Dual Li-Ion cell lights (cpf multi-level)

[mikemo]

I've been playing with my new (to me) CPF multi-level that I got from Wits' End. The light was brighter with cr123 than a 18650. I hooked up a bench supply and took some readings.

V I power
3.0 .074 0.22
3.2 .168 0.54
3.4 .267 0.91
3.6 .392 1.41
3.8 .531 2.02
4.0 .674 2.70
4.2 .827 3.47
4.4 .983 4.33
4.6 1.08 4.95
4.8 1.03 4.92

As voltage increases over 4.8 volts there is a drop in current, with the total power delivered to the light right about 5 watts, all the way to 7 volts (nicely regulated). Brightness appears level over about 4.5 volts (no light meter, just by eye).

So, running the light on an 18650, with its voltage around 3.8 volts, would be (and appears to be) well below the optimum voltage for the light. It appears that running on a pair of rcr123 would be the way to go. What concerns me is this light's flat output as the voltage drops. The light won't start to dim until the voltage each cell is about 2.2 volts, well below the safe level.

I'm wondering how protected cells will behave in this light. A single 18650, while not the brightest, will be safe as the current draw drops to near zero as the cell voltage goes below 3 volts. If I get protected rcr123, what will happen if I run the light until one of the protection circuits opens up? In general this seems like a weakness in any dual cell li-ion light.

Thanks for any advice.
Regards,
Mikemo

 

[mdocod]

this is definitely an issue with buck-style circuits when used with pairs of li-ion cells. As you have said, this thing will run full brightness even when a pair of li-ion cells are severely over-discharged. The issue is not limited to this type of light either, there are numerous 1 cell lights out there than can run on a single RCR123, or single 18650 (designed only for 18650) that are boost/buck style regulation that can also over-discharge the cell, many of the circuits used are designed to stay in regulation all the way down to 0.9V (often 1xAA and 1xCR123 powered lights share the same circuit within a brand), so you can imagine the damage potential....

The trick is to stick to quality protected cells, but on top of that, just be personally mindful of your usage, maybe run it down on the highest setting until the protection kicks in the first time (from a full charge), time it so you know how much runtime you have available on future runs, immediately recharge. Then in the future, try to limit your usage to about 80% (or less) of the available runtime from the cells. On top of that, keep in mind that li-ion can be "topped off" any time, there is no reason to deep cycle a li-ion, so even if you only use it for 5 minutes, it wouldn't hurt to put the cells on the charger for a little while to bring em back up. Shallow cycling li-ion will extend useful life quite a bit. Ideally speaking, you don't want to bump up against the low-volt protection circuit, but it'll be there for you when you accidentally go too long, just try to get the cells back to the charger as quickly as possible if you do trip the protection circuit so it spends the least possible amount of time in deeply discharged state.

Eric

 

[mikemo]

Mdocod,
Thanks for the info. That is what I suspected. I think I'll just stick with the 18650 unprotected cells I have for this light (scavenged laptop cells). By 3 volts the light should be nearly out. I'll have to watch it as I run this light on low a lot during the night.
Regards,
Mike M.