Best18650 battery to maintain 3 amp draw

I just got my nailbender xml dropin designed to use a single 18650 battery, and my tailcap measurements where at 2.8 amp then as the the battery voltage went down to 3.9 v it is only drawing 2 amps. Does this mean that the battery is not capable of sustaining the 2.8 draw for the whole cycle of the battery. The driver is regulated so shouldn't it mean it should draw 2.8 amps regardless of voltage. I also am curious about how voltage sag when the battery is under a 2.8 amp load affects the battery. I am currently thinking about getting new batteries that can handle the high draw better. Thanks.

You can always look at my 18650 battery comparisons to see how the different cells handle a high current load (2011 test, 2010 test).

A regulator/driver has a minimum voltage where it can keep the specified current, this depends on the actual design.

daguy80 said:

I just got my nailbender xml dropin designed to use a single 18650 battery, and my tailcap measurements where at 2.8 amp then as the the battery voltage went down to 3.9 v it is only drawing 2 amps. Does this mean that the battery is not capable of sustaining the 2.8 draw for the whole cycle of the battery. The driver is regulated so shouldn't it mean it should draw 2.8 amps regardless of voltage. I also am curious about how voltage sag when the battery is under a 2.8 amp load affects the battery. I am currently thinking about getting new batteries that can handle the high draw better. Thanks.

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I'm not qualified to answer all your questions, but I do know that LiMN batteries are best suited for high current demand and result in less voltage sag than Li-ion batteries. However it would seem to me that a good quality Li-ion 18650 could handle that current draw without problems unless the cell is old. If you buy new cells, I would recommend AW IMR 18650 batteries. They have the safer LiMN chemistry, and handle high current loads very well.

Most likely, the drop-in uses a 7135-based driver and you are running out of voltage head room to stay in regulation. It's not just Vbatt. It's also how much voltage you lose from various parasitic sources like the tailcap, the springs, and the battery contacts. You might look into vanIsleDSM's FETie switch.

I am curious about IMR cells. How safe are they? Is there any chance at all of these cells venting and exploding like regular lithium cells? Let me know thanks.

daguy80 said:

I am curious about IMR cells. How safe are they? Is there any chance at all of these cells venting and exploding like regular lithium cells? Let me know thanks.

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They're considered much safer than LiCo cells and thus do not need or have protective circuitry. Another plus to them is their capability to handle high current draw. I use an AW IMR 18650 in a Surefire C2 with a Kerberos quad light engine. Works great! The one downside to IMR cells is slightly lower energy density compared to LiCo cells resulting in shorter runtime, but not dramatically so.

daguy80 said:

I am curious about IMR cells. How safe are they? Is there any chance at all of these cells venting and exploding like regular lithium cells? Let me know thanks.

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They are safer, but not completely safe and their capacity is half of the best normal LiIon cells.

HKJ said:

They are safer, but not completely safe and their capacity is half of the best normal LiIon cells.

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What would be some of the dangers involved with IMR batteries. How are they safer then regular LiIon cells?

daguy80 said:

What would be some of the dangers involved with IMR batteries. How are they safer then regular LiIon cells?

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The do not explode if overcharged, but they still has a problem with reverse charge. In the "Smoke and Fire..." there is a thread where part of a big light (6 or 8 18650 IMR cells) went through a heavy glass plate.

Where can I get more info on the shining beam driver that is used in my xml dropin. I can't seem to be able to find it on shining beams site.

I don't know about the particular part you are referring to, but Bryan pulls the listing if he does not have stock of a particular product.

XML's have a very low vF. So any quality 18650 protected cell will have no problem delivering 3A for the majority of the discharge.

jasonck08 said:

XML's have a very low vF. So any quality 18650 protected cell will have no problem delivering 3A for the majority of the discharge.

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Can you please tell me what this means?

The SB 2.8 is out of stock; you can find them listed at illumination supply, but they may also be out of stock ATM. KD also has basically the same driver.

HKJ said:

You can always look at my 18650 battery comparisons to see how the different cells handle a high current load (2011 test, 2010 test).

A regulator/driver has a minimum voltage where it can keep the specified current, this depends on the actual design.

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I was looking at your battery test and to meet it seems like the redilast 2600 was the best one in hold the voltage. Correct me if I am wrong. If it is the one that seems to hold the voltage the best, would that mean that under load, it would be the best battery for my case since my drop in is drawing around 2.8 amp and in order for it to be regulated, the voltage under load must be 3.6 volts or higher. Let me know thanks. I also really like the redilast 2900 mah batteries but they to have more voltage sag when current draw is high.

daguy80 said:

I was looking at your battery test and to meet it seems like the redilast 2600 was the best one in hold the voltage. Correct me if I am wrong. If it is the one that seems to hold the voltage the best, would that mean that under load, it would be the best battery for my case since my drop in is drawing around 2.8 amp and in order for it to be regulated, the voltage under load must be 3.6 volts or higher. Let me know thanks. I also really like the redilast 2900 mah batteries but they to have more voltage sag when current draw is high.

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You are correct, the Redilast 2600 is the best at 2A load, when the load goes up a IMR cell might be better (but has much lower capacity). The 2900 also does well at 5A load.

daguy80 said:

Can you please tell me what this means?

