Relationship between voltage of stored charge and % capacity, Lithium and Lead

[Neeves]

I've been sent this article

Ultimate Guide to Lithium-Ion Battery Voltage Chart - Jackery

Its part advertorial

But they quote, or provide spreadsheets for assessing state of charge by simply measuring voltage. Is this accepted, is the relation ship that simple and do, would, all LFP batteries follow the data.

I ask because if this is correct then the more common method of measuring amps in and subtracting amps out to provide capacity is grossly expensive compared to a simply meter measuring voltage and having a parallel scale indication capacity as a %. Basically I'm suspicious.

Jonathan

 

[PaulRainbow]

I've been sent this article

Ultimate Guide to Lithium-Ion Battery Voltage Chart - Jackery

Its part advertorial

But they quote, or provide spreadsheets for assessing state of charge by simply measuring voltage. Is this accepted, is the relation ship that simple and do, would, all LFP batteries follow the data.

I ask because if this is correct then the more common method of measuring amps in and subtracting amps out to provide capacity is grossly expensive compared to a simply meter measuring voltage and having a parallel scale indication capacity as a %. Basically I'm suspicious.

Jonathan

Had a quick skim, looks like nonsense to me. How is it the two charts and the graph all show different figures ?

Basically, the voltage curve of LFP is so flat that taking voltage readings for SOC is pointless. Even the steeper curve of lead acid needs to be done with care, you can't take a reliable reading with any loads or charging taking place.

 

[Poey50]

Had a quick skin, looks like nonsense to me. How is it the two charts and the graph all show different figures ?

Basically, the voltage curve of LFP is so flat that taking voltage readings for SOC is pointless. Even the steeper curve of lead acid needs to be done with care, you can't take a reliable reading with any loads or charging taking place.

Totally agree.

 

[B27]

It's not helpful to say 'Lithium Ion' when they appear to mean LiFePO4. There are a lot of subtly different Li-Ion cells it seems.
Likewise not all 'Lead Acid' is exactly the same, I think my dual purpose house battery has some calcium in it and tends to rest at 12.8 Volts.

The volts when charging or discharging at various rates are more useful IMHO.

Those Jackery things look like very handy kit though, sadly a bit pricy and not really justifiable for business right now.

 

[Kelpie]

You can get a perfectly functional battery monitor for under £50.
Even a notorious skinflint like me thinks that's not a lot of money.

 

[PaulRainbow]

It's not helpful to say 'Lithium Ion' when they appear to mean LiFePO4. There are a lot of subtly different Li-Ion cells it seems.
Likewise not all 'Lead Acid' is exactly the same, I think my dual purpose house battery has some calcium in it and tends to rest at 12.8 Volts.

Calcium has different charging voltages to other LA batteries, what voltages are you using ?

The volts when charging or discharging at various rates are more useful IMHO.

Volts when charging or discharging have absolutely no meaning, with reference to SOC. Volts at the battery when charging will just be that of the charging source. When discharging it will be affected by the current that's being drawn, bigger the current, lower the voltage reading, but it will "bounce" back up when heavy loads are turned off. The only meaningful voltage reading with LA is when they are at rest (no loads or charging).

Those Jackery things look like very handy kit though, sadly a bit pricy and not really justifiable for business right now.

 

[noelex]

Volts when charging or discharging have absolutely no meaning, with reference to SOC. Volts at the battery when charging will just be that of the charging source. When discharging it will be affected by the current that's being drawn, bigger the current, lower the voltage reading, but it will "bounce" back up when heavy loads are turned off. The only meaningful voltage reading with LA is when they are at rest (no loads or charging).

With a lead acid battery it is possible to compensate for the charge or discharge current (providing it is reasonably steady), and arrive at a rough SOC using voltage. It is more accurate if you take temperature and the exact battery chemistry into account.

