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Topic: 4x AA Ni-Mh voltage drop on load (Read 10796 times) previous topic - next topic

danielb7390

Hi,
So i got this new batteries (http://gp.batteries.pl/collection_uploads/gp_recyko_aa.pdf) already used twice to charge my phone and works great.
Now that i got home back to my electronics i was trying to power a router with them. Here's the info:

Router: 240mA@12V (measured by me with the multimeter between the ground wire)
Batteries: 4x 1.2V 2050mAh(typical)
Dc-Dc boost converter from ebay

They sit around 5.2V unloaded, but as soon as i plug the load the batteries voltage drops to almost 3.5V! Because of this the converter goes mad and drops the output voltage too....
According to the data-sheet they *should* go up to 6A discharge..

And no its not the converter tried with non-rechargeable batteries i have (6x C +-8Volts) and it works...

Any help would be appreciated!

TomGeorge

#1
Aug 14, 2014, 12:04 pm Last Edit: Aug 14, 2014, 12:06 pm by TomGeorge Reason: 1
Hi, show us the DC-DC converter, what are you using for wiring and how are you holding the batteries.
Have you measured each cell when they are under load to check if they are all dropping in output or just one?
Sounds like they are not fully charged.
Can you measure the current being drawn from them when the fault occurs.
Also the current being drawn with the C cells.

A picture of your setup would help.

Tom...... :)
Everything runs on smoke, let the smoke out, it stops running....

danielb7390

#2
Aug 14, 2014, 12:21 pm Last Edit: Aug 14, 2014, 12:23 pm by danielb7390 Reason: 1
The converter: http://www.ebay.com/itm/310717070508

I also thought of the charge I'm changing again just to make sure but i charged them yesterday and stored they should be good...

The current draw when it drops it increases a lot i saw 2A@-+3.5V from the batteries and then disconnected the wires.

Will try again with the AA when the charger gives me a green light!

Tests with the 6x "C" batteries:
I set-up the output around 12V for this test.

Battery  Voltage without load: 8.73V
Battery  Voltage in load: 6.90V - 7.10V (It jumps a bit up and down)
Battery Current Draw: 0.55A max

Output Voltage: 12V
Output Current: 0.26A

As for the picture i'm just using alligator clips to connect the things. Also i was using a battery holder to rule it out i tried with the batteries in a row and connect the alligator clips in the end by hand, same result.

You think this batteries are not enough?
Thanks for the help!

danielb7390

First of sorry for double post but the other one is big enough already...

The batteries just finished charging witch means they were already charged...
Fresh from the charger: 1.45V, 1.45V, 1.44V, 1.44V

Pack unload voltage: 5.77V
Pack Load voltage: 3.35V

Load voltage Bat #1:1.32V
Load voltage Bat #2:1.31V
Load voltage Bat #3:1.30V
Load voltage Bat #4:1.23V
Total sum of bat voltages: 5.16V

This is strange.. the total of each cell is bigger than what i'm getting in the output how can this be?

Load Battery draw: 1.30A
Load  Battery voltage: 3.10V (its starting to drop from the above test...)
Output Current: 500mA
Output Voltage: 6.50V (should be around 12V that's what i set it to)

If i can do any more tests please say.

TomGeorge

Hi, with 4 Nimh batts thats 1.2V x 4 = 4.8V. Thats using the nominal voltage per cell, so thats okay.

Try putting a capacitor across the Nimh, say 470uF or 1000uF, and a 0.1uF as well, you may be getting problems between the ripple current from the SMPS and the internal resistance of the Nimh.
Keep the leads between the Nimh and the supply as short as possible.

http://www.ti.com/lit/ds/symlink/lm1577.pdf

The spec sheet even shows an example circuit, 5V to 12V.

Tom......... :)
Everything runs on smoke, let the smoke out, it stops running....

danielb7390

#5
Aug 14, 2014, 04:00 pm Last Edit: Aug 14, 2014, 04:46 pm by danielb7390 Reason: 1
Still same...

On the output i placed a: 2220uF 35V cap
On the input i placed a: 2200uF 16V AND a 104 ceramic cap

Same, with the big one in the output if i let it charge up before plugging in the router it decreases slower but ends up in the same...

More ideas?

EDIT: Just tried with 4xAA normal ones and same problem happens! I'm starting to think the voltage in the input is too low for the converter... this ones drop from 6.30V(no load) and 3.72V (load)

EDIT2: With 5xAA IT WORKS! (7.80V)
Now its safe to assume that the input voltage is the problem?

polymorph

In my experience, many alligator clip-leads use -very- small wires that are poorly crimped in place. I went through all mine and soldered them, or replaced with larger wires. Some sets of my clip-leads had what looked like about 24 to 26 gauge stranded wire, of not very good quality.
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

danielb7390

#7
Aug 14, 2014, 04:46 pm Last Edit: Aug 14, 2014, 04:49 pm by danielb7390 Reason: 1
But if it was that it wouldn't work with the other "C" batteries...?

With 5x AA making 7.80V input it works, with 5xAA making 7.40V input same problem...

About the alligator indeed they seem very bad crimped when i can i will solder them or maybe change the wire in some of them with wire i got from a pc psu!

polymorph

All the voltage drops add up. So if you start with a higher voltage, maybe it works.

There is also the matter of corrosion on the ends of the cells.
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

danielb7390

#9
Aug 14, 2014, 05:03 pm Last Edit: Aug 14, 2014, 05:12 pm by danielb7390 Reason: 1
Well the rechargeable batteries are new, same for the normal ones.

EDIT:  :0 REALLY??? Just connected with just 4 wires and guess what it works! Even with the rechargeable ones, safe to assume alligator clips are the culprit ...

Its drawing:
2.76V
1.30A

elac

You need higher capacity at 4.8V, those modules will boost 4.8V to 12V, of course the efficiency is very poor.
You will need to provide 600mA @4.8V plus whatever the mA the module consumes for reliably running 240mA @ 12V.
That's over 25% of your total capacity.
To avoid large voltage drops on your supply keep the load to 10% of your batteries capacity.
So to reliably run 240mA @ 12V  from 4.8V you should have a battery pack with an 6aH capacity.
With a booster module it is most efficient to keep the supply voltage as close to the load voltage as possible.
At 9.6V you would need 300mA plus the mA the module consumes, so a 9.6V battery pack with 3aH capacity would also work.
But what about this question, how long do you want run the load for?
It's all about the skills

danielb7390

Its drawing:
2.76V
1.30A

It's wrong! I was still using the aligators to measure the current, changed to my multimeter probes and it gives total different results!

5.12V
800mA

Now thats better!

polymorph

Where are you measuring that? At the power inputs on the regulator, or at the battery terminals?
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

danielb7390

I don't know if i follow you?

The CURRENT I'm measuring in-line between battery GND and the GND input of the converter.
The VOLTAGE I'm measuring in the GND and VCC of the input of the converter.

I just have it like this and its working perfect: ( AA + AA + AA + AA) --- DC-DC --- Router

This alligators really need some better solder/cable they all show the same problem  :P

elac

Better?
Batteries: 800mA @ 5.12V = 4.096W
Load: 240mA @ 12V = 2.88W
Module consumes 1.216W
Not very efficient.
Total power of the "batteries in perfect world" @ 5.12V = 10.24W
Shouldn't drain your batteries more then 50% or 5.12W of power to safely use.
It's all about the skills

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