I have something similar.
However, you have to be careful because it lacks the temperature control pin, which means that you are not safe from something going wrong.
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Thanks for the replies! Also, @irvinewade, I hadn’t read through that thread before. Thanks for so much excellent information about your L5 experience!
Related question: do you find two batteries adequate to get through the day? That is, does the depleted battery charge completely before the other full battery dies? I’m picturing potentially needing three batteries to get through a full day of usage. Curious on your thoughts.
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I’m not actually attempting to do that at the moment. Having the spare battery and external charger was more just for exploring possibilities, and perhaps for occasional circumstances where I really need to double the available time.
Right now I am living with the amount of time on one battery and just putting it on charge once it tells me to.
However my forecast answer to your question would be “no” - if the meaning of your question is: if I had to spend 24 hours off grid would two batteries be enough? However see next.
It’s touch and go. Charging the battery externally takes very close to the same amount of time that it takes for the internal battery to go flat (with the obvious variable that the latter depends on how much I have been doing with the phone and what components are hardware killed and what is connected to the USB-C port).
So if the meaning of your question is: given constant availability of a grid supply could the phone remain available forever with two batteries but without ever putting the phone on charge? then the answer is a “touch and go yes”.
To refine that answer a little: external charging takes about 8h 20m and I will usually put the phone on charge when the battery depletes to 20% which may be sooner than 8h 20m, but if I pushed my luck with the internal battery and let it go closer to 0% then I might be able to go long enough to change the battery after the external one is charged and before the internal one is flat.
Really if I wanted that workflow then I would need to get a better charger (higher current). The one I bought was cheap and just for exploring.
Also, the fundamental problem with this interpretation of the question is that, well, given constant availability of a grid supply, I can just put the phone on charge. I don’t even need a second battery.
There would be a carefully arranged circumstance where you can argue that this situation makes sense i.e. where you are coming in and going out in a complicated dance, while the spare battery is on charge somewhere and you can rush back at the right time to swap batteries. But that isn’t how I want to live my life - hence why I am keen for Purism to implement a sleep mode that is good enough to go 24 hours on one battery (preferably more than 24 hours).
A third interpretation of the question is: getting through the day means getting through, say, a 16 hour period off grid. Then the answer is obviously yes provided that the phone is only subject to “regular usage”. (If I have a peripheral sucking power at the USB-C port e.g. USB-C portable drive, then all bets are likely off.)
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You can also consider an external battery (powerbank).
The advantage is that it is saver to carry, is move convenient than replacing the battery, can be used for other devices too, and can have a much langer capacity than a spare battery.
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Any standard Li-ion charger should do - just remember to not charge the battery above 4.2V.
FWIW, charging extra batteries can be easily done via the phone itself. If you use a powerful enough power supply with high enough input current limit, you should be able to take the battery off while keeping the phone fully operational, powered from USB. Then you can just put the other battery in and it should simply start to charge.
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Well thought out advice, I think 
(as depends on how someone organized).
Are you getting (as no manual provided) this 4.35V intelligent charger’s red LED light back to blue after 8+ hours (if down to 3% while 4500mAh÷600mA requires more than 7 hours for sure, although guessing that up to 5 hours might serve the purpose also) of charging your Librem 5 3.8V battery? Asking while didn’t wait (after first time usage yesterday) for blue LED light on my PinePhone’s one but it was 100% after putting it back to the smartphone (and during last night decreased to 92%, which impressed me positively).
As well, I find your recommendation to use 4.35V intelligent charger as quite better option, kind of native support to this battery itself, for Librem 5 external only charging (yet please be aware/read @dos post, particular request actually, directly above from this one, that Librem 5 battery inside of the smartphone should not be attempted to be charged with over 4.2V in order to reach its higher voltage capability of 17.1 Wh, as I understood this important request) of original Librem 5 battery while the linked one from @JR-Fi provides, charges with using 4.2V (which in my amateur opinion isn’t quite adequate for high voltage 3.8V batteries when outside of this particular smartphone charged, as related to this thread title). @irvinewade, thanks a lot for this advice, link to slow charging external device (and probably, while not 100% sure how to properly explain this, for battery itself somehow healthier type of charging)! And please note that: “the battery capacity can be increased by about 15%.”
