6 volt motor power source

[The Natural Philosopher]

Model train controller. and a

Speed 600 7.2V

[John Rumm]

In the case of power because generally it does to a certain extent...

Higher voltage batteries will have a higher energy density than a lower voltage one of the same cell capacity.

E.g. a 5Ah 18V batt will store 90Wh, vs 60Wh for a 5Ah 12V battery.

Also the higher voltage will allow more motor power while keeping the battery drain to a reasonable level.

Not a concept many are going to understand. Perhaps just "output power" would be slightly simpler, but then again that ship has sailed!

[John Rumm]

Nope, for a number of reasons.

Firstly the micro USB on the power bank is for charging it - not powering other devices. You need to connect to the full size USB A plug for that.

Even with the right lead, Running 5V on a thin lead with a maximum output of 1A may not give enough motor power.

[Fredxx]

Sorry, are you saying I can get more power out of two cells in series than I can in parallel?

In a real world case where operating times matter rather than some sort of fake advertising, many laptop batteries are series parallel, where there are 2 banks of either 3 or 4 cells in series.

Cell voltages are dictated by chemistry and is normally a given rather than choice.

In terms of a lead acid battery Peukert's law, for a given volume of battery, useful power is independent of voltage (no of cells).

However, I am aware that the size AA is about optimum for Alkaline batteries in high discharge environments.

Again a function of battery size, not how many cells it may have.

It is in many instances, my car engine is specified in terms of kW output.

[John Rumm]

via

to power a 6 volt dc motor?

Not sure about Bosch, but Makita make a small circular saw that uses

10.8V batteries.

While their capacity is relatively low at 1.3Ah, their ability to deliver current is quite high - allowing the saw to draw quite a large current (10A+).

This means that you can actually get quite a high delivery rate of energy (i.e. power) although the implication is that you wont actually get much saw time (5 to 10 mins perhaps) before depleting the battery. That is still equates to quite a number of cuts into a sheet for 18mm ply for example.

In your application you won't need anything like that ammout of peak power, although using a 5V 1A supply limits you to 5W... that will spin a motor and may be enough to make a cut - although I expect it would be easy to stall by applying much pressure.

I think Paul's empirical approach will work best in this case, because you have so many unknowns. So a battery pack made up from normal alkaline dry batteries - say 4 AAs powering the motor with a multimeter on a high current range in in series between batts and motor. You can then feel how the motor performs and cuts, and also look at the current drawn.

The you will know if you can get adequate performance, and also what kind of batteries you need, and what likely run time you will get.

[Paul]

I got the impression you had a motor in hand ?

Have you been experimenting yet ?

You have been asking about a particular voltage which suggested you had a motor selected.

Now, this one, the sale item comes with pictures. There's a picture of the underside of the unit, showing how they selected their motor.

9.1 x 7.0 x 2.8 inch 24V 4A to 15A 10000 RPM (printed on motor) 24V 4A (the laptop-style SMPS power source in the product box)

You can see on the size of the pulleys, there's a slight speed reduction, so that the blade does not spin at 10000 RPM. Maybe the blade spins at 6000 RPM or so, unloaded.

A horsepower is 746W. If the motor really did use 24V 4A, that's

96W and about 1/8th of a horsepower. Just to give some idea that the project is a serious saw, and a bit more than a hobby motor. There's probably enough power there to cut 1" soft pine, if a bit slowly.

These two show examples of "too little" and "too much".

1.5" long 1" diameter 6V 140mA no load, 1.4A stall current, 5000 RPM (won't break saw blade) 12V 150mA no load, 3.4A stall current, 10350 RPM (a bit fast)

This is a motor similar to the NovelLife one.

3" long 2" diameter 12V 1150mA no load 12000 RPM (gearing with pulleys) 12V 12.5A rated power (In the NovelLife table saw, you can see they provide 96W of adapter power, to run a motor having a 150W "rated power". So it's not far off from being realistic. The stall current will easily cause the NovelLife wall adapter to shut off, because the current draw would be higher than that.)

