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The battery management can be broken down into two main sections:
1. Charge management:
The Texas Instrument UC3906 smart charge controller optimizes battery capacity and life expectancy through logic control of the charge voltage and current at the charger's output with reference to ambience temperature. During charge, capacitance of the SLA battery will change, in turn causing the load to vary. The job of the UC3906 is to comply with the load variance and constantly monitor the charge state to prevent overcharge or deep discharge. It is coupled to a transistor 2N2955 charge current controller to further ensure a constant charging current best suited to the capacitance. While the charge voltage is regulated between 10 to 14V, the current gradually steps down through a three-state program: 750mA from initial charge to 70 to 75% (in approximately 10 hours), 400mA to the next level of 90% to 95% (15 hours) and then a float charge of 200~400mA to complete charge of 100% (24 hours). After the battery is fully charged, it is advisable to leave the charge switch on. The smart charge controller will maintain a low current (80mA) charge to sustain the charge level. (We've learned that although self-discharge rate is relatively low, SLA batteries should not be left in a discharge state to prevent sulfation.) A charge control relay makes sure that the PSU cannot output DC power during charge to avoid excessive voltage or current reaching the amplifier.

2. Discharge management:
The DC output of the PSU has a cutoff point at 10.5V, which means when the voltage of the SLA battery drops below 10.5V, the low voltage detector circuit will trigger the control relay to switch off the PSU automatically. This is to prevent the battery from entering a deep discharge state, which would jeopardize battery life.


The 12V/7AH SLA battery that comes with SLAP is a Taiwanese-made YUASA NP7-12 which costs about $20 to replace. With sophisticated smart charge management such as this, the life expectancy of the battery, according to KingRex, is 3 to 5 years. All internal wiring is via 18 AWG to facilitate high current flow. An AC input and DC output fuse are in place as safety measures. To satisfy my obsession for bi-amping, I requested two SLAPs from KingRex and they kindly granted my wish.

Ease of use
There are only two buttons. The charge button is solely for charge. You must turn that on when charging, off when powering up the amp. The PSU button actually has a dual function: to output battery power to the amp and to turn on the battery status monitoring circuit. When it is turned on, a dual color LED indicator mounted on the front panel will inform you of the charge status at a glance - red for charge in progress, green for charge complete and PSU in use, red/green flash for low battery (below 10.5A). Since the SLAPs came with the batteries somewhat charged (measured 13.39VDC), I topped up the charge before I used them. After a few trial runs, I preferred to have the PSU button turned on at all times for the status LED to function. (It lights up even when the AC cord is unplugged.) Since SLA batteries have no memory, I recharged every time after use. In other words, using SLAP is worry free. You simply leave it plugged in, turn on the charge button and PSU button at all times and only switch off the charge when operating the amp. (However, the user manual recommends switching off the PSU to conserve energy while maintaining charge since the monitor circuit would be constantly using up some power otherwise.)
To gauge the endurance of the SLAP, I purposely kept a time-voltage log while enjoying off-grid T20 bi-amping over the course of a few weeks. Each T20 was driving a pair of Loth-X BS-1 in D'Appolito array, with the volume knob varying between 11:00 and 12:00, which is my normal listening level. After finishing listening to each CD in full (no skip, no repeat, no fast-forward or
backward), I noted down the voltage reading of the two SLAPs (for left and right channels), which in most cases were almost identical. You'll find the full report in Side Bar 1 but here's the debriefing:
1. When fully charged, a high 14V was recorded. It started to drop (self-discharge) at 0.01V every two or three seconds as soon as the charger was switched off (the T20 wasn't turned on).
Sidebar I: The battery consumption log - continue
2. After playing the first CD (duration 70 minutes), voltage lowered to 12.97V. After playing the forth CD (another 190 minutes passed), the voltage reading remained strong at 12.91V and then plateau'd off for quite a long time. Further listening helped to indicate that 'voltage consumption' hinged more on the playback sound level rather than duration of the CD. After playing a ± 60-minute CD, the drop-off voltage, if any, ranged from 0.02V to 0.07V.
3. Self-discharge had been recorded at the rates of dropping at about 0.15V in 12 hours and 0.9V in 36 hours. It discharged to low level (LED flashing red/green) in about 10 days. A complete charge to 14V (100%) took 22 to 23 hours.
4. A fully charged SLAP should be able to give at least four days' worth of enjoyment of 12 CDs without recharge.