This is what we used before modern battery monitors were available. These days purchasing a battery monitor is much more sensible and accurate, but it is worth having these graphs available in case the monitor dies at an inconvenient time.

https://www.plaisance-pratique.com/IMG/pdf/SOC_vs-Voltage.pdf





I have not seen similar graphs for lithium, probably because voltage is such a poor indicator of SOC unless you are at a very high or low SOC.

 

[Refueler]

For all Li formats - there are what are called 'Fuel Tables' ... which relate rested voltage to state of charge. It is not correct to say Li has flat voltage curve. They DO have a voltage curve that once rested - will give a reasonable indication of charge state.

BUT : What is not given by voltage - is the capability of the cell(s).

With Li ... similar to Lead Acid and other forms - you can have a full charge voltage - but its capability near zero. Put a load on and cell(s) fail to deliver. This is where the monitoring gear really comes into its own ... by monitoring how much goes in vs what comes out.
But that in itself can create false data. If the monitoring unit is not calibrated or reset at intervals as the cell(s) age.

Personally I prefer load testing - to see the voltage drop for a given load. That then tells you if cell(s) are up to the job.

 

[B27]

.....

Volts when charging or discharging have absolutely no meaning, with reference to SOC. Volts at the battery when charging will just be that of the charging source. When discharging it will be affected by the current that's being drawn, bigger the current, lower the voltage reading, but it will "bounce" back up when heavy loads are turned off. The only meaningful voltage reading with LA is when they are at rest (no loads or charging).

That really is quite badly wrong.

 

[Refueler]

PaulRainbow said:
.....

Volts when charging or discharging have absolutely no meaning, with reference to SOC. Volts at the battery when charging will just be that of the charging source. When discharging it will be affected by the current that's being drawn, bigger the current, lower the voltage reading, but it will "bounce" back up when heavy loads are turned off. The only meaningful voltage reading with LA is when they are at rest (no loads or charging).

B27 said:

That really is quite badly wrong.

No its not actually ....

While charging - the volts you read are the volts charger pumps out to overcome battery resistance to charge.
While discharging - the volts you read are based on the load on the battery - which causes voltage drop.

The only time you can read battery voltage with reference to charge state is when rested and no load in /out.

 

[B27]

Discharge:

Charge.


Lead acid cell voltage vs SOC for various C-rates, in charge and discharge.

This is how real-time battery analysis works. By altering the current in and out of the battery, you can discern its capacity, state of charge and state of health. If you drive a stop/start car, it's probably doing that as you drive along the road.

I had thought the quality of knowledge on this forum was a little better than shown by a few posters on this thread.


Similar graphs are available for various varieties of Lithium and other chemistries.

 

[PaulRainbow]

That really is quite badly wrong.

Actually, it's 100% correct.

 

[Poey50]

Discharge:
View attachment 166697
Charge.
View attachment 166698

Lead acid cell voltage vs SOC for various C-rates, in charge and discharge.

This is how real-time battery analysis works. By altering the current in and out of the battery, you can discern its capacity, state of charge and state of health. If you drive a stop/start car, it's probably doing that as you drive along the road.

I had thought the quality of knowledge on this forum was a little better than shown by a few posters on this thread.


Similar graphs are available for various varieties of Lithium and other chemistries.

For fractional C rates which are typical of marine usage the use of voltage as any kind of guide is only meaningful at the upper and lower knees on the charging curve.

 

[Refueler]

Discharge:
View attachment 166697
Charge.
View attachment 166698

Lead acid cell voltage vs SOC for various C-rates, in charge and discharge.

This is how real-time battery analysis works. By altering the current in and out of the battery, you can discern its capacity, state of charge and state of health. If you drive a stop/start car, it's probably doing that as you drive along the road.

I had thought the quality of knowledge on this forum was a little better than shown by a few posters on this thread.


Similar graphs are available for various varieties of Lithium and other chemistries.

Sorry to say this - but you are taking a very liberal view of the tables and interpreting to fit your point. The tables - if you were to delve deeper - are only a guide and subject to the 'elevated' values induced by load in / out ...
Rested cells provide more accurate value of SOC .... but lack as I say the data of capability.

