4x AA Ni-Mh voltage drop on load

Using Arduino General Electronics
#1

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!

#2

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...... :)

#3

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!

#4

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.

#5

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......... :)

#6

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?

#7

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.

#8

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!

#9

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.

#10

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

#11

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?

#12

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!

#13

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

#14

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 :stuck_out_tongue:

#15

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.

#16

If you are measuring voltage at the input of the regulator, then that includes all the voltage drops along the way. I thought it would be unusual for a NiMH cell to drop that far in voltage.

#17

@elac It's better because its working 8) There's any way to make it more efficient?

@polymorph Before i also read the voltages on the battery pack it was same result.

If im making this right this setup should be able to run up to 2h almost? or I'm wrong?

One of the problems is the router(s) i have 2 i can use, both of them have another dc dc converter inside them! They used those linear transformers... And yes tried direct to batteries but the voltage is low... it boots up but when connecting it drops.

#18

It's not good to drain the rechargeable batteries until the router shuts off, that will be well below the recommended discharge. The boost module will keep boosting until there is not enough current or voltage to do so, regardless of the battery pack's state of charge. Go with your assumption that the router will run for 2 hours and see how many cycles you get before the time your load runs becomes dramatically lower or your batteries won't charge properly. Be aware that as the voltage of the batteries drops the modules efficiency gets worse. For better efficiency raise the voltage of your battery pack. 2 x 9.6v battery packs in parallel a 4aH total capacity will give you around 2aH of useable current or 19.2W. That you could safely say will run your router for 2+ hours.

As for adding another router, it might "work" for you, but with your current setup it surely will not be "properly working".

#19

You guys keep talking of battery packs problem is i cant seem to find anything for a good price! On ebay most of them are fake, local stores only have this batteries for digital cameras where can i get nice batteries from??

I wasn't going to run it until it shuts down, i was going for around 1v/battery 4v total pack.. isn't that the correct discharge voltage? Also if i get more 4x AA and connect in series i should get 8x1.2 = 9.6v would that be better??

This is part of a bigger project, im converting a rc car to wifi controlled it already works perfect problem is the batteries! The car itself used a 9.6v 900mAh battery and my friend said it would run for around 30minutes, if i bought 4 more to join like i said above would it work? I don't need huge runtime at least 30min it would be nice :)

#20

danielb7390: You guys keep talking of battery packs problem is i cant seem to find anything for a good price! On ebay most of them are fake, local stores only have this batteries for digital cameras where can i get nice batteries from??

Those designed for digital cameras should work fine for servo application as both are higher current/shorter duration type loads. Any battery will have some limit of number of servos that can be manipulated simultaneously due to the specific battery's internal resistance rating.