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Hacking an Extra Rechargeable NiMh Cell onto an "Alkaline Only" LED Circuit.

LED with Alkalines

Celeste bought some LED lights for my bike trailer this winter (as the second half of my commute is always in the dark). They were about $20 at target and come in lots of cool colors.

They run on 4 AA batteries. I didn't notice the fine print on the packaging until we got them home and I put a bunch of Nimh rechargeables into them. The lights were horribly dim. I looked at the fine print and it basically said, 'don't use rechargeable batteries.'


Well fuck that. I'm getting off the alkaline waste habit. I've recently switched all of our stuff to some fake eneloop batteries that duracell sells. (duraloops, in net forum parlance) So far they've been great!

If you know anything about NiMh batteries though, you'll know that they generally discharge at a lower voltage than alkalines do. A fresh alkaline battery will spit out 1.5V according to their specs, sometimes a little more in reality. A fresh NiMh battery will only do 1.2V according to their specs.

So, a difference of .3V isn't very much.. especially with most devices only using 2 cells. Lots of tech allows some fudge room of up to even a couple volts because battery voltages drop as they discharge. In the case of these cheap LED strings though, a loss of 1.2V across 4 cells was a big deal.

LEDs with NiMh Rechargeables

[ Electrical background for dummies (from a dummy): Most of the devices you use that take batteries have those batteries configured in a 'serial' fashion. That is, they are electrically connected as if they were all stuck together in a line head to tail. (like a flashlight) Even when you have batteries sitting side to side, you'll notice they alternate head to foot. That's to make it simpler for them to wire up serially. The alternative is called 'parallel' where all the positives on all the batteries are connected together, and all the negatives are connected together. With 4 serial alkalines, you'll get an ideal 6V, with 4 parallel alkalines, you'd have a voltage of 1.5V. (the voltage of a single cell). Now, back to our story: ]

So I said to myself, hey.. I've got a whole stack of things that run at 1.2 volts.. One more cell should fill in nicely! : P

NiMh Proof of Concept

So, I hacked on another battery. Most of what I really want to blog about was technique in doing so. So here it goes.

I had a 4 cell battery holder, that has some special circuitry for running the LED's.. (um.. a switch and resistor.. I'm betting) At any rate, I didn't want to heavily modify the whole thing.

So I dug up some old etchable circuitboard I had lying around. It's a standard circuit board material, with copper on either side. The key part for me though, is that the copper on each side isn't electrically connected. So I can insert it between one of the batteries and the spring contact underneath it, and effectively wire in another battery there.

Battery Insert

I'll save you the grinding and cutting. I built a basic circle shape with 2 pins going out such that they were each only connected to one side of the board. I then wired each side to the only battery holder I had in the house such that they corresponded to the correct pins on my battery.

Insert Notch
Insert Installed
Insert Installed
Insert Pins
Battery Mod Interior

When I measured the 4 Alkaline cells I got 6.32V. With the 5 rechargeables, I get 6.39V of cheap rechargeable goodness! Woot!

Anyway, here's the final product:

The kid rig, with running lights

You can get a closer look at the pics over at the flickr set:

Comments step..a picture of Gram in the trailer with the lights.