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Topic: [Solved]Measure a battery amps (Read 8336 times) previous topic - next topic

tsunamy_boy

Apr 13, 2014, 06:49 pm Last Edit: Apr 22, 2014, 04:04 pm by tsunamy_boy Reason: 1
How can i get to know the maximum amps of a battery?
Is it possible to measure that with a multimeter or something?

CrossRoads

You can measure the amps being drawn - put your meter in Amp mode, or mA mode for lower currents.
Red probe connects to + of battery, black probe to where the battery + would have gone, so your meter is in series with the current flow.

Start off in Amp mode.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

retrolefty


How can i get to know the maximum amps of a battery?
Is it possible to measure that with a multimeter or something?


While a simple question there is not a simple answer. The maximum current a battery can supply at a given terminal voltage mostly depends on it's built in internal resistance. This is a specification that most battery manufacture don't publish or make easily available, but if it's a 'name brand' battery you might be able to find a datasheet published somewhere.

Note that where this is a very important specification as in radio controlled electric motor powered aircraft or the such where current demand can be very high and total flight time is measured is mins rather then hours. A battery's total charge capacity is measured in amp/hours or millamp/hours, but that doesn't tell you the maximum current you can draw for a short time. The newer specification for max current draw is so called C rating as specified in some Li-Po cells where they might say it can support a 10C or 20C or even higher value. What that says is that a cell that is say rated as a 1 amp/hour can supply (for a very short time) 10 amps or 20 amps peak current. I've only seen those ratings on some Li-po cells, although auto 12volt batteries often give a short time high current capability as 'peak cranking amps' which is effectively the same kind of spec.

Don't know if that helps, and if you can give us more information about what you require in current demand, duration, and what specific battery you are considering maybe a better answer can surface.

tsunamy_boy

#3
Apr 13, 2014, 08:12 pm Last Edit: Apr 13, 2014, 08:14 pm by tsunamy_boy Reason: 1

You can measure the amps being drawn - put your meter in Amp mode, or mA mode for lower currents.
Red probe connects to + of battery, black probe to where the battery + would have gone, so your meter is in series with the current flow.

Start off in Amp mode.


I did that and i just discharged a Lipo Battery completely (0v now)
Maybe i can't do that with lipo batteries.

Quote
Don't know if that helps, and if you can give us more information about what you require in current demand, duration, and what specific battery you are considering maybe a better answer can surface.

It's a chinese (from ebay) nokia battery but it looks like it's not 750mA because when i charge it with my charger (TP4056 - with a resistor of 1.6k) the final voltage is 4.1v. But when i use the Nokia itself i get 4.2v.
I look up online and those Chinese batteries can stand more than one type of amps

CrossRoads

Charge control chips can be set to different voltage levels to finish at.
MAX1811 for instance, a pin high or low determines what level is used.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

retrolefty



You can measure the amps being drawn - put your meter in Amp mode, or mA mode for lower currents.
Red probe connects to + of battery, black probe to where the battery + would have gone, so your meter is in series with the current flow.

Start off in Amp mode.


I did that and i just discharged a Lipo Battery completely (0v now)
Maybe i can't do that with lipo batteries.

Correct and you most likely destroyed the battery cell. Li-po cells should never be allowed to discharge below 3.0 volts.

Quote
Don't know if that helps, and if you can give us more information about what you require in current demand, duration, and what specific battery you are considering maybe a better answer can surface.

It's a chinese (from ebay) nokia battery but it looks like it's not 750mA because when i charge it with my charger (TP4056 - with a resistor of 1.6k) the final voltage is 4.1v. But when i use the Nokia itself i get 4.2v.
I look up online and those Chinese batteries can stand more than one type of amps

I think you should research more about li-po batteries before you do anymore 'experimentation'. Li-pos have very specific charging and discharging rules and one can easily destroy them and even cause fires.


tsunamy_boy

Ok, so after a lipo battery charging, if the battery is low than 4.2V, this means that the battery is not fully charged right? So in other words, it will not overcharge if i keep charge it the same way over and over?

CrossRoads

4.1, 4.2 - depends on the charger.
4.1 is going to be pretty charged.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

tsunamy_boy


4.1, 4.2 - depends on the charger.
4.1 is going to be pretty charged.


Cool, so it looks to you that there would be no problem? If i got 4.4 for example, this would be a problem (is this what it's called over charging?)?

CrossRoads

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

tsunamy_boy


4.4 would be a problem.

Understood, thank you and thank you all.
Have a nice day guys

jbarchuk

Quote from: tsunamy_boy link=topic=233021.msg1678764#msg1678764
date=1397422989

Understood, thank you and thank you all.
Have a nice day guys


What I'm going to write here may seem a little terse and rude, but I'm trying to get your ATTENTION! Please don't take it personally but this is your life and/or your family's life were talking about.

No, not 'understood' at all. I don't know if realise the implications of '4.4 - would be a problem.' A lipo burns a 2000F and shoots flames up to 6'. Anything within reach that is potentially combustable instantly combusts. That includes your face.

Back in the day lipos got a very bad reputation for causing fires, generally caused by crappy chargers. Lately, most fires are caused by overcharging, or trying to keep using damaged batteries.

