Adding 15s 48v battery in parallel with existing 16s batteries

[The Good Package]

I've acquired a |48v|100ah|15s| lifepo4 battery and I want to add it in parallel to my existing |48v|100ah|16s| x 4 (400ah) array.

- The 15s battery has a recommended charge voltage of 54.75v and BMS over voltage cutoff of 56.25v
- Each 16s battery has a recommended charge voltage of 58.4v and BMS over voltage cutoff of 60v

I'm thinking of configuring a grid / solar charge cutoff of 55.5v for the entire array, so I would be consistently overcharging the 15s unit and undercharging the 16s units. This should produce a loss of about 5% (20ah) from the existing 400ah array (5ah per battery), but I would be gaining another 80ah in return from the 15s.

Is this doable without any long term issues for the 16s batteries? Are my estimates correct? I don't care about the long term health of the 15s battery by overcharging since I acquired it for very little.

 

[bhavin66]

Only suggestion I could think of is reduce the charge cutoff to 55.2v and that would keep both sets within limit and on the lower end 47v or 48v should be good

 

[The Good Package]

Thanks - I'll be using a Victron BMV-712 + 500a SmartShunt to monitor the entire array as a single 55.2v | 480ah battery. The SOC calculation won't be perfect, but it should be within 10%. I'll be using the relay on the BMV-712 to turn on/off the grid charger based on SOC, since I'm thinking voltage alone will be too unreliable and I don't want to discharge the 16s batteries too far beyond 80%.

 

[timselectric]

I've acquired a |48v|100ah|15s| lifepo4 battery and I want to add it in parallel to my existing |48v|100ah|16s| x 4 (400ah) array.

- The 15s battery has a recommended charge voltage of 54.75v and BMS over voltage cutoff of 56.25v
- Each 16s battery has a recommended charge voltage of 58.4v and BMS over voltage cutoff of 60v

I'm thinking of configuring a grid / solar charge cutoff of 55.5v for the entire array, so I would be consistently overcharging the 15s unit and undercharging the 16s units. This should produce a loss of about 5% (20ah) from the existing 400ah array (5ah per battery), but I would be gaining another 80ah in return from the 15s.

Is this doable without any long term issues for the 16s batteries? Are my estimates correct? I don't care about the long term health of the 15s battery by overcharging since I acquired it for very little.

1. I don't like either of those over voltage cutoff's.
At 3.75v you're already doing damage.
2. Your bottom limits need to protect the 16s.
And the top limits need to protect the 15s.
Mixing the two together limits the capacity of both.
But as long as you stay within the middle of both it will work.

 

[Quattrohead]

I don't like it at all.

 

[The Good Package]

1. I don't like either of those over voltage cutoff's.
At 3.75v you're already doing damage.
2. Your bottom limits need to protect the 16s.
And the top limits need to protect the 15s.
Mixing the two together limits the capacity of both.
But as long as you stay within the middle of both it will work.

The over voltage cutoffs are hard coded by the BMS and can't be changed. I've added the battery to my array and it seems to be working well. Keeping the max charge cutoff at 55.2v will keep the 15s cells at/under 3.68v. I'm not going to let the overall SOC drop below 30% to ensure I'm not running the 16s batteries too low. I may not be getting the full potential out of the 16s units, but I'm still gaining another 80ah for next to nothing from the 15s, so overall I think it's worth it.

 

[JohnZEBSolar]

I've acquired a |48v|100ah|15s| lifepo4 battery and I want to add it in parallel to my existing |48v|100ah|16s| x 4 (400ah) array.

- The 15s battery has a recommended charge voltage of 54.75v and BMS over voltage cutoff of 56.25v
- Each 16s battery has a recommended charge voltage of 58.4v and BMS over voltage cutoff of 60v

I'm thinking of configuring a grid / solar charge cutoff of 55.5v for the entire array, so I would be consistently overcharging the 15s unit and undercharging the 16s units. This should produce a loss of about 5% (20ah) from the existing 400ah array (5ah per battery), but I would be gaining another 80ah in return from the 15s.

Is this doable without any long term issues for the 16s batteries? Are my estimates correct? I don't care about the long term health of the 15s battery by overcharging since I acquired it for very little.

Hey, Did you get this mismatched battery system running? I am thinking of doing the exact same thing and was wondering how it turned out? Any additional info would be appreciated
Thanks