Circuit and Converter

 

Schaltung des Batteriekoffers ohne den Boardcontroller.
Power supply part of the box circuit.

The design of the power box circuit and the converter selection is quite straight forward and supplies two different outputs distributed via various connectors. For the voltage conversion I tried to use an appropriated DC converter always. As an addition an arduino based controller is part of the box. Its task is to diplay most relevant voltages and to calculate the current state of charge, i.e. remaining capacitiy.

The voltage is provided by appropriate DC-converters. At all there are three converters in the system:

Charging the battery

An LTC3780 based boost/buck converter is used for charging the battery. Since the module is neither polarity protected at the input nor anti reverse protected at the output diodes are mandatory for an appropriated protection.

At the input is a 15 Amps Schottky diode. The voltage drop doesn’t matter. Only the power dissipation has to considered. At 7  Amps one expects probably 5 W power dissipation. Hence the diode should be mounted with sufficient air flow (see here). Since there is a fan for charging, I mounted the diode close to the converter module in order to have both in the air flow.

At the output a so-called ideal diode is used. Basically this module replaces the diode with a low loss MOSFET to drastically reduce power dissipation, heat generation, and voltage drop. Since the voltage is a critical parameter for Li-Ion charging it is recommended to enable a tight voltage control of the converter.

Das Innere des Batteriekoffers.
The insert of the power box before final Integration. Most of components are positioned already.

Notebook Converter

My notebook computer is an aged HP 2510p which consumes max. 3.3 Amps at 19.5 Volts. As an appropriate converter I’ve found a 60 Watt module wich can deliver this load. During my tests it revealed that this module has a nice feature reagarding over current protection. It seems to regulate the current down, if the temperature get critical. Hence I could omit an additional fuse in the power train. The converter is adjusted for 3.3 Amps, if the notebook draws more the current is limited accordingly.

In order to safe the quiescent current in the case no notebook is connected, the converter is switchable.

Telecope Supply

Telescope Converter

The so-called telescope converter supplies my astro equipment, which is essentially the mount and the cameras. Therefore two aspects shall be in focus: reliabiliy and qualitiy of the DC output.

As the supply should be as failsafe as possible I tried to achieve this by a strong power raiting. The converter is capable to deliver up to 10 Amps. I adjusted it to limit at 8 Amps. Most of the time it will operate at 3 Amps, which is far below the specification limit. This should result in a long life of the converter. With the converter itself I don’t have any experience so far.

The converter PCB is made of aluminium, hence I conclude that it acts as heat sink as well. For that reason in is mounted elevated in order to ensure good air circulation around.

The RC-Filter

The telescope converter is completed by an RC-filter. This filter improves the qualitiy of the output. The 2200 µF at the converter output just adds to the output capacitor of the converter. Usually they are designed a bit small. A larger one may reduce the quality of the voltage control, but reduces ripple significantly. The inductance of 10 µH is specified for 10 Amps.  The currents in my case should always be significant below that value. Needles to say that the connections of the filter shall be as short as possible and have a low inductance…

The Board Controller

Circuit

The main task of the controller is to monitor the capacity of the battery. This is done by two means.

Kappazitätsanzeige.
The controller display and the corresponding button.

One is estimating the remaining capacity based on the off-load voltage. For this the Controller measures the battery voltage continously, calclulates the remaining capacity and displays it in percent.

Usually the battery is under load, hence the voltage measured is no off-load voltage and therefore the estimated percentage is wrong – usually the capacity is underestimated in this case. The second capacity estimation measures the drawn current and calculates the „used“ capacity. This is displayed in Ah.

The photo shows the display of the controller and its corresponding button. Here it is indicated a remaining capacity of 70 %, a battery voltage of 11.2 Volts and a discharge current of 0.01 Amps. The lower right hand part indicates the output for the telescope supply as 13.7 Volts and the capacity drawn since the latest reset is 0.1 Ah.

Circuit of the Arduino pro Mini Controller board.

The circuit is straight forward and shown in the picture beneath.  I set it up on a breadboard carrying all necessary Elements and as well appropriate connectors. The whole arangement is held at the carrier by two notched  POM (Polyoxymethylen) bars (the white bars at the pictures).

Program Code

The Arduino code is listed here for the interested.