Simple/safe charger for internal battery pack?

Looking to make a light with an internal battery pack that the user doesn't need to remove. The pack would be 7 cells, 4/3A, NiMH, probably 4AH.

Would like the use of the charger to be as simple as possible; the light would have a receptacle and simply plug the lead in, and charger let you know when it's charging/trickling(maybe)/done. I.e. I don't want to present the user with dials, multiple switches, LCD readouts, separate power supplies, etc. I'd love to get a nice, universal do-everything charger for myself, but want to be able to give this light to folks in as much a plug-n-play format as possible, while hopefully keeping reliability intact.

Want to have some reasonable assurance that the pack doesn't die young. On the discharge side, will have a cutoff voltage, but want to make sure the pack doesn't die charging. I know that dT/dt is the best method to terminate charging but haven't found anything that would incorporate into the simplistic ideal above. I'd be happy to embed a thermistor in the pack if I could find a charger that uses that as input.

Oh yeah, inexpensive too. /ubbthreads/images/graemlins/yellowlaugh.gif

Charging time is not that important; something like 3 hours would be great, but overnight would be fine if I can meet the other goals.

So far the closest I've found is a -dV wall mount charger from PowerStream.

http://www.powerstream.com/NiMHWMm.htm

One of their other pages that links to it seems to claim there's a time-based cutoff, but all I see is the delta volt termination. The price is nice ($25). There's a mention that these are " OEM only, minimum purchase quantities apply", but quantities listed for sale as low as 1 or 2 units... Anyone dealt with these folks before? Do you need to offer some proof of being an "OEM"?

I'd appreciate any opinions on the suitability of the above charger, or pointers in the direction of potentially better chargers, especially if a DT/dt could be incorporated.

Sorry to make this a new post; surely the information can be gathered from existing CPF threads; but I've been at this for a few hours and eyes are getting bleary /ubbthreads/images/graemlins/duh2.gif

/ubbthreads/images/graemlins/thanks.gif

Hello Nubo,

Powerstream seems to know their stuff and has a good reputation. The charger you listed seems quite good and should work well.

Tom

I looked at their site. That charger will only charge one voltage, that can be set by the OEM buyer. It will not charge, say, 1-10 cells as a stand alone charger.

Bill

Hello: This charger might work. I've been happy with purchases from BatterySpace.

http://batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=1111

Thanks for the responses.

SilverFox, after reading your exhaustive testing of batteries I am encouraged by your statement.

The only doubt in my mind is that I've seen a few posts discussing situations in which delta voltage does not always work and cells get cooked (or worse). That's why ideally I'm looking for something to incorporate a delta-temp detection. Though I'm sure it would be more expensive, I'd be willing to go that route to get to that extra level of certainty. The example I have in mind is the charger that came with my Light&Motion HID bike light. It plugs into the battery pack with nothing more complicated than a couple of LEDs for indicators and it uses the dT/dt sensing method. I've never had anything but perfect results through hundreds of recharges. That charger shows manufacture by "I.T.E." of Korea, but must be strictly OEM, I haven't seen any of their stuff on the web. I guess there's the option of $99 retail from L&M, but I think they'd get curious if I ordered a bunch. /ubbthreads/images/graemlins/thinking.gif

Hello Nubo,

The missed peak detection usually only occurs to new cells, or cells that are already charged. Unfortunately it is difficult to avoid those situations.

Another idea is to incorporate a timer. If the charge rate is not too high, you would only end up cooking the pack for a short time...

Tom