adding 2 new strings to 2 existing battery strings

[jtvt]

I have two 16s 280ah strings and want to add 2 more. What is proper procedure for this? My thought was to top balance the new strings and let them settle. Then charge existing strings to as close to the voltage of the new strings as possible and connect them. Each string would be fused and using 4/0 cable.

thx

 

[DThames]

I think there are many ways you could do it. Myself, I would charge both up, let both settle a couple of hours, then connect them in parallel with a current limit resistor (like 0.5 ohms) in the circuit. Let them set until they were the same and then remove the resistor from the circuit.

A 16ga 100 extension cord is about 0.8ohms if you short the prongs on one end, so you have 200 feet of wire in the circuit.

 

[jtvt]

The two original strings are in use. I won't have a long window to take the ones in use off line. Maybe I should top balance the new ones. Charge to "full" the ones in use. Then swap. Take old ones out of service. Put the new ones in service. Draw new ones down a bit, then recharge them to "full". Disconnect for a couple hrs to let them settle, then add both old and new into the system. That way they will all be charged using the same equipment and settings. Good idea on the resistor.

 

[HRTKD]

I would top balance the new ones, then let them rest. Charge the existing batteries, then let them rest. Add the new batteries to the bank and give the entire bank a charge. You're going to end up relying on the BMS to stop the charge on some of the batteries, while the others catch up. With old/new batteries, that's what will happen unless you put in a battery balancer that keeps all four batteries balanced.

 

[DThames]

While not as safe as with a resistor, this is what I did that one time I did this. I had a Bluetooth BMS on each battery string.
1. Connected first battery, let it fully charge, in the BMS control turn it off.....making the BMS open circuit, taking the battery out of the system.
2. Connect the second battery and let it fully charge, then remove input power from the charger
3. Took measurements of both batteries with my meter and the BMS monitor to understand any meter offsets that might come into play.
4. Put a light load on the second battery, remove the load, and see what the static voltage is....repeat until the static voltage on battery 2 is equal with the voltage on battery #1.
5. Using the BMS control, turn battery #1 back on, which puts the first battery in the charge/discharge mode, parallel to battery #2.

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