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The use of voltage reducers in rectifiers.

The use of voltage reducers in rectifiers.
13 Sep 2018

The use of voltage reducers in rectifiers is appropriate to accommodate the number of cells in batteries or to achieve faster recharge. 

An inevitable problem when it comes to calculating a battery is theoretically obtaining, for example, a result of 167 Ah.

If we are calculating a NiCd battery with medium discharge, perfect, Emisa’s MP160 gives us that exact value, see links: http://norwatt.es/wp-content/uploads/2017/06/Bateria-NiCd-Emisa-LP-HP-MP.pdf and http://norwatt.es/wp-content/uploads/2017/06/Bateria-NiCd-Emisa-LP-HP-MP.p

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However, if the calculation requires 170 Ah, we have no choice but to install 189 Ah (the cell MP180) with a resultant increase in costs.

At times, it is more viable economically to add cells to a series instead of increasing the capacity of all of them.

The problem then, tends to be that the maximum voltage needed to recharge this battery would exceed the maximum value allowed on distribution bars.

Another problem is the need for a very fast battery recharge (as for NiCd batteries), which implies a very high fast charge voltage.  1.6 - 1.65 VDC per cell would be possible depending on the make and model of the battery.

In order to resolve both problems, a voltage reducer is installed (normally inside the rectifier), which is no more than diodes connecting the battery and the output bars, so that under normal conditions (float or discharge) they are short circuited through a normally closed contact, that functions when the device is on fast charge.

In this way we can have the battery with a higher voltage than that of the bars, even though they’re in the same circuit.  The voltage difference is independent of the current (as would happen if we used a resistance).

NOTE: when it comes to calculating heat loss, you must take into account that, although there might be a significant voltage in the reducer, this only works during fast charge, and therefore, the battery voltage will increase soon after, limiting the current, and thus the emission of heat.

See attached diagram of activated and deactivated reducers.

voltage reduction

Example of use of a voltage reducer:

We have a system of 110 VDC with the usual margins of +10% -15%

  • Rated voltage: 110 VDC
  • Maximum voltage: 121 VDC
  • Minimum voltage: 93.5 VDC

And we want to connect a NiCd battery that has 83 cells. Its voltage would be:

  • Float voltage: 83 cells x 1.45 VDC/cell = 120.35 VDC. No problem, this is within the margins.
  • Fast charge voltage: 83 cells x 1.5 VDC/cell = 124.5 VDC. This is above the maximum allowed.

When the battery voltage is 124.5 VDC, we need the output consumption to remain less than 121 VDC, so we need to create a of 124.5 - 121 = 3.5 VDC.

Please note, that it is habitual for manufacturers to have predefined voltage modules and in this case the voltage decreases would be of 3.8 VDC a

Rodrigo Suárez Cueto

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