Proceedure for adding a pack to existing battery bank

[scpanish]

I have an existing battery, 2 yrs old, consisting of two paralleled 48V packs of Eve 280Ah cells, JK BMSs. I am adding a 3rd pack of identical spec cells to get more capacity. I assume the general procedure is to match the voltage of the new pack to the old before attaching the new pack to the busses, to minimize surge and eddy currents. But I don't know the details of the procedure. For instance, how closely do I need to match the voltages of the old and new packs? Should the existing packs settle without load to a resting voltage (tough since they power the house)? Or is it adequate to turn off the house loads for a minute or so, verify there is little voltage change, and then connect the new pack? I have 100A fuses on the positive buss connections, so I assume I should connect the negative first (which is the BMS controlled line) and then the positive, which will also have a 100A fuse. Can I use the BMS to initially block charge and discharge current, and then turn the on after completing the connections?

Am I overthinking this???

Thanks in advance you all!

 

[740GLE]

For the inservice battery, drain it down to under 80% SOC and verify voltage delta of about 1-2v or so with the new battery.

Resting voltage of say between an 80% and 30% should be around 3.25-3.3v per cell.

Some charge will flow from higher to lower charge batteries, a couple cycles it’ll balance out.

The flat voltage curve makes it super easy to connect more batteries together at various SOC levels. Just don’t do extreme SOC.

 

[toms]

@740GLE is bang on with the advice. Only thing i can add is make sure you have a high AIC fuse (eg class T) between the packs. Typically use a Bus bar that inverters / chargers and each fused battery pack connects to.

 

[740GLE]

I crossed my fingers with 200a Mega fuse of my Lynx and the 200a “breaker” that luyuan provided with their battery box.

I forget the voltage delta between the old 230ah 24v and the new 280ah 24v, if I had to guess .3v or so. My initial inrush was about 300w into the new battery and it ramped quickly down to <30w which seemed to last a while.

 

[scpanish]

thanks...seems less critical than I expected. It isn't difficult to normalize the voltages so i'll do that anyway, to maybe .25V.
FYI I use Type S, semi-conductor fuses between the positive buss and the battery inputs, extremely fast and hard to find. Very expensive, but I have a free source from a friend who has closed his electronics business (mfr of custom power-related stuff). Similar fuse type on the output to the inverter, but 400A.