question

florian-koenigk avatar image

Settings for Li-Ion 48V Battery Pack

Hi folks!

I have the MPPT 100/20 48V Solar Charger up and running, charging a 48V Li-Ion battery pack.

The OCV for Panasonic GA cells (13 in series) is going from 42,3V (SOC = 7,5%) up to 53,9V (SOC = 97,5%); This is the working range.

Now I am trying to find the best settings. Could you please help me with that!
How should I set
- absorption voltage
- float voltage
- equalization voltage (really needed for Li-Ion?)

- Re-Bulk voltage offset (what is it for?)
- Absorption duration
?

Thank you very much in advance!

Regards, Flo


MPPT - Solar Charge ControllerBMV Battery MonitorLithium Battery
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

2 Answers
Jason Bolduc avatar image
Jason Bolduc answered ·

How should I set
- absorption voltage

Set this to your high voltage charge cutoff.


- float voltage

Set this to your nominal cell voltage. for example, if using a LiFePo4 16S battery pack it would be 3.2 x 16 = 51.2v. This could be set higher but there are trade-offa.


- equalization voltage (really needed for Li-Ion?)

Never needed on lithium based batteries

- Re-Bulk voltage offset (what is it for?)

Another setting that has a but of personal preference.. typically0.5 to 2v lower than the absorption voltage.


- Absorption duration

Generally not needed with lithium chemistry batteries.

These settings are quite important and setting them wrong can damage the battery and possibly void warranties. Always follow the manufacturer's settings.


2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

mahep avatar image
mahep answered ·

May I ask why the tensions cannot simply be set to the same, cutoff value? And if not, isn't the absorption duration essential to make sure the battery is fully charged? Let me explain the thinking behind my question.

Below a pic from the internet showing the Li-Ion charge curve. The curve is determined by three parameters:

- Charge current Iref (determined by the sun as power / battery voltage, right?)

- Vref : basically the same as absorption voltage, to be maintained constant in the absorption phase as the input current decays according to the battery's state, reaching toward zero.

- Ico : cutoff current, to turn off charging (the "tail current" in expert settings). Actually such a cutoff is not necessary on Li-Ion batteries, and it is used only for diagnostic, for example in this programmable charger to indicate a "charged state", but the current actually keeps flowing in and the BMS may continue working to balance the cells even at this small current. That means, one could theoretically prolong absorption time indefinitely (disable tail current), or equivalently, set the float voltage to the same value as absorption voltage.

charge-profile-of-lithium-ion-battery.png


For reference, the lead-acid charge curve, from which the terminology comes from, looks like this (here without the equalization phase), with an additional "float" storage phase.charging-cycle-of-a-lead-acid-battery-11.jpgSo in my understanding, setting the float current to a lower values (the nominal voltage) would probably stop the current flowing in (assuming that diodes would block the current flowing back from the battery into the charger !), and then become active again when the battery reaches its nominal voltage, meaning 50-70% of charge I suppose? But what's the point of charging the batteries fully to let them discharge subsequently?

In conclusion, in my understanding these concepts from lead-acid battery could be left aside and a single tension could be used (the absorption tension), corresponding to a long absorption phase. In that case, in the end of the absorption phase, a low residual current could keep flowing in to maintain the desired charge.

To say more, manufacturers recommend to charge the battery at 90% or 85% charge to prolong battery life. In that case a lower tension than maximum could be set as input. The simulator linked above can be used to determined the proper tension based on the desired state of charge.


2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.