Is this sound reasoning?

Okay, so I was looking around at the cool ideas and the wife informs me she has lost her company issued 3D Mag. (Possibly stolen from her truck, but just as likely, forgetten on a running board and lost along the road.)

Anyway, I start getting ideas. (Obviously) Some of the criteria is cheap to run over all--rechargables only--bright enough to light up dark areas under a trailer even on a bright day. Her 3D Mag pretty good for what it was.

I was thinking about replacing it with a 2D Mag, EverLED bulb, NiMH bateries, and building my first mod in the form of a 8AA to 2D converter, probably using the RadioShack holder route. This setup would give me 9.6v and after reading that the EverLED seems to be fairly efficient at 9v, I thought this may be the way to go.

Do have a couple of questions though:

- 8 ea. 1800mAh NiMH gives 9.6v and using the JohnK's figures of .13 A at 9v pushing the EverLED, I figure better than 13 hours run time. Can this be right?

- My thinking of using the most capacity of the batteries by using many and keeping the voltage up overall until it dropped to 2-3v overall, means each cell would be putting out only .25v or slightly better each. Plus, the power falloff being fairly dramatic on NiMH (I've read elsewhere), would mean I get good light up until the very end of the charge when it fails pretty darn quick.

Is this sound reasoning? Am I missing something? Would this stress the batteries too much?

I'm thinking of modding the battery holder to be a direct replacement of 2D cells so when the NiMH battery pack fails, she can drop in a couple of regular D cells to get by until the NiMH are recharged. I figure this is a good reason to use the EverLED as it can take just about anything.

So, whatcha think of my idea for my first mod?

CR

you will kill nimh cells by reversing them.the first cell to drop will get charged in reverse.if you use 2 cells you will know when the first cell drops due to the drop in brightness.this is your warning to stop.
an 8 cell setup wont give you your warning since the everled will go on till 7 cells are dead and are being reversed.

You had everything right, until you mentioned running the batteries till the overall output was 2-3V. As you calculated, this will give you around 0.25V per cell. At lower than 0.8V per cell, you start to run the risk of reverse charging any weaker cell.

It is highly possible that if you intend to run the rechargeables till 2-3V overall, you will damage one or two cells and you end up having to replace them. It would be interesting to see if you do get any savings when you have to replace those cells.

However, battery packs like those meant for cordless drills and remote control cars frequently do the same thing. Just imagine how they are able to deliver 7.2V, 9.6V or 12V? Same method you wanted to use, ie bung a whole bunch of cells together until you get the voltage you want. Usually though, they don't run the cells into the ground. Frequently, before you reach those danger levels, the performance of the equipment has already deteriorated to the point where you will immediately recharge them. It won't be the case here as the EverLed will still be as bright till it reaches the 2-3V range.

So you can do what you want to do with those 8 cells. Just remember to top them off at the end of everyday. This will reduce the risk of reverse charging. And if you do find one or two cells getting reversed charged, there is a method to save them. I think it involves a high voltage/current jolt to the cell in question when recharging. Check in the batteries forum for that piece of information.

Okay, thanks. I knew it sounded too simple.

Taking this one step further and suppose I were to put 8AA Alks in this same holder, this would add up to 12v or better with fresh batteries. Would this be too much for the EverLED to handle? I know literature has stated the EverLED is for up to 6 cell lights, I just wonder if 8 cell would harm it.

The reason I'm asking is my wife wants something really low maintanence. If she can get 13+ hours on a set of Alks I think she'll go for it. She only uses the light for maybe 60 seconds at a time, 2 to 4 times a day--it's really short duty. The batteries should last her a really long time.

I wouldn't mind going ahead with what I've said above, just it'd be my light as I would putter with it and make sure it stays charged. The wife wouldn't do that.

CR

Did you ever consider a lower voltage parallel configuration? /ubbthreads/images/graemlins/icon3.gif
IMHO, this would eliminate a lot of potential problems... /ubbthreads/images/graemlins/rolleyes.gif

60 seconds at a time, 2-4 times a day works out to about 4 minutes a day. If you can get those 13 hours, you'd be running the light for about 195 days before you needed to think about a battery change. Over half a year. I think we can forget about running NiMH cells. They would lose most of their charge in a month. But you could keep topping them off every week.

So that's one thing that could work, ie top off every week. You would still be in the green and wouldn't be running any great risk of reverse charging the cells. I thought the light would be seeing heavier duty. That was why I recommended topping off the charge everyday.

Another item to consider is that all manufacturing has certain tolerances. 4 AA cells would be a tight fit in most cases. What if you got a Mag with the minimum size. Then it would be a real bother trying to shake out the 4 AA cells. Why not build it with 3 AA cells side-by-side, ie 6 cells total? And why not use a C-cell body? That way you have a smaller, lighter flashlight which can take alkalines or rechargeables without any change in brightness, or significant runtime loss. Running rechargeables, you would need to top off probably every other week to stay ahead of the self-discharge and in so doing, will also prevent any problems with reverse charging.

But the 6 AA configuration will also allow you to run alkalines without any worries about blowing the LED, in case you're sick of the fortnightly recharges.

And finally, you get to use a C-cell body. Smaller, lighter, but still with that clickie switch.

Anyway, good luck with it, whatever you finally do.

I did think of the 2x4AA and was thinking it wouldn't be that much harder to rig the RadioShack holder to a 2x4 instead of 8. However, the advantage of the higher voltages garnering better efficiency would be lost and still not be able to use the light dimming to know when to stop before risk ruining cells.

Realizing that it's going to be a long time between changing/charging batteries I think going with 6AA to 2D setup running alks is going to be the way to go. With the smaller diameter and much tighter case, plus C cells about the same length as AA's, I'm not confident enough in engineering a 6AA to 2C adapter, though, so a 2D Mag will have to do.

I heard back from the EverLED folks and they said it's rated for 9v, 12v is too much. So much for that!

However, I might go forward with the 8AA holder and put 6 cell bulb in the 2D Mag just to see how well that works. Might not be enough runtime to bother with, though. I don't know, maybe.

Thanks for all of the input!

CR