Continuing with our look into station battery maintenance, let’s explore the value of measuring and/or monitoring flowing currents and connection resistances. Excessive current flow either DC or AC will indicate a problem in either a battery cell or a problem with the charger and rectifier circuit. With loose connections you lose continuity in the battery string and these high resistance areas will become hot spots.
DC Float Current:
Float current is the DC current that flows through the battery string during float conditions. It is the interaction between float voltage and the cell’s internal resistance. Because of Kirchhoff’s Current Law, float current can be measured anywhere within the series string. A difference in measurement of the current at the beginning of the string as compared to the end of the string will indicate current flowing to ground through a damaged cell. The value of ground current can be very small and difficult to measure when located in the string. Normal float currents are typically less than one amp and vary by size and manufacturer. If the manufacturer does not recommend the amount of float current under normal conditions, then values from one test to another should be compared to look for gross increases which have doubled or tripled from the previous measured value. This is especially important in VRLA batteries as increased float current can lead to thermal runaway. Float current should be measured during float conditions and not during discharge or recharge.
Float current flowing through a battery string
AC Ripple Current:
Ripple current is measured with an AC current meter. The measurement should be taken through each series string during float conditions. Ripple current and ambient temperature should be analyzed together because high ripple current in the presence of a high ambient temperature will have more of an adverse effect on the battery than high ripple current alone. The IEEE recommends that ripple current does not exceed 5 Amps for every 100Ah of battery capacity. The less the ripple current is the better the battery life will be due to ripple current effects. Ripple current measurements are suggested in IEEE-1188, as heating is a cause of thermal runaway in VRLA batteries.
Current ripple from a charger
Inter-cell resistance also called strap resistance is an important test and should not be overlooked. There may be those that feel a correct torque of the strap bolt and nut is sufficient to ensure a tight connection. They would be correct; it may be a tight connection but “tight” says nothing about the ability for current to flow through the strap. Strap resistance should be measured across each strap by making contact at the battery terminals and not on the straps themselves. The reason for making contact at the battery post and not on the strap is that the measurement needs to include the contact resistance between the strap ant the post. The space between the strap and the post is most likely where any high resistances will occur due to a loose connection or foreign material in the space between the strap and post.
Proper measurement points for inter-cell resistance measurements
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