|Q:||What is the best battery monitoring system available?
|Q:||How can you say that, I have seen other systems that seams to work in the same way?
|A:||They do not have protective resistors that limits the high battery current to a safe level.|
And they don't work in the same way, for sure.
In Batscan, all parts after the protective resistor including wiring, sockets, measuring modules,
communication bus network etc, meets the specifications for Limited Current Circuit (LCC), or SELV
for personal safety as required by the International and European safety standard IEC/EN60950.
|Q:||I have seen other monitoring systems that use fuses on the connections to the battery terminals. Is that safe?
|A:||Absolutely not! WARNING HERE! These kind of systems are probably not designed by a graduated engineer, and you
should not let them anywhere near your batteries!
A fuse is simply a piece of wire inside a glass cylinder. The wire inside the fuse will melt when heated by a current flowing
trough it that exceeds the rated current of the fuse for some time. Unfortunately, this current is in the orders of magnitudes
higher than the current required for killing a person. Fuses do nothing for protection of humans.
Fuses are mainly intended for protection of wires so that they do not overheat in case of a short circuit. However, all fuses have
limiting values regarding maximum breaking current and voltage that cannot be exceeded for them to work as a protection device.
A standard small fuse of the type you will find in these systems have typically a maximum breaking current specification of 35A.
But the short circuit current in a large UPS battery can easily be in the 5000A range. This exceeds the limits of the fuse
more than 100 times, a short circuit will make the fuse explode and produce an arc that can ignite hydrogen gas, it may also re-melt
and allow excessive current though the wire it was supposed to protect.
In short: Fuses should not be used in secondary connections to a high-energy battery.
|Q:||I don't care about personal safety, I am only interested in the health of my batteries.
|A:||The protective resistors works both way, and they are of equal importance for the battery.
The resistor will effectively protect the battery, not only from accidental discharge in case of a fault in the
wiring, but also from fire and explosion that can be the result from sparks igniting hydrogen gas.
|Q:||Why do Batscan only monitor batteries? I have seen Battery Management Systems that also can make
|A:||We believe that these are two different tasks, and that they do not combine very well into one single system.
There are many reasons for why this is not such a good idea, the most obvious one being that if the Battery Management
System breaks down it may also destroy your battery.
The Batscan Software is running at a standard PC-Computer under the Windows operating system. This gives many advantages
in terms of connectivity to other systems, compatibility with future versions, and a lot of computation power and memory
storage space to Batscan.
However, this may not be the ideal platform for a system that is allowed to shut down
the charger for an emergency battery in a nuclear power plant.
Battery Management activities are best handled by the power system itself and many UPS's already have built in functions
for periodic battery tests. A good Battery Monitor System should be able to take advantage of this feature in the UPS and
detect and record all battery data during such a test.
|Q:||How does Batscan measure internal resistance in the battery cells.
|A:||Internal resistance (IR) is not a measurable physical unit. All resistance (R) 'measurements' are derived
from the actual measurement of Voltage (Volt) and Current (Amp), or one of them if the other is known, and then calculated as:
R = Volt/Amp|
In practice, the IR in a battery cell must be calculated between two pair of Volt/Amp values as:
IR = (Volt#2 - Volt#1) / (Amp#2 - Amp#1)
where both current values are of the same sign.
Unfortunately, the IR is not necessary a linear function, it may vary by the current. IR is also severely affected by many
other factors such as; age, temperature, state of charge (SOC), and internal faults.
The absolute value of IR is therefore not very useful for locating faulty battery cells.
However, the ratio between the cells IR can give a good indication of the battery's condition, provided it is measured accurately.
Calculating IR from individual Volt/Amp measurements can only give a rough indication and confirm that a cell is faulty.
For the high accuracy that is required by an 'early warning system', all measurements must be taken simultaneously.
In the example above, Batscan is detecting a high internal resistance in a cell. This is done by 'zeroing' all
voltages right before a change in current (at +12A), the bar-graph shows the resulting cell voltages when the current
has changed to +37A. This is the actual net result where all errors are cancelled out.
In Batscan the current is known to always be exactly the same (due to the simultaneously sampling technique) for all cell voltage
readings. Therefore, when there is a change in current, the change in voltage over a cell is in direct proportion to its IR.
High internal resistance (IR) as detected by Batscan.
It is unmistakeably that one of the cells has more than double the IR compared to what is 'normal' in this battery string.
This is what really counts, and it is easily observed in Batscan, even for a layman.
For the battery expert who wants to know the ohmic reading of the cells IR, the following formula can be used:
IR = 0.12 Volt / (37A - 12A) = 4.8mΩ
What tells you more?
The red bar in the graph,
or 4.8mΩ ?
... to be continued.