Undervoltage cutback [tour 19]

Undervoltage cutback – or Battery Discharge Protection

When a battery is discharged its voltage falls. It is a widely known fact that discharging a battery too far can damage it, especially if it is left in the discharged state. This is why you really should fit some sort of Battery Condition Indicator.

If you are selling your machines, it is tempting to want a controller that has undervoltage cutback included. The idea is that this will stop users discharging the battery too far, damaging it and claiming a replacement battery under guarantee!

However, as a battery is cycled in normal use, its capacity gradually drops with each cycle: in effect the battery is getting smaller with age. So a new golf caddy (or whatever machine) will have enough capacity in its batteries to do well over 19 hole, but as it ages this will drop off until it only just does 19 holes, then 18, then 17 before it hits this undervoltage cutback point.

So – whilst the cutback may stop some premature guarantee failures – it is certain to cause other customers to complain later in the product’s life. Just where is the minimum point in this complaints curve?

Really the only possible way to monitor battery misuse is to monitor it properly with a microcontroller data-logger. Hardly economically viable! Otherwise I guess one could fit undervoltage cutback with a 9 months time-out, but even if this made economic sense, the ideal time would vary from user to user!

So, although 4QD do fit undervoltage cutback to some controllers, we are not enamoured with it and its only sensible use is, for instance, on the Pro-120 controllers where excessive reduction of battery voltage will cause the main relay to de-power, making the controller switch itself off. Undervoltage cutback will prevent this!

The other disadvantage with battery discharge protection is that it limits the controller to a single battery voltage: our 4QD series are wide range and the 48v version may also be used off 36v or 24v – but not if battery discharge protection is left connected!

User Option

On the Pro-120, undervoltage mechanism is fitted, but is set to a low voltage (around 15v) so it is effectively disabled. It may very simply be enabled by fitting an appropriate resistor in the ignition wire, see A typical wiring diagram.

This gives the option of an OEM supplying new vehicles with protection fitted, then, when a customer complains of problems that indicate his battery is failing, he can be told how to disable the protection. This will give him warning that he soon has to lay out for a new set of batteries!

It also allows a variable resistor to be fitted to give a variable level of discharge protection. This could be used, for instance, to give battery current limit.

If you want a full technical explanation of how the undervoltage works for the Pro-120, see Internal power supply and protection circuitry, Pro-120, DNO and VTX series.


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