charge and discharge voltage limits

Hi guys,

would you please let me know the upper and lower thresholds you are using for charging voltage of your powerwall battery packs?

As far as I know, the upper boundary is 4.2 volts, but from what I understand, no one intentionally charges their powerwall battery packs to that level.

As far as the lower boundry, it seems to me that 3.42 volts would be the lowest level that could be used.

I'm assumingthat a24 volt inverter accessing a 7s batterywouldrequire a minimum voltage of 3.42volts (24/7), similarly a 48 volt inverter accessing a 14s battery would also require a minimum of3.42 volts (48/14).

thanks,
Paul

Yes, that is correct. The primary reason for only going to a max of 4.1V is for longevity. As well as not going any lower than 3.4V. Staying with those ranges gives a huge boost to cell lifespan.

For doing "capacity" checks, we do charge up to 4.2V and allow for discharge to 3.2V (I believe some may only allow to 3.4V during discharge as well). This gives a MAX of what the cell is Capable of doing. By going slightly lower than the max voltage, you know you always have a little headway available.

In general, when sizing for an inverter, you mostly go with 3.6V - 3.8V when doing calculations. As this is the average voltage. Most inverters can go +/- a certain percentage and be ok; and they show that in the ratings range.

Korishan said:

Yes, that is correct. The primary reason for only going to a max of 4.1V is for longevity. As well as not going any lower than 3.4V. Staying with those ranges gives a huge boost to cell lifespan.

For doing "capacity" checks, we do charge up to 4.2V and allow for discharge to 3.2V (I believe some may only allow to 3.4V during discharge as well). This gives a MAX of what the cell is Capable of doing. By going slightly lower than the max voltage, you know you always have a little headway available.

In general, when sizing for an inverter, you mostly go with 3.6V - 3.8V when doing calculations. As this is the average voltage. Most inverters can go +/- a certain percentage and be ok; and they show that in the ratings range.

Click to expand...

Thanks for the advice.

Is there any advantage to having a slightly higher voltage than required?

For example, would there be an advantage to having 15s with a 48v inverter, or would that be a waste of money?

As long as you're with the max of the inverter, the higher voltage will just give you longer pack usage.

Let's say we have 9s @ 3.7V = 33.3V. The inverter will function up to 35V safely.
For simplicity sake, let's say the pack drops .2V/Hr. Our cutoff is 30.6V (3.4V/cell), so it will take 13.5 hrs to cut off.

Now, if we have 8s @ 3.7V = 29.6V
Again, with .2V/Hr, cutoff would be 27.2V. That would make the pack only last 12 hrs.

So there's the big difference between 12Hrs as opposed to 13.5Hrs. And, of course, If you go with a larger string, such as 48V equiv is 13s. So you could get a larger buffer as the inverter will function at a much higher voltage range. This is partly why if you can afford to and have the room for, to go with a 48V system, it is recommended to do so. Everything gets more efficient and battery pack capacity increases dramatically.

However, most of us run 24V as it's a really good balance between cost/efficiency.


*I hope all got all the above correct; please correct me if I missed spoke or missed something *

This is awesome information, I am putting it in my notebook..... I hate microsoft but love one note

I go 14s because thats the max my inverters and charges will do if i want to go to the top of the battery. Thats why i dont go 15s or 16s. Its all about being able to utilize the hardware you have.

I made this little scale based on avg of about 200 cgr18650e cells I tested, this is at 1A discharge




Yes the scale itself is not to scale, just made it easy for me to see exactly what my usablerange should be.

You can get really good charts for that purpose out there. Also comparing some parts of number of cycles. Generally you get most out of the cells between 3.6 to 4V.

BUT. Its generally resting voltages. So therefore you need to go below 3.6V and in some cases down to 3.4 or even 3.2. This depends on the cell and where you meassure. So basically if you set the limits on the Inverter you need to add all the voltage sag on the connectors and cables. in most cases atleast 0.2-0.3V or even more. And then the sag on the cells.

So even if you are not going down below more than to 3.6v resting you need to calculate based on the loaded voltage.

Thanks for all the great info!

I run mine between 3.6v and 4v

Resting voltages or just stopping early to save cycles?

3.6V under high load can easily be 60% SOC.

To save cycles and to be nice to them.
If you charge and discharge them at half the rate and don't use all the voltage they seem to last for ever and never get hot.

I have lots of burner sanyos and they never get hot if you stay under 4.1v

Thats true.

I'm going down to ~3.2V with a 24V system and 7s LiIon bank. My inverter cuts the battery off at 22.8V (/ 7 = 3.25) and my charge controllers are designed for 24V LA batteries, so they stop charging @ ~28.8V, which is 4.11V for each series.