12 VOLT GEL CELL CHARGER

Recently, a fellow amateur was looking for a gel cell charger which would first charge at a fixed rate and then later switch to a trickle charge when the cell was fully charged. After reviewing several catalogs and web sites, the MAX712 IC was discovered. This IC meets all the requirements for almost any type of battery charging system. The circuit in Figure 1 was designed specifically for 12 volt gel cells. 
When a discharged gel cell is connected, the charger goes into a fast charge mode at a fixed rate of 400 ma. After the chip detects the voltage leveling off or when 4 1/2 hours has elapsed. (which ever happens first.) the fast charge will stop. After the fast charge has ended, the IC goes into a trickle charge rate of about 50 ma. This trickle charge continues until 13.8 volts is reached which will stop all charging current since the cell is now fully charged. If the cell voltage should drop for any reason, either a fast charge or trickle charge (IC will detect what is needed) will start again. 
When constructing this circuit, be sure to attach a small heat sink to Q1. Apply a DC (partially filtered) voltage of at least 15.3 volts. The voltage must never go below this level even under load conditions. Many of the DC wall transformers available will work just fine as long as they meet the minimum voltage requirement. The input voltage can be as high as 24 volts. If the input voltage must be in the 30 volt range, increase R1 to about 820 ohms. 
The output voltage must be aligned prior to use. Disconnect the battery from the circuit and apply power. Connect a digital volt meter or other accurate volt meter to pin 2 (positive lead) and to pin 12 (negative lead). Adjust R7 until exactly 13.8 volts is read. 
Because this circuit will not overcharge a gel cell, the battery can be connected indefinitely. This circuit is designed primarily as a 12 backup system and can be connected to the load provided the device to be powered only draws current during power line interruptions. Use a diode from the battery to load if needed. This circuit makes an excellent battery backup to an amateur transceiver.
The MAX712 IC and the .62 ohm resistor are available from Digi-Key, 701 Brooks Ave, Thief River Falls, MN 56701 (1-800-344-4539). Order part numbers MAX712CPE-ND and 0.62W-1-ND respectively. All other parts are available at Radio Shack. 
DE N1HFX 
PARTS LIST 

C1

MAX712 Battery Fast-Charge Controller IC (Cost is $6.27 from Digi-Key)

R1

680 ohm 1/2 watt resistor (Blue Gray Brown)

R2

150 ohm resistor (Brown Green Brown)

R3

68K resistor (Blue Gray Orange)

R4

22K resistor (Red Red Orange)

R5

.62 ohm 1 watt resistor (Blue Red Silver) (Cost is 27 cents from Digi-Key)

R6

1.8K resistor (Brown Gray Red)

R7

10K PCB trimmer resistor (103)

R8

470 ohm resistor (Yellow Violet Brown)

C1

1 microfarad tantalum capacitor (observe polarity)

C2,C4

.01 microfarad capacitor (103)

C3,C5

10 microfarad electrolytic capacitor (observe polarity)

Q1

TIP42 PNP transistor or similar (attach heat sink)

D1

1N4001 Diode (observe polarity)

LED1,LED2

2 volt standard LED (observe polarity)

 

Li-Ion Charger

Description
The LP2951 regulator is manufactured by National Semiconductor. The choice of values is from an application note "Battery Charging", written by Chester Simpson.
Diode D1 can be any diode from the 1N00x series, whichever is conveniently available. It functions as a blocking diode, to prevent a back flow of current from the battery into the LP2951 when the input voltage is disconnected.
Charging current is about 100+mA, which is the internally-limited maximum current of the LP2951. For those wondering, this is compatible with just about any single-cell li-ion battery since li-ion can generally accept a charging current of up to about 1c (i.e. charging current in mA equivalent to their capacity in mAh, so a 1100mAh li-ion cell can be charged at up to 1100mA and so on). A lower charging current just brings about a correspondingly longer charge time. IMHO 100mA is quite low, low enough that the circuit can be used for an overnight charger for many typical single-cell li-ion batteries.
The resistors are deliberately kept at large orders of magnitude (tens/hundred Kohm and Mohm range) to keep the off-state current as low as possible, at about 2μA. Resistor tolerances should be kept at 1% for output voltage accuracy. The 50k pot allows for an output voltage range between 4.08V to 4.26V - thus allowing calibration as well as a choice between a charging voltage of 4.1V or 4.2V depending on the cell to be charged. The capacitors are for stability, especially C2 which prevents the output from ringing/oscillating.
Parts List

