Showing posts with label batteries. Show all posts
Showing posts with label batteries. Show all posts
Friday, September 26, 2014
Voltage Tester for Model Batteries
With a suitable load, the terminal voltage of a NiCd or lithium-ion battery is proportional to the amount of stored energy. This relationship, which is linear over a wide range, can be used to build a simple battery capacity meter.
Circuit Image :
Voltage Tester for Model Batteries Circuit Image
This model battery tester has two functions: it provides a load for the battery, and at the same time it measures the terminal voltage. In addition, both functions can be switched on or off via a model remote-control receiver, to avoid draining the battery when it is not necessary to make a measurement. The load network, which consists of a BC517 Darlington transistor (T2) and load resistor R11 (15 Ω /5 W), is readily evident. When the load is active, the base of T1 lies practically at ground level. Consequently, T1 conducts and allows one of the LEDs to be illuminated.
Voltage Tester for Model Batteries Circuit Diagram
The thoroughly familiar voltmeter circuit, which is based on the LM3914 LED driver, determines which LED is lit. The values of R6 and R7 depend on the type and number of cells in the battery. The objective here is not to measure the entire voltage range from 0 V, but rather to display the portion of the range between the fully charged voltage and the fully discharged voltage. Since a total of ten LEDs are used, the display is very precise. For a NiCd battery with four cells, the scale runs from 4.8 V to 5.5 V when R6 = R7 = 2 kΩ. The measurement scale for a lithium-ion battery with two cells ranges from 7.2 V to 8.0 V if R6 = 2 kΩ and R7 = 1 kΩ.
For remote-control operation, both jumpers should be placed in the upper position (between pin 1 and the middle pin). In this configuration, either a positive or negative signal edge will start the measurement process. A positive edge triggers IC1a, whose output goes High and triggers IC1b. A negative edge has no effect on IC1a, but it triggers IC1b directly. In any case, the load will be activated for the duration of the pulse from monostable IC1b. Use P12 to set the pulse width of IC1a to an adequate value, taking care that it is shorter than the pulse width of IC1b.
If the voltage tester is fitted into a remote-controlled model, you can replace the jumpers with simple wire bridges. However, if you want to use it for other purposes, such as measuring the amount of charge left in a video camera battery, it is recommended to connect double-throw push-button switches in place of JP1 and JP2. The normally closed contact corresponds to the upper jumper position,while the normally open contact corresponds to the lower position.
Parts :
Resistors: R1,R2 = 47kΩ
R3 = 100kΩ
R4 = 500kΩ
R5 = 1kΩ
R6,R7 = see text (1% resistors!)
R8 = 1kΩ5
R9 = 1kΩ2
R10 = 330Ω
R11 = 15Ω 5W
R12 = 15kΩ
P1 = 100kΩ preset
Capacitors:
C1 = 10nF
C2 = 100nF
Semiconductors:
D1-D10 = LED, red, high effi-ciency
T1 = BC557
T2 = BC517
IC1 = 74HC123
IC2 = LM3914AN
Miscellaneous:
PC1,PC2,PC3 = solder pin
JP1,JP2 = jumper or pushbutton
PCB Layout :
Voltage Tester for Model Batteries PCB Layout
Copyright : Elektor
Wednesday, August 13, 2014
Solar Charger for Lead Acid Batteries
This schema is still under development, but works well as shown.
This schema is intended for charging lead-acid batteries with a solar panel. The customary diode that prevents the battery from discharging through the solar panel has been replaced by a FET-comparator combination. The charger will stop charging once a pre-set voltage (temperature compensated) has been reached, and recommence charging when the voltage has dropped off sufficiently. The load is disconnected when the baterry voltage drops below 11V and reconnected when it gets back to 12.5V.
The schema has the following features:
- Charges until Vbat = 13,8V (adjustable), then float charges;
- Shuts down load when Vbat < 11V (adjustable), resets at 12,5V;
- Temperature compensation;
- Will work with cheap and readily available components like LM393 comparators and BUZ11 FETs;
- Uses less than 1.3mA (Attempts to use micropower comparators have failed spectacularly so far, see below);
- Burns less than 20mW in FETs when charging at 0,5A. (More expensive FETs with a lower RDSON will yield even better results).
Note that the charging current is limited only by the solar panel used.
Heres the schema:
Note the funny place of grounding of the first 2 comparators. Theres some weirdness here: this bit of the schema gives me headaches. Two problems:
- If I ground the first two comparators (LM393) in the same place as the third, i.e. not between the FETs, the thing wont work and the battery will discharge over the solar panel. Why? Am I playing to close to the rails? How can this be remedied/improved/redesigned? Do I need a diode between the comparators imputs?
- If I use micropower comparators like the Texas Instruments TLC393, the comparators blow up spectacularly, but with the standard LM393 everything works fine. Why? What did I miss?
Help would be greatly appreciated!
Next attempt
This one works fine and uses about 0.5mA, but that might improve because Im not done tweeking yet:
by Oscar den Uijl, odu@xs4all.nl
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