Rechargeable batteries in WMLs

[RevolverRob]

Surefire was, for many years (and may still be) pretty bad about not building their heads to handle input voltage much above the nominal voltage of whatever the original battery spec was. Plenty of roasted heads from 2xRCR123s.

Streamlight has generally been better about building lights that take a wider range of input voltage. BUT they've been really bad about telling you what the limits of a given head/design are.

Surefire LiFePO batteries have a fairly low capacity (450mah) compared to ~1500mah of a standard CR123. So, they simply will not run as long overall. They do have slightly higher initial voltage to a CR123, so they may have slightly higher initial output, but the simple fact is - they are going to drain quickly. Dimmer lights are almost certainly a function of voltage decrease due to draining the battery.
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RE: Head Design - it's complicated. But as I understand it, most flashlights are designed with variable voltage regulators to make them more-or-less constant current. This is why light output is voltage dependent, not current dependent. Otherwise, you could drop in a 1500mah battery of the same size as a 750mah battery and get more light, instead of what you get, which is more run time. However, most lights also have a direct-drive circuit. So, once the voltage drops below a certain level, the light goes to direct drive and drains the battery until the battery's shut down circuit kicks in (2.5v for the batteries we've been discussing here, about 1.0-1.1v for AA and AAA).

ETA: Direct Drive is just like our old flashlights back in the day. You put in fresh batteries and it starts sucking them down and within a few minutes you've got very dim/no output. The one advantage of LEDs here is they are much more energy efficient than incan bulbs. So you might get quite a bit more life from a light in direct drive today than you did in Grampa's old Everready.

RE: Input voltage and nuking heads - Most heads have controllers and circuits designed to handle a specified range of input voltage but many do not have a built in shut down protection if voltage above that level is input. So, you can get rapid overheating resulting in melt-down that takes time, sometimes you just get insta nuked circuits. Either way, you're probably not having a fun day.

One advantage to this though is...it gives us a quick way to gauge if you're pushing the limit. Higher voltage = More Heat. And circuits that aren't designed to handle the higher voltage heat up faster and hotter. So, if you observe a rapid increase in temperature, that is significantly larger than the stock voltage battery - you're probably pushing the limits input voltage of your head.

Hypothetical example: A light using a 3.2v CR123 it takes 120-seconds of constant on, on High to reach 100ºF on the external surface of the head. Same light with a 3.7v 16340 30-seconds on High to reach 100ºF external temp. That would tell you that the head can handle short bursts of 3.7v input, but it reaches a high operating temperature very fast. Probably not what you want overall. If by contrast you toss in a 3.7v 16340 and it takes 110-seconds to reach ~100ºF - well, that would suggest the light can handle that input voltage pretty well.