunately not by choice just where it had to stand and where the inverter had to go

So i see a bit of voltage drop
And if i aim for 3.45

I don't get them full and sometimes land at 70% soc

Even though the voktage is just about the 3.35v per cell

Hence why i set my charge to 53v to compensate and get a full battery everytime

I should play it down 0.1v at a time to see where exactly i need to be to get it full

But 3.53v per cell isn't to drastic for me

And having a bit higher voltage speeds up the charging
 
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FrozenToast said:
Howzit Everyone

My two Blue Nova batteries in series are reporting 26.4v on the inverter. The inverter is set to 28.8v bulk charge and no float as oddly enough my off grid Growatt provides PV power for load instead of the battery when using the USE2 mode. Ive read USE is normally chosen to have this work.

So the batteries last the full 2 or 4 hours load shedding but its just weird that its spikes up to 28.8v on charge then after an hour or two the inverter stops charging showing the batteries are full and the batteries show 26.4v.

I know its not wise to use the voltage to determine SOC on an axpert type inverter. So I just want to know if its something I should investigate or just leave it if it works type of thing.

Thank you for your time
Click to expand...
13.4 is around the resting mark so 13.2 is a bit low imho. Double check with multimeter on battery to confirm? Are you using a balancer? How are the battery cables? Cable length and gauge? If it charges to 14.4V and is giving you your runtime then I don’t think there’s anything to worry about and it is what it is.
 
Yea it is what it is , it is not like you can open em up and check individual cell voltages
To see which cell is making you rest lower

Why don't we get batteries with copper pads number that correspond to cell 1-3 positive to be able to check individual cell voltage/balance

Or batteries with bluetooth bms so you can actually see the state of balance , they exist we just don't see them in SA

At best you have to wait for every miniscule balance session to shave off the imbalance that exists if ever
 
And naturally not every battery will be exactly the same

.maybe your battery rests at 3.31v and mine at 3.35

Because of some slight chemical differences in your batch of batteries vs mine

And naturally individual battery voltages can cause this ie are they balanced
 
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So I emailed Blue Nova and told them about my setup, and they recommended running a bulk of 28v and a float of 27.6v. So far it seems the voltage is back to where it used to be when LS starts so thats a good sign I guess.

@wingnut771 Yeah it seems to be working fine so I will just follow the Voltages that Blue Nova recommends and see what happens.

@leon.davibe In hindsight I should of gotten a battery with either Bluetooth BMS or one of those batteries that can communicate with the inverter. But you live and you learn.

Thanks for the input everyone.
 
FrozenToast said:
So I emailed Blue Nova and told them about my setup, and they recommended running a bulk of 28v and a float of 27.6v. So far it seems the voltage is back to where it used to be when LS starts so thats a good sign I guess.

@wingnut771 Yeah it seems to be working fine so I will just follow the Voltages that Blue Nova recommends and see what happens.

@leon.davibe In hindsight I should of gotten a battery with either Bluetooth BMS or one of those batteries that can communicate with the inverter. But you live and you learn.

Thanks for the input everyone.
Click to expand...
No worries :)

Just want to clarify: What do you mean by "it's back to where it used to be"?
 
wingnut771 said:
No worries :)

Just want to clarify: What do you mean by "it's back to where it used to be"?
Click to expand...
Well it used to sit "fully charged" at 26.6v or 26.7v. And on all the voltage charts I have seen that .2v means to me it was dropping down to 70% instead of sitting at around 90%.
 
FrozenToast said:
Well it used to sit "fully charged" at 26.6v or 26.7v. And on all the voltage charts I have seen that .2v means to me it was dropping down to 70% instead of sitting at around 90%.
Click to expand...
Have you confirmed battery voltage with multimeter?
 
FrozenToast said:
So I emailed Blue Nova and told them about my setup, and they recommended running a bulk of 28v and a float of 27.6v. So far it seems the voltage is back to where it used to be when LS starts so thats a good sign I guess.

@wingnut771 Yeah it seems to be working fine so I will just follow the Voltages that Blue Nova recommends and see what happens.

@leon.davibe In hindsight I should of gotten a battery with either Bluetooth BMS or one of those batteries that can communicate with the inverter. But you live and you learn.

Thanks for the input everyone.
Click to expand...
Dependant on inverter behaviour

ie if it
Does float properly ie maintain the voltage at 27.6 i would only run that for a while if i believe there is some balancing issues inside the battery

The battery won't accept any more charge if full

So there is no point in keeping it at an elevated voltage in float

ie 3.45v per cell

If you get it charged full enough to pull through loadshedding i would keep the 28
And lower the float to 3.35v per cell ie 26.8

If you struggle to get enough charge in you can go 28.8-29.2v on the charge

The charge rate at the top doesn't slow down if the voltage is a bit higher.

But would never leave the float above the resting point of lithium imo
 
leon.davibe said:
Dependant on inverter behaviour

ie if it
Does float properly ie maintain the voltage at 27.6 i would only run that for a while if i believe there is some balancing issues inside the battery

The battery won't accept any more charge if full

So there is no point in keeping it at an elevated voltage in float

ie 3.45v per cell

If you get it charged full enough to pull through loadshedding i would keep the 28
And lower the float to 3.35v per cell ie 26.8

If you struggle to get enough charge in you can go 28.8-29.2v on the charge

The charge rate at the top doesn't slow down if the voltage is a bit higher.

But would never leave the float above the resting point of lithium imo
Click to expand...
I'm just trying to get to the bottom of why it has dropped "0.2V" first before moving forward.
 
wingnut771 said:
I'm just trying to get to the bottom of why it has dropped "0.2V" first before moving forward.
Click to expand...
Internal balance of battery
And then the suggestion of higher float to sustain 3.45v per cell would make sense to force more bms balance time
 
leon.davibe said:
though

3.45+3.45+3.45+3.05= 13.4

3.35+3.35+3.35+3.35= 13.4

Hence why it settles cause the 3.45cells will settle down to 3.35
Click to expand...
We are talking resting voltage which has dropped 0.2V. Please explain to me how it can bulk to 14.4V with unbalanced cells?

There is not enough info here.
 
There is always a settle down

Post charging

If you were at float for long enough time to have the cells settle down to the 3.35

The float would sustain the one low cell at 3.35 and whe removed it would settle down to reflect it's state of charge and thus affect pack voltage
 
wingnut771 said:
We are talking resting voltage which has dropped 0.2V. Please explain to me how it can bulk to 14.4V with unbalanced cells?

There is not enough info here.
Click to expand...
A good bms will allow up to 3.75v per cell a kak one will allow up to 3.8-9v

Hence

3.75+3.75+3.75+3.15=14.4

Bms dependant ie if it allows a pack voltage of 4*3.75 ie 15v

It will be possible to get 15v before charge cuts out if cells is properly balanced
 
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