Charging a Li-ion to a certain level, can this be done?

[cernobila]

I would like to keep my mods simple and low cost, so I am trying to work out if I could charge my Li-ion cells to a given voltage to suit a particular bi-pin lamp. (5761 with 2x Li-ion cells charged to 7.6 - 7.8V.....if left alone, these will charge to 8.3 - 8.4V) I have the usual digital multi-meter and the basic Li-ion chargers. Can I measure the voltage of a cell while it is being charged? and if so, how? Would this affect either the charger or the cells?......as you can tell I am electronically challenged...

 

[DM51]

You could do it accurately with a hobby charger, but with an 'auto' 2-bay one it would be trial and error, timing the charge. By no means impossible to do, but a bit of a PITA.

Bear in mind a 2s Li-Ion pack at 7.6V - 7.8V means each cell is only charged to 3.8V - 3.9V, which means they are only ~50% full. You would probably do better if you could incorporate a regulator of some sort, such as AW's new one.

 

[VidPro]

what he said ^
a diode in series could potentially put a drop in of .5-.7 volts.
and the volt drop by the diode goes out the diode as HEAT, so you would need a good diode for hotwire, and way to remove the heat.

7 ways to do anything, leave it to me to find a cheap easy one
even a "bundle" of 1amp diodes i have used to get a drop, each item of a bundle doesnt get the exact same power or heat, but enough of them way over need, and you can get some passive air cooling of the drop.

 

[mdocod]

I have a MUCH easier solution for ya:

Use a pair of lithium-nano-phosphate or lithium-iron-phosphate cells instead. They have lower initial voltage (~3.6Vish each) and flatter output and are MUCH safer for use with a high current lamp like the 5761, and since you will be able to charge it "all the way" up before use you will get about the same runtime as you would have gotten from a Li-Ion only partially charged to avoid insatflash.

Here's a LiFeP04 18650 that can handle ~15-20A safely.
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=3737

Also you can pick up a pair of A123 systems nano-phosphate cells by picking up a VPX battery from black and decker and cannibalizing it for cells. The A123 systems cells are going to be the better option but more expensive...

Either cell you can technically charge on a regulator li-ion charger, it will overcharge them if you leave em on too long but LiFeP04 and/or nano-phosphate cells are very tolerant to overcharging. IMO just put a wall timer on the charger to kill the charge prematurely without having to monitor closely. Something like a WF-139 should be fine, just set it to about 3 hours for dead cells. shorter charge times for partially used cells.

 

[cernobila]

mdocod, the more I look into this the more I am convinced that the easiest solution is to use the good old ROP High lamp instead.....

According to LuxLuthor's tables;

5761: applied V 7.8, measured Amps 5.8, lumens 1916, temp Fº 167.
ROP High: applied V 8.4, measured Amps 4.8, lumens 1948, temp Fº 110.

Why would I want to go to all that trouble to use the 5761 if the ROP High would give me slightly longer run time at a lower temperature at more or less the same output.....the added bonus is that the ROP will not fall "out" like a bi-pin bulb could.

 

[mdocod]

the 7.8V and 8.4V to the bulb there is right before insta-flash. in those ranges, you have to keep in mind that when dealing with the instaflash territory there is a lot of variation. If he were to test 100 of each bulb I'm sure there would be an "average" insta-flash point different than those represented by a single bulb.. what you need to look at is how much voltage you are actually going to get TO the bulb. generally speaking, the ROP will always be a little dimmer than a 5761 with the same power supply. You won't be driving these bulbs anywhere near the "1900+ lumen" range on any realistic flashlight setup anyways.

If you are dealing with 18650s, you can probably just direct drive a 5761 without much trouble (except the fact you'll be running the 18650s beyond their safe limit), they sag to about 3.6V fresh off the charger under the 5+A load of the 5761 anyways. after a short time they will be below 3.5V each. The problems with instaflash arise when you are dealing with larger scale li-ion cells, like Emoli or AWs "C" size Li-Ion. Those hold their voltage under those loads a lot better and need some kind of soft-start/regulation to fire up 5761s without problems.

Overall though, I think the differences between having an ROP or 5761 mod aren't enough to worry about the difference, I've mentioned this before I'll say it again, it takes large differences in output to really bother "going for it." You can hardly see the difference between 1000 and 1500 lumens. In fact we're more likely to detect differences in color temp than we are in total lumens when dealing with lights that are within 50% of each-other. In practical use, you have to ask yourself what the extra 50% output is really going to do for ya.

 

[cernobila]

the 7.8V and 8.4V to the bulb there is right before insta-flash. in those ranges, you have to keep in mind that when dealing with the instaflash territory there is a lot of variation. If he were to test 100 of each bulb I'm sure there would be an "average" insta-flash point different than those represented by a single bulb.. what you need to look at is how much voltage you are actually going to get TO the bulb. generally speaking, the ROP will always be a little dimmer than a 5761 with the same power supply. You won't be driving these bulbs anywhere near the "1900+ lumen" range on any realistic flashlight setup anyways.

If you are dealing with 18650s, you can probably just direct drive a 5761 without much trouble (except the fact you'll be running the 18650s beyond their safe limit), they sag to about 3.6V fresh off the charger under the 5+A load of the 5761 anyways. after a short time they will be below 3.5V each. The problems with instaflash arise when you are dealing with larger scale li-ion cells, like Emoli or AWs "C" size Li-Ion. Those hold their voltage under those loads a lot better and need some kind of soft-start/regulation to fire up 5761s without problems.

Overall though, I think the differences between having an ROP or 5761 mod aren't enough to worry about the difference, I've mentioned this before I'll say it again, it takes large differences in output to really bother "going for it." You can hardly see the difference between 1000 and 1500 lumens. In fact we're more likely to detect differences in color temp than we are in total lumens when dealing with lights that are within 50% of each-other. In practical use, you have to ask yourself what the extra 50% output is really going to do for ya.

I read you loud and clear on the inability to see the difference when the output is close......originally I was excited with the 5761 but that has kinda worn off considering that I am looking for a bulb to run in a standard 2x C and 2x D Maglites with only the lens and reflector being changed, running on 2x C and 2x D protected Li-ion cells which read 8.3 to 8.4V at the bi-pin adaptor fresh of the charger. I think the good old ROP High has become the bulb of choice in this instance. I don't think I should waste my money and time trying to use the 5761 in my Maglites.

 

[LuxLuthor]

Also, I made this thread in response to your PM to help you take into account the resistance issue. I made it VERY simplistic so people just getting started can start to get their hands around the concepts. Hopefully it can stay simple and easy to read. :kiss:

 

[cernobila]

I read you loud and clear on the inability to see the difference when the output is close......originally I was excited with the 5761 but that has kinda worn off considering that I am looking for a bulb to run in a standard 2x C and 2x D Maglites with only the lens and reflector being changed, running on 2x C and 2x D protected Li-ion cells which read 8.3 to 8.4V at the bi-pin adaptor fresh of the charger. I think the good old ROP High has become the bulb of choice in this instance. I don't think I should waste my money and time trying to use the 5761 in my Maglites.

Well, after LuxLuthor's instruction on here, I have changed my mind on the 5761, I am going to give it a good workout in my 2x D Maglite with Li-ion D twinpack, once I receive the parts.......