IC1	LP2951, voltage regulator
D1	1N4002, General purpose diode
R1	2M, 1%, metal-film
R2	806K, 1%, metal-film
P1	50K, potentiometer
C1	0.1uF, polyester
C2	2.2uF/16V, electrolytic
C3	330pF, ceramic

Multi-Charger (Multi-purpose charger)

To keep the lights on and the camera going I have a multi-purpose battery charger. This contains around 24 Wh of energy storage in eleven NiMh AA cells and circuitry to charge my laptop, satellite phone, mobile phone, camera battery and a PP3 for Ju's Dog Dazer. It can also power a white LED tent light that sheds just about enough light to cook by. A cunning design means that some of the AA cells can be removed from the internal battery stack and exchanged with flat ones from a torch or whatever. The battery stack can be charged from a DC supply of between 10 and 30 V (which includes the laptop's mains adapter) and from my Schmitt Dynohub and from a 5 W solar panel. I seem to be able to generate about 8 Wh from a day's pedaling, which is supplemented by whatever the solar panel can gather.

It seems that from a typical day of pedaling and weak sunshine I can garner enough power to run the laptop for about an hour. I try to keep the laptop's battery fully charged so that the power from the charger goes straight into doing something useful in the computer. It is of course possible to use it to charge the laptop's internal battery but this is less efficient because it involves an extra two conversions between chemical and electrical energy.

The display shows the charge status of the internal batteries and how much power is going in or out. You can also switch on and off the various outputs and set a timer for the battery charger. There are over-ride switches in case of software problems!

Here's a picture of me testing the dynamo side of things using a 'rolling road'.

I used an oscilloscope to monitor the current through the batteries and found that it was a series of pulses at the frequency of the dynamo (not surprising as there is no smoothing). However it still seems to charge them OK.

There is a circuit diagram here. Basically it contains a stack of eleven 1800 mA AA cells. These may be charged by either the dynamo via a bridge rectifier and a current limit, and/or a solar panel also via a current limit. The current limit uses a normally closed relay to provide a zero-resistance current path when the current is less than the limit. This relay is then opened by the micro-controller if the current exceeds the limit, thus forcing the current to flow through the limit circuitry. The batteries can also be charged by a SEPIC DC-DC convertor (LT1512) which produces a constant current of 200 mA from a voltage in the range 10-30 V, eg. a car cigarette lighter socket or the laptop mains adaptor.

Taking power from the AA cells are three DC-DC convertors. First is a step-up convertor to 16 V to power my laptop. Second is a step-down convertor for the mobile phone, tent light and whatever else might need power. Third is another SEPIC convertor which provides a constant current to charge the digital camera battery, or a PP3. Also taking power from the AA cells is the guts of a Motorola satelite phone charger. There is something complicated about how this works so it seemed easiest to just use the charger wholesale.

Solar Inverter Battery Charger

Here is an Energy saving battery charger. It harvests solar energy to replenish 12 volt Inverter battery. It has auto cut off facility to stop charging when the battery attains full charge. The charger uses a 24 volt solar panel as input.
The circuit uses a variable voltage regulator IC LM317 to set the output voltage steady around 16 volts. Variable resistor VR controls the output voltage. When the solar panel generates current, D1 forward biases and Regulator IC gets input current. Its output voltage depends on the setting of VR and the output current is controlled by R1.This current passes through D2 and R3. When the output voltage is above (as set by VR) 16volts, Zener diode ZD2 conducts and gives stable 15 volts for charging. Charging current depends on R1 and R3. Around 250 to 300 milli ampere current will be available for charging. Green LED indicates charging status. When the battery attains full voltage around 13 volts, Zener diode ZD1 conducts and T1 forward biases. This drains the output current from the regulator IC through T1 and charging process stops. When the battery voltage reduces below 12 volts, ZD1 turns off and battery charging starts again.
Solar Inverter Battery Charger Circuit

Connect the circuit to the solar panel and measure the input voltage. Make sure that it is above 18 volts. Connect the circuit to the battery with correct polarity and adjust VR till LED lights. This indicates the conduction of ZD2 and output voltage. Use heat sinks for LM317 and TIP122 to dissipate heat.
Note : The same circuit can be modified for charging different types of batteries. The only modification required is the change of ZD1 and ZD2. Select ZD2 value for the required output voltage and ZD1 for cut off voltage level. For example for 6 volt battery, ZD1 should be 6.1 volts and ZD2 6.8 volt. For Mobile battery, ZD1 should be 4.7 volts and ZD2 5.1 volts. All the other components remain same.