So I've read through the forum posts on this topic and understand that the 3Gs is really finicky when it comes to charging/power. I've been burned by accessories that no longer work so I'm willing to give a little leeway. That said here's my problem:
I have assembled two MB V3.0 kits. One reliably charges my and my wife's iphone 3Gs and the other pops up the dreaded yellow triangle. I've triple checked my work and put the two side by side to make sure absolutely everything was in it's rightful place - it is. Reading here, I tried jumpering the middle data pins but no luck. I have not yet tried grounding the usb shield. I should note that this charges my daughter's itouch just fine, so I'm doubting it's assembled improperly.
If we assume that both are assembled correctly, and I'm using the same batteries in both (swapping them in and out for tests), and both perform consistently with 2 different 3Gs devices then I'm led to believe there is a component problem with the second MB. The only different between it and the first is that I had to get an extra resistor sent out as one was missing from the kit. As with all devices I realize that each component is engineered with certain tolerances in mind and I'm wondering if perhaps those sometimes add up to something the 3Gs just won't accept when it goes to negotiate the handshake.
Before you ask, I have not tested with a multimeter yet. I'll try to get one and post results. In the meantime, I'll take some pics of the non-performing unit so at least you can look for any assembly problems. Please help! All I really want is a working MB v3.0 for my 3Gs as we go backpacking and this is is a lightweight life saver!