Increasing the current capacity of a NiMH battery pack?

[Pauldesign]

Wow, a lot have changed since i last visited the forum until it took me about 30mins to figure out how to post a new thread

New view, new features and new moderators. Great!!!, but my only suggestion is the "post a new thread" should stand clear and big not embedded in a drop down arrow. otherwise great stuff.

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Well, esp to the power electronics gurus, i'll like to know whether any of you guys have increased the capacity (current size) of a battery by just using transistors (BJT, MOSFET) or step-up current IC besides connecting more batteries in parallel.

I've been wondering if it's practically possible to increase the current capacity of a Ni MH 4V8, 5AH, 0.5C max AAA battery pack to say 10AH by only using power transistors, without connecting more batteries in parallel?

Basically, my setup is as follows; the battery will power about 3 power transistors connected in cascade whose output will be connected to ~10A load (0.5 Ohms) and if possible isolated from the battery ground (-ve).

It seems theoretically possible but have some of you old folks practically achieved that without compromising the manaufacturer specs and performance of the battery (any rechargeable battery).

Off cource, a PICAXE chip will do all the monitoring and control.

Thanks and please let know if more info is needed.

 

[westaust55]

With most batteries, the Ah rating is based on a specified discharge rate (eg C/5) where the Ah is the capacity when discharged at a given current and it will thus last for a given duration.

Accordingly you can discharge at a higher current rate but the time is also reduced. The time duration factor is often not proportional to the current change factor.

Thus doubling the current does not halve the time but just by way of example it may be 40%.

This is (or should be) all covered in battery datasheets if you get the one for the battery you are considering.

 

[nick12ab]

New view, new features and new moderators. Great!!!, but my only suggestion is the "post a new thread" should stand clear and big not embedded in a drop down arrow. otherwise great stuff.

The Post new Thread button is still standing clear above the sticky posts in the forum.

 

[SAborn]

IMO the answer is no its not practical, for short run times you can exceed the battery rating but not for long or cell damage will result.

Im a little miffed on how you intend to isolate the battery ground and still have power flow.
In many ways you seem to want to get blood from a stone or have your cake and eat it too, i do not see it feasible.

 

[srnet]

I've been wondering if it's practically possible to increase the current capacity of a Ni MH 4V8, 5AH, 0.5C

...

It seems theoretically possible

If the discharge capacity is rated at 0.5C = 2.5A, then that is likley the discharge rate for which the capacity (5AH) is quoted.

Most NiCd, NiMh and Lipo batteries are reasonably linear with regards discharge rate and capacity, Alkalines on the other hand are most definetly not.

However as you increase the discharge rate the power lost due to internal resistance increases. How much power is lost depends on the type of battery, for instance high capacity AA NiMh batteries loose output volts fairly rapidly beyond about 1 or 2A. High power AA NiMh can however supply 8A or more without significant voltage drop.

But you actually quoted;

"Ni MH 4V8, 5AH, 0.5C max AAA"

If these are AAA, they wont be 5AH at all, more like 500maH to 1000maH max, and domestic grade AAAs wont produce much more than 1A ........

 

[srnet]

In many ways you seem to want to get blood from a stone or have your cake and eat it too, i do not see it feasible.

If one wants 10A from an AAA, then I would agree .......

 

[Pauldesign]

Thanks Nick; I now see it (top left) but before it wasn't there. U can see, i'm still figuring out this new forum layout. I'm now like a rooki trying to catch up

Thank Srnet for the reply; i'm combining 1V2 1.25A batteries in series and parallel combination to get the 4V8 and 5A.

I've came across some pricey AAA that can push up to 2500mAH.

For clarity sake, i don't wanna draw 10A directly from an AAA or any rechargeable but indirectly via a power converter (amplifier/IC) of some sort connected to a rechargeable battery.

Thanks once more for all th replies.

 

[srnet]

It would make more sense to us if you could confirm;

What is the input voltage and current capacity of the battery you are using ?

What is the voltage and current out that you want ?

 

[papaof2]

If your output voltage is less than your input voltage, you might get more amps at fewer volts via a DC-DC converter.
Example: 12 volts at 5 amps in, 5 volts at 10 amps out (60 watts in, 50 watts out with an efficiency of ~80% - probably good for a home brew device)

Does NOT apply for low voltage in, higher voltage out.

John

 

[MartinM57]

Thread title changed to something more appropriate and useful for future searching...

 

[BeanieBots]

I've came across some pricey AAA that can push up to 2500mAH.

No you haven't!
Even if you have seen something that makes that claim, quite simply, they are lying.
It's impossible (with today's technology) to get that amount of chemical into an AAA case. End of.

