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Two Parallel battery strings?

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  • #16
    Cable matching certainly has something to do with it, but is just a moot point because we can somewhat manage cable impedance. What you cannot manage is the batteries internal Impedance, and no two batteries are equal. 2, 3, or 4 strings makes no difference, parallel circuit laws apply. The string with the lowest impedance does most of the work.

    Where parallel strings work, and works very well is in Float Service for emergency standby power used Telecom. Data, and Military. where outages cannot be tolerated. They are never cycled, and use a Constant Current / Constant Voltage charge algorithm set to Float Voltage. The type of battery used is extremely low impedance that can tolerate high C--Rates of 30 minutes if needed. (Lead Calcium) heir cycle life is poor of less than 300 cycles, and cost a lot more than Deep Cycle Batteries solar uses. .

    One method you can use to parallel Pb cells is to use 2-volt-cells and connect them in parallel at the cell level like lithium batteries. Bu there is the catch. If you are using 2-volt cells, means you are building a monster because 2-volt cell capacity ranges from 2000 AH to 8000 AH with each cell weighing some 400 to 2000 pounds each x 24 cells for 48 volts.

    Best advice anyone can give you is if you need 400 AH, buy 400 AH batteries. That is going to be either a 4 or 6 volt battery.

    Additionally parallel strings can be extremely dangerous. Example if you do maintenance on one string, like replace a bad battery, you are going to have one bad day when you reconnect and melt some connectors and smoke wires. Never occurred to you the two battery strings were no longer at equal SOC. When you made the final connection, the two strings are going to Equalize almost instantly when the battery on-line supplied thousands of amps of current to the one you closed in, thus melting connections and smoking wires between them. It did not have a rectifier or charger to limit current between the two stings. Thought never crossed your mind you need to have a second charger on hand, to bring the off-line battery up to equal voltage. You failed to test voltage to make sure the batteries are .25 volts or less difference. Today you learn the lesson when that final connection is made. Too late now, pay up.
    Last edited by Sunking; 06-09-2020, 07:35 PM.
    MSEE, PE

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    • #17
      Originally posted by Sunking View Post
      Cable matching certainly has something to do with it, but is just a moot point because we can somewhat manage cable impedance. What you cannot manage is the batteries internal Impedance, and no two batteries are equal. 2, 3, or 4 strings makes no difference, parallel circuit laws apply. The string with the lowest impedance does most of the work.

      Where parallel strings work, and works very well is in Float Service for emergency standby power used Telecom. Data, and Military. where outages cannot be tolerated. They are never cycled, and use a Constant Current / Constant Voltage charge algorithm set to Float Voltage. The type of battery used is extremely low impedance that can tolerate high C--Rates of 30 minutes if needed. (Lead Calcium) heir cycle life is poor of less than 300 cycles, and cost a lot more than Deep Cycle Batteries solar uses. .

      One method you can use to parallel Pb cells is to use 2-volt-cells and connect them in parallel at the cell level like lithium batteries. Bu there is the catch. If you are using 2-volt cells, means you are building a monster because 2-volt cell capacity ranges from 2000 AH to 8000 AH with each cell weighing some 400 to 2000 pounds each x 24 cells for 48 volts.

      Best advice anyone can give you is if you need 400 AH, buy 400 AH batteries. That is going to be either a 4 or 6 volt battery.

      Additionally parallel strings can be extremely dangerous. Example if you do maintenance on one string, like replace a bad battery, you are going to have one bad day when you reconnect and melt some connectors and smoke wires. Never occurred to you the two battery strings were no longer at equal SOC. When you made the final connection, the two strings are going to Equalize almost instantly when the battery on-line supplied thousands of amps of current to the one you closed in, thus melting connections and smoking wires between them. It did not have a rectifier or charger to limit current between the two stings. Thought never crossed your mind you need to have a second charger on hand, to bring the off-line battery up to equal voltage. You failed to test voltage to make sure the batteries are .25 volts or less difference. Today you learn the lesson when that final connection is made. Too late now, pay up.
      Just curious, are there commercial products available that can be used to ensure safe parallel connections? I imagine a module that connects to 2 banks and checks for safe voltage difference before closing a contactor to parallel the banks. There would likely be some loss through the contactor but more safe.

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      • #18
        Bus bars and matched factory crimps are about as pristine as you can get. Very hard to DIY.
        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

        solar: http://tinyurl.com/LMR-Solar
        gen: http://tinyurl.com/LMR-Lister

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