LT1070-based 48V solar trickle charger

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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 ( 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 LT1070-R1.png

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

Now although I started this as a trickle charger, the LT1070 series can handle up to 2.5 amps depending on the version, and so it could serve as the real charger.

Now a 200 watt solar panel would be pretty large (around 116 x 116 cm; 46 x 46 inches) and at current prices, around US$1000 -- so it isn't in my budget. But one can always dream!

p.s. This hilarious article is a must read before using the LT1070:

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This page contains a single entry by Karen Nakamura published on January 1, 2010 8:18 PM.

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