I can produce tables of Li capacity based on voltage - but the voltages and SOC are RESTED values ... not under load / charge. The same for Lead Acid ...

If this were not true - then why is it advised to meter battery's once rested after use / charging ?? If your stance was correct - then there would be no need to wait - we would just meter at any time ....

Battery analysis ie BMS monitors work on time vs power out / in ..... that is why they need resetting at times.

Here's a LiFePo4 table ... not that the LOWER charge rate shows an increased level through each stage - based on that fact of internal resistance of cells ... distorting the figures .....

Here's the rested voltage vs SOC table for typical LiFePo4 cell :

SOC (%)​	Voltage (V)​
100​	3.60 - 3.65​
90​	3.50 - 3.55​
80​	3.45 - 3.50​
70​	3.40 - 3.45​
60​	3.35 - 3.40​
50​	3.30 - 3.35​
40​	3.25 - 3.30​
30​	3.20 - 3.25​
20​	3.10 - 3.20​
10​	2.90 - 3.00​
0​	2.00 - 2.50​

One of the enemies also of metering while a cell is active - is heat. The cell creates heat as well as electrical power ... that heat further distorts the readings ...... and is why temp is usually mentioned.

 

[Refueler]

For fractional C rates which are typical of marine usage the use of voltage as any kind of guide is only meaningful at the upper and lower knees.

And only as an indication .....

 

[B27]

Here's a typical device that people use to measure battery SOC and SOH:
https://cpc.farnell.com/duratool/d0...8754&s_kwcid=AL!5616!3!426684131180!!!network}!1732284433399!&gclid=EAIaIQobChMIhrab5O6WggMVld_tCh3iJgqcEAQYBiABEgLCQfD_BwE
"

Product Overview​

12V battery tester is designed to test the condition of the automotive battery using conductance method. Unlike the conventional method of draining the battery by applying resistance load to it and obtain the result from the meter gauge; this analyser utilizes a series of pulsed voltage across the battery cells and observes the AC current that flows in response to it. It tests 12V sealed, lead acid batteries down to 2Ah and 12V Lithium batteries for vehicles, motorcycles and scooters.

• Accurate results in milliseconds

That's pretty basic compared to what a modern petrol car is doing, or a mobile phone.

 

[Refueler]

Oh Dear .....

 

[vas]

Here's a typical device that people use to measure battery SOC and SOH:
https://cpc.farnell.com/duratool/d0...8754&s_kwcid=AL!5616!3!426684131180!!!network}!1732284433399!&gclid=EAIaIQobChMIhrab5O6WggMVld_tCh3iJgqcEAQYBiABEgLCQfD_BwE
"

Product Overview​

12V battery tester is designed to test the condition of the automotive battery using conductance method. Unlike the conventional method of draining the battery by applying resistance load to it and obtain the result from the meter gauge; this analyser utilizes a series of pulsed voltage across the battery cells and observes the AC current that flows in response to it. It tests 12V sealed, lead acid batteries down to 2Ah and 12V Lithium batteries for vehicles, motorcycles and scooters.

• Accurate results in milliseconds

That's pretty basic compared to what a modern petrol car is doing, or a mobile phone.

and your point is?

 

[Sandy]

I often scratch my head when I see this mythical 'state of charge figure'. What are they measuring? How are they measuring it? Does it relate to any internationally agreed procedure?

I have 390 Ah of AGM house batteries monitored by Victron kit (MTTP, Shunt and temp sensor) thus should be pretty good at telling me what my SOC is. Yet according to their instrumentation with a discharge of 33 Ah it tells me I have a SOC of 99%.

I am more interested in the voltage and amp hours used*.

*Used being the difference between actual usage and generated.

 

[Neeves]

Those Jackery things look like very handy kit though, sadly a bit pricy and not really justifiable for business right now.

I did note their portable, large scale, power bank. It seems my idea of building a portable Lithium unit is not original and is well established.

Jonathan.