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My astronomy project:
Lithium Battery power


Content:

  1. Astroserver and powerbox, ver 1.0
  2. Astroserver and powerbox, ver 2.0
  3. Balcony outdoor outlets
  4. Lithium Battery power
    1. Lithium Battery power
    2. DC/DC converter
    3. To be continued

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!


Page IV.2: DC/DC converter

Why 24 Volt ?

It was difficult to find a 12 Volts Lithium battery with enough energy. The ones I found was very expensive and no charger included or the safety electronic and enclosure. That was one reason to why I choose to go for 24 Volt, but there is something more. A battery doesn't hold its voltage during discharging even if Lithium batteries are better than most other type of batteries. When I use lead acid batteries they don't have 12 Volt, they hold more like 13.7 Volts when newly charged and when it's discharged closer to 11 Volt. Some of my equipment works better with 13.7 Volts and others must have 12 Volts. With a 24 Volt battery and a DC/DC converter that take 24 Volt down to a lower voltage I can choose more exact which voltage I will have and it will be stable at that voltage independent of what voltage the battery deliver. With an adjustable converter I can set the output Voltage to 13.7 Volt or as I plan, 12.5 Volt. You have always some voltage drop through the wires, in my case from 0.1 to 0.2 Volts. It's very important to investigate what the different equipment that connects to this battery can handle in input Voltage, more then 12 Volts can destroy them in some cases, others are made for 13.7 Volts.


DC/DC converter:

My latest equipment I have setup is optimized to operate with a low power demand. Typical 1.6 Amp at 12 Volt, that is only 20 Watt. I guess this battery will last for 10 hours with that load, but I have to do some test before I'm sure. Now I use a DSLR camera, if I in the future replace it with an active cold camera the power goes up, only the cold camera can take 5 Ampere at 12 Volt, then this battery for sure is too weak. Batteries of this kind with higher power use higher voltage batteries, 36 Volts, 48 Volts or even higher, that higher voltage make it more difficult to find a DC/DC converter.

When I choose a DC/DC converter to this 24 Volts battery the demands was:

  1. At least handle 35 Volts input voltage
  2. Have an adjustable voltage output
  3. Have the capacity to deliver 5 Ampere.
  4. Have a high efficiency, at least 95%

I have ordered two different DC/DC converters to test with.

Note:
Lithium batteries stores a lot of energy and can be dangerous, and a big battery like this is totally forbidden to bring on a flight !


Astroequipment powered by Lithium Battery

I bought these two DC/DC converters, both has a maximum of 35 Volts input and can deliver up to 5 Ampere if equipped with a cooler. I chose the one to the left which included the cooler.


Astroequipment powered by Lithium Battery

I also bought this tiny DC/DC converter, it has an adjustable output voltage and can deliver 1.8 Ampere. I plan to use it later to power the camera with 8 Volt.


Astroequipment powered by Lithium Battery

I must also have an enclosure to the DC/DC converter. I designed a box that integrate with the battery's cassette. There are also a place for a fuse and a raised fundament for the circuit board. The rectangular hole in the middle is for the battery connector. What's left is to add a lot of holes for the air circulation to not over heat the device, the corners will also be rounded off. In this case with 12 Volt output and maximum 5 Ampere it deliver 60 Watt at maximum load, with an efficiency of 95% it will be power losses of 3 Watt that must be cooled off.

Note:
I'm living in a very cold climate, in hot environment it maybe need a more effective cooling !


Astroequipment powered by Lithium Battery

It's the all small details that take all the time when designing something with CAD drawing. Now ventilations holes and rounded corner are at place. If it fit well it's now only a lid that is missing.


Astroequipment powered by Lithium Battery

Direct out from the 3D-printer, as usually I made some mistakes. The rectangular opening was off by 3 mm, I forgot a hole for the out going power wires, but I can still use it. File the opening bigger and thread the holes with M3, M4 and M5. This box is printed with ABS Pro filament, much more difficult to 3D-print compare to PLA filament, this time it lost its connection to the printer bed and got warped.


Astroequipment powered by Lithium Battery

Very important, a fuse direct on the 24 Volts battery output cable. I start with a 3.15 Ampere fuse, should be enough to deliver 5 Ampere at the 12 Volts output.


