I was playing around with creating a PWM for power saving (and getting a stable light output) from a LED chain in the past; see here for an article about that.
|TPS61070/MCP1640: Cheap and very efficient StepUp ICs.|
When trying different step up ICs I finally ended up with the TPS61070 and MCP1640 which are remarkably similar with their specs and pin configuration. Those turned out to be the most efficient step ups available for hobbyist DIY projects.
They tend to discharge the cells below 1V which is expected according to the data sheets. While this is no problem for a short time on two serial NiMH batteries, a longer time in that range surely degrades the cells. Even if a pair of batteries lasts for around a week with my LED chain now, this led to two of my Duracell brand NiMH cells being damaged by deep discharge. I had to bring them to the recycling facility.
|Duracell LSD vs Sanyo Eneloop|
I bought 8 peaces for about 16€ at Amazon now. They came precharged but I charged them all to be really full. And then I plugged the first pair into the battery holder to see how well they fare. Unfortunately I didn't write down when this was, but after more than a week a battery change was due. The cells were down to 0.8V and 0.4V each; still too deep discharged - but the cells charged just fine after that, taking a proper load of more than 2000mAh. With the Duracell batteries, the lower discharged cells only took a few hundred mAh of load until the gave their "fully charged" dV signal. Sometimes a refresh cycle helped, but the broken cells didn't recover their capacity anymore.
Now the second pair of Eneloops runs the LED chain for 10 days already and there's no end in sight yet! This is far longer than anything I reached before. 7 days was the usual time frame for my Duracells. They are Low SelfDischarge (LSD) type cells as well, but what a difference the Eneloop make! The Duracells claim 2400mAh capacity, the Eneloop only 1900, and still, they greatly outperform the Duracells! They also cope way better with deep discharge.
I plan two additions to the LED chain StepUp and PWM circuit to get rid of the deep discharge problem and prolong the runtime even more: Adding a Schmitt Trigger after the oscillator stage for less leakage - and placing a NCP303 1.8V as undervoltage lockout to put less stress on the batteries.