Undocumented JTV battery charger behavior

Started by vtgearhead, October 13, 2017, 07:50:22 AM

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vtgearhead

Definitely a "top thing to know".  A few weeks back I inserted one of my JTV battery packs and noticed LED stayed solid red for a short time (charging state) before going off completely.  The battery was not at full voltage (typically around 8.4v no-load).  Long story short:  It turns out that battery was defective and had apparently developed a high internal resistance on one or more cells.  So it appears that the combination of charge current falling below a certain threshold while the battery voltage remains low results in the LED going out completely.  When I retried with my other battery everything worked as it should (solid red while changing, then slow blink).

Would have been nice if they documented the behavior, but hopefully this will save others some grief.

GiRa

That's actually documented: when the charger thinks that the battery is fully charged (100%) it will provide just a tiny stream of current to keep it there, turning off the led.
When it starts blinking it means that the battery is nearly 100% charged.

vtgearhead

Quote from: GiRa on October 14, 2017, 01:02:26 AM
That's actually documented: when the charger thinks that the battery is fully charged (100%) it will provide just a tiny stream of current to keep it there, turning off the led.
When it starts blinking it means that the battery is nearly 100% charged.

I may not have explained it clearly.  When using a good battery, the LED has two states:  Steady on while charging and slow blink when at or near full charge.  With a properly functioning battery I never see a situation where it's inserted in the charger and the LED is completely off.

With a defective battery, the charger shows a steady (on) LED for a while then goes out completely.  The battery is not at full voltage and shows only 3 of 4 indicators when plugged into the guitar.  It also drains quite quickly.

GiRa

Quote from: snhirsch on October 14, 2017, 07:37:16 AM
With a properly functioning battery I never see a situation where it's inserted in the charger and the LED is completely off.

If you leave the battery in the charger for some hours after it starts blinking, the led will shut off. This happens when the battery truly reaches a full charge.

vtgearhead

Quote from: GiRa on October 15, 2017, 12:53:27 AM
If you leave the battery in the charger for some hours after it starts blinking, the led will shut off. This happens when the battery truly reaches a full charge.

I'll take your word for it, but have never seen this happen on mine - even after the battery is inadvertently left in for days.

jerseyboy

Using 3 JTV battery packs here and only one will completely turn off the charge LED when full.  I also happen to be an EV battery engineer type and have worked with literally 1000's of 18650 lithium can cells. 

Regarding why some packs never turn charge LED completely off I'm guessing probably increased IR or perhaps some imbalance between the 2 cells wired in series (2S)? 

I haven't opened a JTV pack yet but I seriously doubt there's a balancing BMS inside the pack case.  Surely some BMS charge/discharge cutoff protection but probably no balancing/equilibrium function. 

Dang, I should probably just measure IR of these packs to see if that is indeed the reason for the different behavior using the same chargers across 3 packs?  Unfortunately, I'm traveling right now and unable to drag out test gear to measure DCIR. 

Until then if someone desires more info about battery cell IR search youtube for "lipo internal resistance testing"