Re: JetBeam Backup Series (XP-G R5) BC10 & BA20 Reviews: RUNTIMES, BEAMSHOTS, and mor
Hi selfbuilt, many great thanks for your advice and reply. I don't really understand how you translate it into 5C?
By using a very simple metric of working back from time. For a 750mAh battery (as many RCRs are rated), a 1C discharge rate would mean that the cell provides a current of 750mA for one hour (*under ideal conditions*). 2C discharge would mean the cell would deliver 1500mA over 30 minutes, and so on (i.e. 5C would be 3750mA for 12 mins). Of course, to actually determine C, you would normally use a power supply to discharge the cell at a set current.
There are a lot of caveats here. First off, rated capacity of cells is rarely accurate (e.g. AW protected 14500 consistent have 25-30% greater capacity that AW protected RCR in my testing- despite both being rated at 750mAh). Moreover, discharge rates are not linear over time (i.e., cells don't give up energy linearly). Also, in reality, internal resistance turns some of the energy into heat (and thus lowers the resulting capacity as you raise the discharge rate).
Since I can directly measure none of this, I simply go by the simple rule-of-thumb based on runtime - if it takes less than 30 mins to discharge the cell, we likely talking >2C (which is my max comfort zone). Below 20 mins (i.e. >3C estimate), I am very uncomfortable.
There's a good explanation of C-rate at
battery university.
I did another test running the BC10 at low mode brightness with RCR123 and it managed to last 55mins on the stop watch. It felt pretty hot to touch, and I was wondering if it would cause any damage to the led chip? And lastly ICR battery do not have protected circuit so if I deplete it completely the voltage will drop to 0V. Why would most people recommend the battery to be thrown away? Is it because the battery cant hold any more charge well?
~1C discharge should be fine.
As for a fully drained Li-ion, it MUST be thrown away. For safety purposes, you should never run a cell down below ~2.7V for any sustained amount of time. You should not rely on protection circuits, as they typically trip at the low-mid 2V range (e.g. 2.4V). The reason for this is that under a heavy load, the voltage will drop rapidly once you reach ~3.2V or so, and will hit ~2.4V within seconds. But once the circuit trips and power is cut-off, voltage quickly recovers to >2.7V (i.e typically within a few secs). This likely won't be a problem occasionally (only spending a few secs below 2.7V), but repeated trips to the protection cut-off are not healthy in the long run.
The real danger with protection circuits is in lights that drop to a low moon mode as the battery nears exhaustion. In these cases, you could be running many hours at <2.7V before you ever reach the cut-off feature at ~2.4V. If you let that happen, the cell may never recover back to >2.7V (or it could take hours). Either way, that cell should be tossed, as its storage capacity is permanently damaged.
All this to say, the true danger is in unprotected cells.
These were developed for high-power incadescent operation - where by time you noticed a drop in output, you werel still within a safety margin voltage for the cell. On modern circuit-controlled LED lights, output won't drop until you near that ~3.2V level - at which point it will drop to below 2V in a matter of seconds! The further it drops, the slower will be its recovery to minimum voltage levels (if at all).
The reason you have throw such cells away is not because they can't hold a charge well - it is because the cells become unstable and unpredictable during charging. A high-quality charger will hopefully recognize the low voltage, and refuse to charge. Most chargers (which frankly have charging rates too high for low-capacity RCRs anyway - but that's another matter) will blithely go ahead and hit with a lot of juice. You run the risk of a catastrophic failure during charging (i.e. a fire erupting in the charger!).
The relative risk of that may be low, but it's worth remembering the differing safety profiles of primary lithium cells and rechargeable Li-ion. For primary lithium, your greatest risk is under a current discharge (especially in multi-cell setups where differences in capacity could lead to reverse charging are potentially "venting with flame"). For Li-ions, your greatest risk is when under a current charge, especially if the battery has been damaged by a sustained <2.7V stay.
This is why it's a good idea to invest in a DMM if you plan to use Li-ions. You should always check the resting voltage level of your cells before plunking them in a charger.
Many of the experts in the batteries subforum can explain all this better than I, so I suggest you check that forum out.
