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Topic: Calculating amps!! (Read 2 times) previous topic - next topic

FelixFelicis

Hi,

I have simple questions calculating the amps in a circuit. I have a 1.5v 2100 mAh AA battery. For my circuit, I have added 4 similar rating aa batteries in series. My question is, what is the maximum current that can be drawn from this setup? or is the current drawn directly proportional to the load in circuit?

My geared motors work at 32 mA and their stall current is around 500 mA @ 6v. If I limit the current to 32 mA, and if the load in the motor increases, will it slow the motor down?   

Also, What is the max current drawn, if I add battries with 2100 mAh and 1600 mAh together. Say, 1 setup has 1.5 v each and the other setup with 1.5v and 1.2v.

I know that these questions are stupid!!!
If some one could explain these, I would really appreciate.

Thank you,
Regards.

DVDdoug

#1
Mar 07, 2012, 01:37 am Last Edit: Mar 07, 2012, 02:00 am by DVDdoug Reason: 1
Well...  mA-hours doesn't tell you how much maximum current you can draw.   It's a measure of how long the battery will last.   Here is a datasheet I found for a AA battery.   It says:
Quote
2,850 mAh (to 0.8 volts)
(Rated capacity at 25 mA continuous drain)
So in this case, 2850mAh means they measured it at 25mA, and it took 114 hours for the voltage to drop to 0.8V. 

On this particular datasheet, I didn't see a maximum current rating, but the discharge graphs show dischage current up to 1000mA, so I assume that's "safe".

Quote
For my circuit, I have added 4 similar rating aa batteries in series. My question is, what is the maximum current that can be drawn from this setup?
The total current rating should be the same, but the voltage is summed.   With 4 batteries the total energy available (milliwatt hours) will be 4 times that of one battery.

Quote
My geared motors work at 32 mA and their stall current is around 500 mA @ 6v. If I limit the current to 32 mA, and if the load in the motor increases, will it slow the motor down?   
Right!  With limited current, the motor will slow-down (or stall) with a lighter mechanical load than the motor can otherwise handle (at around 500mA).  "Conservation of energy"...  If you want more mechanical power out, you need to put more electrical power in. ;)

Also, if the motor draws 32mA with no-load (or with a given light-load), it probably won't start with 32mA.  You generally need a "kick" of current to overcome the initial friction and inertia.

Quote
Also, What is the max current drawn, if I add battries with 2100 mAh and 1600 mAh together.
In series?   Probably unpredictible results...  The 1600 mAh will most likely die first, and then you'll mostly be getting whatever voltage remains from the higher capacity battery.

If you've got a flashlight that takes two batteries, and you put-in one carbon-zinc and one alkaline, nothing bad will happen, except the carbon-zinc battery will die first and the light will go very dim with only the alkaline supplying power.

oric_dan

These are non-obvious questions. so not stupid.

1. what kind is your 1.5V 2100 maH battery? 1.5V usually indicates non-rechargeable Alkaline,
and none of the ones I have have the energy capabilitiy indicated. Rechargeable always have the
energy capacity marked, but they're always indicated as 1.2V.

2. the maximum current batteries can provide varies by battery type. I often run 6-8 R/C servos
off NiMH AA-cells and they do ok. That's at least 2-Amps, but when you're drawing such high
currents, the energy available from the batteries will probably be less than the energy ratings.
In regards non-rechargeable Alkaline, there are 4-5 types and they all handle higher currents
differently. The manufacturers provide information regarding different load types for the
different Alkaline types.

3. you probably cannot limit the motor currents to 32-mA and expect to get much torque out.
Rather vary the speed and torque by using PWM, and let the current be as it may.

4. all manufacturers warn AGAINST mixing different types of batteries in the same series circuit.
You especially have problems if the capacities of the batteries are different, if the health of the
individual cells are different, and even if the charge on the batteries are different.

Eg, with rechargeable cells at least, if you have a bad cell in the series pack, the good cells can
overwhelm it, in effect "reverse-charging" it, and actually push its voltage into the negative
direction. I once built a circuit that tracked individual cell voltages and then mixed in a few
weak cells with strong cells, and watched in realtime as the weaker cell voltages flattened and
reversed.



FelixFelicis

Thanks for the reply guys.

I think I am beginning to understand this. (Just beginning..  :))

Let me try to summarize..

1. For a  2850 mAh , 1.5v battery, it takes 114 hours to drop down to 0.8v if it works at 25mA..
Lets say 25mA for 100 hours. So Can I utilize 2500mA for 1 hour!!?? (Maybe not practically, but theoretically is this  rite!!??) Adding 4 batteries and making it 6v should work for 2500mA for 4 hours. So basically it depends on the load.. rite??

2. Regarding my motor, I am using a L293D driver IC. I have already connected them to the PWM pins. The pulses are like 0 and 255. Can I remove the resistors with this setup??
" "Conservation of energy"...  If you want more mechanical power out, you need to put more electrical power in. ", Now this is greek. How to add more electrical power in?!!