As second solution, above mentioned 4.2V external charger (with Librem 5 battery outside of the smartphone itself) would live fully charged battery at 3.7V Max., I believe, and with quite less time needed as it provides 1250mA, I think (and perhaps prolonge its expectancy life). What you think/know about? Just for the purpose of dialog/discussion here, until some expert shows/points out to exact differences/advantages of two mentioned external charging methods (as those universal devices for external charging of LiPo or LiHV batteries look the same but they aren’t, 4.2V vs 4.35V).
And least but not last would be to thank to Purism for choosing 3.8V Li-ion battery! The one that gives users two choices of external charging (as 3.7V battery should not be charged with 4.35V, someone might try this just for one time, if insured, but don’t think that such kind of insurance exists), but 3.8V battery can be charged with 4.2V, externally as well as internally (therefore calling other members within this Forum that already tested those simple external chargers to provide some additional input).
It doesn’t matter whether it’s charged externally or internally. The battery is fine with 4.35V, but you shouldn’t put a battery charged above 4.2V into the phone at all. The internal charger is already configured to not go above 4.2V, while when charging externally it’s up to you to ensure that it doesn’t go above 4.2V.
If you do happen to charge it above 4.2V, you should discharge it a bit before putting into the phone so it won’t provide voltage higher than 4.2V to the phone.
(as already noted somewhere else, the phone should actually be fine with 4.35V, but IIRC one of the M.2 cards is not; plus I haven’t checked the whole power path to be sure that there are no other components that won’t like voltages above 4.2V, so can’t do any claims about that either)
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Limited charge voltage of 4.2V/4.35V external charging device output and rated voltage of Li-ion battery itself are two diferent numbers. Therefore my guessing in above post that to 3.7V or 3.8V charged (externaly) Librem 5 battery should not make any damage when put back to the particular Librem 5 smartphone. And sorry for my confused up and down writing.
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As @Quarnero pointed out above, a battery charged with 4.35V should actually provide something around 3.8V when fully charged, which does make some sense according to my (very limited) understanding of how batteries work 
Making my statement above not quite accurate.
So, to err on the side of caution, I’d say that when charging externally you should verify the voltage that the battery provides after being fully charged before putting it into the phone. As I said above, we don’t use voltages above 4.2V for charging because some components specify 4.2V as their maximum operating voltage.
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Are there any concrete devices for charging externally which
- fit to the pins of the L5 battery
- which have some kind of (configurable) intelligence stop charging when reaching 4.2V
?
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Within this thread: External battery charger for Librem5 are two links (to aliexpress.com and to amazon.com) provided for the very same charging device, pin compatible device.
This thread now has more todo with flashing a L5 and not with charging. It’s nearly impossible find something there 
It is compatible.
EDIT:
“Input: The length of the battery you are trying to charge should be between 42 and 70mm”
The charger that I have is adjustable to the width of the battery and adjustable to the position of the pins.
If you want something that is out-of-the-box specifically and only for the Librem 5 battery width and pin position (or has no width constraint and is specifically for the pin position) then I am not aware of such a device.
Could you please share the exact model and from where you’ve bought it. Thanks.
@guru, you can see very well related pin connectors within this post: https://forums.puri.sm/uploads/default/original/2X/f/f49c192f919b68c6ffb4aed8343f74828017a8c8.jpeg.
Second post in this topic - but I’ll save you the bother of scrolling and clicking: https://www.amazon.com.au/Onite-Universal-Battery-Charger-High-Voltage/dp/B005REEWYY
(has US power pins - will need some sort of adapter for your local power socket)
Thank you! The problem is that I can’t see any dealer for this in Germany, or at least in EU
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Yes, same product exists on the main Amazon site (amazon.com) but for some reason won’t ship to Germany but will, for example, ship to France. Might depend on how close to a border you are and whether you know someone in the neighbouring country and whether Amazon ships to that country. 
No need to be concerned as from aliexpress.com belonging Price includes VAT (still payable if you do not need it right away, before tomorrow), and that is and calculated as 19% in advance for your particular purchase, every product purchase. No need to worry about particular transport cost/pollution either as most packages will be delivered to Amazon shop(s) anyway. Also, it is not Made in Germany product, punkt.