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Old Aug 30, 2015, 02:57 PM
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Battery Charge won't charge at 1C

Can someone explain why these RC chargers won't charge at the rate you program them to??

Ive been trying to charge a single LiFePo cell, 26650 format, to 3.6V and 3300 mA

I've been trying to find a charger that will charge LiFePo cells at 1C, but no success so far. The

first charger I tried was a UP120AC. It would only charge at 0.5C at best, but after a few minutes

it would drop down to 200-300mA and take forever to charge the cell. That unit had a bad discharge

channel, so I sent it back.

The current charger I have now is a SkyRC D100. It more or less does the same, starts out charging

at around 1C, but quickly drops off in charge I. Although this unit is better than the UP120, I

would like to b able to charge at 1C. I am going to be testing LiFePo cells for an electric bike

that are 10A cells, and I would like to be able to charge at least at 0.75C so it doesn't take

forever.

This unit is good for 50W on each channel, and I'm not pushing that limit and 3.6V * 4A, but it

just won't put out that much current for any length of time. The cell does eventually reach a

3000ma charge, but more than two hours to achieve it. Fast or Regular charge are the same, no

difference in charge rate. Am I missing something here???

By the way, the D100 Discharges OK, up to it's max ability of 10W.

I haven't tried any type of balance charge as I'm only charging one cell on each channel.

Thanks for any insights you may have.

Mark
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Old Aug 30, 2015, 04:06 PM
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When charging, are you in the Constant Current or Constant Voltage phase.

Sounds to me like you might be trying to charge a pack that is 80% charged and the charger is in the Constant Voltage phase which limits the charge current.
As can be seen in the graph below, the charge current drops rapidly once the CV phase is reached.

Does this fit your problem? The fly in this ointment however is " The cell does eventually reach a

3000ma charge, but more than two hours to achieve it" which doesn't fit well with the starting with charged cells.
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Old Aug 30, 2015, 04:24 PM
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Does your charger show voltage out? Without a balance cable to verify cell voltage the charger cannot compensate voltage drop in wires and start CV phase early.
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Old Aug 30, 2015, 06:34 PM
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Like hoppy says. You don’t have any current flowing if you don’t have a voltage differential.

So what is the voltage of the cell before you connect it to the charger and what is the voltage the charger is trying to apply to the cell?

If you don’t have enough voltage difference you will not put any current into the cell.
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Old Aug 30, 2015, 06:49 PM
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I put the cell through a discharge phase, I assume it discharged it fully, down to 2.6V, then went to the charge phase. Not sure if or when the the charger is in CC or CV. I'll attach a graph made by the software.

Don't use balance wires on a single cell, so the charge is not in balance mode.
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Old Aug 30, 2015, 06:51 PM
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From your graph, it's clear that the charger is immediately entering CV mode (holding voltage at 3.6V) and starting to taper current from there. This is due to resistance somewhere - either internal to the cell or in one of the connections between the charger and the battery.

Minimizing resistance (voltage drop) in all connections between cell and charger can shorten charge time by keeping charger in CC stage longer.

How is the battery connected to the output of the charger?
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Old Aug 30, 2015, 08:27 PM
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Everything looks (kind of) what would be expected except the voltage curve. How is it that it shows no change from start to finish? I would expect it should show something like the inverse of the current curve.
I’m not sure what the “Voltage:2.97V” and “Voltage:2.99V” are on the key at the bottom of each display.
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Old Aug 30, 2015, 08:35 PM
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Quote:
Originally Posted by Rhea View Post
Everything looks (kind of) what would be expected except the voltage curve. How is it that it shows no change from start to finish? I would expect it should show something like the inverse of the current curve.
I’m not sure what the “Voltage:2.97V” and “Voltage:2.99V” are on the key at the bottom of each display.
The voltage shows no change as the charger is in CV mode throughout the entire charge cycle. CV for LiFEPO4 is 3.6 volts.

The 'voltage:2.97V' and '2.99V' is the resting voltage of each cell upon charge termination. This seems rather low. Calibration issue? It'd be wise to compare with a DMM of reasonable accuracy.

5A charge with 1/5th CV termination is likely hitting these particular cells pretty hard, especially if the charges represented in the graphs had robust connections between charger and cells. If the connections were sound, these cells have rather high internal resistance and achieving the desired 1 hour charge ain't gonna happen.
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Old Aug 30, 2015, 08:59 PM
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The cells are in a battery holder and the leads to the charger are very short, 5 inches or so, with alligator clips.

About two hours after the charge was finished, the charger shows that the cells have 3.35 and 3.34 V (separate meter shows 3.32) and an internal resistance of 91 and 79 mOhm. I think those V readings are correct for LiFePo, not sure about the internal R.

It does seem like the charging is in CV mode, is that normal or not? Would having a balance lead make a difference even with only 1 cell ??

Lots to learn here :>)
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Old Aug 30, 2015, 09:01 PM
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I chose 5A for the "Fast" charge, but it never really went close to 5A during the charging.
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Old Aug 30, 2015, 09:12 PM
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Quote:
Originally Posted by markjenkins View Post
The cells are in a battery holder and the leads to the charger are very short, 5 inches or so, with alligator clips.
What gauge wire are the charging leads?

Pete
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Old Aug 30, 2015, 09:14 PM
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Here is the graph of the Discharge, stops at 2.6V and 3000 ma A input to the cell.
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Old Aug 30, 2015, 09:15 PM
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Well, they came with the charger, I would guess 16 or 18, probably 18
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Old Aug 30, 2015, 09:17 PM
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I think I know that adding a "Balance" lead won't do any good, how could it tell the difference between the cell V and the charger output voltage !
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Old Aug 30, 2015, 09:37 PM
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The cells are in a battery holder and the leads to the charger are very short, 5 inches or so, with alligator clips.
The battery holder and alligator clips give me pause. There can be dramatic variability in the loss characteristics (current handling) of battery holders and alligator clips.

Got a photo?

Quote:
Originally Posted by markjenkins View Post
About two hours after the charge was finished, the charger shows that the cells have 3.35 and 3.34 V (separate meter shows 3.32) and an internal resistance of 91 and 79 mOhm. I think those V readings are correct for LiFePo, not sure about the internal R.
Voltage readings are right where they should be, if perhaps a slight bit on the low side. Internal resistance numbers are through the roof for a 26650 LiFePO4 cell of that capacity. By comparison, my highly abused 7-year-old 26550 2300mAh cells from A123 Systems still have internal resistance of 11 to 14 mOhms.

Whether your losses are internal to the cell or somewhere in your connections between cell and charger is still an unknown but I'm leaning toward cells of questionable quality and discharge capability.

Can you solder wires to both ends of the cell to eliminate the cell holder / alligator clips?

Quote:
Originally Posted by markjenkins View Post
It does seem like the charging is in CV mode, is that normal or not? Would having a balance lead make a difference even with only 1 cell ??
Entering CV mode immediately at initiation of charge is indicative of a bad connection, a bad cell, or far too high charge current for the cell in question. Did the cell get warm during charge? If so, this points to high internal resistance and too high charge current for that cell.

Adding a balance lead likely won't help as I suspect that your charger does not have remote sense capability. Additionally, this would only help if you have a bad connection somewhere in the circuit between charger and cell. Also, the balance lead would need to be connected directly to the cell to effectively null the losses.

What specific cells do you possess? Are they new or did they have a previous life?
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