A compact power bank

Although there is always a mains connector nearby, the garden in urban area is not the best place to watch DSO's. So to release from the needs for a mains connector, this power bank has been realized. In itself it is quite simple, consisting of a battery, a charger, a Volt/Ampere meter and some connector stuff. All fits in a sturdy watertight PolyPropylene tool case measuring 30x35x15cm (BaseTech 1310219).

 

Schematic


The schematic diagram is quite straightforward. The main parts are:

  • Charger: 12V/5A HF charger and maintenance charger. This type can be left connected because it switches to maintenance state when the battery is full.
  • Battery: 20Ah deep cycle AGM, meant for golfcarts. For this type of use you need a cyclic battery and not a car battery.
  • V-A meter: compact panel meter with integrated volt and ampere measurement circuit.
  • Connectors: three weipu 2-pin connectors and accompanying fuses. This may not seem a lot, but my mount and everything that rides on it has only 1 power input.
The switch has been added to disconnect the battery completely when it is not in use. As a consequence, when switched off the panel meter is also off.

Construction

The case is slightly tapered, so I decided to make a tight-fitting wooden inner box.This is also needed because the battery is quite heavy (6.5kg) and therefore has to be fixed. The front panel screws down to this inner box, and fits neatly against the walls, exactly flush with the edge of the case shell.

 

As a bonus, I bought an adjustable 3A boost converter, to crank up the 12V to the 19.5V required to charge the small netbook (Acer Aspire Happy 2). This laptop has low power requirement, since it only runs a VNC client. But a dead battery during the observation session would still be annoying, since you would lose control over the mount.

 

Only when charging, so the mains is connected and the switch is on, the current flowing into the battery is shown. When the current switches off and the voltage is 14.5-15V, the battery is full.