For me, when fooling with motors, it's the shaft diameter and "finish" that is a key item. Many times a motor is just fine for a job, but the shaft isn't keyed, doesn't have a flat, and then I can't fasten the load onto the shaft. Most annoying. There is a limit to how much torque you can extract from a motor with a perfectly circular shaft, before the set screw lets go. Some shafts have a flat, and then when you affix a pulley or chuck or other kind of item, you have something to work with.

The end result is, "picking motors is a bitch". You can waste hours and hours shopping for one. It's just as irritating as selecting connectors for electronics projects, where the catalog has a million items in it, and none of them is just right. If I was some kind of machinist, this wouldn't be important, as I'd just machine components to fit my motor.

Paul

[newshound]

Even for coffee stirrers? I reckon my weedy little USB charged portable (3cm dia, 12cm long) would cope with them.

But I agree for more serious work. Can't see why OP doesn't just get one of these

[John Rumm]

Yes in the sense that power = volts x amps... so in the case of a battery pack, more volts = more individual cells and hence a higher total battery capacity. Remember battery capacity is *not* measured in Ah, but in Wh - i.e. for modern packs NiMh / LiIon packs its dependant on the number of series cells.

Individual cell capacity can be quoted in Ah, but that is only half the storey with a battery made up of several cells.

Tools its generally series only. Not sure why you think marketing a tool as an 18V drill rather than a 12V drill is fake advertising? Since one its true, and two it will perform better (generally, other things equal etc)

Indeed.

Not many lead acid power tools about though.

Yeah, the 18650 rules the battery world...

For clarity I am talking about battery drain in terms of peak current. Most tools will keep that under 20A, and quite often half that. A 18V Drill at 20A will be drawing 360W, so you might get 15 mins of run time out of a 5Ah pack.

Now in theory you could build a 9V drill that draws 40A and gives the same performance, but in real life physics is a bitch, and 4 times the heat losses in the drill, and much thicker windings in the motor etc, make it a non starter before you even get to the question of the quality of cells you will need that can supply the 40A on a regular basis.

So as a first order approximation, the bigger the voltage on the drill, the better it will perform - other things being equal.

And even that is only part of the storey - kW/Tonne is a more useful real world metric.

[newshound]

Just what I was thinking! Perhaps we should write a generic DIY FAQ for this, because this problem seems to come up about once a week!

Can anyone figure what the OP means by sliding a coffee stirrer in laterally? And, having slid it into this slot, what is the saw supposed to do to it? Cut it into shorter lengths, just cut limited depth slots into it, slit it axially? What sort of accuracy / repeatability?

[Fredxx]

I am very aware of that, but generally energy is a function of volume. Not voltage, in that you could have a load of bigger cells leading to a lower voltage pack, or small ones which in marketing speak is more power.

By saying, "Higher voltage batteries will have a higher energy density than a lower voltage one of the same cell capacity" you mean a bigger battery is better, and not directly due to the voltage.

I think you're falling into the trap of more volts is more power. It isn't. There is no reason why a drill with a small 18V pack is going to be better than a drill with a hefty 12V pack. Assuming of course the same chemistry and energy density.

Except I might be able to get the smaller 18500 cells more cheaply and get more perceived performance if I made an 18V pack.

We're falling into the trap that the winding will be the same diameter wire and the same number of turns. The difference between a 6V motor and a 12V motor is that one will have twice as many turns with half the wire diameter so that I^2R losses will be the same. Fill factor is improved with thinner wire but insulation becomes a larger proportion of the overall diameter.

No, assuming all is equal and designed to suit the voltage.

Quite.

[tabbypurr]

almost, absence of dividers will help if you drop from 12v to 2v.

Surely alkaline D cells do better.

NT

[John Rumm]

Given they (nearly) all use the same sized cells, volume and voltage are proportional to each other.

Yes, agreed, but also in the real world bigger will usually also be higher voltage.

