Megger BITE 3, Инструкция по эксплуатации

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AVTMBITE 3 Rev 8 April 2014
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The BITE 3 generates data that describes an overall condition of a battery. Weak
batteries are due to a number of reasons, some of which are sulfated plates, dry-
out (loss-of-compression), loose intercell connectors, grid growth, etc. The BITE
3 also measures float current which increases over time as batteries degrade. In
the case of VRLAs, increasing float current can indicate impending thermal
runaway. [Flooded batteries can’t runaway thermally due to the large volume of
acid which merely boils off limiting battery temperature to about 260
F (125 C).]
Also measured is ripple current which is an indicator of charger output
condition. Battery chargers convert ac into dc but no charger is 100% efficient.
Some ac carries over into the dc network and is called “ac ripple”. If that ripple
current is above about 5% (5A rms per 100Ah) then battery heating can occur
thus shortening batteries’ lives. Normal aging of chargers causes a slow and
tolerable increase in ac ripple. But if a diode blows, ripple current can increase
three to four times which can heat the battery. Measuring ripple current helps to
identify the general condition of the charger output.
Electrical Theory and Practice
A battery string is a series circuit of cells that look like resistors to the applied
current. Current flows due to a voltage applied from the charger. In a series
circuit, Kirchhoff’s law states that the current is the same everywhere in that
circuit. Ohm’s law states that each resistor will have a voltage drop in response to
the applied current regardless of whether that current is ac, dc or both.
Impedance works by applying an ac current signal to the battery and measuring
the resulting voltage drop. Impedance is then calculated using Ohm’s Law,
Z = E/I. To get accurate internal ohmic values (impedance, etc.) the current
must also be measured. A battery is connected in parallel with a load and the
charger and frequently it is connected to other parallel strings. The actual current
can vary based on relative condition and parallel paths for the current. Hence, it
is necessary to measure current that is causing the voltage drop in order to obtain
accurate impedance values.
How the BITE 3 Works
The BITE 3 works by applying an ac current signal across an individual cell/jar
and measuring the ac voltage drop caused by that ac current as well as the
current in the individual cell/jar. It will then calculate the impedance. The
standard lead set used is dual-point, Kelvin-style. One point is for applying the
current and the other for measuring the potential.
In addition to the standard impedance cell voltage and intercell connection
resistance, the BITE 3 uses a patent pending technique to measure float and
rippled currents. The best method to measure current is by measuring the voltage
across a shunt. Megger uses the idea that there are many shunts within a battery.
By first obtaining a strap resistance value, then using that as a shunt, the float and