but read my answer of dec 21. Having read that, people made this with more success.
After reading and considering this, I had a few thoughts for alternatives. The first was to consider extending a light up beside the Tempest on a pole to boost the solar panels charge rate. The ideal range of light wavelengths for solar panels to generate the best charge current is typically from about 300 to 1100 nanometers, so a full spectrum grow light may fill the bill. The second thought was that Ali Express seems to have a plethora of 2.4v NiMh rechargable battery packs available if the LTO is hard to find.
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FWIW, I placed a couple of (protected) 18650 cells in parallel into a generic bike battery case I had laying around:
I injected the voltage into the PCB via one of the JST connector’s the solar panels use. Then mounted it to the mast:
After 24 hours, I still seem to have sustained voltage in the dreary weather you can see in the picture above.
BTW, I know there is a PBA available from the accessories section of the website, so this post is for educational and entertainment purposes only.
injecting power via the solar panel connectors seems a good idea!
Thanks. If any aspect of this project shit the bed in the long term, I’ll update it here.Lots of variations on this basic concept, probably the most preferable would be using a battery pack that has more mAh potential. I just used what I happened to have laying around. I used protected cells because they have an internal shutoff somewhere around 3V, whereas I believe the Tempest BMS, programmed to manage LTO chemistry, probably has too low of a shutoff for Li-Co chemistry. Also, if you could purchase a generic battery holster for 21700 cells from AliEx that would give you greater storage than 18650 cells give you. If the Tempest were my long term station, I’d probably just spot weld 4 21700 cells in parallel and be done with it. I only have unprotected 21700 cells here and do not have a 3 or 4P BMS handy that I could use, so again the low voltage cutoff that the Tempest algorithm uses could be problematic. Any estimates as to the total mAh required to get a Tempest through 3-4 months of no sun (so you don’t have to either get up, or lower the mast to change batteries) would be appreciated.
The only disadvantage I see is that the solar panels probably don’t contribute anything anymore to the unit.
For what’s worth, the older design with sky & air unit, could run for about 9 months on 8 high quality aa batteries, without solar. In case it gets cold in your place, you need some kind of battery that also runs while it is cold.
Thanks for that,
The operating temperature range of lithium-cobalt oxide (Li-Co or Li-cobalt) batteries that I used here is typically between -20°C to 60°C (-4°F to 140°F).
It can get colder here, but the long range forecast is not indicating anything approaching that as yet. If that does arise, I can either make other arrangements (such as plug it into an AC adapter I made up that also works for this), or simply do without. An Ecowitt WS90 is my main station, and I’ll have to swap out the batteries in it to Energizer Lithium primary AA cells at my first opportunity anyway.
Just as an update to this topic, here is the latest status of the Tempest station in question (done right now), it would appear that the battery pack is working so far:
Thanks for the feedback.
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As I did not disconnect them, three of the four panels are still functioning (each has their own connection to the PCB). In fact, with a little bit extra work (made easier if you had some of those female Nano JST connectors so you could make up an alternate line), or, you could ‘T’ into one of the solar lines and have all 4 working. This methodology is the functional equivalent of having sunlight for the months that the battery pack lasts.
I noticed earlier this summer the battery not keeping up its charge so much. Today I was going to do this project… but upon opening the unit, to my surprise there’s already a bigger LTO battery in there.
Unless I missed it, I had not read about this change in this thread. Anyone around Montreal needs a LTO battery? 
it’s a big vague, but i think I can read that it is only 1300 mAh compared to the 2500 mAh of the one I inserted.
Yes, I also see that this battery in the picture is rated at 1300 mAH. It would appear that the battery supplier that Weatherflow uses simply changed the wrapper on the cells from green to blue (this is common with OEM manufacturers). It may also be worth an update for me to mention that the battery pack DIY shown above, (which my load test indicates has a capacity of 6200 mAH), still seems to be holding my Tempest unit at 2.8V. As winter equinox was 6 days ago, this represents the period for those of us in the Northern Hemisphere when I need it the most.
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Thank you for pointing out the battery on the picture was 1300mAh, I had not even check that. The new battery doesn’t even have the capacity written on it.
I’ve already put the new battery in place. Before doing so, I weight the original and the new battery. The new battery weights ~39.05g and the old one ~38.9g , delta is ~ 1.15g. Hopefully that gram accounts for the additional +1700mAh (3Ah total).
I’ve put back the station on its pole. The station is reporting 2.50v.
I found on the net specs to float charge at 2.63 but another site says 2.80. To be safe I went with 2.63 but looking at the reported voltage, perhaps 2.8 would have been safe.
[edit] Per this comment, I should have used 2.8 to charge. At least solstice is behind us.
Charging: Typically, the cutoff voltage for charging a single LTO cell is around 2.85 to 2.9 volts per cell.
Capacity: I do not have any experience with Li-Titanate, but the theoretical specific capacity of LiCoO2 is approximately 140-160 mAh/g. However, when accounting for practical cell design, electrolyte weight, and other components, the actual specific capacity might be slightly lower.