Do you mean "increase capacity". If yes, that too is impossible.
However, it is possible to convert any voltage to any other voltage whilst keeping the POWER (ignoring efficiency losses) constant.
So, if you had say 10 AAA cells in series of say 500mAhr capacity and you fed that into a buck converter to reduce the nominal 12v down to a nominal 1.2v it "look like" a single cell of 5000mAhr capacity. Also, if the max discharge current of each cell was say 1A, after the buck converter you could draw 10A. (again ignoring any losses).
Put simply, you can't get something for nothing.
If your massive current requirement is short duration, the normal trick is to parallel each cell with a supercap.

 

[srnet]

It's impossible (with today's technology) to get that amount of chemical into an AAA case. End of.

Indeed, there is no possibility of getting 2500mAhr out of an AAA, 1000mAhr is about the tops from these NiMh. 2500mAhr is close to the limit on the much larger AA size NiMh.

I cant work out if there is confusion between AA and AAA sizes or a confusion of capacity (as in mAhr) and the amount of curent cells are capable of.

 

[flyingnunrt]

Do you mean "increase capacity". If yes, that too is impossible.
Put simply, you can't get something for nothing.

Notwithstanding the above, LiPo's Might be a better choice.

 

[srnet]

LiPo's Might be a better choice.

In terms of size versus weighrt versus capacity versus output current capability then yes, no contest.

But OTOH, more complex to charge, not so PICAXE friendly (choice of 3.7v or 7.4v really), and capable of dangerously high cuurents in fault conditions (as in Boom !)

 

[BeanieBots]

I think there is indeed a lot of confusion going on. In particular, what the REAL quest really is.
Perhaps Pauldesign will come back with a clearer explanation of what he is trying acheive.

 

[Pauldesign]

Thanks for all the replies, i really appreciate the time and effort.

You're right MartinM57, i was beginning to wonder about the title myself.

My apologies to have created some confusions (as usual!). I was in a rush and i didn't present my facts correctly. My bad!

As BB suggested, i think it's best i clear all the cobwebs esp before Dippy boo me.

First thing first, please find attached the datasheet of the battery i intend to use.

Plan of action:

- I'll hook up 8 of those in a series/parallel combination setup to theoretically deliver ~5V and 5AH (5000mAH) under no load conditions.

- Although the theoretical max discharge current in my setup will be 10mAH at 1C for ~30mins (alright in my apps), I don't wanna connect this directly to a 10A load as it will damaged or reduced the life span of the battery if always discharge at the rate as it might be the case.
- An option have been to use more battery packs or a high capacity battery, but that comes with a price, not to mention the bulkiness.

Now the question i've been trying to ask is:

Will it be practically possible to discharge the battery at a standard rate via a DC to DC step-up/boost converter or preferably via a power transistor (i don't mind about the heat dissipation), which will in turn increase the output current say 10A (by using x4 2N3055 power transistors in parallel). My thought or assumption has been, the battery will be able to provide enough drive power or an appropriate base current to the transistors which will be further ampilfied (using the beta factor/gain) to a higher collector current capable enough to deliver 10A (to a 0.5 Ohms 50W constant resistive load) for about say 30mins or more at a safe discharge rate to the battery.

Basically, the transistor/DC to DC booster, will serve as a driver or buffer between the battery and the load.


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Part II:

Now the concept of energy conseravtion, kicks in; "power can neither be created nor destroyed but changed between forms".

My biggest doubt now has been, is it possible to supply ~25W (5V x 5A) of battery DC input power hoping to deliver ~50W (5Vx10A) of output power based on the transistors gain (assuming a non-ideal situation i.e. neglecting power losses in the transsitors and elsewhere).

I hope i'm a bit clearer now and please let know if further clarity is needed.

Oops, for some reasons, i can't attached a file. I click the "Manage Attachement" and nothing happens, i'm unsure if the issue is on my side or PICAXE forum related.

So, please follow the below link to access the battery tech specs.

http://za.rs-online.com/web/p/aa-rechargeable-batteries/7033515/


Thanks once more for the concern.


Paul

 

[SAborn]

I still fail to see where or how you think a transistor will magically increase the power, its not much more than a electrical on/off switch so how can it increase something that is not there to start with?

 

[techElder]

To put it bluntly in the form of a question ... Do you also believe in skyhooks and automobile engines that run on water? That's the area where you are playing now.

Transistors do not contain magic fairy dust.

... assuming a non-ideal situation i.e. neglecting power losses in the transsitors and elsewhere ...

Neglecting power losses would most certainly be the ideal situation!

 

[kevrus]

quote by PaulDesign... is it possible to supply ~25W (5V x 5A) of battery DC input power hoping to deliver ~50W (5Vx10A) of output power based on the transistors gain (assuming a non-ideal situation i.e. neglecting power losses in the transsitors and elsewhere).

It sounds to me like you expect the transistors to pull 25w of power fom thin air 'because of their gain'. They cannot mgically produce an additional amount of power. It may be an idea to read up on transistor operation...

