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Topic: Max amp load on LiPo battery (Read 903 times) previous topic - next topic

Amartin2

Hey everyone!

I'm running into an issue where I have some servos hooked up to an Arduino, but they aren't drawing enough amps from the LiPo battery. I can hear and see them act much faster when I hook it up to my desktop power supply. The reading on the desktop power supply is telling me it's consuming around 5Amps, but when I use my Lipo battery and try to test it with my multimeter, the max amperage is 1.25 (also it feels like testing amperage with the multimeter is making my servos noticeably slower, do I have a shitty multimeter that is introducing more resistance than normal??) 

I'm using a 7.4v 1800mah 25C LiPo battery which from what I read on the internet, it should have a max amp draw of 45?

Capacity * Discharge rate / 1000 = Max amp draw
1800 * 25 / 1000 = 45 amps

I'm powering my motors separately than the Arduino so that shouldn't be a problem. I tried switching to a battery with a higher discharge rate of 65C and everything seemed to work fine, but I'm wondering why the other battery can't output as I expected...

Thanks for the help!

-Alonso

retrolefty


Hey everyone!

I'm running into an issue where I have some servos hooked up to an Arduino, but they aren't drawing enough amps from the LiPo battery. I can hear and see them act much faster when I hook it up to my desktop power supply. The reading on the desktop power supply is telling me it's consuming around 5Amps, but when I use my Lipo battery and try to test it with my multimeter, the max amperage is 1.25 (also it feels like testing amperage with the multimeter is making my servos noticeably slower, do I have a shitty multimeter that is introducing more resistance than normal??)
Mutlimeters in current mode do add a shunt resistance which cuts down the voltage a little that the servos will receive, it's called it's 'burden voltage'. Is your battery under servo load putting out the same amount of voltage as your desk power supply?

I'm using a 7.4v 1800mah 25C LiPo battery which from what I read on the internet, it should have a max amp draw of 45?
The so called C rating (as in 25C) is a somewhat variable rating, kind of like gas mileage ratings on cars, your batteries C rating may vary with age and usage.

Capacity * Discharge rate / 1000 = Max amp draw
1800 * 25 / 1000 = 45 amps

45 amps discharge but only for like 2.5 mins, and there is no condition where your servos will be trying to draw 45 amps. So the question is when on the bench supply what amps do the servos draw at what voltage output, compared to what amps they draw at what voltage when on the battery?

By the way most servos want voltage anywhere from 4.8 to 6.0 volts. What voltage are you trying to feed them and is it above the recommended maximum voltage the servos are rated for?

I'm powering my motors separately than the Arduino so that shouldn't be a problem. I tried switching to a battery with a higher discharge rate of 65C and everything seemed to work fine, but I'm wondering why the other battery can't output as I expected...

Could just be a tired battery not able to output it's rated current any longer?

Thanks for the help!

-Alonso

Amartin2

Hi Retrolefty,

I have an adjustable power supply which I'm setting to 7.4v, and the LiPo battery also shows 7.4v when tested with the multimeter. The batteries are brand new so there shouldn't be any problems with the discharge rate being lower than original.

I'm using three of these high voltage servos:

http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18399

retrolefty


Hi Retrolefty,

I have an adjustable power supply which I'm setting to 7.4v, and the LiPo battery also shows 7.4v when tested with the multimeter. The batteries are brand new so there shouldn't be any problems with the discharge rate being lower than original.

I'm using three of these high voltage servos:

http://hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=18399


All I can suggest is that you measure voltage while the servos are being powered by the battery both at the servo connection and at the battery terminals to see if you are suffering some voltage drop across wires, connections, etc.

Lefty

Amartin2

Hey Lefty,

I will definitely check the voltage as you mentioned when I get home, but in theory that battery should be able to handle those high-voltage servos no problem right???

You don't usually have to worry about wire gauge when dealing with amperage below 15 right??

-Alonso

retrolefty


Hey Lefty,

I will definitely check the voltage as you mentioned when I get home, but in theory that battery should be able to handle those high-voltage servos no problem right???
Time will tell, make your measurement tests.  ;)
You don't usually have to worry about wire gauge when dealing with amperage below 15 right??

No, any given wire size might be too small for some amount of current, that is why you will measure to see if you are dropping voltage on the way from the battery to the servo positive and back from the servo negative to the battery negative. It's a case of measured data out weights opinion and guesses.

-Alonso


michinyon

You have done something wrong there,  because LiPo batteries have a huge current capacity.

Chagrin


You have done something wrong there,  because LiPo batteries have a huge current capacity.

Or might be a bad battery / bad battery protection circuit.

MarkT

You might have a poor connection somewhere - can't really imagine a LiPo worrying about 5A unless something
is really wrong.
[ I won't respond to messages, use the forum please ]

shadowjk



You have done something wrong there,  because LiPo batteries have a huge current capacity.

Or might be a bad battery / bad battery protection circuit.


RC LiPo batteries typically do not have any protection circuits.

If the user has drained the battery too low, it will have lower current capacity, and also added instability.

I would recommend to the original poster, that he also order one of those rc low voltage alarms, so he doesn't accidentally overdischarge.

As for voltages, a 7.2V RC Li-Po should come off the charger at slightly less than 8.4Volts. and can be considered empty when the voltage starts going below 7V..

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