Something From Nothing
Part of Chapter Charger Output Current Limit
BUT WHAT DOES this limit really mean? In many users’ minds, it has become a minimum value that the user could expect the charger to deliver. Remember that, originally, it was a guarantee that the current wouldn’t exceed 120% of the rated value. You could still expect that the charger, or dynamo, would deliver 100% of its rating, and the current would be limited in all cases to somewhere between 100% and 120%.
Some users thought, of course, they could always depend on that 120% - that they were getting a 120 Adc charger for the price of a 100 Adc charger. Do you really think that you’re going to get that for the price of a 100 Adc charger?
Consider the variations in the operating conditions for a battery charger. It’s called upon to charge a battery that’s been discharged anywhere from a few percent down to almost zero volts, up to its maximum permitted equalize voltage, for a stated maximum number of cells. And it must do that even with an ac power brownout (12% below nominal ac voltage) or up to 10% over the nominal voltage. Over a temperature range of 0 °C to 50 °C. With a connected load ranging from nothing up to 100% of its rating.
A combination of extreme operating conditions – ac brownout (-12% voltage), maximum equalize voltage for the highest possible number of cells, and a full load condition – defines the target for worst-case charger design. The charger is guaranteed to deliver 100% of its rated current under those conditions, but it isn’t guaranteed to deliver more than 100%.
When a battery charger reaches current limit the dc bus voltage is determined only by the battery, not by the charger. When the ac power is restored after a failure, the charger will be in current limit, because the battery has been partially discharged, if only by a little, and its terminal voltage will be below the float voltage. Until the battery gets recharged to the float voltage, the charger remains in current limit. The charger output voltage will match the battery voltage. Period.
Typically, a battery charger doesn’t run at the extreme conditions noted above, and so it may be able to provide more than 100% rated current. What we’re saying is that you shouldn’t count on it.