 

[hippy]

My biggest doubt now has been, is it possible to supply ~25W (5V x 5A) of battery DC input power hoping to deliver ~50W (5Vx10A) of output power based on the transistors gain (assuming a non-ideal situation i.e. neglecting power losses in the transsitors and elsewhere).

I hope i'm a bit clearer now and please let know if further clarity is needed.

I'm still confused but I think that's because of the terminology and the way you describe the transistors working. What I think you are really asking is ...

1) I have batteries of 5V, nominally rated for 5A, which I sometimes want to power a load which will need 10A; can I do that ?

2) Can I control the provision of power to the load via transistor switches ?

Is that it ?

If you are asking can there be a passive electronic circuit which can provide 5V @ 10A and be powered from 5V @ 5A then, no.

 

[Dippy]

"is it possible to supply ~25W (5V x 5A) of battery DC input power hoping to deliver ~50W (5Vx10A) of output power based on the transistors gain"
- NO chance. You can't create energy. Forget it. Schoolboy Science.
Gain means voltage or current, not magic power or energy.


Posts 17 to 19 are bang-on.

Basically, forget it, move on to Plan B. Or some other Plan which , hopefully, obeys the Laws of Physics.
I don't mean to be blunt, I am just trying to save your (and our) valuable time.

 

[srnet]

This bit does not make sense at all;

"Although the theoretical max discharge current in my setup will be 10mAH at 1C for ~30mins"

For one the discharge current is measured in mA, its capacity thats measured in mAH

And two, when the discharge is quoted at 1C, that is taken to mean the discharge is such that the cell is discharged completly in 1 hour. Thus a 5AH cell discharged at 1C, is discharged at 5A and will supply that current for 1 hour.

 

[BeanieBots]

Oh Dippy! play the game
We all know that a bipolar transistor is a current amplifier. You put current into the base and get more out of the collector.
The proposed 2N3055 has an hfe (gain) of about 15, so 5A into the base is 75A at the collector.
(a little knowledge is such a dangerous thing...)

@srnet, I didn't have a clue what that was all about either.
"10mAhr at 1C for 30mins."
So, 10mAhr is consumed in half an hour which means 20mAhr in one hour which is a current of 20mA.
at 1C must refer to the calculated 20mA, so now we know the battery capacity to be 20mAhr.

 

[westaust55]

Transistors do not contain magic fairy dust.

Only one portion of "magic smoke"

 

[westaust55]

So, please follow the below link to access the battery tech specs.

http://za.rs-online.com/web/p/aa-rechargeable-batteries/7033515/
Paul

from the datasheet:

cell size: AA​

which is more reasonable/expected

"1C" is the discharge rate ==> 1 hour

cell capacity is 2100 mAh at 1C and from the datasheet, if discharged at 2100 mA the cell will be discharged to 0.9 V in about 55 mins


cell capacity is 2500 mAh at 0.2C so if discharged at 500 mA the cell will be discharged to 0.9 V in about 5 hours

 

[Pauldesign]

Thanks for all the replies and sorry to have created some more confusions due to my typos (esp the 10mAH; i meant to say 10AH=10000mAH) and maybe the way i'm explaining it.

Nevertheless, i got the drift from the various replies and the thread seems to be opening some can of worms (blunt moments )

With due respect, i really do understand the tech specs of datasheet and need not to be further explained and sorry to have made u guys explaining it.


Well, it seems as if only BB seems to have figure out what i've been trying to explain and taking it from there (based on that); using 8 of those batteries in the attachedment and arranging as i've previously explained, is it feasible to get continous 50W of output power for a duration of about 30mins; from 25W of input power (from those batteries) using that power transistor ( 2N3055 as drivers) and neglecting all heat losses? If you now understand, please just say Yes or No

I hope there are no more typos, otherwise i'm getting blind/blunt.

Thanks once more.

 

[SAborn]

Sorry to say BB was taking the piss out of you and the rest of us in a cheeky way, and the only place you will find magic fairy dust is maybe with transvestites and not transistors.

FOR THE LAST TIME.... Transistors dont work the way you think they do, and if they did we would all have free energy

 

[hippy]

using 8 of those batteries in the attachedment and arranging as i've previously explained, is it feasible to get continous 50W of output power for a duration of about 30mins; from 25W of input power (from those batteries) using that power transistor ( 2N3055 as drivers) and neglecting all heat losses? If you now understand, please just say Yes or No

Yes, I understand. No you cannot do that.

25W in and 50W out cannot be achieved unless you can hold 25W to add to the input by storing it elsewhere. So for example the 25W in could charge something to 25W, then you could use the 25W in and 25W stored to deliver 50W out for a period of time.

