J720 Battery Rebuild

I'm dangerously close to buying many of those ultrafire batteries. So many of my devices need cells. Probably need about about 30 cells all told. Are they good or rejected sanyos? or?

Ok, no answer from anyone so I got off my lazy backside and googled.

It appears they are either really good or junk. Not much middle ground. I bought 24 so after weeding out the duds if I get 18 I`ll be pleased.

$5 each / net if I get my 18 working cells. ha. I wonder how many hours my 6651 will run with wifi on? 8-10-12? we'll see.

I have a few ultrafire batteries in various flashlights....though not the cells that have tabs attached. My experience so far has been 100% +ve, though the sample size is small.
I bought mine through dealextreme.com, and they replace faulty items, and shipping is always free.

I hate to revive such an old thread but I am having a problem after replacing the cells in my J720 pack

The 720 shows full but after an hour or so of use it shuts off as if the batteries are dead. I plug it in to charge and within a few seconds the charge light goes green.

I watch a video from handheld computing where he mentions how to reset the battery by removing all the batteries, then putting the battery back in and then plugging in to charge but same results.

There is also mention from MrSB of a "diagnostic mode" but I can't find a reference to how to access it or if it would do any good.

Im stumped

If you drop a multi meter on the cells once fully charged and then again after it blinks out, what does the voltmeter say? Have they actually discharged?

The Jornada diagnostic mode is entered by soft resetting and immediately jabbing S on the keyboard.

Did you connect all three wires properly?

Sorry, just asking. Sometimes I overlook the obvious in my own projects, too

Think you need to determine that the problem is in fact the battery.

Does unit shut off when battery is removed and running on AC power? Do you have another battery that you can test to see if that one too shuts off?

Did you test the cells before installing them in the rebuild? I actually buy my batteries these days from Vape shops...they tend to have good quality capacity for their ecigs and the prices is usually cheap. And I then charge them externally before rebuilding my pack and then test them using this device I bought off eBay for less than $15.





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Following up on this, I was able to resolve the problem by going into diagnostic mode using the “S” after soft reset. It showed 7.x volts resting and 8.x volts while charging.

I decided to set several tests to loop over and over and left it overnight to deplete the battery fully. The next morning I plugged it in before work and it powered on showing 0-1 charge bar in the OS. I reset in diag mode again and left it charging all day and now it shows a full charge in the OS . I won’t be charging it again until I let it deplete all the way again and see how it goes.

Thank you for the responses everyone. For whatever reason I’m not getting notified when there is a response. Looking into it

Edited by OJLukeSkywalker 2023-05-13 11:29 PM

Came across this thread while recelling my 900C and was shocked to see so many people buying ultrafire lithium-ion cells as replacements. Please never buy or use ultrafire cells; they're either cheaply made and dangerous or harvested from old packs, at the end of their lifespan, and dangerous. Any 18650 cell that says it can store over 4000 mAh is lying to you, and quality cells made by Samsung, LG, Panasonic, Vapcell, or Molicel from a reputable seller are only around $6-$8 each, which is much less than you'll lose if an ultrafire lives up to its name.

Edited by Fauxreigner 2023-10-09 4:50 PM

Interesting links. The more interesting ones might be the articles that point out how to find a good cell https://lygte-info.dk/info/battery%20protection%20UK.html and https://lygte-info.dk/info/isMyBatteryProtected%20UK.html