why battery charger voltages are high 1

[E2005]

Recently I bought a battery charger(Fast Ni-cd/ Ni-mH) from Radio shack(Catalog # 23-422). I was wondering why it has an output of 20V(0.8A) for charging a 9V battery(why not 9V exactly). Another thing, I have another 9V battery charger(from a different company)which actual gave me reading of 15V(300mA) on the output. Do all battery chargers always give out voltage higher than what the charging battery rating is. Is there a reason to it and how much it should exceed by.

 

[IRstuff]

How did you think it was achieving its "Fast" charging?

TTFN

 

[Skogsgurra]

Yes, they are all above the nominal voltage. How much? That differs a lot. Some (lead battery chargers) often work with an unsmoothed rectified voltage and the voltage measured depends a lot on what you use to measure the voltage. Most DMMs have such a high input resistance that they measure the peak voltage (capacitance in charger and cables serve as peak voltage storage). If you measure with an old moving iron meter, you will probably measure a much lower voltage. There is also the TRMS vs AVG thing to be aware of.

Then again, there are modern NiMH and LiIon chargers that are well above the nominal battery voltage. Simply because current like water only flows from higher levels (voltage) to lower. You simply need a higher voltage to charge the battery. With the same voltage on charger and battery there would not be any current flow.

There's a lot more to read about this and the web is full of good documentation. Try it!

Gunnar Englund

http://www.gke.org

 

[VEBill]

I assume that you measured the output voltage with the charger not connected to a battery. It is an interesting measurement, but more-or-less useless information.

Wait until the charger thinks that the battery is fully charged, and then measure the voltage at that time (when they're still connected). The voltage at that time should (ideally) be something close to the proper value for the battery.

The fully charged voltage is likely something higher than the nominal voltage. '12-volt' batteries are something like 13.8 or 14.2 when fully changed. A '9-volt' NiCd might be 10.2 or so, but certainly not 15 !!!.

 

[itsmoked]

To add to the already good comments here E2005, the higher the voltage the faster the charge rate. However the faster the charge rate the hotter the batteries will get so sometimes fast chargers need to watch the temperature of the pack.

 

[Paulusgnome]

It needs to be noted that battery chargers are designed to be controlled current sources, and the voltage required to deliver the current is, within limits, not controlled.
If your battery charger develops 20V across a 9V battery under charge, this is simply the voltage needed to drive the design current into the battery.

Mark aka Paulus

 

[Chuckles159]

More simply put, if the voltage from the source was the same as the battery voltage, the battery wouldot charge. You nead a "head". That is, a difference in voltage level to get current to flow from the charger to the battery. I know there is a lot more to it than this but this covers the basic idea.

 

[ScottyUK]

Some chargers are constant current sources. It depends on the cell chemistry. Most (all?) of those designed for lead-acid cells are constant voltage, although they may well enter current limit during the early part of the charging cycle and behave as a constant current type until the cell voltage comes up.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[RFTrax]

Your charger is smart enough to be able to handle the (slightly different) charge needs for the two different types of battery, though I don't know if it is adaptable enough to figure out cell-pack voltages and combinations.

Choosing NiCd as an example. The fast-charge type use "sintered plate" or similar tricks to vastly increase the area available to the chemical mechanism. The so-called mass-plate and others may be engineered for long periods between use, or low temterature. They are made to a purpose.

A NiCd is charged using a constant current because the voltage will not substantially rise to self-limit the current like happens with a lead-acid battery. Cheap wall-plug chargers use a single resistor, and a current typically C/20 to C/10 where C is the "capacity" in mA.hr or A.hr. This is a safe enough level that will not cause overheating or unfortunate chemical change. It takes ages, and if used carelessly to keep "topping up" a part discharged battery, will result in the battery aquiring a "memory" of its usage. Parts of the cell never exercised will deteriorate. From here comes the tip to let the cells discharge as much as possible before charging. Modern builds are less prone to this, but it still makes the charger requirement tough.

Modern chargers use smarter control chips, much developed from the heavy use and casual charging of batteries used for small power tools. If you just charge away at a fast rate, and deliver the constant current x time to get to the nominal "capacity", you then need to charge about a third beyond that because the charge efficiency of the battery chemistry is only about 65%. Depending on the previous usage, you may get there sooner than you think! There needs to be *some* way of detecting that the battery is full. In the case of NiCd, the full point is accompanied by a sudden and dramatic rise in temperature, along with the cell voltage (while charging!) starting to reduce. This change of slope is only millivolts, but it is the signal to cut way down to a few mA maintenance current. Electric drill charger packs often include a diode temperature sensor taped onto a cell, to give a second signal to the charger control chip.

Some chargers will automatically discharge the pack before putting in a charge. These are sometimes touted as being "good" for the cells in eliminating the "memory effect" and recovering cell capacity. Probably the real motivation was they allowed a timed charge without trying to detect full charge state, but I could be wrong for some products. I once abusively overcharged a 1800mAh pack so the plastic covering melted, and the whole lot was too hot to hold. It still worked, though it needed a new covering which I made from heatshrink. I have been known to pulse a 12V car battery in series with a headlamp bulb into dead AA cells. The nickel crystals within had grown to short-circuit the cell. A few tries of this, and the crystals burn out, and the cell accepts a charge. The 50mA ignored, I give it several minutes of 2A. A cell in this state is not worth this sort of attention. You can repeatedly charge/discharge to bring the capacity back somewhat, but I never do. I need very high reliability from batteries.

Forgive that this may have rambled on more than appropriate for your Radio Shack product, but I don't know the item, and I have been the distance with NiCads.