lipo
Two parts to my question.
Seeking safety…
1 cell chargers can't be used for multiple cells in series as their charge voltage won't be enough. I don't have experience with chargers designed to charge multiple cells in series, but I assume such charger would have a separate input for each cell's thermistor if present or at least its voltage: without some kind of load balance circuit you'll end up with different voltages across your cells, so you have to make sure none is particularly stressed at the time of charging - lithium batteries are dangerous if charged improperly.
The usual solution (mine anyway, used it in a product no later than last friday) is to use a boost DCDC converter: if you look up the definition, you'll see it's a converter that's similar to a pump: it increases the voltage on the output. However, this is at the expense of:
A formula doesn't cost much: $$Power_{in}=Power_{dissipated}+Power_{out}$$ $$\Leftrightarrow V_{in}*I_{in}=\frac{V_{out}*I_{out}}{efficiency}$$ $$\Leftrightarrow I_{in}=\frac{V_{out}*I_{out}}{efficiency* V_{in}}$$ $$BOLbatterylife=\frac{capacity\times depthofdischarge}{I_{in}}$$ BOL states for beginning of life. The deeper the discharge, the more the capacity will drop with cycles. Here capacity is in Ah => battery life in hours, the rest is SI.
Example of efficiency curves of a boost converter with and without PFM (source).
Illustration of waveform of a PWM+PFM [Edit: actually it seems to be PFM only. PWM would modulate the width as well] boost converter output voltage before filtering (source).
I would suggest using a boost converter (you can buy plug and play modules) with pulse frequency modulation (as you'll certainly be using low currents if it's a watch) and a high efficiency curve, and outputting on the Vcc/5V input of the Arduino. Not the RAW because you'll end up unnecessarily losing power in the integrated regulator as well (even more, since it's a linear regulator that is made to dissipate the difference in power). Add additional filtering (called decoupling caps here) to be sure everything's smooth (switching converters are quite noisy) though. You'll still need a single cell charger though; most of the chargers I've seen around which enable simultaneous charging are doing nothing more than connect devices directly to the battery while it's charging, but I think it all depends on the power drawn. If the power draw is ten times less than the charging current, I reckon it's fine (anyone can confirm?).
I have randomly purchased several of these relatively cheap lithium ion batteries from ebay, originating from China. I have a fairly high quality battery charger/analyzer that I use to test these batteries.
They have all been the advertised voltage, but he capacity is nowhere near what they claim. A 5000 mAh battery tested at 333maH. Another 5000 mAh (claimed 5000 mAh) battery came in at 571 mAh. Clearly these inexpensive batteries are closer to 1/10 the claimed capcity. I have managed to get a refund when I complained to the seller.
I saw a video on YouTube where a person took an 18650 battery bought off of ebay from China (that had a tested capacity significantly lower than claimed) and he took the battery apart. Inside was a minature battery that looked like a triple 'A' battery surrounded by a powder that looked like flour!
In other words you get what you pay for. From my experience, it appears as though batteries manufactured in Japan tend to be of higher quality/capacity, although that can change quickly. If the battery you are looking at has a 'C' and mAh rating, then you might be able to get a refund from ebay if the battery doesn't perform as advertised. Today's lithium batteries appear to be relatively safe as long as you don't use them beyond the abilities or 'C' rating. Fire proof boxes are not necessary for storage, but recommendations from manufacturers still tend to include the instruction to "not leave the battery charging unattended". I suspect that this has to do with the fact that there are a lot of really cheap and poorly made chargers that potentially slightly overcharge the battery (usually through trickle charging which you should not do with lithium ion) which is what leads to fires and explosions.
Best Answer
To store? I never heard anything about storing but I guess it doesn't hurt. Those bags are more for charging when the battery is most likely to go off. They prevent fires by containing the fireball so it doesn't spread while relieving the gas pressure of the explosion (if you just contain the pressure then you end up with a shrapnel-producing bomb). This is also why you leave a crack in the lid and do not seal it shut if you are charging in an ammo box.
Even if the steel in the ammo box can burn, surely it's more difficult to set off than everything else around it? And I don't think we're talking about something like thermite here. The steel shouldn't ignite.