3.2v 26650 LiFePo4 testing


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rearden

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Feb 11, 2019
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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
 

OffGridInTheCity

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Dec 15, 2018
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floydR said:
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 boardthere is a markedslide switch to change it over :)
 

floydR

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Aug 23, 2017
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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
 

OffGridInTheCity

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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

image_ushufx.jpg


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 forLifePO4.
 

rearden

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Feb 11, 2019
Messages
35
OffGridInTheCity said:
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

image_ushufx.jpg


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 forLifePO4.
Anyone found a similar switch for the LiitoKala Lii-500? Just wanted to make sure before I shell out cash for the Opus.

rearden
 

gauss163

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Jun 28, 2020
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It deserves to be better known thatif weonly need toto know the capacity under reasonably low currentloads then wecan simply use the (full) charge capacity, because (unlike NiMh), Li-ion chemistry has very high (Coulombic) charge efficiency (> 99%), socharge_in =charge_out(within 1%,for low rate discharges).

Otoh, for high rate discharges we cannot access the full (chemical) capacity becausehighcurrent I causes a high voltage drop I*R (R = total internal resistance), whichcauses the discharge to terminate before the cell is actually empty. In this caseweneed to do a dischargeat the high rateto determinethe 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 remarksmade by Henry Xu (Opus lead engineer), my impression is that the reason this switch remains undocumentedand unexposedis that it was not fully tested (and possible other issues). Iirc they did fix some problemswith LiHV (4.35) in some later versions, but I don't recall much feedback onLiFeP04, 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.
 

jonyjoe505

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Feb 28, 2018
Messages
229
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.
 

rearden

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Feb 11, 2019
Messages
35
gauss163 said:
It deserves to be better known thatif weonly need toto know the capacity under reasonably low currentloads then wecan simply use the (full) charge capacity, because (unlike NiMh), Li-ion chemistry has very high (Coulombic) charge efficiency (> 99%), socharge_in =charge_out(within 1%,for low rate discharges).

Otoh, for high rate discharges we cannot access the full (chemical) capacity becausehighcurrent I causes a high voltage drop I*R (R = total internal resistance), whichcauses the discharge to terminate before the cell is actually empty. In this caseweneed to do a dischargeat the high rateto determinethe 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 remarksmade by Henry Xu (Opus lead engineer), my impression is that the reason this switch remains undocumentedand unexposedis that it was not fully tested (and possible other issues). Iirc they did fix some problemswith LiHV (4.35) in some later versions, but I don't recall much feedback onLiFeP04, 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
 

Redpacket

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Feb 28, 2018
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Discharge rates will depend on the manufacturer specs - some list higher currents, some lower - so it'd be relative to those specs
 

Crimp Daddy

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Feb 21, 2018
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Opus is you want a multi-bay charger, or a RC/hobby balance charger would be my top pick... and it's also a really nice item to just have around for battery work.
 

gauss163

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Jun 28, 2020
Messages
278
rearden said:
What discharge rates do you consider to be high current?
When does the effect of the Internal Resistance start to become significant?

If we fully charge an empty cell then the reported mAh is very close to its (current) chemical capacity (assuming that the charge termination current is not set unreasonably high). But the actual usable capacity that wecan extract from that charge depends on various factors, esp. the discharge rate (current), health (esp. R = total DC IR) and temperature, because - by Ohm's Law - the voltage while (steady-state) discharging is dragged down roughly by I*R at discharge current I, which causes it to sooner reach the termination voltage. If the product I*V is not too large then we'll be able to discharge almost all the charged capacity (typically over 99%), so the charged capacity yields an excellent estimate of the discharge capacity. But this close correspondence breaks down when I*R gets too large. This is easiest to comprehend by looking at discharge curves at various rates, e.g. below, for two identical NMC Li-ion cells. For each rate there are two similarly colored curves - one for each cell.


image_itmgsf.jpg


His charge curve shows they charged about 1044mAh, and discharged 1044mAh @100mA and 200mA, and 1039mAh at 500mA, all > 99.45% of the charged capacity. But at higher rates the curves 2.8V termination points move leftward, yielding lower capacity, e.g. at 15A (two lowest curves)the stronger cell yields about 923mAh(92%) and the weaker peters out much earlier at around 416mAh(40%). This is because the I*R drop has essentially pulled down the voltage curve so much that its termination point is now very close to the "knee" of the curve - where it has much smaller slope, so even a small voltage change causes a large capacity change.

To see this more clearly, suppose our termination voltage is 3.0V instead of 2.8V. Then the lowest 2 curves in theabove graph show thatat 15A both cells peter out very early - at around 255mAh(24%) and 310mAh(30%) when they hit 3.0V. As we can see from the stronger cell's curve, most of its energy is delivered in the flat part of the curve below between 3.0V and 2.9V, but we cannot access it because it liesbelow our 3.0V termination voltage. [Note: the reason this curve has flatter shape than the lower rate curves above it is due to self-heating effects. The large 15A rate generates sufficiently more internal heat that it decreases the cells internal resistance I, which decreases the voltage drop I*R. This was enough rise for the stronger cell to deliver most of its energy above 2.8v, but the weaker cell couldn't recover in time].

Those were new cells. If they were more aged theirinternal resistance might be twice as large, so we'd notice the same effects as above at about half the current,since (I/2)(2R) = IR yields the same voltage drop. But stillthe discharge capacity remains above 99% of the charge capacity at the lower rates, so wedon't need to do a discharge to know the (chemical or low-rate)capacity - we can simply use the charged capacity.

NotesFor simplicity I have mostly ignored temperature effects above. But these can play a large role - esp. at the extreme ends of temperature ranges. e.g. in coldwinter termperatures the self-heating effect may make or break whether or not a cell delivers decent capacity (IR greatly increases at cold termperatures, which is why EV packs often employ heaters).

The linked tests useprofessional equipment.Consumer-level (dis)chargers are less precise and accurate so they may reportslightly more variation between charge and discharge capacitydue to their limitations.
 
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