I'm designing a radio that can take dry cells or rechargeable cells. I want the radio to have a charging point but don't know how to detect that the batteries it may have in it are not rechargeable and so shouldn't be charged. Is there a simple way to do this or do I just put a disclaimer on the radio not to try to recharge dry cells? Thanks for your comments. Dave
What sort of rechargeables?
With nicad batteries there isn't much to worry about. The charger's maximum voltage is well below the voltage for a zinc-carbon. I'd still advise putting a warning label on the thing though.
If you use the temperature rise method of stopping the charging, it would be hard to imagine a non-lithium cell that could get into trouble being charged.
You can measure the internal resistance. The internal resistance of the rechargeables is several times less then that of the alkalines. This fact is used by the chargers, so they don't attempt charging alkalines. However the internal resistance of the deeply discharged NiMH can be quite high too. So, the idea is the following: try charging with a small current for several minutes, then measure the resistance.
Vladimir Vassilevsky DSP and Mixed Signal Design Consultant
Put your batteries inot a holder that has some way of indicating ot the device that they are rechargable or not. Seal the rechargable batteries in the holder so the end user can't change them withut destroying the holder.
All dead batteries look the same, dead.
t
Some battery packs have circuits built into this to accomplish this tasks. Search some of the Maxim and Benchmarq products (if memory serves..)
Also, I know some batteries have form factors that already take care of this. For example, back in the days of pagers (remember those?), several models used "N-cell" batteries (also known as 2/3-AA ?). The NiCad versions of these had little tips on the positive terminal which would engage with a charger. The Alkaline versions did not have the tips, and when inserted into the charger would not make contact with the charging voltage.
I don't recall seeing this on other batteries, and certainly not the more common consumer sizes. And you'd have to have your head (and calculator!) examined if you were to design something new these days using type-N Nicads...., but maybe its worth a look.
Otherwise, I'd use a disclaimer anyway. There might also be a fancy way to monitor charge rates, and derive something from that. Good luck. -mpm
My CCRadio has an AC power cord but to charge NiCds an external wall- art is needed in addition to the AC power. I though it pretty "cheap" for a $175 radio to not include a battery charger powered off the AC source, until I thought about how much trouble 12oz. of coffee can cause for a company. Use the disclaimer.
-- Keith
It is pretty hard to measure the internal resistance of a battery unless you really pull some juice out of it. [I've done this for battery evaluation with some custom circuitry. In my case, I used the battery as a source, pulling both DC and AC out of it since I wanted to examine the impedance over voltage. ] Some companies tried alkaline detect by observing the battery voltage under charge conditions. I really advise against doing any of those games and just use a different battery holder for each type of battery.
You may want to consider end of discharge voltage. You can take an alkaline down to at least 0.5V and have it be useful, but that would kill a secondary cell.
Nope! First, no assumption can be made as to the condition / charge of the battery, so a simple unloaded voltage measurement will do. *IF* one knew the battery was fresh, a Leckance cell measures about
1.55V open circuit, and different (fresh) rechargeables measure different voltages depending on the type, but the majority of themseem th be in the 1.25V region. Oh, yes...attempting to re-charge an unknown battery could lead to disastor.
Not practical due to size standards ("A", "AA", "C", "D", etc).
There is less risk than it first appears with standard shaped cells. This assumes that you have a temerature rise detector on the cells:
A Lithium produces way more voltage than a nicad or zinc-carbon. If a Lithium is down that low, it is junk. Refusing to attempt to take a battery from less than 1.6V as a Lithium protects the charger against trying to do a Lithium charge on a nicad.
If a completely dead Lithium has a current applied that stops before the cell gets above 1.6V nothing very interesting will happen.
A nicad charger that limits the voltage won't do anything bad to a good zinc carbon cell. Zinc carbon cells that are really messed up heat up when you connect 1.2V onto them but if you stop on detecting the rise, nothing comes of it.
The usual way I've seen / used is to have the batteries in a subassembly or carrier which comes in two flavours. The dry cell version simply doesn't have a charging contact. Ideally the rechargeable battery pack uses cells of a different shape to the non rechargeable cells so it's not physically possible to stuff non rechargeable cells in it. If you can't make the cell shapes incomatible, you cover the rechargeable pack with warnings like "not suitable for use with non rechargeable cells" etc (disclaimers in case some idiot puts some in and charges them.)
Last time I designed an instrument, we managed to convince the Marketing department that it would be a lot smaller, simpler and cheaper if it just had one type of battery, and lithium ion were now good enough that you no longer needed to worry about shelf life from self discharge (which was the big problem with NiH).
-- Nemo
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