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According to the datasheet, a Cree XM-L LED has a forward voltage of about 3.3V at a forward current of 2.8A. 3.3V is fairly low. If the driver is 7135-based, the driver's voltage overhead to reach regulation is only about 0.1V, so in theory you should stay in regulation with Vbatt down to ~3.4V under load. In reality, this doesn't happen because the flashlight has real-world parasitic resistances (e.g., tail switch contacts, spring contacts, battery contacts), which reduce the available voltage to the driver.

When you cite your 3.9V figure in post #1, is that a Vbatt under load, or did you pull out the cell and measure its voltage without a load applied?

What brand of 18650 are you using?

Have you cleaned all of your contacts, including the tail switch? If your 3.9V measurement is an open circuit voltage measurement, then you may very well have well over 50% cell capacity remaining. If we look at hkj's 2011 discharge curves for 2A draw and assume that your cell is approx 50% discharged, we see that voltage under load could range from a high of about 3.6V and a low of about 3.4V, with the SpiderFire and AsuCell curves being outliers and even lower in voltage under load. Depending on the actual Vin to the driver at which if falls out of regulation, a Redilast 2600 might help a lot or not much at all.

I'd clean the contacts first to try to eliminate parasitic resistance, and thus voltage drop, to the extent possible. If you follow the link I provided above for vanIsleDSM's FETie switch, you'll see his curves that demonstrate the value of reducing the tail switch resistance essentially to zero.

The fact that your tail current measurement drops to 2A suggests that for your 3.9V measurement for your 18650, the actual Vin to the driver may be down about 0.05V to 0.07V under the voltage overhead required to stay in regulation (assuming that your DMM measurement didn't add too much resistance, such as from the probe wires, to skew the results). That is, instead of delivering at least ~3.4V to the driver, you may be delivering ~3.35V. As you can see, it doesn't take much to fall fairly far out of regulation.

HKJ said:

The do not explode if overcharged, but they still has a problem with reverse charge. In the "Smoke and Fire..." there is a thread where part of a big light (6 or 8 18650 IMR cells) went through a heavy glass plate.

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I belive that was a TK Monster equipped with Sony Konion cells. At least that's the only thread I found with an exploded light that went through heavy glass. I remember reading that the first time around and assuming they were LiCo cells. Definitely not the expected behavior, I wonder if they were genuine sony IMR cells or fakes with regular LiCo chemistry. Still with 8 batteries I suppose even relatively "minor" venting across all 8 batteries could cause a large pressure buildup.

For the person who asked how much safer IMR is, I'd say an order of magnitude. Those and LiFePO4 batteries are the only ones I'd use in series configuration. The most important thing though is making sure that you're using cells of good quality and practice safe charging/maintenence habits. At 3 amps I'd feel uncomfortable with a LiCo cell.

Justin Case said:

According to the datasheet, a Cree XM-L LED has a forward voltage of about 3.3V at a forward current of 2.8A. 3.3V is fairly low. If the driver is 7135-based, the driver's voltage overhead to reach regulation is only about 0.1V, so in theory you should stay in regulation with Vbatt down to ~3.4V under load. In reality, this doesn't happen because the flashlight has real-world parasitic resistances (e.g., tail switch contacts, spring contacts, battery contacts), which reduce the available voltage to the driver.

When you cite your 3.9V figure in post #1, is that a Vbatt under load, or did you pull out the cell and measure its voltage without a load applied?

What brand of 18650 are you using?

Have you cleaned all of your contacts, including the tail switch? If your 3.9V measurement is an open circuit voltage measurement, then you may very well have well over 50% cell capacity remaining. If we look at hkj's 2011 discharge curves for 2A draw and assume that your cell is approx 50% discharged, we see that voltage under load could range from a high of about 3.6V and a low of about 3.4V, with the SpiderFire and AsuCell curves being outliers and even lower in voltage under load. Depending on the actual Vin to the driver at which if falls out of regulation, a Redilast 2600 might help a lot or not much at all.

I'd clean the contacts first to try to eliminate parasitic resistance, and thus voltage drop, to the extent possible. If you follow the link I provided above for vanIsleDSM's FETie switch, you'll see his curves that demonstrate the value of reducing the tail switch resistance essentially to zero.

The fact that your tail current measurement drops to 2A suggests that for your 3.9V measurement for your 18650, the actual Vin to the driver may be down about 0.05V to 0.07V under the voltage overhead required to stay in regulation (assuming that your DMM measurement didn't add too much resistance, such as from the probe wires, to skew the results). That is, instead of delivering at least ~3.4V to the driver, you may be delivering ~3.35V. As you can see, it doesn't take much to fall fairly far out of regulation.

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The measured voltage at 3.9 was under no load. I would assume that under load it may be much less. According to my understanding of your answer above, the driver would stay in regulation down until 3.4 volts because the forward current is 3.3 volts for a xm-l led even though the driver is rated to stay in regulation until 3.6 volts only. I am using cheap blue trustfire 18650 batteries. So when the driver is out of regulation does it just run direct drive after that until the battery is too low? Do you have any suggestions to best clean my contact points?

I use isopropyl alcohol. Others swear by the various DeoxIT products.