Project #2 Part 2

As part of the grand scheme for project #2 (cell phone/ipod back up battery) I built a charger for rechargeable AA batteries (NiMH).

NiMH batteries are 1.2V as opposed to the 1.5V for normal AA. So one thought was just put 4 in series, then there's no over voltage fear. But if I'm using rechargeable batteries I want to be able to charge them and preferably over USB (I don't feel like getting into building something that plugs into the wall). But the USB voltage can be as low as 4.75V - tough to charge up to 4.8V off 4.75V (not impossible, but would require a lot more parts). Also 4 AA doesn't fit in the size I want.

Now the thought is use two rechargeable AA and use a boost converter to charge the phone (sort of like mintyboost except I want to provide more current and have rechargeable batteries). Charging two AA off USB isn't too tough since Linear makes a part (LTC4060) which is designed to cover this situation. So I ordered two free samples from Linear. They got here on Friday. Unfortunately the part has some really small pins (the tiny black part on the big brown board).



Fry's sells a protoboard that fits the part's package so I bought one of those and soldered on the part and the bit of surrounding electronics. Technically there should be a ground connection to the bottom of the chip, but the protoboard isn't set up for that. It is not a big deal because it is mostly for heat conduction and I'm setting the charger to use much less current than normal (because of the limitations of USB) so there shouldn't be any thermal issues. Of course the first time through soldering as I was almost done I messed up one of the holes so I got to de-solder everything and do it again on the other side of the board. The big part hanging off the side is a pnp which is what the current actually flows through to get to the batteries. This is one I just had around - it is massively oversized. What I ended up doing for the tiny pins was to put a big blob of solder over all the pins on a side, then use solder wick to remove all the excess.

The main thing I was learning about when building this was how to solder such small pins (key lessons being: light helps, magnifying glass helps, pay attention, move slowly, check connections with multimeter often, solder wick is my friend). However there were two other bonus lessons. One is those 9V battery connectors aren't great for connecting two sets of wires because if on one red is positive then on the other black is positive. This occurred me as I noticed the LTC4060 getting really hot after connecting the batteries. Luckily it didn't break the part (good job Linear on the reverse protection). The other lesson was what it feels like when a 700 deg F solder blob jumps off the soldering iron and lands on your finger, especially when it takes a moment to flick it off because you have stuff in your hands. It makes a cool little blister in the shape of the blob.



Here's the charger working (see the red LED is on, so it must be working). I don't think it is perfect - it took a long time to charge and I had to unplug it and plug it back in to get it to charge all the way (at least I think so, it is so hard to know what is going on with batteries).



Here's the state of my beautiful card table work bench:



I'm going to go ahead and call part 2 a success. Eventually I will need to find something over than that big protoboard to put everything on, but first I'm going to get the boost converter built (I'm waiting on some samples from Maxim) and make sure the electronics work before working too hard on the physical design. I am starting to think designing a PCB is the way to go, batchpcb looks cheap enough to be reasonable.

You might have noticed the battery holder is different than the normal plastic kind - I ordered some fancy ones because I thought they would fit in a smaller space, but they don't and they make getting batteries out a huge pain. So I'm using them for prototyping, but I'll be switching back to the cheap plastic kind.

It might seem like lithium ion batteries would be better for this project than NiMH. They do have some advantages. But using NiMH means they can be replaced by normal AA if needed. And lithium ion batteries are a bit more prone to exploding if charged wrong.

Some of you might notice this a really simple project. That is true. But I've noticed that what has kept me from making projects in the past is my design skills are way beyond my building skills. So I decided to work on stuff more on the level of my building skills.