Easy SKY+SPA battery expansion (unofficial)


First off, this idea came to me after seeing this post, Sky Solar charger failed - again - #59 by lewtesco, by @lewtesco where he used AA sized LiFePO4 cells.

This is a simple way to increase the battery capacity of a SKY+SPA (Solar Power Accessory). Note that this is not an official modification sanctioned by WeatherFlow, so do this at your own risk!

Needed for this modification:

  • 4 - 14505 or 14500 LiFePO4 cells
  • 4 - AA “dummy” cells
  • DC Volt meter

Some people live where the SPA does not get enough sun to make it through the winter. This very likely could be helped if the battery capacity of the SPA were more than the 800mAh/1000mAh of the factory version. Since the SKY was originally designed to use eight AA batteries in a 2s4p (2 series then 4 parallel) arrangement it can be used to expand the battery capacity provided that exactly the same type of battery used in the SPA is also used in the SKY. For this modification to work you need to get four AA sized LiFePO4 cells and four AA sized “dummy” cells which merely have the + terminal connected to the - terminal. This is needed so that the SKY sees the proper voltage and that voltage is the same as what the SPA puts out.

I purchased these items:

Since these are listed as unavailable an alternate source I have ordered from is AA Portable Power Corp.

from Amazon.

Installation instructions

  1. First measure the voltage of each AA LiFePO4 cell to make sure they are all at the same voltage or very nearly so. If they are brand new they should be at about 3.2V and be very close in voltage to one another. If they are very far apart in voltage it could indicate a defective cell. The ones I received for this modification were all within 0.02V of each other. If you are using an analog meter make sure the red test probe is placed on the + terminal and the black probe is placed on the - terminal.
  2. Remove the SPA from the SKY and measure the voltage of the SPA at the pogo pins as shown in this picture: If the voltage of the SPA is within 0.1V of the AA LiFePO4 cells then it is safe to continue with the modification. If not, and you don’t have the proper equipment, you will need to charge or discharge the SPA to get it close to the voltage of the batteries. The SPA can charge without the SKY attached. If the SPA voltage is low you could also run the SKY just on the new LiFePO4 cells until the voltage comes close to that of the SPA.
  3. Install the AA LiFePO4 cells and the dummy cells in the SKY as shown: Battery install It is imperative that all the cells shown in blue are LiFePO4 cells and the cells shown in yellow are the dummy cells, or vice versa. In other words, each “layer” of cells must be the same type of cell. (Picture modified from Batteries & Solar Power Accessory – WeatherFlow Support)
  4. Install the SPA to the SKY, remount the SKY.

That is it! Due to the nature of LiFePO4 cells they will all stay balanced with each other since they are all in parallel (all + terminals are connected and all - terminals are connected). As long as the voltage when they are connected in parallel is sufficiently close it doesn’t matter what the original SOC (State of Charge) is or what the relative capacities are.

If you suspect that your SPA has an issue with the internal battery, you could open the SPA and disconnect and remove the battery from the SPA and just use the new cells you installed inside the SKY. Understand that the modification described above and also removing the internal battery may/will void any warranty from WeatherFlow. Also, if you choose to just cut the battery leads inside the SPA, make sure you cut one at a time so you don’t short out the battery or SPA.

The new capacity with the cells I used and the most recently shipping SPA would be 650mAh4+800mAh=3400mAh=3.4Ah or about 3.4Ah3.2V=10.88Wh, up from the original 2.56Wh.

Hope this helps someone.