@jcs613, I got the one linked above from Amazon. Only used it once and worked fine for up to 110W load. Was heating a bit too much above. Not sure how comfortable I am leaving it unattended however, cheap electronics and a short could be catastrophic. Will probably want to add an inline fuse, but most of what I find on Amazon is low voltage DC fuses.
For long-term storage, storing at 40% battery capacity is best for maximizing the lifespan of the cells. For the SP140, 40% battery capacity is at roughly 86 volts.
(You must also realize that roughly 60 volts on the sp140 is considered 0% battery capacity as the BMS should have a hard cutoff at or before it gets that low. Additionally any lower voltage would severely damage the li-ion cells anyway.)
The sp140 battery is at about 100.8v fully charged, the nominal or average voltage of the battery is 86.4v, and the cutoff is 60v if I remember correctly.
Is there anything specifically you want to learn? Some resources are better for certain aspects additionally we have a lot of people on this forum very experienced with every aspect of battery pack design and production.
As for charging, at least the 4kWh battery should be capable of charging faster than what your 110v wall outlet can provide, let alone what the charger can give. So I wouldn’t worry. Just set at whatever charging speed works best for your needs and forget it.
Optimally for long-term storage, discharge or charge (whichever needed) to about 86 volts and store at room temperature.
Overall the sp140 is a pretty fool-proof battery. So as long as you don’t subject it to both extreme voltages and temperatures for long term storage and don’t charge it directly after a flight where you pushed it really hard, you shouldn’t need to worry about much else.
Apologies if this is something you already knew, but just in case… The amount of oomph you can draw from a battery is Voltage * Current (or maybe current-squared, depending on what you care about). As voltage goes down, your oomph goes down … but so does the amount of current you actually draw, so your oomph goes down faster than the voltage does. (Thus Bob’s comment about 60V being the same as empty.)
Using the water analogy, if you had a big lake at the top of a hill, the voltage goes down as the water level goes down, but there comes a point when no more water is flowing. It is still way above the bottom of the hill, and might flow if you could use a siphon or something, but it won’t flow on its own any more, so it has no useful oomph, despite plenty of height (voltage).
The oomph/total power output is Wattage. Of course wattage is equal to voltage x amperage. Additionally because voltage x resistance = current. When your voltage is cut in half, the power output of a given motor will be cut to approximately one-fourth.
Anyway, the 60 volts is not related to that. The sp140 could be designed to climb at 50 volts if it would extend the flight time. The 60 volt limit (2.5 volts per cell) is the minimum voltage the battery cells can safely go to before you start to significantly reduce the packs life span, risk damaging the pack, and increase the fire hazard as it can promote the formation of dendrites.
Based on what I read of max voltage of the pack being 100.8v and max cell voltage being 4.2, it sounds like there are only 24 cells in series. How many bundles are in parallel if any? Because if there were only the 24 cells in series they would be really small and light compared to the full packs size and weight, lol.
Also is there a minimum and maximum ambient temperature for long term storage that should be avoided?
Does anyone else have experience using the load tester linked above to drain the SP140 batteries? If so how did it go? And how complicated was it to perform?