Variable Voltage

[lnvisiblemanParticipant @lnvisibleman]

I am looking into the possibilty of being able to increase the voltage to boat motors by switching on three battery packs, in series, so as to change the supply from 4.8v to 9.6v and then to 15.4v.

The motor will take 18v with no problem.

I have loads of different rotary switches that I can rig up to a servo but cant quite get my head around the required wiring.

Can any of you geniuses (or is that genii? ) come up with a wiring diagram that may help me out ?

Thanks all

[lnvisiblemanParticipant @lnvisibleman]

[Malcolm FraryParticipant @malcolmfrary95515]

Switching between 2 in parallel to 2 in series requires a 2 pole change over. Putting 3 in series and tapping off the voltages is just a wiper selecting the connection points, BUT the wiper must never connect two adjacent connections, and there is the probability that the battery nearest the ground (-ve) will flatten before its mates. Just leaving them in series and using the right ESC makes much more sense and will work. Not many rotary switches are designed to carry serious current, much less switch it.

[lnvisiblemanParticipant @lnvisibleman]

I don't actually need to have them in parallel, all I want to do is to increase the voltage in equal steps.

This is mainly to restrict the speed when being used by a very young child.

[Malcolm FraryParticipant @malcolmfrary95515]

Your proposed system would give you the problem of uneven battery charge states, which is undesirable since nice even recharging becomes a problem. This is the basic cause of the myth of battery memory which has circulated unimpeded for many years regarding NiCad packs.

If there is a need to restrict performance, just wire a nice big power resistor in series with the motor and use a switch to short it out when you want the full power back. The batteries get the care and attention they need, power restriction can happen when its needed and the resistor can be fairly cheap. For half speed, measure the resistance of the motor, take about 2/3 that value, thats the resistance wanted to give half the power to the motor, thus half the speed. Assume that the resistor will get the full supply voltage and use that to work out the power rating of the resistor. This will give a generous safety margin.

[Michael SheridanParticipant @michaelsheridan92093]

For series switching you could do this (but I'm not saying you should). I think this circuit is correct, but if I've missed something then someone please let me know:

I've shown four packs but you can do three or two by shortening it.

As shown NO battery is selected. You can then switch on as many and whichever you want to get the volts, and it will also allow you to perhaps balance the discharge of the packs a bit.

Note the fuses. There is a real risk when building and testing this that you could short a pack out, so you really, really do need a fuse in line with every pack. Obviously the fuses and switches need to be rated for the maximum motor current.

Edited By Colin Bishop, Website Editor on 18/07/2013 07:14:47

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