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Dual batteries or high-output alternators? - part 1

To make things easier think of the battery as a water tank and the alternator is the punp that fills it. The more water the pump can provide the less the chance your reservoir will run dry. For some applications a short term drain will be acceptable. Winching, in analogy above, is like pulling the plug out of the reservoir. Adding a second reservoir is quite feasible but it will only double the time you can 'drain' them - and also double the time needed to refill.

An heavy electric winch on full pull takes between 350 and 450 Amps at 12 Volts, that's roughly 5000 Watts (12V x 450A=5400W). That's almost like shorthening the 2 posts and this puts an tremendous load on all parts concerned. Those 5000 Watts are required all the time.

At the start at 13,6V you have about 370 Amp draw.

If you pull hard the voltage starts slowly to drop from the 13,6V as it is without a charge to 12 Volts and below. At 12 V you have already 415 Amps draw. Under 12 Volts discharge begins.

The required voltage your starter needs is around 9,6 Volts and without that it will not engage. Once your battery drops there the winch will draw 520 Amps.

At 8 Volts the ignition switches off on most petrol models. First missfire occurs and then the engine dies. It's slightly better on diesels as they requiere no electricity. But wait, that's not right. Modern diesels have an diesel switch off valve required to stop the engine. This needs only a small amount of juice to keep open but if this is no longer available your diesel will stop as sure as any petrol engine. On elder diesels you had an manual shut off valve. In that case you can pull until your battery burns out. At Those 8 Volts the draw from the battery is 625 Amps! Those over 600 Amp draw is a tremendous stress for the battry as well as for the lines.

That's it pretty much. If you stalled the engine by running under this elusive 8V barrier you will need a jump start or a tug to restart it. You can wait for half an hour for the battery to recover as they are supposed to do.

In all this time the alternator tries to refill the battery. The standard alternators used to be way too small for the requirements of this. Land Rover used 35 and 45 Amps models. Today with the increase in electronics they have at least 100 Amps. Now let's start with such an 100 Amp unit. If your winch pulls 2 minutes on full load (which isn't much) your alternator needs 10 minutes to recharge the battery. This however is an ideal figure not reached under normal conditions. It's much more likely you will need 15 minutes to recharge for every 2 minutes of pull. If you have an 45 Amp unit you will need at least half an hour for this - this makes an 7% resp. 14% duty cycle.

That's not too bad for the winch too. The winch, as any electric motor, puts out only a fraction of the input as mechanical energy. In good circumstances not less than 2/3th are put out as heat. Imagine an 3000 Watt heater inside the small winch and it becomes clear why most electric winches are burned out under heavy work and why you msut give them time to cool down.

In fact the whole scenario is even worse. The resistance inside the winch increases while the temperature increases so producing even more heat. The alternator gives it's rated output only above an certain rpm and if the battery is discharged enough - and if it's not overheated.

Part 2