3. Regarding the batteries with different ratings, I rather not mix them.

Thank you
Please do reply for the other queries
Regards.

retrolefty

Quote
Lets say 25mA for 100 hours. So Can I utilize 2500mA for 1 hour!!?? (Maybe not practically, but theoretically is this  rite!!??)


Unfortunately no, not in the real world. You may only get half the total mah capacity when discharging at the 2.5 amp rate Vs the 25ma rate, maybe even less! The battery manufactures often give a graph of total mah capacity Vs current rate discharge in their datasheet. Otherwise you can only determine that by testing and measurement. Also a charger has to put back more then rated mah back for rechargeable batteries as the chemical process is not 100% efficient.

Lefty

kf2qd


Adding 4 batteries and making it 6v should work for 2500mA for 4 hours. So basically it depends on the load.. rite??



Not quite ... You are using the term battery to mean 2 different things.

A single "battery" is properly a CELL. Several cells in series of parallel is a BATTERY. Comes from the idea of artilery. 1 gun is just a gun, a group of guns is a battery.

Cells in series add their voltages so 4 cells in series for 6V is still only 2500mA for 1 hour. 4 cells in parallel would add their current capacity and provide you with 1.5V at 2500mA for 4 hours.

FelixFelicis

Hmm..

kk.. I get it. lets say its not 2.5 Amp for 4 hours, Assume if its around 1 amp for 4 hours..

My questions is..

The current flowing through the circuit, is it a constant 1 Amp, or does it depend on the load in the circuit..
I have a 32mA 6v motor connected to the supply. Should I limit the current flow to 32 mA or is it that the motor itself will draw only 32 mA from the supply??

Please suggest..
Thank you..

FelixFelicis



Adding 4 batteries and making it 6v should work for 2500mA for 4 hours. So basically it depends on the load.. rite??



Not quite ... You are using the term battery to mean 2 different things.

A single "battery" is properly a CELL. Several cells in series of parallel is a BATTERY. Comes from the idea of artilery. 1 gun is just a gun, a group of guns is a battery.

Cells in series add their voltages so 4 cells in series for 6V is still only 2500mA for 1 hour. 4 cells in parallel would add their current capacity and provide you with 1.5V at 2500mA for 4 hours.


May be I should start learning Electronics from the Scratch Again..  :~
Ok..
Reframing it, If I add 4 cells in series I will still get 2.5A for 1 hour, and in parallel for 1.5v I get 2.5 A for 4 hours.. Can I get 10A from the same setup for 1 hour??

Nick Gammon

I suggest you look up the videos by Afrotechmods on YouTube.

He talks about that, amongst other things.

One problem with batteries is that they have an ESR (equivalent series resistance). At high current drains this inbuilt resistance itself lowers the voltage output and makes the battery get hot.

Nick Gammon


retrolefty

The current flowing through the circuit, is it a constant 1 Amp, or does it depend on the load in the circuit..

The load resistance always dictates how much current will flow, in the classic ohms law formula current = volts/ohms, ohms of the load will determine the amount of current at any specific voltage, provided the voltage source can provide it. Batteries have 'internal resistance' which limit their maximum current flow capacity depending on the size and chemistry used for a specific cell or battery of cells.  

I have a 32mA 6v motor connected to the supply. Should I limit the current flow to 32 mA or is it that the motor itself will draw only 32 mA from the supply??

The motor will draw what it requires. However how much current it draws will depend on the mechanical load placed on it. If yours draws 32ma with no load, try holding the armature fixed and see what the 'locked rotor' current draw is, much higher, no?

Lefty



FelixFelicis

@ Nick Gammon... Thanks a TON for that Video. It was awesome (and a bit scary.).. I got more than enough information from it...


@ Lefty ... Ya the full load current is around 500mA.... Got the point.. Thanks for the reply ..

At the bottom line, my project will utilize 2 geared motors, a controller, and few Leds and Irs. So its safe for me to power them with 4 batteries in series....

Assuming that the circuit utilizes 1A current from the 6v, The calculated heat  is around 48mW .. Which I suppose is not harmful... I would love to look at my pop corn pop, but not my controller 

Thank you
Regards..

oric_dan

Quote

Lets say 25mA for 100 hours. So Can I utilize 2500mA for 1 hour!!?? (Maybe not practically, but theoretically is this  rite!!??)


Unfortunately no, not in the real world. You may only get half the total mah capacity when discharging at the 2.5 amp rate Vs the 25ma rate, maybe even less! The battery manufactures often give a graph of total mah capacity Vs current rate discharge in their datasheet. Otherwise you can only determine that by testing and measurement. Also a charger has to put back more then rated mah back for rechargeable batteries as the chemical process is not 100% efficient.


The 3rd graph in the battery paper linked in post #2 is very illustrative of battery capabilities,
Constant Current Discharge vs Service, in hours.

At 10 mA load, the battery is good for almost 300-hrs operation, or roughly 3000 mA-hr.

At 500 mA load, the battery is good for about 2.5-hrs operation, or roughly 1250 mA-hr.

At 1-Amp load, the battery is good for only about 1-hr operation, or roughly 1000 mA-hr.





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