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Why SLA battery
You can say that the battery gene runs in the Tripath family. The very early Sonic Impact T Amp can run continuously for 8 to 10 hours on eight AA batteries as claimed by the manufacturer. The Trends Audio UD-10 DAC operates on four rechargeable Ni-MH batteries. (Okay, that has no Tripath in it but is marketed as an accessory.) Then of course, Red Wine Audio has fame and glory built upon the clever partnership of Tripath chips and on-board rechargeable SLA batteries. Personally, my experience with a UPS (uninterruptible power supply) has somewhat left me ho-hum about SLA batteries in general. Even with market leaders like APC and Belkin, their SLA batteries last about 3 years at best, under 2 years on average. So I decided to learn more about the reliability of SLA batteries and googled to find one interesting web article "NiMH vs. SLA" by Mr. Steve Duncan. The article, which appears to be a high school science project performed in a very serious manner, set out to determine if Nickel-Metal Hydride (NiMH) is a superior battery chemistry to the older Sealed Lead-Acid (SLA) type. Although the application was based on a 36V pack for personal light electric vehicles, there's something we can learn about SLA batteries, especially those who intend to build their own battery power unit for Tripath amps that require higher rail voltages to operate. (For instance, Winsome Labs' Mouse can handle up to 36VDC.)

Let me just highlight some of Mr. Duncan's findings, focusing on SLA since that might interest audiophiles most.
Overview: Of the four popular types of rechargeable batteries, SLA is the economical and traditional choice, NiMH (nickel-metal hydride) the newer, lighter and reliable alternative, Lithium-Ion batteries such as those used in notebook computers requiring more complex charging systems and careful handling (one should recall that certain notebook computers issued recalls on their batteries due to danger of catching fire or exploding), with Zinc composition batteries being less common and more expensive.

Strengths of SLA: (1) mechanically simple and low failure rate, (2) very low self-discharge rate, about 5% per month compared to NiMH's 30% per month, (3) affordable price at about $20 per 12V battery, (4) can be recharged 200 to 300 times though NiMH beats it by 300 to 500 times, (5) ability to maintain potential voltage under load, (6) recycle programs readily available.

Weakness: SLA battery cannot be left at a discharged state or sulfation (corrosion of battery plates) will begin.

When I asked Christine Wu of KingRex what other batteries they had considered, her answer coincided with the above, thus confirming SLA to be the best choice, all things considered. But it didn't stop there.

Perfecting SLA
The real challenge for the KingRex R&D team was to enhance a commercially available SLA battery to meet audiophile expectation. From an engineering standpoint, that goes beyond sonic performance and involves reliability, maintenance and safety issues. Although the packing was very secure, removing the 10.8 lb. SLAP out of the double box and thickly padded foam was a breeze. (There are very rigid safety code and environmental requirements for shipping hazardous materials like SLA batteries.) The SLAP has the same footprint as the KingRex amps and PSU, measuring 7" wide x 5" deep but stands three times taller at 6", making it almost cubic. The black anodized aluminum front panel is branded with two tiny logos, KingRex and SLAP! Centered between them is one solitary LED that changes colors to indicate charging (red), charge complete (green) and low voltage (red/green flashing). Turning to the back, the first thing I noticed was the big SLAP! Somehow I saw the pun: a small slap in the face of the competition and a big slap on the back for KingRex. Two rocker switches with integrated indicator lights take up the bottom left corner and top right corner. The bottom one, next to the IEC receptacle, controls battery charge. The top one controls the power supply to the amp. Connection to the amp is achieved through an 18-inch cord with 3-pin XLR connection to SLAP and a coaxial plug to the amp, a standard accessory. The XLR connector is made by US Contech and rated at 6 amp maximum current flow. Everybody knows the shorter the DC power cord, the better. But I still asked the obvious question in case some users would prefer to place the battery away from the amp and make their own power cords. Christine's response was "make it no longer than 1 meter".

As a KingRex design goal, efficient battery management came before they'd even consider sonic performance. Opening up the chassis of SLAP was just as easy as other KingRex products but the internal view was far more captivating in the sense that so many circuits and parts could be so tidily arranged into such a compact chassis. I requested a more detailed description of the topography and James did it with charts and pictures. The SLA battery occupies the bottom 2/3 of the internal space, providing a low centre of gravity while the battery management PCB sits atop. The battery is secured with metal plates on all six sides and user replaceable. The replacement procedure was so straightforward that even dummies like me didn't need instructions. First I had to remove the П shape metal cover by removing 12 screws. Then I had to slip off four wired connectors and unscrew four screws to remove the PCB. The battery is locked in by a metal plate held down by four screws. Unscrew them and the YUASA NP7-12 is at your disposal so to speak. It should be noted that the terminals are protected by rubber covers to avoid shorting. The whole process took me under five minutes.

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On the upper deck, two aluminum plates work as heat sinks and form three partitions. On one side are the charge control circuits, on the other side the charge current control and low voltage control circuits. In the middle is the charger power supply which looks decidedly more complex than the usual power supply for the Tripath amps. So I asked the KingRex team if this was the usual transformer-diode-capacitor AC/DC conversion affair. The answer came back as:

"No, they are different. The main difference being the switching power supply for the Tripath amp is using a constant rate of voltage flow under full load so when the load varies, the output amperage fluctuates. SLAP uses a 19V/3.16A switching power supply regulated to output 10 to 15 VDC at a constant current (750mA) to the charge controller IC. Why 750mA? We try to keep it at around ¼ of the power supply's full output to avoid overheating. Users can plug and forget. At the same time, 750mA is about 1/10th of the full capacity of the SLA battery. 1/10th is the recommended charge amperage, which is critical for the longevity of most SLA batteries. On the other hand, if the current is too small, the charge time could be incorrectly determined by the IC, which could be tricked to misjudge the capacity of the battery. However, when the capacity has reached a certain level, the current will be reduced accordingly to promote a complete healthy charge. And that is the three-state charge management."