At first, I thought about building MintyBoost-like circuit for this, but then I actually measured Nokia charger's voltage. With no load, both old ACP-7E and new AC-4 SMPS charger puts out 6-7(8)V. No idea what happens under load. It probably drops. Old is labeled 3.7V and new 5V.
Actually I should retrieve my somewhat better multimeter and drop that poor 1V resolution current clamp to see correct results.
Well, at first I tried to charge my phone with 12-24V car charger and cheap 2*9V batteries. Not so good results. I guess that more than half of energy went to heat. Batteries were very warm and charger was also a little bit warm (no wonder, because of the current). And per-battery voltage dropped from ~10V to ~3V. Poor batteries.
Well, I even managed to charge a phone little bit. According to not very accurate battery charge indicator, I calculated that I managed to squeeze out about 150mAh. Then I disconnected it. I don't know what was charger's output voltage, but probably under 5V. I also tested it without load. Outputs ~6V if input is >=6V. But phone was still showing that it's charging until I disconnected it.
Second time I connected my poor charger solution again, because there was still energy in those batteries. This time, I let it run for hours. When I finally checked what it's doing I saw that charger LED was very dim (yeah, I know that LED isn't voltage measurement device, I'll use multimeter next time) and phone charge indicator was acting very weird. It seems like it's "half-charging". Indicator rolls to half and stops there. Then repeats.
I have no idea, if undervoltage can damage something in phone. Currently I expect that it can. Or it's just not very good.
Then... I thought that, oh wait... there are actually emergency chargers based on 9V, or better, on 4*AAA batteries.
I thought that maybe I can build a charger with just 4*AA batteries. Or C/D size 1.5V batteries. Just 4 batteries and one inline shottky diode, which gives me about 0.5V voltage drop and also protects my phone from wrong polarity. Which, I guess can surely damage it. And it's a little bit over month old too...
And what about undervoltage? Can it damage my phone or not? Any ideas? I also heard that some 1.5V cells can have voltage up to 1.7V when not under load. But that's not very much and will drop.
Well, so, I though that I can make charger out of 4*1.5V batteries, a shottky diode and according to some reports, I also need current limiting resistor. Some Nokia's wont charge when they receive too much or current. Example was Nokia 6610. So maybe I need about 10-15 ohms resistor.
BTW, can anyone confirm if it's true that alkaline cells maintain their nominal voltage until they're almost empty, but regular zinc-carbon ones just continue to drop voltage through their lifetime, until it's zero? Haven't measured it yet.
I guess that alkalines make better charger because voltage is correct for longer period? But I also read that I can't draw too much current from alkaline because then large part of precious energy dissipates as heat. Is about 355mA too much for them? Actually, it's value on charger, I have no idea how much it actually takes.
And of course I can'd design a emergency charger that runs only on alkalines.
Or I should forget this idea and go for boost converter because of dangerous undervoltage?
But, when using some high-power D-size (alkaline) cells I must have a lot of power. They're heavy, I know... but not so much when in backbag.
So, there are my ideas... Just some random ramblings... :( I guess it will fit to this category.