Ibex Battery Systems

Application Note 5
So How (And Why) Do You Cascade Chargers?

Lead-Acid batteries are made up of series-connected 2V cells. Minor variations occurring between cells usually don't interfere with charging when the charger is in constant-current (bulk) mode or elevated constant-voltage (absorption) mode (see App Note 4 for a discussion on charging modes).

However, if the number of cells is appreciably higher than 6 (for a 12V battery), then variations in cell internal leakage current may cause the cells to be maintained at different levels of charge while the charger is in float mode. This is because the small float current passing equally through all cells may not be sufficient to maintain the charge of the cells with the highest internal leakage currents.

If this happens, it is possible that, during deep discharges, cells that have not been maintained at 100% charge will become reverse charged. This usually destroys the cell and renders the battery unfit for use. This is seldom a problem for 12V (or lower) batteries. For a higher voltage battery, partially discharging and recharging every 3 months or so will re-equalize the charge on each 2V cell and allow the battery to be maintained by a float charger.

A way around these problems is to create a high voltage battery with a series string of 12V batteries - each of which has its own charger. Each charger will supply the proper float current to maintain its own battery. The individual batteries may have widely different leakage currents without affecting the other batteries in the string.

Voltages over 100V may easily be obtained. Note that the chargers are not simply connected in series - that would not work. Each charger is connected to its own battery with a positive and negative wire to form a 12V module. The modules are then connected in series to form the high voltage battery.

When cascading batteries and chargers in this way, it is completely acceptable to form a plus and minus battery by connecting the center of a two-charger cascade to ground or, as shown in the diagram, to tap off at a lower voltage in a multiple-charger cascade (optional load).
Series connected battery chargers

Note that in the example shown in the diagram, chargers N and N-1 must supply the main load with current I1 Charger 1 however, must supply the optional load with current I2 as well as supply the main load with current I1 so its total current draw is I1 plus I2.

When putting large numbers of chargers and batteries in series, be mindful of the charger's output isolation specification. Ibex Battery Systems specifies the output isolation of its chargers at 150V (output terminals to chassis).

When current levels are small, it may be cheaper to obtain voltages higher than 12V by using a DC/DC converter. App Note 7 discusses this.