Quote
How can i get to know the maximum amps of a battery?


The mere fact that you ask question says that you are not an engineer, and are therefore not qualified to 'know' (measure) the A of a battery.

The mAh of a battery is a shell game. It is essentially determined by how many potential -cycles- a battery will have.

The cycles are determined partly by how hard you draw the current, and how far down you run the voltage.

A lipo is fully charged at 4.2V. Notice though that when you buy one brand new it's at 3.65-3.75V. That's the nominal voltage, the 3.7 that's printed on the label. That's what the factory charges the battery to to give the longest life when sitting on a retail store shelf for a year or two.

When they say that 4.2 is the maximum, they mean that anything beyond that risks fire/explosion. (OK 4.25 or whatever but who wants to risk it.)

Beyond 4.2 the battery doesn't necessarily immediately go 'poof.' It starts to get warm, probably it puffs, the chemicals degrade, it gets hotter, there's a thermal runaway and -then- it goes poof. The only lipo fire I ever saw personally was a 'moderate impact' helicopter crash. walking over to it, and then looking for a bit, it had to be somewhere between 1 and 2 minutes before it actually caught fire, 3' flames. A multicell battery fire can -appear- to go out, then catch fire a minute later as another cell catches.

These numbers aren't cast in stone either. Manufacturers vary, chemistries vary, production processes vary. Stay on the *safe* side of all the numbers and you'll have the best chance of not suffering a disaster. But the reason I'm being LOUD about all this is that so far you're playing on ALL the UNSAFE sides of the numbers.

The point being that once you go past 4.2x, or give it a severe shock/impact, you're holding a grenade in your hand, the pin is pulled, you just haven't released the  yet. But the the detonator is not under your -control-, it's a chemical reaction that will light up sooner or later.

Quote
...i just discharged a Lipo Battery completely (0v now)


BTW you just destroyed the lipo. Yes there're potentially ways to 'revive' them but it is severly damage, will have a drastically shortened lifespan, and every time you charge it you're playing 'pull the pin on the grenade and see if it lights up this time.'

Quote
Ok, so after a lipo battery charging, if the battery is low than 4.2V, this means that the battery is not fully charged right? So in other words, it will not overcharge if i keep charge it the same way over and over?


Not necessarily, and probably not, because you haven't mentioned what you're charging it with, also given some of the treatment you give them such as discharging down to 0V. A damaged battery -may- not have the ability to be fully recharged to 4.2. The fact that it won't go to 4.2 -may- be a sign of damage. A reputable charger, first, won't let a battery charge past 4.2, and if it doesn't reach 4.2 won't let it charge past a certain time limit.

The C rating of a battery is not cut and dried. Again, it is a shell game number picked by the manufacturer to get an 'OK' number of cycles from the battery by the average consumer. 100 cycles is generally assumed to the target cycles. (As was mentioned, manufacturers don't publish -real- specs.) You can, for instance, discharge a 10C battery at 20/30/40/whatever rate you want, but it may work only 20 or 5 times, or only once. OTOH I've heard of rare instances of mocel aircraft catching fire in flight because they pushed a battery too hard.

Battery usage isn't supposed to be measured by mA, it's supposed to be measured by V. The 'ABSOLUTE maximum discharge' is -generally- touted at 80% or about 3.4V. But, that will give you the fewest usable cycles. One fellow I read about, whose -job- it is to manage battery systems, said he got -600- cycles by dischargong to the nominal 3.7 / 50%.

Regardless the topic really was about charging. You didn't mention what size batteries you're using. Common consumer toys come with a charger, and shouldn't be messed with by either using a different charger or using the charger on other batteris unless you *KNOW* what you're doing. Here's a good reference about overcharging and the 4056. http://www.rcgroups.com/forums/showpost.php?p=27249411&postcount=334

One more BTW. NEVER leave a charger unattended. There was a story this past Xmas about a new toy, got used a few times in the morning, and was left on the charger while the family went out. They came home and the house was burned down. It was -probably- a 10-15 minute charger, that -probably- had a warning label on it that said 'do NOT leave on charge for more than one hour.' Misuse = 'poof.' Junky charger = 'poof'

cjdelphi

I ended up building my own lithium ion charger circuit using simply 5v, arduino powered.

polymorph

I think there was a misunderstanding and the OP connected his meter set on Amps directly across the battery.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

tsunamy_boy


The point being that once you go past 4.2x, or give it a severe shock/impact, you're holding a grenade in your hand, the pin is pulled, you just haven't released the  yet. But the the detonator is not under your -control-, it's a chemical reaction that will light up sooner or later.

Ok, i'm scared now!!
So by this, tell me what's the best way to charge a lipo battery.
I've followed this topic: http://forum.arduino.cc/index.php?topic=230870.0
Do you think i'm ok with that?


I ended up building my own lithium ion charger circuit using simply 5v, arduino powered.

It looks scary for me :P do you have a schematic and the source that you can share with this community?
It would be awesome.
Also tell me, can you charge any kind of lipo battery (i have an idea that each mA need a different situation)?


I think there was a misunderstanding and the OP connected his meter set on Amps directly across the battery.

Well, yeh  :smiley-eek:

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