dissipative battery balancer

Discussion in 'General Electronics Chat' started by casey.mynott, Jan 2, 2013.

  1. casey.mynott

    Thread Starter New Member

    Dec 30, 2012
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    0
    Hello all!

    I have done quite a bit of searching on the topic of charging a series string of lead acid batteries and can't really come up with a good plan of attack for a solution that fits my idea of a shunt style balancer with precise control.

    I have some experience bread boarding simple circuits. I have never really designed a circuit with this level of complexity but would love to get more involved with circuit design and implementation!

    My example is for a 48 volt series string of 12v agm 16ah batteries. So, 4 batteries wired in series. http://www.odysseybatteries.com/battery/pc680.htm



    What I understand:

    When charging an AGM style 12v battery the charging profile follows Constant Current (CC) to bring the battery voltage up and then Constant Voltage (CV) to finish the charging. For this situation the AGM's I am using can see high charging current, about 25 amps. So empty, the batteries will move from an individual voltage of 11.8 to a Constant Voltage of 14.8. When all batteries are sitting at 14.8 volts with 1 amp of charging occurring the batteries are considered fully charged.

    Unfortunately, with AGM lead acids, or any for that matter, over time and use, they get "out of balance" when charging a series string. So, by applying an overall charge voltage to the pack, in this case 59.2 volts (14.8v*4) total, some batteries reach absorption phase before others and their voltage is higher than it should be. Which, leads to gassing of the battery that is over voltage OR batteries that are under charged OR both.



    What I am attempting to do:

    Shunt the excess current in heat to allow all batteries to reach absorption phase never exceeding the max allowable of 14.8 volts per battery.



    What I would like to see in the circuit design:

    -Precise control of the shunting "turn on" voltage of 14 volts.

    -Precise control of the ramp of how many amps are shunting at 14 volts all the way to 14.8, no battery can exceed 14.8 volts. (I have done a bit of PIC programming in that far past but need a refresher course big time!)

    -Max amps shunting of 3 amps or for this purpose, lets say 50 watts

    -2 indicator LED's. One for the circuit is active at 13.9 volts. Green. The other, the circuit is shunting from 14.0 to 14.8, Blue.

    -Draw on the battery when not in use in the 5 uAmp range.

    -Some sort of fuse in the circuit to protect it.


    Some batteries will shunt longer than others as these are the ones that reach the peak 14.8 volts first. The charger I am using ramps down the amperage when "peak" pack voltage is reached. So if charging to a peak voltage of 59.2 volts at 25 amps, when the pack reaches peak voltage the 25 amps begins to ramp down in a fashion that the pack voltage can be maintained, this is the absorption phase.



    Some resources I have found:


    A shunt regulator using zener diodes and an incadesecent lamp

    http://www.evdl.org/pages/hartregs.html


    A simple method to ensure balancing is to place a clamp regulator across each cell which shunts current when voltage reaches the constant voltage charge point.

    http://electronics.stackexchange.com/questions/34748/how-to-make-a-balanced-battery-pack


    Could be useful!

    http://www.re-innovation.co.uk/web1...l-power-monitor-96/70-design/design-documents



    It seems to me that the ideal solution for precise control could utilize the 50watt resistor attached to a fairly big heat sink. When the voltage of 13.9 is reached the Green LED lights up and to show that the circuit is ready. Any drop below 13.9 and the LED turn's off. When the voltage of 14 is reached the circuit begins to pass current through the resistor. Max shunting should occur at 14.8 volts.


    Thank you for any feedback you can provide. I have a grasp of what I would like to see this circuit do. Implementation has me stopped cold. I am in the process of attempting to draw up a rough schematic to get a visual idea of what I am trying to do. ;)

    Casey
     
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