Internal Resistance Tester For Batteries

This circuit is designed to check the condition of lead-acid and gel cell batteries with capacities greater than 20Ah. It switches a load of about 18A at a rate close to 50Hz so that the internal resistance of the battery can be measured using a digital multimeter across the battery terminals. The measured AC voltage in millivolts divided by 10 (ie, a shift of the decimal point) is approximately equal to the batterys internal resistance in milliohms. As shown, the circuit is quite straightforward and is based on two 555 timer ICs (IC1 & IC2) and power Mosfet Q1. IC1 operates as a monostable timer with a period of 10s.

When switch S1 (Test) is pressed, IC1s pin 3 output goes high for 10s and this enables IC2 which operates as a 50Hz astable oscillator. IC2 in turn drives power Mosfet Q1 which is connected across the load in series with three 0.22W 50W resistors. IC2 then turns off again after 10s - ie, at the end of the monostable timing period. LED1 provides power indication when the circuit is connected to a battery, while LED2 (green) comes on during the test period. The thermostat is not necessary unless the unit is to be used repeatedly (the Jaycar ST-3823 70°C unit is suitable) and you want to protect the output circuit against overheating.

Note:

The power Mosfet does not need cooling but the thermostat and the 0.22W 50W resistors should all be mounted on an aluminium heatsink at least 2mm thick. In practice, the internal resistance of car batteries can vary from about 15mW down to about 3mW. Before testing the battery, check that the electrolyte level is correct and that the voltage across its posts exceeds 12.5V for a nominal 12V battery; ie, close to full charge. That done, switch on the cars headlights and measure the DC voltage between each battery post and its connecting terminal. It should be less than 10mV in both cases; if not, the terminals need cleaning. Once youve done that, you can turn off the headlights, connect the tester and proceed with the internal resistance test. Be sure to connect the multimeters test probes directly to the battery posts, to read the internal resistance (not the battery terminals).

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Measuring motor insulation using HI POT tester

What is HI-POT tester?
When we need to evaluate motor condition,HI-POT tester or High Potential tester is the common equipment are used.As we know,a multimeter or ohm meter reading only can determine if a motor winding is open or shorted but cannot checked winding insulation condition if shorted with ground or phase.

The main purpose of the Hi Pot tester is to verify the quality and integrity of the electric motor insulation materials used on the armature or stator and also safe to using it without any defect.
Sometime we not take a serious attention or not know about it to checked winding insulation.This step is important to ensure motor can running in good condition without any breakdown.
Normally electrical motor may have a weakened winding that shorts to ground when it running.It have several caused for the winding breakdown due to the value of system voltage,winding heat build up or overloaded mechanical stress.For other caused it will come from motor terminal board that has an insulation failure from the terminal to ground (earth) or from other terminal for winding connection.

Basic operation of HI-POT Tester
A HI-POT tester produce power source with a high voltage (Volt) and low current (ampere) and it is used to check for an insulation failure for motor winding that the ohmmeter cannot measure with its low voltage power source may miss.
It measures winding insulation resistance to ground (earthing) and terminal board resistance from terminal to ground and from terminal to terminal ( phase to phase )
To get a best result and for safety reason when perform measurement for winding insulation resistance,all wiring is removed from the motor winding terminals.It also same when perform measurement for terminal board resistance, all wiring is removed from the terminal board.
Below is several rules when using the Hi-Pot tester.It is to avoid any serious damage or injuries during insulation measurement and should never be :

1. Operated alone or by an inexperienced service person.
2. Set at a voltage of more than twice the motor operating voltage plus 1000 volts.
3. Be set for more than 5 milliamps without consulting or refer user manual from motor manufacture
4. Applied for more than the time it takes to obtain a reading (no more than a few seconds).
5. Used as a phase-to-phase winding check.
6. For air condition motor compressor,used when the system is in a vacuum.

* SAFETY NOTE : Please read HI-POT tester user manual before using it and refer to the trained or qualified person for assist during measurement.It the best way to avoid any unexpected issued and for safety reason.

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