Testing AW P18650 -31 High Current Draw

[Mattaus]

Hi all,

I received today 3 AW P18650-31 cells. These are my first AW cells. The main reason I got them was because I wanted some higher quality 18650 cells than the cheap DX ones I currently have. Another main selling point for me was the 6.2A max continuous discharge.

HOWEVER when I hook them up to some 7135 based drivers I am modifying, I cannot seem to get more than 2.7A out of them. I am expecting 4.5A to the driver when I measure the current draw at the tail. Oddly enough they start at 2.7A and slowly creep up to 3A+ if I leave them running long enough. Do I need to run them through a few cycles first? Or is the quoted 6.2A under different circumstances?

I know the driver is good for 4.5A because my other test pack (4s2p eneloops) can do the rate fine.

Any ideas?

Thanks,

- Matt

 

[Th232]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

What kind of voltage droop do you get when it's hooked up to the driver? Bear in mind that your 4s2p Eneloop pack is a nominal 6V while the li-ion is 4.2V. That might have something to do with it.

 

[Mattaus]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

The Vf of the leds are only 3.4V, so the voltage of the single 18650 should be plenty especially as the drop across the driver is about 0.2V....unless I'm missing something fundamental here?

EDIT: You are correct Th232. I need IMR cells to sustain 4.5A at this voltage from a single cell. Wish I had grabbed one in the last order!

 

[HKJ]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

The Vf of the leds are only 3.4V, so the voltage of the single 18650 should be plenty especially as the drop across the driver is about 0.2V....unless I'm missing something fundamental here?

EDIT: You are correct Th232. I need IMR cells to sustain 4.5A at this voltage from a single cell. Wish I had grabbed one in the last order!

Even with IMR you will be hard pressed to get enough voltage, except for a short time.

Try playing with my battery comparator and the 5A curve.

 

[Mattaus]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

Oh! Nice calculator. Thanks for that.

 

[Justin Case]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

Both the IMR and regular AW 18650s can hold above Vf + 7135 overhead (which BTW is 0.12V, not 0.2V) for plenty of time. In principle, you should easily reach ~4.5a draw. Most likely, your setup suffers from excessive parasitic resistance, which adds to the voltage overhead needed to reach full regulation. Could be the DMM. Hard to say. You also probably have a contact resistance between Batt+ and the driver center spring. And any DMM resistance might be matched by the tailcap resistance when you assemble the light.

I've been saying this for a while. IMO, the 7135 can be a problematical driver when trying to obtain high drive currents. The system parasitic resistance can make it challenging to match battery voltage to LED Vf to maximize driver efficiency while ensuring running in full regulation for a long period of time. Li-ions give you a very coarse voltage increment to play with: 0, 4.2V/3.7V, 8.4V/7.4V,.... So basically, it's 1xLi-ion or bust. With NiMH, you have finer granularity in Vbatt: 0, 1/5V/1.2V, 3.0V/2.4V, 4.5V/3.6V, 6.0V/4.8V, 7.5V6.0V,.... So you could use 3xNiMH or 4xNIMH, depending on the exact light configuration. As discussed by Th232, 4xNiMH gives you a lot more voltage leeway to play with than 1x18650, but with perhaps acceptable driver efficiency.

 

[HKJ]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

Both the IMR and regular AW 18650s can hold above Vf + 7135 overhead (which BTW is 0.12V, not 0.2V) for plenty of time.

This depends very much on the load at 4.5A very few batteries can do it. You are right about the parasite resistance, at 4.5A there will be a voltage drop over any connection and wire. Using a DMM to check the current will be problematic.

For the 7135 the 0.12 V is typical dropout voltage, the output current specification is at 0.2 volt. The dropout voltage is at 90% of current.

 

[Slickseth]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

You may want to consider editing the title of this thread. Since the batteries are clearly not the problem.

 

[Justin Case]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

This depends very much on the load at 4.5A very few batteries can do it. You are right about the parasite resistance, at 4.5A there will be a voltage drop over any connection and wire. Using a DMM to check the current will be problematic.

For the 7135 the 0.12 V is typical dropout voltage, the output current specification is at 0.2 volt. The dropout voltage is at 90% of current.

When I look at the output current vs. dropout voltage curve on the 7135 datasheet, it looks like you get full output to me at less than 0.2V overhead (it looks like full output at 0.12V).

Regardless, even if it is 3.6V min for 7135 full regulation, your data shows that an AW black label 18650-3100 can stay above that for at least 1.25 Ah (excluding the one cell that seems excessively weak, and ignoring statistics of very small numbers), or over 15 minutes. My point was that there should be plenty of time to make a steady state current draw measurement. It's not as if you have a 3 sec window to see if the AW18650 might reach 4.5A draw, after which it's all downhill. Thus, if the OP is not measuring full power even from the start, it probably isn't the fault of the AW18650.