Of course, my findings were far from scientific. So I tried to compare notes with KingRex and ended up learning more. First, they confirmed my finding #2 that a 12V SLA battery can only maintain output above 13V for a short time and then it keeps going strong between 12.8V to 12V for a very long time, which is the prime window of the battery. Second, the YUASA NP7-12 is rated at 7AH (20-hour rate of 0.35A to 10.5V) which means when outputting at a regulated amperage of 0.35A, the battery capacity will drop from 14V to 10.5V in 20 hours (0.35A x 20 hours = 7AH). But the T20 won't require a constant 0.35A. With the volume knob set at 12:00, it hardly uses 0.3A. And music output is of course not constant. During the quietest passages, the amp could be using as little as 0.1A. That's why on one single charge, the SLAP can easily provide 30 hours of playback. Having said that, it should be noted that the T20 might consume more than 3A current during extremely loud passages. So much for the endurance test.

The measurable benefit
Some casual listening while comparing PSU with SLAP left some lingering questions in my mind. What happens if the T20 doesn't use more than 3A current? And what happens if it does? Has that anything to do with why sometimes I found the PSU to sound better than the SLAP? From James came the answers. "That's because the power settings are different. The PSU is +13V and the SLAP is +12V. So for dynamic range performance, the PSU will be a little bit better than SLAP. However, for long-term use, SLAP will have the more transparent background than the PSU." Exactly how and why? Both the SMPS and the PSU are capped at 3A but the PSU is designed to accommodate instantaneous high current by slightly lowering the voltage to boost amperage. The +13V rail gives it a little bit of headroom to manage the juggling act. But this is not enough to handle extreme situations whereby the tradeoff would be audible sonic distortion due to low voltage. That's when the 105A maximum output (for up to 5 seconds) of the SLA battery comes to the heroic rescue. It can meet the challenge of that transient peak, which usually lasts for 0.01 seconds and hardly requires the full 105A (otherwise the output fuse would blow). Yet the reserve is there. And that's what matters to audiophiles.

To show me the difference between SMPS, PSU and SLAP, James did some bench tests and emailed me the oscilloscope pictures. He reminded me that the T20 was originally created for desktop audio application or small area listening. "Generally speaking, users of this kind of product will not need huge current or wide dynamic range for their kind of music. In most cases, demand on the current would be below 2A. Therefore we believe the 3A ratings of the SMPS and the PSU are adequate. The PSU is of course better because of its reserve amperage in the capacitors. However, when the amp is drawing excessive current from the capacitors, the capacitors need to recuperate from the diodes. But if the transformer cannot replenish them in time, the capacitance will fluctuate, causing the DC current sine wave to ripple. At the same time, the diodes are frantically supplying the capacitors, rationing the charging time, breaking down the charge pulse from half-cycle to quarter cycle and resulting in 120Hz noise. The SLAP is basically a source of abundant current with big capacitance that is never short on supply and therefore impossible to induce rippling noise. The two significant benefits of the SLAP are maximum amperage and noise control."


The oscilloscope pictures above demonstrated James's point very well. Note that 'standby' is at 0A (without load) and the rest are tested at a dummy load of 4 ohms. Only the SLAP remains flat and free of ripple wave at 3.2A. I was curious about why the PSU was tested at 3.25A and the SLAP at 3.2A. "Ohm's law: I = V/R. PSU output voltage was 13V, hence 13/4 = 3.25. SLAP output voltage was taken at its prime time 12.8V, hence 12.8/4 = 3.2." James always has the answer. All I have to do is ask.