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Battery Voltage drop
#11
Speaking about voltage drop, what is an acceptable voltage drop while discharging,
I discharge my individual 18650 cells with 1A or 0,5A.
I also have a battery pack that I am testing right now that had four 18650 cells in parallel, the pack drops from 4,0V to 3,7V when discharging with 1A.
Seems to me that the cells are dead (the pack also didn't charge above 4,0V with a TP4056 module), but what are your opinions about voltage drop while discharging?


(My first post since a couple of years, good to be back!)
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#12
(04-24-2020, 09:44 AM)Lux_Gamer Wrote: ... the pack drops from 4,0V to 3,7V when discharging with 1A....

It should not drop instantly. Of course it's normal to gradually drop under load from full to empty per the usual voltage curves.
Some of your cells in that pack are likely poor capacity &/or have bad IR (internal resistance).
Running off solar, DIY & electronics fan :-)
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#13
(04-24-2020, 10:09 AM)Redpacket Wrote:
(04-24-2020, 09:44 AM)Lux_Gamer Wrote: ... the pack drops from 4,0V to 3,7V when discharging with 1A....

It should not drop instantly. Of course it's normal to gradually drop under load from full to empty per the usual voltage curves.
Some of your cells in that pack are likely poor capacity &/or have bad IR (internal resistance).

It drops instantly when i connect the load. Of-course the voltage will drop overtime, but the voltage of the pack drops instantly when I connect the load. The pack is a 7s4p prebuild pack from a machine, I don't have fancy equipment to test the pack in one go so I discharge each parallel block on its own trough the balance connector.
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#14
(04-24-2020, 10:44 AM)Lux_Gamer Wrote:
(04-24-2020, 10:09 AM)Redpacket Wrote:
(04-24-2020, 09:44 AM)Lux_Gamer Wrote: ... the pack drops from 4,0V to 3,7V when discharging with 1A....

It should not drop instantly. Of course it's normal to gradually drop under load from full to empty per the usual voltage curves.
Some of your cells in that pack are likely poor capacity &/or have bad IR (internal resistance).

It drops instantly when i connect the load. Of-course the voltage will drop overtime, but the voltage of the pack drops instantly when I connect the load. The pack is a 7s4p prebuild pack from a machine, I don't have fancy equipment to test the pack in one go so I discharge each parallel block on its own trough the balance connector.

When I've seen this 'instant drop' in my 18650 battery experiments it was its because the cells cannot (easily) support the power drain being placed on them.   The max discharge specs of a particular cell don't always mean that they support that max discharge (or even 1/2 of the max discharge) 'easily' (e.g.without significant voltage drop).
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#15
(04-24-2020, 02:27 PM)OffGridInTheCity Wrote:
(04-24-2020, 10:44 AM)Lux_Gamer Wrote:
(04-24-2020, 10:09 AM)Redpacket Wrote:
(04-24-2020, 09:44 AM)Lux_Gamer Wrote: ... the pack drops from 4,0V to 3,7V when discharging with 1A....

It should not drop instantly. Of course it's normal to gradually drop under load from full to empty per the usual voltage curves.
Some of your cells in that pack are likely poor capacity &/or have bad IR (internal resistance).

It drops instantly when i connect the load. Of-course the voltage will drop overtime, but the voltage of the pack drops instantly when I connect the load. The pack is a 7s4p prebuild pack from a machine, I don't have fancy equipment to test the pack in one go so I discharge each parallel block on its own trough the balance connector.

When I've seen this 'instant drop' in my 18650 battery experiments it was its because the cells cannot (easily) support the power drain being placed on them.   The max discharge specs of a particular cell don't always mean that they support that max discharge (or even 1/2 of the max discharge) 'easily' (e.g.without significant voltage drop).


I don't know what the discharge specs are for the cells that I'm using but the battery pack is used in a electric lift with a motor, and I only discharge them with 1A per pack of four 18650 cells in parallel so about 250mA per cell in theory if the cells where equal.
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#16
Voltage drop is normal during discharge. Look at this curve for a panasonic. At 650mA during the start it drops to 4V quickly from what I assume is the full charge of 4.2V. At 3250mA it is almost .25V less than 650mA.

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#17
4.0V to 3.7V drop with only a load of 250mA/cell sounds like a steep drop to me?
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Running off solar, DIY & electronics fan :-)
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#18
(04-25-2020, 12:31 PM)Redpacket Wrote: 4.0V to 3.7V drop with only a load of 250mA/cell sounds like a steep drop to me?

yes that was my taught exactly.
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