So I’m going to assume that there’s no way you’ll get an additional 1700 mAH from 150 mg ( I think your decimal place is at the wrong spot, so the difference is milligrams, not grams). I have done some work with building battery packs, and I do know that the weight difference between my 18650 and 21700 LiCo cells is 22 grams, and the extra capacity I get from the 21700 cells over the 18650 cells is 2,000mAH, so a seat of the pants estimate could be: expect an additional 100mA of capacity for each additional gram of weight.
Oh, and I just noticed you mentioned ‘Float’ charging your cell.
Float charging is a method primarily associated with lead-acid batteries, where a continuous voltage is applied to keep the battery fully charged while preventing overcharging. When it comes to lithium-ion batteries, including various chemistries like lithium cobalt oxide (LiCoO2), lithium iron phosphate (LiFePO4), and lithium manganese oxide (LiMn2O4), float charging is not the standard or recommended method for long-term maintenance, so perhaps it is also not the best for Li-Titanite either.
It may have just been an oversight in terms of mentioning your charging method. The advantage of using a booster pack that connects to the Tempest PCB is you now can take advantage of the BMS that is integrated into the unit, and will have the correct charging algorithms already in place for proper charging of your integrated LTO battery.
@jmkellas you are right there’s no floating charge, my brain was tricked how I read the specs on 1 site. For the battery weight, yes my bad again. The small difference proves the 3Ah battery can’t have such high capacity.
This video tests the capacity of a 3Ah LTO but runs short at 1.5Ah. Perhaps his tests isn’t precise, could that battery actually have a capacity of 1300mA like the oem one? Based on the weight, it’s very likely. Not sure if this could explain why @sunny battery test didn’t go so well?
[edit] at 10:10 he’s testing max current capacity : It supports 10C but with a capacity of 1500mAh.
Sourcing batteries from AliExpress or any non reputable vendor is never a good idea. Often they advertise AA or AAA batteries claiming numbers that don’t even exist from best known manufacturers that have rock solid reputations. After looking around, batteryspace has 18650 LTO rated 1300mAh… the only place to find higher capacity is on Ali or any odd sites that ends up shipping their battery from China. If someone here found anything above 1300mAh from a reputable source, please let us know where.
In the end, using an external battery pack or a permanent charger seems the best approach. Worst case, if the battery pack fails under deep freezing temperature, the LTO should be able to keep up. Deep freeze days usually come with clear sky, the solar panels can do their job.
I feel that I wasted my time replacing a 1300mAh with one possibly not from a reputable source… I hope Weatherflow’s OEM battery isn’t from China. Gotta wonder if I should put back the original battery.
There are many good battery manufacturers in China, it is the sellers that can be sketchy. Eve and Lishen manufacture very good batteries on par with LG cells. I recall that folks in the BudgetLight Forum recommended a seller in Shenzen that I used years ago that was very good. There is a saying in the enthusiast communities, “there are milliamp hours, and then there are Chinese milliamp hours”! Same goes for Lumens. We use digital hydrometers that have Eve batteries that are made in China and their characteristics are on par with Samsung and LG in my opinion.
If I were you, I’d just leave the battery that you replaced in situ, and go ahead and connect an external power pack of some sort, either DIY, or the OEM product, but then again, others may have a different opinion, and I respect that.
Bear in mind that when someone load tests a battery, their low voltage cutoff will determine the final capacity. I think in the video you reference, he tests down to about 1.75 Volts, whereas the Tempest cuts off much higher than that I believe.
I did make a video on how to connect an AC Adapter to the Tempest before I made the battery pack. I considered also doing a follow-up video for the battery pack, but there seems to be very little interest in it so I haven’t bothered. If it is of any interest, the video can be found here:
I should also note an update in this respect. I notice at night now (with my battery pack attached), my Tempest reports a voltage of 2.76v, and then when the sun comes out, it returns to 2.8v.
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i still have some left, but couldn’t find them quickly enough. Mine was really a lot bigger. The diameter was probably around 23mm. Is yours the same size?
your battery is labeled 1300mAh, if you bought it like that, then ali is still selling the correct thing to you. If you were going for an 18650 battery, that means it is supposed to be 18 mm in diameter, not like the 23mm I have.
@sunny I had look at the Ali link you provided and days or weeks later while looking for the best price, I focused on LTO + 3Ah. The model to purchase is the 23680, I clearly got the wrong one even if it claims to have the same capacity. I’ll remember that mistake for a long time! After you still happy with your battery? I read above you had charging issues, I wonder what you’ve done about this? [edit] On Ali the vendor claimed this 18650 was 3Ah… bad vendor!
@jmkellas agreed there are great battery manufacturers in China, it’s some vendors. Thanx for the video, I’ll sure look later today.
Yesterday I was really happy not to have to cut out plastic. Today is cloudy and based on what I see, the battery won’t keep up if it’s too cloudy too long. I think what I’m seeing is Deja Vu. Makes me wonder if I accidentally put back the oem battery by mistake 
my unit was replaced (broken sensor) This new one has the same battery issues… not enough power during the long gray dull winters without sunshine. So various sensors are turned off in order to save power. Too bad.