# Battery voltage and discharging

5255 Views 11 Replies 9 Participants Last post by  pm_dawn
Hi,
I am trying to judge state of charge for a my lifepo(TS90Ah,38 cells, ~120vpack) batteries and looking at the graphs from thundersky I am having issues....
I basically want to know the top voltage (100% charged) and the bottom voltage (70% discharge) range for the 90Ah units.

The Thundersky docs show that these voltages (working off a 3C discharge) are from 3.2v (100% charged) to 3.1v (30% charged). This sounds really quite wrong. Am I being a graph retard???
I thought that after a full charge the cells should start at around 4.2v ish and then slowly drop down to 2.5v (around 70-80% discharge).

If the voltage curve changes for different 'C' discharge rates does anyone have use a formula to calculate more accurate pack charge percentages?

Can anyone provide a graph of a cells discharge please?

Thanks!
1 - 12 of 12 Posts

#### favguy

· Registered
Daily: Tesla Model S, EV Build: The Magnificent Favelec!
Joined
·
658 Posts
Under discharge conditions, you have no chance of measuring state of charge by voltage, the thundersky graphs should also be taken with a pinch of salt!

You can measure state of charge at rest, as the drop in voltage is quite linear between around 95% charged and 30%, below 30%, you'll struggle to measure it accurately using voltage as it's falling so fast.

#### Travdude

· Registered
Joined
·
84 Posts
This graph might help.

As you can see there is a huge difference between 3.1 and 3.2 volts!

Favguy is right, it is difficult to tell SOC by voltage alone.

I am waiting for someone to make a good battery monitoring system.

#### Attachments

• 92.2 KB Views: 112

#### illuminateddan

· Registered
Joined
·
118 Posts
Discussion Starter · ·
So what I really need to do is measure the voltage drop under multiple load conditions and then chart all this data to then calculate a formula for a discharge curve, and then use this formula with the packs 'trip discharge history' to calculate what I have used and what I have left based on my recent driving style. Oh.

How long do the batteries take to spring back to 'no load' voltage conditions?

How have other people estimated battery percentages on their BMS systems?

#### frodus

· Registered
Joined
·
4,065 Posts
a better way would be to measure voltage AND Amps, calculate watts and then subtract that from a full charge. Then you'd know SOC.

Voltage alone is not a good measurement of SOC. If you measure energy in and out, it can provide a far better idea on how much energy is left.

#### illuminateddan

· Registered
Joined
·
118 Posts
Discussion Starter · ·
So current BMS's also measure current use, or do they just look after the charging?

#### favguy

· Registered
Daily: Tesla Model S, EV Build: The Magnificent Favelec!
Joined
·
658 Posts
You could use an Ah counting meter to show energy used. This will measure your voltage and current and calculate it for you.

I'm personally using a bargraph style meter built and calibrated to show state of charge between 95% and 30% at rest (30% will show empty) one for each pair of cells, in an array. It's still not ideal, joe public would no doubt still manage to wreck the battery, but it'll be ok for me along with some form of BMS yet to be decided.

#### CroDriver

· Registered
Joined
·
887 Posts
a better way would be to measure voltage AND Amps, calculate watts and then subtract that from a full charge. Then you'd know SOC.

Voltage alone is not a good measurement of SOC. If you measure energy in and out, it can provide a far better idea on how much energy is left.
How to count in looses like heat? Since we're pulling high amounts of power from our pack the heat looses are quite high and can be over 10% of the overall stored energy. This could cause some problems white trying to measure the SOC.

I wonder how Tesla's BMS manages that problem.

#### peggus

· Registered
Joined
·
308 Posts
Just integrate the current, not the power! The energy charge efficiency and discharge efficiency are not the same and varies with temperature, amp draw, SOC, phases of the moon, etc.

Coulombic charge efficiency (Ah in / Ah out) is much more reliable and consistent.

#### MN Driver

· Registered
Joined
·
870 Posts
So current BMS's also measure current use, or do they just look after the charging?
Most BMS's do not measure Ah consumption but there are some that do, if you are willing to build your own based on instructions and can know electronics decently, can solder well, and have the patience you can put together a very nice BMS with a small display that indicates Ah consumption, amps, voltage, voltage deviation between cells, temperature, etc using a programmable PICAXE setup. Likely overkill for most and pointless for some but I'll probably go with that sort of system.

#### glaurung

· Registered
Joined
·
153 Posts
Blue Window dashboard computer has a"fuel gauge". User manual might give a hint how it calculates available energy.Any user have feedback on how accurate it is? Might want one
Harri

#### pm_dawn

· Registered
Joined
·
251 Posts
Hello Harri !

Have you taken a look on the Swedish Forum ?
If you can read Swedish there is a lot of info there for you.
One of the builds are using the Blue Window.