How do chargers determine what battery pack is installed?

[Ian Stirling]

I read in sci.electronics.design that DaveC <dave-
[email protected]> wrote (in <0001HW.BD2123CB000124FD09E9C0F0@ne
ws.individual.net>) about 'How do chargers determine what battery pack
is installed?', on Mon, 19 Jul 2004:
No. A theoretical current source develops an infinite voltage on no-
load. This is inconvenient, so practicable current sources have an
internal impedance Z in parallel with the current generator, so that the
no-load voltage Vo = IZ, I being the constant current. Vo is the
compliance voltage.

The current I can be achieved (approximately) only if Vo is very much
larger than the battery voltage.

With all modern semiconductor based power sources, the compliance
voltage is a few tenths of a volt below the maximum output voltage.

The number of powertools whose multi-battery constant current charger uses
a resistor, and a power supply of many times the voltage (apart from the very
cheapest untimed low power ones) can be probably counted on the fingers of
one ear.

 

[Ted Edwards]

No. A theoretical current source develops an infinite voltage on no-
load. This is inconvenient, so practicable current sources have an
internal impedance Z in parallel with the current generator, so that the
no-load voltage Vo = IZ, I being the constant current. Vo is the
compliance voltage.

Don't know what sort of constant current sources you use but the ones
I've designed and used have a fixed voltage supply, Vs, driving an
electronic current limiter. The output voltage remains approximately
constant at Vs as long the load draws less current at that voltage than
the rated current, Io. Any attempt to draw more than Io results in the
voltage dropping. You thus get a nice square V/I curve. For a charger,
you simply need to choose Vs greater than the max battery voltage you
wish to charge.

Ted

 

[John Woodgate]

I read in sci.electronics.design that Ian Stirling
reader04.plus.net>) about 'How do chargers determine what battery pack
is installed?', on Mon, 19 Jul 2004:

With all modern semiconductor based power sources, the compliance
voltage is a few tenths of a volt below the maximum output voltage.

The number of powertools whose multi-battery constant current charger
uses a resistor, and a power supply of many times the voltage (apart
from the very cheapest untimed low power ones) can be probably counted
on the fingers of one ear.

Sure, but I was responding to a purely technical point, not a practical
one.

 

[John Woodgate]

I read in sci.electronics.design that Ted Edwards
'How do chargers determine what battery pack is installed?', on Mon, 19
Jul 2004:

Don't know what sort of constant current sources you use but the ones
I've designed and used have a fixed voltage supply, Vs, driving an
electronic current limiter. The output voltage remains approximately
constant at Vs as long the load draws less current at that voltage than
the rated current, Io.

That isn't a constant-current source. It's a constant-voltage, current-
limited source.

Any attempt to draw more than Io results in the
voltage dropping. You thus get a nice square V/I curve. For a charger,
you simply need to choose Vs greater than the max battery voltage you
wish to charge.

Read all the thread. Your comment is true but irrelevant.

 

[Bob Wilson]

I read in sci.electronics.design that Bob Wilson <rfwilson@send_no_spam.
intergate.ca> wrote (in <[email protected]>) about 'How
do chargers determine what battery pack is installed?', on Mon, 19 Jul
2004:

Only if the compliance voltage of the current source substantially
exceeds the battery voltage.

Naturally. That was the assumption.

Bob.

 

[Bob Wilson]

I read in sci.electronics.design that DaveC <dave-
[email protected]> wrote (in <0001HW.BD2123CB000124FD09E9C0F0@ne
ws.individual.net>) about 'How do chargers determine what battery pack
is installed?', on Mon, 19 Jul 2004:
No. A theoretical current source develops an infinite voltage on no-
load. This is inconvenient, so practicable current sources have an
internal impedance Z in parallel with the current generator, so that the
no-load voltage Vo = IZ, I being the constant current. Vo is the
compliance voltage.

[Snip]

Another way to state John's point is that a theoretical current source
generates whatever voltage it needs to (no matter how high) in order to push
the specified (constant) current through the load. If the load's resistance (or
in this case its terminal voltage) is very high, the current source
automatically adjusts for this.

A high "Compliance voltage' is just another way to say that the current source
is capable of a very large range in output voltage, should this be necessary to
ensure meeting the required output current.

Bob.

 

[DaveC]

Another way to state John's point is that a theoretical current source
generates whatever voltage it needs to (no matter how high) in order to push
the specified (constant) current through the load. If the load's resistance
(or
in this case its terminal voltage) is very high, the current source
automatically adjusts for this.

A high "Compliance voltage' is just another way to say that the current
source
is capable of a very large range in output voltage, should this be necessary
to
ensure meeting the required output current.

Could this possibly be one way a charger designer determines that a battery
pack is faulty? If the internal battery resistance rises with age (true?),
and the compliance voltage is high as the charge current is applied, the
charger's controller could determine that this pack is out-of-spec, and
decide to not charge it?

 

[Rich Grise]

The current I can be achieved (approximately) only if Vo is very much
larger than the battery voltage.