Based on experience, I can't agree. On just about any performance metric (save for size and weight) the 18V drill will out perform the lower voltage one (comparing similar quality products, battery chemistry etc).

For example I have a Makita 10.8V ID, and a 18V ID. The small one is light and very usable, but only has fraction of the ability to drive a screw in terms of speed and available torque, and both beat the 7.2V ID. They all use the same sized cells.

You remember when TNP was trying to convince us it was impossible to drive a screw into seasoned oak without a pilot, and I did a quick demo?

Both IDs did the job ok, but it fairly obvious which has the higher performance.

I have also used friends Makita 12 and 14.4 combi drills - they are fine and usable tools, and better suited to some jobs than the 18V, but when push comes to shove they aint got the oomph you get with the 18V :-)

You would get more actual performance (if you equate that to torque and RPM) if you go for a higher voltage on *exactly* the same tool. However shorter run time and motor life.

[Fredxx]

Usually, but is not a given.

You've fallen into the marketing hype; where manufacturers sell performance on voltage. It's a bit like selling a Ford Escort XR3i that has less acceleration than a Ford Escort 1.6 van. It doensn't bode well for sales.

We seem to be misunderstanding the comparisons.

Using exactly the same tool I entirely agree with you. Bu then it's a bit like using the same kettle and wondering why on 110V it takes more than 4 times longer to boil than 230V.

[Fredxx]

Some Duracell datasheets:

A D cell is 5.8x the weight of an AA cell. Volumetric ~ 6.1

If you compare constant current discharge rates a D cell will last < 2hr at 2,000mA where an AA will last ~3.5A at 500mA (to the same end point voltage).

Similar story at lower currents.

4 x AA are far better than 1 x D. They are awful!

[tabbypurr]

That's odd, and news to me. Have you any idea why that is?

NT

[Paul]

Some D cells have a C-sized cell inside the shell.

Which still isn't an excuse.

See post #10 at 80% down this page.

Paul

[John Rumm]

You think?

I would have thought you would have realised by now that I select my tools with care and attention. When I selected my "move to LiIon platform" drill, I went specifically for the specs that match the jobs I am likely to want to do with it. Its why for example I did not go for the brushless version since features like the better battery life or constant speed control, mattered less to me than the peak low gear torque for example (there are times I know I want the torque for use with hole saws and expansive bits in particular).

Here is the kit I selected:

(and yes the side handle on the drill really is about a foot long!)

However if you wish to convince me, find me a lower voltage Makita combi drill, with a two speed gearbox, that can match the 91Nm of torque on my drill.

If one were comparing a cheap as chips supermarket own brand power tool, against one of the more recognised quality manufacturers. Then yup they probably do sell a 20V drill (or whatever), that will under perform some lower voltage tools from one of the more recognised quality kit manufacturers.

However within the range of tools from one maker, targetting the same market, they will typically all use battery packs that use the same Samsung / Sony / Panasonic cells, and will design to keep peek current at a level that will not kill battery packs after only a few cycles.

[Fredxx]

That's not the case for Duracell:

[Fredxx]

I think at low rates of discharge the full capacity of the battery can be realised. At higher rates a similar effect to polarisation occurs near the electrodes which are a further distance from each other in the larger battery, so less diffusion to uniformity.

You may also notice the internal resistance of the D cell is higher in the first place, again I assume from the battery geometry.

One last thing is that batteries shouldn't explode if shorted and perhaps the higher resistance is designed in to limit current and therefore heat?

[Fredxx]

I'm sorry, but you are missing the point. Manufacturers sell their tools with a consistent market strategy, more volts = better performance.

And you are reinforcing that with examples.

Quite.

Which is just confirming the point I am making.

There are some interesting reviews of the Makita 18V LXT and 40V XGT systems. I can't find anywhere that says the 40V system is better. I have even read there is little difference between the batteries which both have 10 cells. One has 5 x (2 in parallel) and the other having all in series that really make 36V and not 40V.

Which shows nominally the same size motor and same size battery. I presume you would agree that power and duration of use should be broadly similar.