High drain (30A+) IMR's in medium/low drain applications (8v)

reo-nagumo

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Jun 28, 2014
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2
Hey all, please let me describe my current situation. But first off, if this has been discussed before, please just point me in that direct and tell me to shut up, since i searched but did not see what I need.

I am currently replacing a custom made battery pack made with two 18650s almost exactly like this: http://www.amazon.com/dp/B002Y2LJW0/?tag=cpf0b6-20

As the batteries on the inside of the pack have gone bad, and the pack uses a specific protection IC, temperature control as well as plug onto the device's PCB, so I was hoping to just replace the batteries. The device always runs at 7.5v to 8v, never any higher or lower.

My plan was to take two 18650s to a Batteries Plus and get tabs spot welded on so I could solder that batteries to those after disposing of the dead ones.

Now the stock batteries in the pack are an unknown brand but they are both 2200mAh IMR 18650 in series.

Now my dilemma is whether I prefer to use some VTC5's or Panasonic NCR18650BD. The Panasonic has the benefit of being 3200mAh over 2600mAh and I would really love the extra power to be able to use the device longer without charging. However I have read that the composition of the NCR18650BD and similar batteries are not IMR and thus not safe for high draw applications.

I suppose one of my two questions here is if the "low draw" Panasonic 18650s with 3200/3400mAh are not safe for constant use at 8v~ in series. I know the VTC4/VTC5/LG HE2 very well and like all three of those a lot.

My second question is if high drain batteries (VTC4/5/LG HE2 and other 30A/35A batteries) actually last longer when pulling less amps. This sounds too good to be true. I'm not sure I am convinced if I took a 10A 18650 @ 2500mAh and a 30A 18650 @ 2500mAh and drained them both at the same rate (2A, ~7.4v) that the 30A one would last notably longer. I would tend to think a 10A battery with bigger capacity would provide more life than a 30A with a smaller capacity if the device in question does not pull more than 2 amps.

Thanks for all your help very much in advance guys.
 
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vicv

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Mar 22, 2013
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2,969
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Southern Ontario
You're basically right. If the device only draws 2A you're better off with higher capacity batteries. Especially if it's a regulated circuit. If the device depends on the voltage and isn't regulated you're probably better off with the Imr cells and they'll provide higher voltage and better voltage throughout the discharge and they'll be safer to boot
 

ShineOnYouCrazyDiamond

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Aug 18, 2009
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4,442
Location
CT, USA
I suppose one of my two questions here is if the "low draw" Panasonic 18650s with 3200/3400mAh are not safe for constant use at 8v~ in series. I know the VTC4/VTC5/LG HE2 very well and like all three of those a lot.

This part of your statement make no sense at all. Voltage is different than current. Any two 18650 batteries in series will be safe for constant use in series. Their cumulative voltage will be about 8.4v at full charge, but nominally closer to 7.4v (3.7v*2).

You don't really state how much your application draws throughout the whole post which is really what you should be focusing on rather than voltage. So what is your amp draw? Is it 2Amps? That's not really "low" as it will drain these 18650 batteries in 1-1.5 hours. What level of LVC drain protection does the electronics have in place to prevent over draining the batteries? You have two batteries in series and IMR cells have no protection so it would be possible to over-drain one of the cells during the run. Protected 3400 batteries would give you better performance.

Now, it sounds like a no brainer that 3400mAh cells would be the best bet for your application. But - based on the higher voltage you are talking about maybe you'd be better off in this application with some of the higher capacity 3.35V cells. You'd get more usable capacity.
 
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