For my "prolonging life of old batteries" project I decided to try a switching DC-DC converter. I've got a couple of Recom's R-7805s (datasheet: http://www.recom-international.com/pdf/Innoline/R-78xx-0.5.pdf ), which are capable of 5V at 500mA output. It is choosen over linear regulator, because I want to lose less energy to heat.
However, when connecting 8 old AAA batteries (about 8.6 V) in series to Vin, the converter works fine only with approx. > 100 ohm loads (50 mA current output), and already 50 ohm causes oscillations (I have tried 100, 50 and 10 ohms), because input voltage drops below the minimum. (I am not using blocking diode on the output, if this matters.)
The question is, how can I deduce transfer characteristics of the converter from the datasheet, needed for further circuit analysis? Is my approach somehow flowed, and if not, am I better off with getting values empirically instead of trying to make calculations first?
My current understanding is that at this rate I will need to connect 40 batteries to get at least 250mA, which means I need to add a second converter, because this one is max 34V.
A subquestion I have is how to combine two converters in parallel. My guess is I need blocking diodes to prevent current going back into R-7805. Any hint on this is also welcome.
I have already asked about a different strategy, but I want to try step down approach first, because I need relatively high current.
Also, I do not have a good understanding of internal resistance of old batteries yet. I made some experiments, which showed about 20% of the nominal energy can be had from 1.3V used AA battery (about 500mAh), however, the current should not be too high, certainly not 250 mA. As I am concerned with efficient power transfer, textbooks are recommending matching impedances. I can make more experiments to have some idea of the battery internal resistance, but I'd liked to see if I can get the converter's input/transfer characteristics from specs, regarding input resistance given the 5 V, 250 mA (ideally 5 V 500 mA) DC load.
UPDATE: Some measurements:
- 1M (scope's input): 7.8 V input, 5 V output
- 1k load: 7.4 V input, 5 V output
- 100 ohm load: 6 V input, 5 V output
- 47 ohm load: oscillating, hard to measure.
After heavy load, input drops to 7.4-7.6 V and need some time to recover.