 

[HKJ]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

When I look at the output current vs. dropout voltage curve on the 7135 datasheet, it looks like you get full output to me at less than 0.2V overhead (it looks like full output at 0.12V).

Regardless, even if it is 3.6V min for 7135 full regulation, your data shows that an AW black label 18650-3100 can stay above that for at least 1.25 Ah (excluding the one cell that seems excessively weak, and ignoring statistics of very small numbers), or over 15 minutes. My point was that there should be plenty of time to make a steady state current draw measurement. It's not as if you have a 3 sec window to see if the AW18650 might reach 4.5A draw, after which it's all downhill. Thus, if the OP is not measuring full power even from the start, it probably isn't the fault of the AW18650.

We do not read my data the same way:

 

[Justin Case]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

We do not read my data the same way:

Oh, now I think I see what the issue is. Your battery comparator doesn't color code the curves properly to distinguish what current draw is being tested. So the "stronger" black label cells aren't stronger. They are being tested at 0.2A and 1A draws. The "weak" cell is being tested at 5A draw.

On my screen, all of the black label 18650 curves are red, so I assumed they all represented 5A draws. I didn't pay attention to the check boxes indicating which current draws the user wished to have displayed (apparently I had the default of showing curves for 0.2A, 1A, and 5A).

 

[HKJ]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

Yes "we" do. I specifically stated "excluding the one cell that seems excessively weak".

There are no weak cells in that chart, it has the trace from two different AW18650-3100 batteries and they do match very well.
Using my comparator and comparing them to other 3100 mAh batteries, the AW has a average performance at 5A.

 

[Mattaus]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

OK, so is it now relatively safe to assume that it's not he cell nor the driver that is causing my issue but rather the way in which I am measuring the current draw? I know all DMMs introduce this parasitic effect but my DMM is not exactly top of the range either. The more I think about it the more I see issues with my test rig as well - relatively long connecting wires, poor contact points (alligator clips) and so on.

I've edited the thread title accordingly.

Batteries are not my strong suit. I'm still learning in this department.

Thanks for the replies. It's been much appreciated. If you have anything else to add please do so!

- Matt

 

[F250XLT]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

Are you testing cells for the triples you are currently making? If so, I'd like to know if the best was to go is IMR or Protected 3100's.

 

[Mattaus]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

Are you testing cells for the triples you are currently making? If so, I'd like to know if the best was to go is IMR or Protected 3100's.

Yes - I've replied to the sales thread with my opinion on the matter which is based heavily on this thread.

Honestly my personal preference is to use an IMR cell, but several people are reporting no issues with the same cell as I am testing here and some of the popular quad drop-ins.

- Matt

 

[Justin Case]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

Post deleted.

 

[Mattaus]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

Now that I think I understand how to use HKJ's excellent battery comparator tool, it seems clear that the AW18650-3100 isn't up to the task of delivering 4.5A at a min of Vf+7135 overhead (much less Vf+7135 overhead+parastic resistance overhead). Even if you go with IMR18650s, you'll stay above just Vf+7135 overhead ~ 3.5V for about 1Ah worth, or about 1/4.5 hours ~ 13 min. If you need to hold above 3.6V, that corresponds to about 0.63Ah, or a run time of about 0.63/4.5 hours ~ 8 min. Any appreciable parasitic resistance, and you're cooked. Just 0.1 ohms translates to 0.45V, and you'll need a Vbatt of at least 3.4V + 0.12V + 0.45V ~ 4v. Basically, IMR18650 run time in regulation under those conditions is zero.

You will probably have to stick with 4S NiMH cells or possibly bigger IMRs.

Bigger IMR's it is. The eneloops are purely for testing - I have no host that can take them. Thankfully people are fully aware of what will and will not work now so my issue is resolved and I at least have a much better understanding of what's happening here!

 

[qwertyydude]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

One thing many people don't consider is the resistance of their multimeter. The leads are usually made of small gauge wire. I replaced mine with ones I made with just 14 gauge wire soldered to banana plugs with just a bare soldered wire on the end. Each lead is only 1 foot long. Lights I previously thought consumed only 2.5 amps and thought they were running weak ended up running at 3.5 amps with the new leads.

 

[Justin Case]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

Post deleted.

 

[Mattaus]

Re: AW P18650 -31 not even able to supply 4.5A continuous...

If no other part of the lighting system adds more parasitic resistance, then Vbatt of about 3.5V or so should be enough to reach full regulation. Even the AW18650 holds above 3.5V for a short period of time -- about 0.5Ah worth, or about 6 min. In theory, that should be plenty of time to get a good current draw reading. Since the DMM measurement came in low, indeed the measurement issue could be the DMM. But the larger issue is that the AW18650 cell is just not up to the task, unless a 6 min run time in full regulation is acceptable.

It's not great, but it's better than NEVER hitting my desired goal. More experimentation is in order of course, but for now it's IMR all the way for small advantage it will offer.