Motivation
Searching for power supply for my new UMI 17 Lite system I didn’t find the matching system. Current Li-Ion based power banks mostly end up at approx. 200Wh. The reduced capacity due to low ambient temperatures is not even taken into account. A LiFePo power banl is much more resiliant against low ambient temperatures. Eligible LiFePo power banks are quire expensive. Since I had good experiences with the 12V LiFePo battery of my Battery Box MkIII, I decided to build a suitable LiFePo power bank myself.
Design
Acquiring a photo the UMI 17 Lite system takes about 20 W electrical power. During download this increases up to 32 W. During GoTo it takes about 25 W are drawn. I assume 25 W as average load and 12 hours for a long photo night. Hence the battery has to provide at least 300 Wh.
Beneath the above given energy the box shall fullfill some more requirements:
- It shall be water resistent as IP65 or similar during transport and during use.
- It shall be as small as possible.
- It shall be suited for „one hand transport“.
- It shall indicate the energy level of the battery.
- The entire set-up shall be able to be disassembled without tools if necessary.
Cells
At a large trading platform I found an opportunity to aquire 16 LiFePo (LFP) cells of type 21700 for a good price. This cells matched, as these set of cells represents the maximum which I see realistic with respect to requirement 2 and 3. Additionally these cells in a 4s4p configuration deliver enough energy (approx. 350 Wh) and by for more current than I ever need at a telescope. My measurements showed an inner resistance of approx. 30 mOhm and capacities at a discharge current of 1 A above the specification. Hence I would note this internet by as a goot deal.
The cells were spot welded to a 4s4p block and became a battery management system. I chose a XT-60 connector for the output of the battery pack.
Set-Up
To get reliable data of the LiFePo power bank SoC, a battery capacity tester is used. Although it’s more of a sensor than a tester, it seems to provide a realistic SoC. To use this little device, it must be adjusted to the battery size before use. The parameters are stored and remain even after a complete power failure. In case you don’t want to rely on this little thing, the voltage is also displayed.
The complete set-up including the wooden carrier board for the current sensor and the IP65 XLR connector is shown at the next picture. The whole inner is form-fitting. This means that the battery block is fixed by the wooden carrier and the walls of the box. The carrier itself is fixed by the wall and to the top by the wooden rectangular prism which can be seen at the lower left of the photo. The 20 amps car fuse in an insulated flying holder should not be overlooked. There is even space inside for a ziploc bag with a spare fuse.
Technical Data
They are not really validated all, but should meet the real values quite well.
| nominal capacity | 350 Wh or 16 Ah at 12.8 V |
| effective capacity between SoC 10% and 90% | approx. 290 Wh or 13 Ah |
| protection class | IP65 |
| size | 240mm x 180mm x 100mm |
| weight | 3.25 kg |
| connector | XLR |