Charging Rechargeable D Batteries
 

My garbage can is one of those hand motion lid opener kinds and needs 4 batteries to operate. :eek:

I used one set of batteries when we first got it, but I don't want to keep replacing them. I want to get some rechargeable batteries, probably Duracell because my other rechargeable's are. The problem is finding a charger that can support the amount of charge a D cell can hold. From what I have read in reviews, even if it "fits" a D cell, that doesn't mean it will fully charge it to capacity (meaning I have to charge more often).

So I'm looking for (and thought of a few) options to charge the batteries as necessary. Here's a few I thought of:

1) Use a power supply/wall wart and cut the end off, strip the casing and attach the positive/negative to a case which will hold the D cells. Never doing this before, I don't know how best to do it. One D cell is 1.5 volts. If I charge them in series, that is 6 volts. Should I get a 6 volt power supply and charge them in series or get a 1.5 volt power supply and charge them in parellel? Or maybe charge a pair in series and another pair in series? Or maybe a pair in parallel and another pair in parallel? In either case, should I get the exact voltage (6 or 1.5) or should I bump it up a little (say, 7-8 or 2-3)?

2) Another option and one I would really like, is a small solar panel. The issue I seem to find is how long it would take to charge the cells. I can find a panel that will output the right voltage, but the mA of output would take daaaaays to charge them up fully. I would need a second set and I don't really want to do that. If I could figure that issue out, the same question applies here as well, how best to charge them (in volts, 1.5, 3, 6, series, parallel)?

Anybody have any ideas or a thread link that I missed?

You can remove the batteries, let them cool off for a few minutes, and put them back in for a second charge cycle. And only using part of the battery capacity makes them last longer - that's what the Prius does. (That applies to all types of rechargeable batteries I know of. You'll want to keep them somewhere in the middle of their SOC range, except for lead acid that likes to stay near full charge. Every once in a while, you'll want to do a full charge to allow the individual cells to rebalance.)

I would recommend ditching the batteries all together. A 5V cell phone charger should be fine to power it. That is 1.25V per cell which is about what NIMH cells run at, and they work in nearly everything. Only the pickiest of electronics don't like them.

Cool project though, I wanna see some pics. :)

I think I get what your saying... plug in the power supply, then strip the wire to attach to each side of the battery terminal... only problem is that there is no nearby power outlet. I like the idea though, that way I don't have to worry about it at all.

I'm still not sure how best to charge the batteries. Series, parallel, two pairs...

Here's what the battery compartment looks like if it helps:
http://i1289.photobucket.com/albums/...psa705997d.jpg
Dimensions are 9.375" X 1.25".

I also had another idea, which wouldn't require batteries or home power at all, but it would be more of a research and learn then put it back to normal type of thing... more to come on that concept!

Since you're DIYing, there is nothing stopping you from extending the wires... Unless you are really WAY too far from an outlet.

If you charge them individually each battery will get it allotted charge, when charging multiple batteries at once the charger will send a charge to equal the combined voltage of in your case 6v or 7v to the set of batteries.

Problem is one or two batteries could receive a overcharge at the same time 1 or 2 more batteries could be undercharged but the charger will not know it, it will happily charge until the whole pack of batteries reach its 6-7v, possible cooking a battery or two in the process.

For RC battery packs and the like , they use balance chargers which allows all batteries to be individually monitored while being group charged.

In your case charge them individually and the batteries will thank you.

Regards, Doug / ecomodded

A balanced pack, in practice, will stay balanced until near end of life. And unlike lithium batteries, NiMH will tolerate a slight overcharge if not done too often - that's how balancing is generally done.

I was thinking about the solar panel idea for charging, then I remembered something I saw here talking about by-pass diodes.

http://www.solar-facts.com/panels/bypassdiode.png

So it got me to wondering if I could install a by-pass diode between each battery (effectively each "solar panel"). If one battery becomes a higher resistance, then the current will flow to the next one until the second one becomes too high resistance and then so on and so on.

Here's what I am thinking:
http://i1289.photobucket.com/albums/...psa2f8e7cb.jpg
You can see the by-pass diodes between each cell, effectively making a path of less resistance to the next one.

Will this work, or is my electrical theory incorrect somehow?

I've been told this won't work at lower voltages (I looked into it while developing a lithium battery management system for my PHEV Prius kit a while back). Zener diodes do not work as well at low voltages as they do at higher voltages. Their transition from blocking to allowing current to flow is more gradual at lower voltages. At higher voltages (like 13V or so), its more like a switch, and that is what you want.

I was actually thinking of using a Schottky diode for it's relatively low voltage drop. I can't find anything about their characteristics based on the voltage of the circuit, would they have the same characteristics as the Zener diodes?