DC motor not starting when using batteries to supply it power

Thread Starter

jamesd256

Joined Sep 19, 2018
14
I have a 12v DC 36w high volume low pressure inflator, and I'm trying to power it with 18650 batteries. The batteries when fully charged will provide ~12v when used as a pack of 3. The inflator is working fine when connected to its own PSU.

When I connect the battery pack up to the inflator and turn on switch, the motor fails to start. If I leave the motor switch on, and touch wires, the motor gives a slight twitch, then nothing. If I kind of rub the wires over the terminals of the battery, the twitches build over a second or two, and the motor can start. If I then fix the wires in place while it's running, the motor continues to work and the units functions.

Is this because the initial starting current required by the motor is more than the battery pack can provide?

If I put my multimeter in parallel with the power termimals while in current mode with the motor running as described above, it registers the current drawn as 0.6A, and the motor slows down. If I disconnect the multimeter, the motor stops. The interesting thing is, if I reconnect the multimeter to the circuit while the motor is powered up but not starting, it causes the motor to start working at this lower speed. So the motor can in fact start from the batteries with a certain circuit in place, albeit at a lower speed.

Is this because the initial current required is less in this arrangement, and within what the batteries can supply? If this was the case I would expect that once the motor is running at this lower speed, removing the multimeter would mean it would result in the motor ramping up to full speed because less initial current was needed.

What to do? Adding a variable resistor in parallel to emulate the multimeter came to mind. How is it done properly? Do I need a capacitor to discharge into the motor to get it going?

Thanks for taking the time to read.
 

Alec_t

Joined Sep 17, 2013
11,414
If I put my multimeter in parallel with the power termimals while in current mode .....
:eek: In current mode your meter behaves like a short circuit across the supply. You're lucky that your meter survived and the battery didn't catch fire. For current measurements the meter must be in series with the load.
 
If you place the multimeter in parallel to the power terminals, you are not really measuring the current circulating through the motor, your are deviating it. Put it in series with the battery and the motor and read the value. I think that it is the reason why it is provoking a strange behaviour when you connect it afterwards.

Sometimes, a Peak and Hold waveform is used in motors and valves: you inject a peak during some milliseconds and aftwerwards the current is regulated using a PWM. In this way the current is high at the beginning but later it can be much lower.

Do you have the datasheet of the inflator?
 

BobTPH

Joined Jun 5, 2013
2,433
A large capacitor (10000 uF?) in parallel with the battery might get you the starting current. Not sure what value is needed. If you could get a picture of the startup current using a sufficient power supply we could estimate what is needed.

Bob
 

KeithWalker

Joined Jul 10, 2017
1,055
Measure the voltage across the motor terminals when you connect the battery. I think you will find that it is much lower than 12 volts.
What condition are the 18650 batteries? are they new and fully charged? I think they are probably old ones from some other device because new ones would have blown the fuse in your meter or burned the insulation off the test leads.
The wires between the battery and motor must be thick enough to carry the starting current or their resistance will drop too much voltage.
A 36W motor will draw 5 amps from the supply under normal load.
To find the initial starting current, measure the resistance across the motor terminals (without the battery connected!). Divide 12volts by that resistance in ohms. The answer will be the start current in amps. It will be quite high and you will need a battery or power supply with very low internal resistance and wiring thick enough to provide that.
Regards,
Keith
 

Thread Starter

jamesd256

Joined Sep 19, 2018
14
Thanks to all for all the helpful replies.

@KeithWalker

The batteries are nearly new Ultrafire 4000mAh, and they are fully charged, or were when I started trying things. I get over 12v from the pack itself, but with the motor connected it drops to 4v, and this is while the motor is powered up but not moving.

The wires are not an issue. The wires going directly from battery pack to input plug are the same ones I connect to a mains power supply.

I did measure the resistance of the motor, it is 1.9 Ohms, so the starting current seems to be 6 and a bit amps.

I do have a digital PSU here: https://uk.farnell.com/gw-instek/psp-603/psu-1ch-60v-3-5a-programmable/dp/4911751

And an oscilloscope: https://www.gwinstek.com/en-global/products/downloadSeriesDownNew/2538/190

All the gear, no idea. Happy to entertain suggestions for how to use the kit to spot the current peak.
 

KeithWalker

Joined Jul 10, 2017
1,055
What exactly is the battery pack? I was assuming that you were just using three 18650 cells wired in series. The pack may contain something that is limiting the current.
 

Thread Starter

jamesd256

Joined Sep 19, 2018
14
I tried the inflator on a 12v lead acid battery, it worked fine

The 'pack' I refer to is a simple 3 battery holder with nothing but connective wiring, so yes, it's 3 18650's in series. These cells have the standard over charge protection circuit built in. Battery holder shown below, and the inflator is the very common mass produced type with a car 12v input adapter.

