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3.2v 26650 LiFePo4 testing
#1
Question 
Anyone have any recommendation on equipment to test 3.2v LiFePo 26650 batteries?  I have a Lii-500 but it only seems to do 3.7v testing.
I have looked on Amazon and Ebay, but everything I have seen so far only seems to charge 3.2v but not do capacity testing.

rearden
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#2
OPUS BT-C3100 and 3400
set to 3.7 v max charge
later floyd
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#3
(07-15-2020, 11:59 PM)floydR Wrote: OPUS  BT-C3100 and 3400
set to 3.7 v max charge
later floyd
Yea you can open the OPUS and inside on the circuit board there is a marked slide switch to change it over Smile
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#4
I cut the opening without opening up the opus cut most of the way though with a rotary cutter(harbor freight)then used a razor knife to cut the rest of the way on three sides and wiggled the piece of plastic until it broke away. Floyd ends up with extra screws or too few screws often enough if i don't need to take something apart I don't.
later floyd
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#5
I'm talking about the OPUS 'hidden switch'.   Just remove the 4 screws on the bottom and life up the bottom plate to see it.   Here's a pic 


and the procedure is detailed in this youtube from one of our all time favorites @AverageJoe - https://youtu.be/GDUikK7jQoo

The 4.2 is 18650.  The 3.7 is for LifePO4.
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#6
(07-16-2020, 01:20 AM)OffGridInTheCity Wrote: I'm talking about the OPUS 'hidden switch'.   Just remove the 4 screws on the bottom and life up the bottom plate to see it.   Here's a pic 


and the procedure is detailed in this youtube from one of our all time favorites @AverageJoe - https://youtu.be/GDUikK7jQoo

The 4.2 is 18650.  The 3.7 is for LifePO4.
Anyone found a similar switch for the LiitoKala Lii-500?  Just wanted to make sure before I shell out cash for the Opus.

rearden
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#7
It deserves to be better known that if we only need to to know the capacity under reasonably low current loads then we can simply use the (full) charge capacity, because (unlike NiMh), Li-ion chemistry has very high (Coulombic) charge efficiency (> 99%), so charge_in = charge_out (within 1%, for low rate discharges). 

Otoh, for high rate discharges we cannot access the full (chemical) capacity because high current I causes a high voltage drop I*R  (R = total internal resistance), which causes the discharge to terminate before the cell is actually empty. In this case we need to do a discharge at the high rate to determine the usable (vs. chemical) capacity at that rate (or else use simulation software and adequate data - which is what fuel gauges do to dynamically estimate remaining charge/time).

Re: Opus BT-C3100/C3400 hidden internal chemistry switch. Based on remarks made by Henry Xu (Opus lead engineer), my impression is that the reason this switch remains undocumented and unexposed is that it was not fully tested (and possible other issues). Iirc they did fix some problems with LiHV (4.35) in some later versions, but I don't recall much feedback on LiFeP04, so it may actually be little tested (by both Opus and users). So proceed with caution - you may be a guinea pig. While LiFEPO4 is generally more tolerant of abuse than other Li-ion chemistries, it is not immune to venting and flaming.
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#8
the opus works extremely well on the 26650, I tested over 200 cells (A123 cells). The only drawbacks is opening the unit to reach the switch, and also you have to use external 26650 holders and connect them to the opus so you can do 4 cells at a time. I have 2 opus and would definitely use them again to test lifepo4 cells.
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#9
(07-16-2020, 04:22 PM)gauss163 Wrote: It deserves to be better known that if we only need to to know the capacity under reasonably low current loads then we can simply use the (full) charge capacity, because (unlike NiMh), Li-ion chemistry has very high (Coulombic) charge efficiency (> 99%), so charge_in = charge_out (within 1%, for low rate discharges). 

Otoh, for high rate discharges we cannot access the full (chemical) capacity because high current I causes a high voltage drop I*R  (R = total internal resistance), which causes the discharge to terminate before the cell is actually empty. In this case we need to do a discharge at the high rate to determine the usable (vs. chemical) capacity at that rate (or else use simulation software and adequate data - which is what fuel gauges do to dynamically estimate remaining charge/time).

Re: Opus BT-C3100/C3400 hidden internal chemistry switch. Based on remarks made by Henry Xu (Opus lead engineer), my impression is that the reason this switch remains undocumented and unexposed is that it was not fully tested (and possible other issues). Iirc they did fix some problems with LiHV (4.35) in some later versions, but I don't recall much feedback on LiFeP04, so it may actually be little tested (by both Opus and users). So proceed with caution - you may be a guinea pig. While LiFEPO4 is generally more tolerant of abuse than other Li-ion chemistries, it is not immune to venting and flaming.

What discharge rates do you consider to be high current? 
When does the effect of the Internal Resistance start to become significant?

rearden
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#10
Discharge rates will depend on the manufacturer specs - some list higher currents, some lower - so it'd be relative to those specs
Running off solar, DIY & electronics fan :-)
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