Voltage Readings
After taking a specific gravity reading, a voltage reading of each cell should be taken. Voltage readings taken on open circuit are of no value, so while taking these readings the battery should be on discharge, having at least a discharge of 15 amperes. A good way to get this discharge is to hold the starting switch in and set mixing valve lever at lean point or wide open.
A low or defective cell will show a voltage reading.10 to.20 volts lower than the other cells on discharge, while a reversed cell will show a reading in the reversed direction when on discharge, especially on heavy discharge.
The voltage readings are a sure check if taken in connection with the specific gravity. When you have low specific gravity and low voltage on the same cells, it is a sure indication of low cells. These cells should be inspected for the probable cause of their being low. Shorting of the lugs at bottom of plates and moss bridging across at bottom of the elements, or possibly a split separator, will generally be the main trouble.
When any of these conditions exist, it is best to take the low cells back to your shop for repairs.
When there is absolutely no indication why the cells are low, they can be cut out of the battery on discharge and put in on charge, until they come up.
The following is a good example of readings taken on a battery with a 10-ampere discharge and having four low cells, 4, 8, 11 and 16. The battery had been giving poor service, due to insufficient charging:
| Cell No. | Specific Gravity | Volts |
| 1 | 1.200 | 1.98 |
| 2 | 1.180 | 1.95 |
| 3 | 1.205 | 1.98 |
| 4 | 1.150 | 1.75 |
| 5 | 1.190 | 1.95 |
| 6 | 1.195 | 1.98 |
| 7 | 1.200 | 1.98 |
| 8 | 1.130 | 1.70 |
| 9 | 1.200 | 1.95 |
| 10 | 1.205 | 1.98 |
| 11 | 1.100 | 1.40 |
| 12 | 1.190 | 1.95 |
| 13 | 1.180 | 1.95 |
| 14 | 1.195 | 1.98 |
| 15 | 1.190 | 1.95 |
| 16 | 0.000 | zero or reversal |
The main thing to consider in checking voltage readings is the variation from the average. The average voltage readings will vary, depending on the state of charge of the battery when the readings are taken.