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Topic: LiPo power supply only works when wired through voltmeter (Read 275 times) previous topic - next topic

Michael-jc

I am building a remote control robot arm on wheels. lots of components, big complicated circuit. I'm trying to power it through a 2C 4000mAh (2000 per cell) LiPo battery. When I switch the circuit on, the power LED flashes once then goes dim, when I measure the voltage across the battery its dropped from ~8.4V to ~1.5V.

Weirdest thing is, when I tried to measure the current using a voltmeter, everything works fine. Everything is spinning and the current draw is ~700mA. Turns out the voltmeter doesn't even have to be switch on. 

The motors are powered straight from the battery (through an L293) and everything else goes through a step down voltage regulator.

Would anyone happen to know what the voltmeter is doing to make it work?

I'm not sure what extra info you might need so if you need a schematic, let me know.

zwieblum

It adds a series resistor, that compensates a voltage spike and/or a broken wire.

Michael-jc


slipstick

Exactly what is the configuration of the battery you are using? "2C 4000mAh (2000 per cell)" doesn't make much sense. How many cells are there in total? Are they connected in series or parallel? What is the overall voltage of the battery?

Steve

Michael-jc

Yeah sorry, reading that again I have no idea what I was thinking.

1C 2000mAh 3.7V per cell. 2 cells in series so 7.4V total. 

For bonus points do you know why my charger charges them to 4.2V per cell when at the 3.7V setting? 

vinceherman

For bonus points do you know why my charger charges them to 4.2V per cell when at the 3.7V setting?
Adafruit has a nice article describing LiPo voltage range.

Michael-jc

Adafruit has a nice article describing LiPo voltage range.

Perfect, very helpful thanks.
Now I just need to know how to get that voltage out of them.
I tried putting increasingly small resistors in series with the battery but it only got the power supply up to 2.5V (with a 10 Ohm resistor which got very hot in a couple of seconds) so there's definitely something else going on here.

slipstick

For bonus points do you know why my charger charges them to 4.2V per cell when at the 3.7V setting?
That's how lipos work. 3.7V is the nominal voltage that the cells hang around at for most of the time. 4.2V is fully charged, 3.0V is empty and anything much lower than 3V means you've probably damaged the cell.

But why on earth are you putting resistors in series with the battery? I think it's time we had a schematic/circuit diagram (hand-drawn is fine, Fritzing is not!). A resistor in series, other than a very small value like 0.1 Ohms for current measurement, is usually a no-no.

Steve

Michael-jc

ok, so in the first schematic, when the circuit is switched on, I get a voltage reading of about 1.5V.

Then I tried measuring the current using a multimeter in series with the battery as in the second schematic and suddenly everything worked as it should. The multimeter doesn't even need to be switched on. (I realise I was calling it a voltmeter earlier which may have added some confusion as I was using it as an ammeter). 

So I'm just wondering what is it inside the multimeter that changes things?

But why on earth are you putting resistors in series with the battery? 
I tried the resistor in series as per zwieblum's suggestion that that is why the multimeter made it work but perhaps I misinterpreted. 




zwieblum

Quote
I tried putting increasingly small resistors in series with the battery but it only got the power supply up to 2.5V (with a 10 Ohm resistor which got very hot in a couple of seconds) so there's definitely something else going on here.
So you have a short in the part of your circuit you don't show :)

Michael-jc

But why would the multimeter fix that? (just noticed it only works on the unfused connection if that changes anything?) 

Also it works with a 9V battery.

zwieblum

It reduces peak current to a value your battery can handle.

slipstick

So let's be clear. If you measure the voltage directly across the battery it measures about 8V until you close the switch, then it drops to 1.5V? And when you measure the current what value of current does it show?

Try disconnecting all the components and adding them back one at a time until you see what's causing the voltage to drop. That's one way to find which is causing the problem.

Steve

Michael-jc

So let's be clear. If you measure the voltage directly across the battery it measures about 8V until you close the switch, then it drops to 1.5V? And when you measure the current what value of current does it show?
Yep. when I measure the current, it all works fine and the current draw is 700mA (thats just the draw from 4 DC motors and an arduino uno, everything else is idle at the moment).

Try disconnecting all the components and adding them back one at a time until you see what's causing the voltage to drop. That's one way to find which is causing the problem.
Well this worked! All it takes is disconnecting one of the four servos from the servo driver before switching the circuit on. I can reconnect it after and everything keeps running. Doesn't work with disconnecting any of the other components though. I know servos have a really high current draw and they twitch when I first switch them on so I guess that's overloading something? 
After noticing this I looked at what the current spiked to at the start but I don't know how fast the multimeter can read. It didn't go above 1 Amp and the battery should be able to manage 4A right? (2 cells x 2000mAh each x 1C safe discharge rate = 4A safe current draw?)
So what is it overloading? why does it stop the circuit from stabilising all together? what was it about having the multimeter in there that let it work and what can I replace the multimeter with?

slipstick

It didn't go above 1 Amp and the battery should be able to manage 4A right? (2 cells x 2000mAh each x 1C safe discharge rate = 4A safe current draw?)
No. You're still getting your battery parameters confused. If one 2000mAh 1C cell can provide 2A max then if you put 2 of them in SERIES you still only get 2A max (but twice the voltage). If they were in PARALLEL (so only 3.7V not 7.4V) then you would have 4A max. With only 2 cells you can't get both twice the voltage AND twice the current at the same time!

And multimeters can't read short current peaks, like servos always take when they first start up.

You haven't shown us any code or told us anything about your servos but if you change your code to put delays between the servo start ups so they're not all starting at once that may help with your problem.

But the real answer is a better battery. Most modern Lipos have much better ratings than 1C, 20C+ is normal these days.

Steve


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