View attachment 212093View attachment 212094
I put a power limit on the desktop PSU of 36w. The blower starts fine at 36w, but what about lower? Starts all the way down to current limited to 6w, runs slowly. Apparently this motor starts with low power.

Maybe the PSU is not able to limit the current and there is a spike that is too fast for it. I doubt that very much as this is a professional tool.

I also tried running two of the 3 battery packs in parallel, same behavior.

But why can I get it started by rubbing the wires? Is it something to do with the arcing?

Also with my little mistake with the multimeter, it seems that if you divert some current from the batteries away from the motor, the motor is happy to start with the slower speed also.

So we know the blower runs on low current, both from the batteries and the PSU. These batteries I'm fairly sure can supply more than enough current to drive the motor, as I've seen batteries in this
 

KeithWalker

Joined Jul 10, 2017
1,055
You posted:
"The 'pack' I refer to is a simple 3 battery holder with nothing but connective wiring, so yes, it's 3 18650's in series. These cells have the standard over charge protection circuit built in."

So the pack does have more than just connector wiring. You state that is has overcharge protection built in, so no doubt, it will have over current protection built in too. That is what is causing the problem. You are managing to fool this circuitry by making intermittent connection etc. but under normal conditions it will not provide enough current to start the motor.
You can solve the problem by either removing the electronic protection circuits from the pack, or by using a different battery.
Regards,
Keith
 

Thread Starter

jamesd256

Joined Sep 19, 2018
14
Thanks @KeithWalker once again.

The pack does not have any overcharge protection, the batteries do.

I'm still wondering though, if a benchtop power supply can move the fan at 6w, is that what's really going on? As mentioned, the batteries can start the fan when you divert current away.
 

KeithWalker

Joined Jul 10, 2017
1,055
Ultrafire 18650s do not have any charge protection built in. They do have a positive temperature coefficient washer built into the positive end cap of some of their cells but it only limits the current if it gets hot (273 degrees F). 4000 mAH batteries should safely supply up to 8 amps (2C) continuous current. If there are no electronics in the pack you have a bunch of dud cells.
Keith.
 
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Thread Starter

jamesd256

Joined Sep 19, 2018
14
Which ones are you referring to? All Ultrafires? See the following link which seems to make a critical assessment of these cells, and they state that the do have a protection PCB. Do you have any resources to show that the don't have protection?

https://lygte-info.dk/review/batteries2012/UltraFire BRC18650 4000 mAh (Brown-Gold) UK.html

They are cheap cells, but they are not duds. Remember that 3 of these cells can happily run the motor, just not start it.I bought 9 of them and have been using 6 for months to power various things without any issues.

I took 3 of these from an un-opened pack to start this experiment, and have since checked the voltage when fully charged and it's the same as the first 6. The link referenced states there is a protection trip current of 4.1A. I think I read somewhere else the discharge rate was 1C. Therefore it seems they should be able to handle the steady state current, especially since we know that the motor will start and run with a power limited to 6W. It also seems that the motor does not demand a high inrush current to start from what I've seen using both the PSU and the battery with current diverted via the multimeter.
 

KeithWalker

Joined Jul 10, 2017
1,055
I checked the spec sheets. The built in current limiter is the TDR washer built into the positive end cap that I mentioned previously.
These cells have an internal resistance of 0.21 ohms. That is high for Li-ion cells. That will be 0.63 ohms for three cells in series. With a starting current of 6.3 amps, there will be a voltage drop of 4 volts. That only leaves you with 8 volts across the motor.
Another factor that can have a very big effect on battery performance at medium to high currents is how you are connecting the batteries in circuit. If you are using a clip-in style battery holder they are not good at handling high currents and will drop a lot of voltage through contact resistance. You need to have the batteries soldered or welded to the wires for reasonable results.
I hope this sheds a little light on the problems you are having.
Regards,
Keith
 

Thread Starter

jamesd256

Joined Sep 19, 2018
14
Thanks very much Kieth, I really appreciate you taking the time to share your skills and knowledge.

Is there any reason not to use more than 3 of these in series to increase the voltage across the motor?
 

KeithWalker

Joined Jul 10, 2017
1,055
Is there any reason not to use more than 3 of these in series to increase the voltage across the motor?
You could add another cell but with that high internal resistance, the batteries might get hot enough to temporarily shut down. It's worth a try but keep an eye on the battery until you know how it runs.. Keep the voltage at the motor close to 12 volts so you don't overload it and cause permanent damage.
Good luck,
Keith
 
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