What to do with 29.6v batteries (8s Lithium Ion)

spectre338

New Member
Joined
Nov 15, 2021
Messages
3
I bought some used battery packs to harvest some cells and discovered that they are in great working order. It would be a waste to salvage them, their formfactor is great for my overland trailer. Only problem is that their nominal voltage is 29.6v which I think is outside of the spec for my Victron SmartSolar MPPT 100V 20 amp (48v). Is there a MPPT charge controller that will work with 8s Lithium Ion?
 
Last edited:

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
1,918
Location
USA
Using VC in demo library, the 100/20-48 can be set to 24V nominal, user defined and expert mode and up to 34.78V absorption:

1637033616927.png

This is more than enough to get to 4.2V/cell for Li-ion.

So your existing controller should have no issues properly charging your 8S Li-ion.

Your bigger concern is many 24V inverters have a 32V upper limit.
 

spectre338

New Member
Joined
Nov 15, 2021
Messages
3
Thank you for the info.
My apologies for the dumb question but how do I work out the Float and Absorption values?

I know that 4.2v per cell is 33.6v, should that be the float?
 

spectre338

New Member
Joined
Nov 15, 2021
Messages
3
Using VC in demo library, the 100/20-48 can be set to 24V nominal, user defined and expert mode and up to 34.78V absorption:

View attachment 72401

This is more than enough to get to 4.2V/cell for Li-ion.

So your existing controller should have no issues properly charging your 8S Li-ion.

Your bigger concern is many 24V inverters have a 32V upper limit.
You are correct my inverter (Quite Power QP-1800 ) is 20v-32v, are you aware of one with a wider input range?
 

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
1,918
Location
USA
Victron Multiplus are 33V.

Absorption is determined by your desired peak voltage. Li-ion cells are typically MUCH shorter lived than LFP with 100% depth of discharge cycle lives of ~500. You can notably increase cycle life by limiting voltage extremes.

3.92V is a magic number. If you never charge above this, cycle life will be dramatically improved; however, you're giving up about 30% of your capacity.

4.05V is about 80-85% SoC, and can also significantly improve cycle life, but that is still above your 32V inverter limit.

Circling back to before my first post:

It's critical that these batteries have a built-in BMS, or you have fitted one to it to provide cell protection. These cells are typically a fire hazard and will do harm to persons and property if not properly protected with a BMS.

Assuming you have a properly functioning BMS, since you already have the hardware, I recommend you set Absorption to 31.84V (3.98V/cell), float to 31.74V (arbitrary 0.1V below absorption). Disable equalization. Do not discharge below 24V.

You should be able to use about 60-70% of the available capacity of the battery. See if that works for you.
 

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
1,918
Location
USA
Additionally, with Li-ion (3.6-3.7V nominal cells), BMS cell balancing should always be enabled UNLESS you have known issues with some cells being weaker than others. If that's the case, then you need to experiment with BMS balancing during only charge and only above a certain value.
 

NVS

Solar Enthusiast
Joined
Sep 14, 2021
Messages
97
With lithium ion you could only charge them to 4v per cell or 32v per pack. It will extend their life. Just run their cycle 22 to 32volts or 2.7 to 4.0 volts per cell
 
Top