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.



This is a clever solution. My SPA failed while still under warranty. Would come on during the day but then lose power at night which told me the solar panel was working but the battery wasnt. Tempst wouldnt repair or replace it so I requested and got a refund. I found there was water inside so obviously the seal didnt do its job. I have access to a battery tester and the battery itself tested OK, but the protection circuit seemed to have failed. I was able to repair it by replacing a mosfet and putting it back together. Also sealed it with some silicone to try and prevent water ingress. But I like this solution, much simpler, I will use it if it fails again. Good job on the coloured diagram showing the battery assembly.

1 Like

Hi @gizmoev, I have in recent weeks undertaken this conversion and am extremely pleased with the results. Cannot thank you enough.

I used to suffer from the Sky Solar panel dropping large amounts of battery (about 3 times a day) for no apparent reason. I have now converted and have NO drops what so ever.

Still perplexed as to why the sudden drops before but super happy now and looking forward to the winter.
Thanks again for the tip.

1 Like

After some years, my SPA unit has also failed - the Sky unit goes offline frequently but then comes back online after some hours of sun. Weatherflow support pointed me towards this forum post and I decided to try the repair and mod. However, I have several questions: When I disassembled the unit and cut off the outer plastic wrap around the battery I see that there is a tiny circuit board connected to the battery and the leads to the main circuit board. I’ll guess that this board is the modification to the original SPA design to prevent charging of the battery when the temperature is below freezing? Any suggestions for the best way to get this attached to the new battery? It looks like there are solder pads at each end of the board, so desolder the old battery tab pieces, then resolder short lengths of bare stranded wire to these and the new battery tabs? Next, if the tiny on-battery circuit board is to prevent cold charging and damaging the SPA battery, will the additional, new batteries in the Sky be similarly protected against cold-charging by this circuit or not? Thanks

More details: Since the SPA had worked flawlessly until the recent failures/dropouts, I assumed that the internal SPA battery had gone bad and therefore was planning to replace it as well (with an identical 1000 mAh battery from AA Portable Power Corp). Interestingly, when I measured the voltage across the SPA or the internal battery leads, I got 3.49-3.50v, even with the solar cell covered and waiting several days - significantly higher than the 3.2v expected. Therefore, even more reason to replace the SPA battery as well…

3.2V is the nominal voltage, not the fully charged voltage. It may not be the battery that is the issue. Make sure the contacts to the SKY are clean. Check both the SPA and SKY contacts.

The solution I provided in the original post is still working great for me and I recommend it over the external battery method I’ve posted elsewhere or the replace battery method. You can also just remove the battery in the SPA and put the batteries in the SKY like I show above.

Would you share your Station ID or a link to your station so I can take a look at it? Maybe there is something else going on. Also, you could just run the SKY on regular batteries for a while to see what it does. If it still goes offline periodically then it may not be a failed SPA you are dealing with.

1 Like

Gizmoev, Thanks for picking up the thread again and apologies for omitting some important details in my first posts: The contacts on both the SPA and Sky are clean and shiny. There was a small amount of moisture in the bottom part of the SPA when I first disassembled it, but seemingly not enough to short anything out (the SPA would power the Sky, even if there were intermittent dropouts and no burn marks around the tiny battery circuit board). The Sky works fine - online and no offline dropouts - with 8 regular alkaline batteries. My station ID is 2395.

I understand the difference between nominal and no-load voltages. The old (original) SPA battery, completely disconnected, reads 3.49v. The new, “to-be-replacement” LiFePO4 18500 battery reads 3.29v; the 4 Shockli AA/14500 batteries read 3.32-3.33v.

Unless I’ve missed something, the problem seems to be the SPA and/or the original battery.


Great. Then I would just try disconnecting the SPA battery and only use the ones in the SKY. You should be able to just desolder one of the leads to the battery to test it. The connections to the SKY are in parallel with the battery, so that should work fine. Does your SPA have the drain holes or is it one of the older models?

Any reason not to also replace the internal SPA battery as well, to get a further boost in total Whs available? Other than the difficulty in soldering the tiny battery circuit board to the tabs on the new battery?
My SPA is one of the older ones - no drain hole(s). I assume I should drill one hole in each of the two slight battery depressions? Large enough to let any water drip through and not get held up by surface tension? Thanks

Only reason not to replace the internal battery is convenience and cost. If the SKY can last through a long period of total darkness without it, then why add to the complexity.

