Thread Tools
Mar 08, 2016, 08:43 AM
Registered User
Thread OP

RCX 3 in 1 Discharger/Balancer


While discharging Unprotected 18650 cells I have found it difficult to know when the discharge if finished. It seems there is always a cell or to still discharging while the others are getting too low in voltage.

In addition the voltage taken with my DVM as soon as the cells are removed from the discharger are a little lower than that shown on the discharger's LCD.

Bill
Sign up now
to remove ads between posts
Mar 08, 2016, 10:16 AM
Space Coast USA
hoppy's Avatar
I'll have to check mine out the next time I do a discharge and see if it reacts the same way.


Quote:
In addition the voltage taken with my DVM as soon as the cells are removed from the discharger are a little lower than that shown on the discharger's LCD.
Usually the voltage increases when the load is removed. Problem probably is with the accuracy of the two measuring devices. One or both are out of calibration? Have to check that out also.

Checking now.....
Last edited by hoppy; Mar 08, 2016 at 10:42 AM.
Mar 08, 2016, 10:26 AM
Registered User
Thread OP
Quote:
Originally Posted by hoppy
and?
I read the manual and found that when the flashing battery sign is off on the LCD the discharge is complete. I was confused because the LCD battery voltage readings were dropping down below the 2.75 volt setting before the discharge was complete.
Bill
Mar 08, 2016, 10:45 AM
Space Coast USA
hoppy's Avatar
2.75V? Seems low but......
What chemistry are these cells?
What are they used for?
Why are they being discharged to 2.75V?
Mar 08, 2016, 12:00 PM
Registered User
If these are regular Li-ion 18650 batteries, I would not discharge them below 3-volts. The real issue is.... why are you discharging them? Li-ion batteries do not suffer from storage at near a fully charged state like Li-po's do.
Mar 08, 2016, 02:07 PM
Space Coast USA
hoppy's Avatar
Bill,
My test results.
#1 Even after several hours, RCX-3 still going, but all cells at set 3.78V, so disconnected.
#2 Check with Multimeter:
RCX
3.78, 3.78, 3.78
Multimeter
3.792, 3.791, 3.789 (Pretty good balance)
Mar 08, 2016, 03:37 PM
Registered User
Thread OP
Quote:
Originally Posted by hoppy
2.75V? Seems low but......
What chemistry are these cells?
What are they used for?
Why are they being discharged to 2.75V?
I'm using them in my flashlight. I'm discharging them to 2.75 so I can recharge and see what their capacity is.

They are various Unprotected LapTop pulls:
Sony US18650GR
Samsung 1CR18650-20F
ASO GL2K241EG

After letting the discharger complete a discharge the voltage is lower than the setting of 2.75vdc on some cells, i.e. 1.04vdc but after a rest it goes up.

Bill
Mar 08, 2016, 04:02 PM
Space Coast USA
hoppy's Avatar
1.04?
Something is amiss here.

I had something like that happen to me twice. During the discharge, I made a change in the setup ( don't remember what it was) but the battery discharged into the 1.XX range.

Did you make any changes during the discharge?
Mar 08, 2016, 04:21 PM
Registered User
Thread OP
Quote:
Originally Posted by hoppy
1.04?
Something is amiss here.

I had something like that happen to me twice. During the discharge, I made a change in the setup ( don't remember what it was) but the battery discharged into the 1.XX range.

Did you make any changes during the discharge?
No, What I do now is check the voltage of the cells while they are still in the discharger's cell holder. I do this when the voltage reading on the charger's LCD is within the range I want. Works better this way.

I also found that if I stop the discharge process the volt meter on the discharger is accurate. Reads higher while discharging.

Bill
Last edited by ko4nrbs; Mar 08, 2016 at 04:32 PM.
Mar 08, 2016, 04:31 PM
Space Coast USA
hoppy's Avatar
BTW, what kind of capacity are you seeing from those cells?
I'm using some from a laptop that are at least 12yrs old in my flashlight. Never checked the capacity however.
Mar 08, 2016, 05:16 PM
Closed Account
Yeah something is amiss there. Voltage should drop when load is applied.

Standard 18650s are no less vulnerable to over-discharge than LiPos are. When you look at data sheets for the cells mentioned, you'll see a discharge spec typically of 2.7V, but it can vary a bit between makes. That doesn't mean you ~should~ discharge them that low. That's also a voltage under load. If you're seeing voltages like that with no load, the cell is over-discharged. Generally standard 18650s can tolerate slightly lower discharge voltages than LiPos, but not a lot. You should probably not see a resting voltage any lower than 3.0V otherwise the cell may suffer damage.

There are 18650s that can tolerate discharge voltages much lower than LiPos. For example the LG brand 20A high drain 18650s can tolerate voltages as low as 2.0V. However the standard 18650s you're pulling out of laptop packs are going to be in the 2.7V area, again that's a voltage under load. Resting voltage should be higher.

When manufacturers specify capacities for 18650s it's typically at a drain rate of 1/5C and they take them all the way down to cell tolerance. In use they should not be discharged that deeply in order to minimize wear. If you use discharge rates higher than 1/5C you'll get lower readings on capacity compared to the manufacturers rated capacity, that's not accounting for normal loss of capacity due to cycle use or age.

You should do your capacity checks without taking the cell all the way down to tolerance. There's not a lot of capacity to be accounted for in the steep part of the curve after the knee and it just puts unnecessary wear on the cells. When a cell has been deeply over-discharged it can make it unsafe to use at worst and at best will cause a large loss in capacity.
Mar 08, 2016, 05:26 PM
Registered User
Thread OP
Quote:
Originally Posted by hoppy
BTW, what kind of capacity are you seeing from those cells?
I'm using some from a laptop that are at least 12yrs old in my flashlight. Never checked the capacity however.
Still testing them. I'll post my findings.
Bill
Mar 09, 2016, 08:53 AM
Registered User
Thread OP
The cells all have capacities from about 1400 maH to 1800 maH.

I found that the low limit for the discharger is 3vdc. Below that and it will not discharge correctly.
Thanks for all the replies.

Bill
Mar 09, 2016, 01:30 PM
Space Coast USA
hoppy's Avatar
"I found that the low limit for the discharger is 3vdc. Below that and it will not discharge correctly."


I know who to go to now for RCX-3 questions as you are now undoubtedly the go-to RCX-3 expert......
Mar 09, 2016, 08:48 PM
Closed Account
Quote:
Originally Posted by ko4nrbs
The cells all have capacities from about 1400 maH to 1800 maH.
Laptop pack cells tend to be in the 2200mAh to 2600mAh range. Though you can find standard 18650s up to 3600mAh (Panasonic NCR18650G), they tend to use the lower capacity ones for whatever reason. Maybe it's a cost thing, maybe something else.

It looks like those cells have a lot of wear either from cycle use or age. That's assuming you're doing your discharge tests at a rate less than 1C. Capacity falls off pretty quick with standard 18650s at discharge rates above 1C.

If you have the test equipment you should do an IR check on those as well. It goes up with wear as capacity falls. A fresh standard 18650 is 50 to 75mΩ. If a cell has measured IR a lot higher it may have trouble providing the power required.


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Wanted Rcx 3 in 1 battery balancer/discharger JimClark Aircraft - Electric - Batteries & Chargers (FS/W) 1 Jan 23, 2017 06:14 PM