John Woodgate, I'm shocked! Please check your maths here[0]. I'll let
this slide for now, because it's very late. ;-)

--
Cheers!
Rich

[0]
For those who don't get it:
On my constant-current charger with sufficient voltage compliance,
V0 adjusts itself to Vbatt + (Ibatt * Rcontact), so unless your
contact resistance is something like a light bulb, Vo doesn't
need to be very much greater than Vbatt at all.

 

[John Woodgate]

I read in sci.electronics.design that DaveC <dave-
[email protected]> wrote (in <0001HW.BD220DC80036CB3605DAA1F0@ne
ws.individual.net>) about 'How do chargers determine what battery pack
is installed?', on Mon, 19 Jul 2004:

Could this possibly be one way a charger designer determines that a
battery pack is faulty? If the internal battery resistance rises with
age (true?), and the compliance voltage is high as the charge current is
applied, the charger's controller could determine that this pack is out-
of-spec, and decide to not charge it?

Yes, it could, but it won't charge it anyway, because the battery
voltage is presumably too high to allow the normal charging current to
flow, unless the charger accepts batteries with a large range of nominal
voltage.

I have an old charger for wet lead-acid batteries that works from a
single 2 V cell up to 24 V. It has a 'constant-current' transformer
(high leakage inductance) and the compliance voltage is 32 V.

 

[Keith Williams]

I don't know, and I strongly suspect that it depends on who's in
'charge' of the of the algorithm. It's on the order of 10-100mV per
cell for NiCd and a couple of seconds for NiMH -- and I think you could
go cheap and use the same algorithm for both; just detect the top of the
peak and call it a plateau.

I believe this method will either leave both under-charged or destroy a
NiMH cell. The generally accepted (rapid) charge method for NiMH is a
constant current with a temperature cut-off. During charge perhaps 10%
of the energy goes into heating the cell. After charge 100% goes into
heat so it's rather easy to sense this difference. This method works
well for both technologies. V or delta-V sense works for NiCd, but is
highly "not-recommended" for NiMH.

Gates used to put out a very nice battery book on Pb-acid and NiCd
batteries, I don't know if it's still in print.

Yes the "Rechargeable Batteries Applications Handbook", originally
published (and given away[*] to customers) by Gates Energy is/was a
very good reference manual. I believe the rights were sold (Stoneham?)
and Gates itself bought (?). The book is available on Amazon:
http://www.amazon.com/gp/product/offer-listing/0750670061//002-6795559-
1989637?condition=all

[*] who swiped my copy?!

 

[Ted Edwards]

I read in sci.electronics.design that Ted Edwards
'How do chargers determine what battery pack is installed?', on Mon, 19
Jul 2004:

That isn't a constant-current source. It's a constant-voltage, current-
limited source.

Any constant current source that does not have infinite compliance is
exactly that except your scheme results in a much sloppier arrangement.

Read all the thread. Your comment is true but irrelevant.

I bet you say that to everybody who disagrees with you.

Ted

 

[John Woodgate]

I read in sci.electronics.design that Ted Edwards
'How do chargers determine what battery pack is installed?', on Wed, 21
Jul 2004:

I bet you say that to everybody who disagrees with you.

I don't just disagree with your opinions; your definition of 'constant-
current source' is simply and completely wrong. For load where the
voltage is constant, the current is ipso facto not constant.

 

[Ted Edwards]

I don't just disagree with your opinions; your definition of 'constant-
current source' is simply and completely wrong. For load where the
voltage is constant, the current is ipso facto not constant.

Quoting from your post:
"No. A theoretical current source develops an infinite voltage on no-
load. This is inconvenient, so practicable current sources have an
internal impedance Z in parallel with the current generator, so that the
no-load voltage Vo = IZ, I being the constant current. Vo is the
compliance voltage.

The current I can be achieved (approximately) only if Vo is very much
larger than the battery voltage."

So your description has an open circuit (as seen externally) voltage
Vo. As you apply load, the voltage out drops in rather sloppy voltage
as the external load current increases. Eventually, when the load
current forces the output voltage down so that V<<Vo, the current will
approach a limiting value.

There is no real difference between a current limited voltage source and
a voltage limited current source other than the portion of the V/I curve
over which it is designed to operate.

Ted

 

[John Woodgate]

I read in sci.electronics.design that Ted Edwards
'How do chargers determine what battery pack is installed?', on Wed, 21
Jul 2004:

As you apply load, the voltage out drops in rather sloppy voltage as the
external load current increases.

I expect you know what that means, but since I *defined* Vo as the no-
load voltage, your assertions that Vo changes when a load is connected
is not sensible.

 

[Ted Edwards]

I expect you know what that means, but since I *defined* Vo as the no-
load voltage, your assertions that Vo changes when a load is connected
is not sensible.

Are you incapable of discerning the difference between "Vo" and "output
voltagge"?

This is getting rather silly and I'm rapidly becoming convinced that it
has been many years, if ever, since you designed and _built_ a practical
current source.

Ted