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Circuits designed by David Johnson, P.E.
Last Updated on: Wednesday, October 18, 2017 05:04 AM

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More  Lithium Battery Chargers
Solar Cell Circuits

Solar Lithium Ion Battery Charger Using LT1129
December 7, 2012

I have been designing a number of solar powered devices lately.  Many of them use just a single 3.6v lithium ion battery.  These batteries like to be charged from a fixed 4.2v source with a charging current limited to about C/8, where C is the milliamp-hour rating of the part.  When being charged from a small solar panel, the panel should be selected so the charging current does not exceed C/8.  In such cases, there is no need for additional charging current control.  However, some mechanism is still needed to prevent the battery from discharging back into the solar panel, when the panel is in darkness.  Usually, a diode is inserted between the battery and the charge control circuit.

I have been designing a number of solar powered devices lately.  Many of them use just a single 3.6v lithium ion battery.  These batteries like to be charged from a fixed 4.2v source with a charging current limited to about C/8, where C is the milliamp-hour rating of the part.  When being charged from a small solar panel, the panel should be selected so the charging current does not exceed C/8.  In such cases, there is no need for additional charging current control.  However, some mechanism is still needed to prevent the battery from discharging back into the solar panel, when the panel is in darkness.  Usually, a diode is inserted between the battery and the charge control circuit.  But, I wanted something with a lower voltage drop.  After looking around at various charging circuits, I decided to try using a low power voltage regulator from Linear Technology.  Their LT1129 has the right characteristics I needed.  It has a maximum current rating of 750ma and a reasonable 400mv voltage drop when delivering such current. The voltage drop is much less, as the current tapers off, when the battery reaches a full charge. Two resistors program the part for a fixed 4.2v output voltage.  Finally, the back feed current, when the input is at zero volts, is a very low level, perfect for solar power applications.  A couple tantalum capacitors, one at the input and on at output complete the circuit. 

If a 10 cell 5v solar panel is used, no heat sink is needed.  The TO220 package is big enough to dissipate the 0.6 watts.  A solar panel which can produce 750ma of current would be big enough to charge some of the newer 4 Amp-hour lithium ion cells appearing on the market.

4 Amp-hour Lithium Ion      
5v 750ma Solar panel

Click on Drawing Below to view PDF version of Schematic

Solar Lithium Ion Battery Charger Using LT1129 designed by David Johnson

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