I think I can understand where the misconception comes from as transistors are often called current amplifiers which seemingly allows then to magically boost current but that's not the reality, They can be thought of as electronic flow valves, current into the base controls current sucked in through the collector, and that current is a multiple of base current, hence "amplified", but they don't create current, it's still the case that, for the same voltage in and out, you can only get at most as much current out as is supplied in.

 

[srnet]

You cannot get 50W of output power from 25W of input power.

Think about it, where would the extra 25W come from ?

 

[Dippy]

I reckon BB is having a titter right now as most of us realised his irony. It certainly gave me a good laugh.

It's not a can of worms at all because no-one here knows what you are really on about.
The way you have written your posts suggest that you are confused.

I get the feeling that you are trying to ask how you get X Amps from Y Ah for a short duration.
Is that right?
If not then I wish you luck with the project.


Give us some numbers and get your terms cleared up. I can't see how anyone can give an answer until you clarify what you are on about

 

[BeanieBots]

@Pauldesign, I have to confess that partly for Dippy's amusement, SAborn is correct about post #23.
However, the main objective of that post was to extract where you were comming from and what thought process was going based on a hunch I had about what you might have read somewhere.

Sit and think about it for a while.
Imagine you could extract more capacity with a transistor.
Simply feed that into another transistor, then another, then another.....
Before long, you could power the entire planet from a single hearing aid cell. Global warming and all energy issues solved.

Consider a rock concert. In particular an electric guitar.
The current comming from the guitar pickup is very small.
That small current is fed into a series of transistors until it is strong enough to drive a set of speakers with enough power to make your ears bleed.
However, nothing comes out unless you plug the anplifier into the mains. That's where the extra power comes from.

You CAN get more current out than the battery can supply but at the expense of voltage as described in post #11.
You can NOT get more POWER out than you put in.

 

[Pauldesign]

Thanks to all for all the replies, your valued critisms, explanations and clarification.

By the way wrt SAborn, i value your replies and i never meant to sidelined any and those of others and i hope

and the only place you will find magic fairy dust is maybe with transvestites and not transistors

u're not also saying that (in quote above) out of experience .

You guys (i won't mention names this time ) all did made some good points which boils down to the samething, although in different ways and i must admit; i created most of the confusions due to my typos and the way i was trying to explain it, but nevertheless, mistakes and bluntness comes with a price and i applaud you all for waking-me up the hard way (which if not of my thick skin, i must have surrenderred long ago), esp after my long absence on the forum .

Nevertheless, an educational misconception of 'law of energy conservation' i was once wondering; i now know and i'll never considerred or even thought of something silly (against renouned laws of physics) like that anymore! .

Cheers

Paul

 

[srnet]

Usually best to think in terms of Watts, as in current x volts, in these cases.

Then its simple, Watts in must equal Watts out, and thus Watts out cannot be more than Watts in - ever.

Of course normally apparent Watts out (as in measured current and volts) will be less than Watts in, because some power is inevitably lost as heat, as even wires have some resistance.

 

[techElder]

You got guts, Paul. You got guts. Don't stop!

... i applaud you all for waking-me up the hard way (which if not of my thick skin, i must have surrenderred long ago), esp after my long absence on the forum ...

 

[SAborn]

u're not also saying that (in quote above) out of experience .

I can assure you it was just an expression and NOT first hand knowledge.

Your theory was off with the fairy's not me.

Good to see you have come to grips with the theory of transistor operations and limitations.

Now lets get back to desiging the flux capacitor.

 

[westaust55]

Heres an idea for PaulDesign to investigate further for "free" power . . . .
http://www.zen22142.zen.co.uk/Circuits/Misc/potplant.htm

 

[srnet]

Now lets get back to desiging the flux capacitor.

Too late maybe ?

http://uncyclopedia.wikia.com/wiki/Flux_Capacitor

 

[SAborn]

The flux capacitor requires 1.21 gigawatts

So whats the problem with that, if we use a dozen transistors off a watch battery we should be able to make it with a picaxe controlling the circuit. (the picaxe is the answer to all problems)

Can you imagine having the task of writing the description of what a flux capacitor is for Wiki, a good sense of humor involved in the description.

 

[srnet]

Can you imagine having the task of writing the description of what a flux capacitor is for Wiki, a good sense of humor involved in the description.

There is a more detailed Wiki entry for Dilithium crystals;

http://en.wikipedia.org/wiki/Dilithium_(Star_Trek)

But Dilithium crystals are real, I recall at one point you could find them either in the RS or Farnell on-line catalogue.

 

[westaust55]

But Dilithium crystals are real, I recall at one point you could find them either in the RS or Farnell on-line catalogue.

http://en.wikipedia.org/wiki/Dilithium

Li2 is known in the gas phase.

The April 1 edition of the catalogue?

Although dilithium is a real molecule, it is a gas with none of the properties of the fictional crystals