I know I have posted pictures on the forum of the drain holes in the newer version and my modifications to the older ones, but I don’t remember what threads they are in. When I get home I can post pictures if you want. It is worth adding the drain holes.

Yes, please on the photos of where and how big the new drain holes are!

Here are some pictures of the new SPA showing the drain hole and a picture of one I added a drain hole to. It looks like I used an 1/8" bit. It won’t hurt to have a larger hole.

The newer SPAs also had a design change in the gasket.

If the spa battery is still taking a charge but on its way to failing do you have to remove it? Just picked up the lipo and dummy batteries to do this mod as every fall the charge circuit cuts out once our temps get too low. Got a notification that sky stopped communicating the other night, but we aren’t that cold yet so I’m assuming my spa battery is on its way out. If I can get recharged to match the new cells will it stay matched with them or completely fail and cause issues down the road? Is there photos anywhere if the spa battery and board to see if I’m up for replacing that rather than just removing? Searched a bit but didn’t come across any yet.


That is what I would do. An old failing battery can bring down the other batteries with it. As long as the voltages between batteries is within 0.3V they can be paralleled without issues. The first full charge will bring them all to the same state of charge. Especially considering the small size of these batteries and small currents involved.

I hope you got LiFePO4 (AKA LFP) batteries and not LiPO batteries.

It sounds like you need to defeat the temperature compensating circuit. I haven’t publicly posted this until now but figured that the SKY PBA warranty is out on all units now, so here it is: short the thermistor and remove the 5.1k resistor.

The resistor is right beside the thermistor shown in this picture:

Here is a picture of a modified SPA. I removed the thermistor but you could just as well short it on the back side of the PCB.


Reporting back on my successful repair of the SPA: I went the full Monty, replacing both the SPA internal battery and adding the four extra LiFePO4 batteries plus dummies in the Sky. So now I have 4 x 650mAh + 1000 mAh = 3600 mAh instead of the original SPA battery only 800 mAh (Note the replacement battery availablef rom AA Portable Battery Corp is now 1000 mAh).
To attach the small circuit board strapped to the battery to the new battery I punched holes in the small solder tabs on either end of the circuit board, then ran short lengths of bare stranded wire through the solder tab holes and the holes in the battery tabs and twisted the ends together to make a solid physical connection. After a quick solder of the stranded wire “links” I trimmed the twisted ends to make them flush with the battery and circuit board, then wrapped narrow lengths of silicone tape around the battery and circuit to hold the board securely to the battery plus one additional length to secure the wires from the main SPA circuit board to the battery end as it was originally (first photo). A final, lengthwise wrap of silicone tape “waterproofed” the battery ends, again as it was originally (photo 2)
Since the inner battery case was “glued” in place by the black caulk around the wires from the circuit board to the photocell, I didn’t attempt to remove it as Gizmoev did, just drilled a drain hole (1/8") through the inner and outer cases, angled toward the underside of the photo cell (photos 3 and 4)
Finally, I squeezed the new battery in (a slightly tight fit with the silicone tape wrap), mounted the circuit board in the cover, added a generous spread of petroleum jelly around the rubber gasket and its mating/sealing surfaces, then screwed the cover back on.
Then I remounted the SKY and SPA and checked SKY voltage in the app (screenshot). Afternoons around 3:00 PM the voltage peaks between~3.45 and 3.55v, then falls off as the lux goes down - indicating that the SPA is charging the batteries. There is no peak on 11/6 because the temperature was below freezing all day (my house is in the Rocky Mountain foothills at 6400 ft, so cold here already), indicating that the SPA thermistor is working correctly and preventing battery charging below freezing.
Since the SPA repair, the SKY has been working perfectly - no dropouts or offline episodes. So a big thank you to Gizmoev for his clever repair and the advice he provided as I was getting started.


This is where the drain hole needs to be.

You may wany to enlarge the hole a little bit.


Thanks for posting all the tips. I just finished. Working great so far…

1 Like

So if I didn’t jumper the thermistor would the spa not charge at all? Removed it and the resistor a few weeks back and batteries are dead the other day. Don’t think I jumpered it in my haste to get everything back on the roof before rain. Looks like they died at 1.75v so will they recharge or do I have to put another set in?

You don’t use the SPA with batteries in the SKY(Never)
The SPA is not meant to charge batteries in the SKY. The SPA has a battery in it and it might be that it is bad or the charge controller has an issue.

Might want to start at the top, this whole thread is about putting batteries in.

1 Like