On this cold New Year's Day, I've been thinking about the problem that NiMH batteries like to self-discharge. I'd love to trickle charge them with a low current but my current NiMH charger seems like 1) overkill; 2) liable to slip into the wrong charge mode and boil them dry; 3) too big of a hammer for this little nail.
So I thought of the 1.5 watt 12 volt solar chargers I had bought for my car back pre-Prius. These would be perfect except for the voltage. I think they are actually around 15 or 16 volts nominal, but my packs are 43.2 volts. So I need to boost the voltage....
Scrounging around the web, the best solution appears to be the LT1070 chip from Linear Technologies (www.linear.com). It requires minimal external components and comes with a through-hole TO-220 package for us non-SMD people.
So... how to design the right circuit..
This is my back of the napkin calculations using the design notes and should be taken with a huge grain of salt. Do not trust my calculations!
Vin = 14 volts
Vout = 48 volts
R1 = R2 * ( 48v / 1.244 - 1) = 46.606 k ohms
R2 = 1.24 k ohms
Duty Cycle = (48v - 14v) / 14v = 70.8%
L = (14V * ( 48V - 14V)) / (0.5 A * 40 kHz * 48 v)
L = 476 / 960,000
L = 495.8 uH
C1 = arbitrarily 100 uF with a low ESR
C2 = (48v * 1A) / (40kHz * (14V + 48V) * (0.33 Vpp))
C2 = 48 w / 818,400
C2 = 58 uF