There has been some misinformation circulating lately concerning UC3906 based charger operation.
UC3906 Smart Battery Chargers have three distinct operating modes: BULK, ABSORPTION and MAINTENANCE. In the BULK mode, the charger limits the maximum charging current to a preset Imax value while monitoring the battery voltage. In the ABSORPTION mode, the charger attempts to elevate the voltage to Voc while monitoring the current. When the current decreases (tapers down) to a preset value Ioct, the charger enters the MAINTENANCE mode. In our smaller 1 Amp or less chargers, Ioct is set to 1/10 (10%) of Imax. In our larger 5 Amp and 3 Amp chargers, Ioct is set to 1 Amp. In the MAINTENANCE mode the charger will issue from ZERO to Imax current while maintaining the battery at Vf.
NOTE: the MAINTENANCE mode IS NOT a trickle mode, the charger will issue ZERO current if the battery accepts proper charge during the first two modes. This feature allows the charger to be connected indefinitely.
Although we commonly refer to a battery as a 12V battery, meaning a battery having 6 cells at 2.0V per cell (12.0V). The actual Lead-Acid battery voltage is in the range of 2.1V / 2.3V per cell (12.6V / 13.8V).
For our standard 12V @ 1 Amp Smart Battery Charger:
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Typical operation - charging modes are sequential. Connect a battery to the charger first, then turn on the AC and the charger will enter the BULK mode and limit the current to a maximum of 1 Amp. As the battery accepts charge, the terminal voltage rises and when the voltage reaches 95% of Voc (13.7V), the charger enters the ABSORPTION mode where it tries to increase the battery voltage to Voc (14.4V). A VERY IMPORTANT point is that the battery voltage does NOT change abruptly. Assuming a good battery, as the voltage approaches 14.4V, the current decreases. When the current tapers down to Ioct (100 mA), usually before reaching 14.4V, the charger enters the MAINTENANCE mode. The voltage is then lowered to Vf (13.8V). There is some residual charging going on when first entering the MAINTENANCE mode but the current is well below Ioct (100 mA).
If a load is applied during the BULK mode, say 220 mA (like a K2 in RX mode), the charger will supply both the load and the battery. Since the BULK current is limited to 1 Amp, 220 mA flows into the load and the remaining 780 mA is available to flow into the battery. If the battery accepts charge, the terminal voltage will continue to increase and the battery current will continue to decrease. If the battery voltage reaches 95% of Voc (13.7V), the charger enters the ABSORPTION mode. In the ABSORPTION mode, the battery current continues decreasing because the battery is still being charged. The system voltage is in the range of 13.7V (95% of Voc ) heading upward toward Voc (14.4V). Under these conditions, the battery current will continue to decrease and the battery continues toward being fully charged. The current flowing into the load does not effect the decreasing battery current.
If a larger load is applied, say 2.5A, in excess of the 1 Amp Imax, the charger will self limit and contribute only 1 Amp to the load. The remaining 1.5A must come from the battery. Now the battery is being discharged which will cause the system voltage to decrease. The charger WILL NOT hold the voltage at the elevated Voc. System voltage at this time is determined by the BATTERY, the CHARGER and the LOAD. When the large load is removed, all of the charger current is available to flow into the battery and charging begins immediately.
If the total load (current X time) draws the battery down to 12.4V (90% of Vf), the three mode charging sequence will repeat. Otherwise, the charger remains in the MAINTENANCE mode and the charger will issue from ZERO to Imax current while maintaining the battery at 13.8V, Vf.
Our 1 Amp charger can be set to other, lower current levels and our QRP version is switchable between any two of the following current ranges.
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The current has to decrease to Ioct in order for the charger to enter the MAINTENANCE mode. If the load is always greater than Ioct, then charge the battery first without any load, allowing the charger enter the MAINTENANCE mode, then apply the load.
Some QRP operators have questioned whether or not the MAINTENANCE mode will be reached. We assume that most QRP operators are human and occasionally have to attend to other functions - such as: eating, sleeping, visiting the wife / kids, etc. During which time the RIG is turned OFF (load below Ioct), allowing the charger to complete the charging sequence and enter the MAINTENANCE mode.
73
Stas, W6UCM