Astroequipment powered by Lithium Battery

Install the DC/DC converter and the cables to check that there are space enough for them.


Astroequipment powered by Lithium Battery

I solder all the wires and install the cooler on the power circuit. I let the 12 Volts wires go out through two of the ventilations holes. The cable area is 2.5 mm2 because I want the power loss trough the cables to be low, but it isn't easy to solder them on the tiny circuit board.


Astroequipment powered by Lithium Battery

Later I did some functional test and were satisfied, after that I designed this lid. I made four feet at the bottom but nothing that should be placed on the wet ground. Because of earlier problem with the ABS Pro filament I did something new. My friend told me that he apply some glue stick on the print bed. This time I did so too, and it looks to print the bottom layer much better now.


Astroequipment powered by Lithium Battery

Install it on the rail which the battery slide on.


Astroequipment powered by Lithium Battery

Maybe I install this rail at one of the legs of the mount.


First power on:

Astroequipment powered by Lithium Battery

I do this first test with my 13.7 Volts power supply. A carefully check and measure that all the minus and plus wires are correct connected. Power it up and adjust the voltage a bit lower than the input voltage, just as a test. As you can see there is a red LED on the DC/DC converter. I count the number of LEDs I now have on my Astro equipment, 17 LEDs ! It's like a Christmas tree in the night, and they take power too. If the current to each LED is 0.005 Ampere at 5 Volt, it gives a total of 0.5 Watt, 1/40 of the total power.


Astroequipment powered by Lithium Battery

The new lid mounted, it's not water prof so it should not stand on the ground outdoors.


Astroequipment powered by Lithium Battery

Waiting for a clear sky !


Astroequipment powered by Lithium Battery

Slide the DC/DC converter box on the battery, but first I measure that the plus and minus terminals are correct against the battery output terminal.


Astroequipment powered by Lithium Battery

First test with load, yes it works ! Now I must test it outdoors some night when it's cold and then measure how long the battery last with these conditions. It's a 300 Wh battery and my equipment load it with 20 Watt, can I hope for 10 hours of operation at 0 degree Celsius before the battery gives up ? It's good if I have time to park the mount and shut down the Raspberry before the battery shut down by automatic. I saw something about that it can use 90 % of its energy before this occur. A more precise calculation of the energy I can use, 24x12= 288 Wh, minus the losses in the DC/DC converter and the losses in the cables, 288 - 14 - 2 = 272 Wh. 90 % of that is 245 Wh. With that the battery should last for 245 / 20 = 12 hours. That give some margin when the battery ages.

As I wrote in the beginning don't do this by your self, buy something that already are made for this purpose.


Astroequipment powered by Lithium Battery

I just simple hang the battery in a carbine hook under the mount.


Out door capacity test:

Outdoor test in progress, temperature +2 degrees Celsius, mean current at 12.4 Volt, 1.6 Ampere. Charge at beginning 75%.

Hour Charge Comment
0 75 % +2.2 Celsius
1 75 % +1.3 Celsius
2 75 % +0.6 Celsius
3 75 % +0.6 Celsius
4 75 % +0.6 Celsius
5 50 % +1.0 Celsius

This little simple test gave me some knowledge how well this battery perform. If I read the charge display correct, 5 hours of use only consumed 25 % of it's energy, it's promising.

Second night, this time I started earlier to get some more hours. This time the battery was charged to 100%.

Hour Charge Comment
0 100 % +0.1 Celsius
1 100 % -0.5 Celsius
2 100 % -0.5 Celsius
3 100 % -0.6 Celsius
4 100 % -0.9 Celsius
5 75 % -0.6 Celsius
6 75 % -0.2 Celsius
7 75 % +0.0 Celsius

With these data I can give a rough estimate, after 12 hour the battery will still have 25% energy left. Very good and the battery, charger and DC/DC converters cost me about 170 Euro. But more use of it will show if it work as good as it looks to do now, what happen if the DC/DC converters fail and it deliver too high voltage ? Then I have to buy a lot of new expensive astronomy equipment.

Dependent of what type of Lithium battery, some of them cannot be charged at cold temperatures because it destroy them. This battery is told to have some built-in protection for that.

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To page IV.3, to be continued

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