Discharging 18650 at 2C?

[jsr]

I know the typical max continuous discharge for Li-Ion cells is 1.5C, but I've read posts from various people saying how the RC people will discharge their cells at much higher rates. I have a project in mind that I may need to discharge the cells at near 2C (about 4-4.2A). Would this high a discharge pose a danger? The cells are protected and rated at 2200mAh.

Thanks.

 

[barkingmad]

I know the typical max continuous discharge for Li-Ion cells is 1.5C, but I've read posts from various people saying how the RC people will discharge their cells at much higher rates. I have a project in mind that I may need to discharge the cells at near 2C (about 4-4.2A). Would this high a discharge pose a danger? The cells are protected and rated at 2200mAh.

Thanks.

Should not really be an issue - especially if they are protected as the protection 'should' kick in if the current draw is too great - but worth checking with the cell supplier for the particular cells you are using...

 

[LEDite]

jsr;

A few top brand unprotected cells will tolerate 2C; but you are pushing the maximum limit.

If they get too hot, the internal pressure disconnect will activate and the cell will no longer operate at all.

I have been most successful with Sanyo 18650 2400mah cells in my RC trucks. They should handle 4 amps OK.

Larry Cobb

 

[jsr]

Thanks guys. My cells are Protected, so does that make a difference on the max limit? The retailer I purchased them from spec'd it at 1.5C max discharge, but that seems to be the standard (everyone's, even AW's high drain cells, are rated at 1.5C...seems a generic spec that doesn't really tell the true max).

 

[LEDite]

jsr;

I don't think the protected cells will handle the discharge.

Sanyo has a discharge graph @ 4.2A on their web page for the cell I mentioned.

Larry Cobb

 

[DM51]

It depends on the discharge profile. If you are going to be discharging the cells in a series of short bursts, then you should be OK. However if you are going for continuous discharge from full to empty, which I think is what you have in mind, you will be pushing the cells hard. The manufacturer puts those specs there for a reason, although they are bound to err on the side of caution.

You could try it once, to see what happens, as long as you monitor things closely — watch out for overheating etc. Take a note of the discharge time, from full to empty. You say the rated capacity of these cells is 2200mAh; you will get a better idea of the true capacity under a heavy load when you time the discharge. It is likely to be quite a bit less than the rated figure. This is not implying that the manufacturer has given false specs — it is normal that the capacity under load is different.

Discharge rates with times from full to empty are as follows:
60 mins – 1C
40 mins – 1.5C
30 mins – 2C
24 mins – 2.5C
20 mins – 3C
15 mins – 4C

My guess is that you will end up with a true figure of ~2.5C, which is high, and would imply a cell capacity of ~1,750mAh under this load. It should not be actually dangerous if you are careful, but your cells will be short-lived and you should be particularly careful when recharging them, as that is when most problems occur.

 

[Fallingwater]

I know the typical max continuous discharge for Li-Ion cells is 1.5C, but I've read posts from various people saying how the RC people will discharge their cells at much higher rates

RC people (of which I am one) discharge at high rates lithium polymer packs, not 18650 LiIon cells.
LiPo cells for RC use are made specifically to support high power drains (the cheapest LiPos are usually rated at 6 to 8C, the more expensive ones can get to 20C and more), and lack any protection that could interfere with that.

The protection in your cells will probably kick in too early for the use you're planning.
You can either use unprotected 18650s or RC grade LiPo packs (which won't fit in a cylindrical body, of course)...

 

[barkingmad]

Or why not just use larger cells in the first place - think the AW 'C' cells are rated at 3300mah so even at 1.5C it should give you about 5A?

 

[Paul_in_Maryland]

The retailer I purchased them from spec'd it at 1.5C max discharge, but that seems to be the standard (everyone's, even AW's high drain cells, are rated at 1.5C...seems a generic spec that doesn't really tell the true max).

Wolf Eyes, Pila, and AW's larger protected cells made since 2006 can all sustain at least 2C.

 

[jsr]

I would *like* to discharge at 2C from full to empty, but it's not necessary. I'm looking to do a Tri-Cree setup, but the light does have a 2-stage switch, so it doesn't have to be only on high at 2C discharge all the time. I'm looking to use 3 of the 1.4A AMC7135 drivers (1 for each LED), so that equates to 4.2A or almost 2C of a 2200mAh rated cell (which will be lower than that at 2C discharge I'm sure).

What do you guys think the longest continuous runtime would be to stay safe (I know, it's just a guesstimate)? 5min? 10min? Otherwise, I will just use the 1A drivers instead, but was thinking 3 Crees running off 1.4A each would be insane in a pocketable torch.

The host light fits a 18650, so a C-Li-Ion is a no go.

 

[barkingmad]

I would *like* to discharge at 2C from full to empty, but it's not necessary. I'm looking to do a Tri-Cree setup, but the light does have a 2-stage switch, so it doesn't have to be only on high at 2C discharge all the time. I'm looking to use 3 of the 1.4A AMC7135 drivers (1 for each LED), so that equates to 4.2A or almost 2C of a 2200mAh rated cell (which will be lower than that at 2C discharge I'm sure).

What do you guys think the longest continuous runtime would be to stay safe (I know, it's just a guesstimate)? 5min? 10min? Otherwise, I will just use the 1A drivers instead, but was thinking 3 Crees running off 1.4A each would be insane in a pocketable torch.

The host light fits a 18650, so a C-Li-Ion is a no go.

Is a single larger cell (i.e. AW 'C' cell) not an option - or perhaps run each LED off a separate cell?

 

[LuxLuthor]

It depends on the discharge profile. If you are going to be discharging the cells in a series of short bursts, then you should be OK. However if you are going for continuous discharge from full to empty, which I think is what you have in mind, you will be pushing the cells hard. The manufacturer puts those specs there for a reason, although they are bound to err on the side of caution.

You could try it once, to see what happens, as long as you monitor things closely — watch out for overheating etc. Take a note of the discharge time, from full to empty. You say the rated capacity of these cells is 2200mAh; you will get a better idea of the true capacity under a heavy load when you time the discharge. It is likely to be quite a bit less than the rated figure. This is not implying that the manufacturer has given false specs — it is normal that the capacity under load is different.

Discharge rates with times from full to empty are as follows:
60 mins – 1C
40 mins – 1.5C
30 mins – 2C
24 mins – 2.5C
20 mins – 3C
15 mins – 4C

My guess is that you will end up with a true figure of ~2.5C, which is high, and would imply a cell capacity of ~1,750mAh under this load. It should not be actually dangerous if you are careful, but your cells will be short-lived and you should be particularly careful when recharging them, as that is when most problems occur.

Wolf Eyes, Pila, and AW's larger protected cells made since 2006 can all sustain at least 2C.

Good information! :thumbsup: