A question about voltage fall

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Borromeo
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A question about voltage fall

Dear people: I noticed that when I twist the throttle rapidly the 4-leds digital voltmeter indicates the voltage lows, even with full batteries. Before disassembly the batteries case I like to ask you about this.

I weld all the main connections from the battery to the controller and replaced that original weak fuse holder with a round one and a 15A tubular fuse.

I built a resistive load of 1.2 ohms with some Ralco resistances and I
connect it at the output of the batteries case. The voltage is 38.4 V
without load and 35.5 V with it. The intensity is 11.5 A.

Could be the internal resistance of batteries or any parasitic resistance of the key switch or fuse holder?

reikiman
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Re: A question about voltage fall

You didn't explain the battery chemistry. It's very normal for battery voltage to dip when they're delivering current.

Borromeo
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Re: A question about voltage fall
garygid
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Re: A question about voltage fall

Most likely the voltage drop is all caused by the normal battery "internal resistance" (which varies with the cell chemistey and cell construction details).

Yes, poor connections, bad switches, and long wires can also contribute, but usually only a little.

What you see (1v per 12-v battery) is quite normal, I believe.

Cheers, Gary
XM-5000Li, wired for cell voltage measuring and logging.

steamboat
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Re: A question about voltage fall

I think an electric motor is nearly a dead sort until it begins to rotate. (use ohm eter). As the motor rotates, a back electo-motive force is created by the windings which acts like a resister to control the current. I am sure someone here can explain this better than me. Jim

antiscab
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Re: A question about voltage fall

I think an electric motor is nearly a dead sort until it begins to rotate. (use ohm eter). As the motor rotates, a back electo-motive force is created by the windings which acts like a resister to control the current. I am sure someone here can explain this better than me. Jim

for a direct online (DOL, no controller) motor, this is correct.
If you are using a controller, the controller will act as a buck-converter and step the pack voltage down, to regulate the current the motor actually sees.

back to the original question:
you will see voltage sag under load on *all* batteries.
how much will depend on the particular batteries you have.
Gel lead acid batteries experience alot of sag. The amount amount of sag you have witnessed is typical of Gel batteries.
if the sag you are experiencing is too great (ie the performance of your setup is degraded to far) then id suggest getting AGMs (or silicons).
even better, upgrade to a more advanced chemistry.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

steamboat
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Re: A question about voltage fall
I think an electric motor is nearly a dead sort until it begins to rotate. (use ohm eter). As the motor rotates, a back electo-motive force is created by the windings which acts like a resister to control the current. I am sure someone here can explain this better than me. Jim

for a direct online (DOL, no controller) motor, this is correct.
If you are using a controller, the controller will act as a buck-converter and step the pack voltage down, to regulate the current the motor actually sees.

back to the original question:
you will see voltage sag under load on *all* batteries.
how much will depend on the particular batteries you have.
Gel lead acid batteries experience alot of sag. The amount amount of sag you have witnessed is typical of Gel batteries.
if the sag you are experiencing is too great (ie the performance of your setup is degraded to far) then id suggest getting AGMs (or silicons).
even better, upgrade to a more advanced chemistry.

Matt

As I understand it, controllers(PWM) put out pulses but the motor sees RMS voltage so while the motor is stationary, it appears to be nearly a short to the controller.

IMO If you lock the brakes on your vehicle then blip the throttle, the engine will not move and your voltmeter will drop. If you open the throttle enough, your fuse/circuit breaker may blow.

antiscab
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Re: A question about voltage fall

the motor sees a triangle wave of voltage and current (its discontinuous if the motor has insufficient inductance)
you use the average to work out mechanical power (since is linear proportional)
you use the RMS to work out DC resistance loss (since its quadratic proportional).

Battery side voltage sags depending on how much power is drawn from the battery.
at stall, a controller draws very little power to power a stalled motor.
this is because the controller enforces a motor side current limit (those that dont blow up if the motor is held at stall too long).
since there is very little voltage, there is very little power.

a fuse/circuit breaker blowing due to powering a motor at stall is due to poor controller design (at a minimum, no motor side current limit).

As the motor rotates, a back electo-motive force is created by the windings which acts like a resister to control the current.

some of the older motor theory books model back emf as a varying DC resistance.
in reality, back emf is a voltage.
both calculation methods will arrive at the same answer, but conecptually, modeling it as a voltage is more correct.

to measure back emf, you need a voltmeter (which a DMM has anyway)
its an AC wave.

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

steamboat
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Re: A question about voltage fall

You said "some of the older motor theory books model back emf as a varying DC resistance.
in reality, back emf is a voltage."
I'm an old coot so the old books apply to me. I know enough about electronics to get me in trouble. Thanks for the new info.

I have a Chinese trike with a 24v 600w motor. There is a 35a circuit breaker between the 2 12v batteries which trips when I try to take my big butt up a steep hill. I just wait until the
the breaker cools, then I go again. Jim

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