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Originally posted by gardner
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Constant Current is just what it sounds like. You pump in X amount of current without a respect to a voltage source. If you have a 100 volt power supply that you can control current limit say 20 amps, you can pump in 20 amps on a 12 volt 200 amp battery with no problem until the battery voltage reaches 15 volts. The battery voltage will rise to 100 volts if it does not blow up first. So in a Bulk or CC mode you pump in X amount of current until the battery voltage reaches a set point of say 2.4 vpc or 14.4 volts on a 12 volt battery. At that point the charger turns off and the battery should be at roughly 80 to 90% SOC.
Constant Voltage is mostly like it says, but also has a Constant Current aspect to it. Lets say you use that bench power supply with current limit set to 10 amps, and voltage to 15 volts. With me so far?
Now let's say we have a 12 volt 100 AH battery that is fully discharged and an OCV of 10.5 volts, and an Internal Resistance of .02 Ohms and connect it to the Power supply. Still with me?
Initially when we connect the battery and if we monitor the voltage and current we will see a current of 10 amps, and a voltage of 10.5 volts + (10 amps x .02 Ohm's) = 10.7 volts. Not the 15 volts you set the voltage too on the DC power Supply. The reason is the Power Supply is acting like a Current Source of 10 amps. That 10 amps flowing through .02 Ohms is only .2 volts rise in the battery voltage. Understand so far?
As we charge the battery voltage slowly starts to rise and the current remains a constant 10 amps. It does so until the battery voltage reaches 14.8 volts. At that point the transition from Constant Current to Constant Voltage begins. When the battery voltage reaches 14.9 volts the current has tapered to 5 amps. As the voltage continues to rise, the current tapers down and will continue to do so until we hit equalization saturation voltage occurs and the battery voltage can go no higher. At that point you are now equalizing cells and the current will be very small. It can take several hours to bring the weak cells up at such a slow pace.
So don't be afraid to set the current limit above c/20. You can if you want, but it will just take a lot longer. Just don't set it any higher than C/8. Only other thing to guard against is over heating above 120 degrees which would be very rare under the right conditions on a hot summer day in the 100's, or low water levels.
OK?
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