Question about hobby charger amperage fluctuation

reptilian

10 mW
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Jan 4, 2009
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27
Hey ES gang, I have a question that I can not seem to find an answer to on my own. Hopefully someone can help me out.
So I built my first li-ion battery made up of 18650 cells. it's a 15s26p pack that breaks down into 6, 5s13p packs for charging.
I am using hobby balance chargers to handle the charging. Is it normal for the charger to fluctuate the provided amperage? My charger will fluctuate up and down from 0 to .3amps and it will take days to get a full charge.
Does anyone have any idea why that might be happening?
 
In the final stage of charging, it is common for chargers to cycle on and off. They turn on and battery voltage rises to the maximum value, and then the charger turns off automatically. Then the battery voltage falls a little bit, and the charger turns back on. This cycle continues until the battery voltage stays at a high enough value that the charger no longer turns on.

Those of us who have been doing this since the days of SLA batteries can well remember the red/green light dance on our "smart" chargers.
 
Chalo said:
In the final stage of charging, it is common for chargers to cycle on and off. They turn on and battery voltage rises to the maximum value, and then the charger turns off automatically. Then the battery voltage falls a little bit, and the charger turns back on. This cycle continues until the battery voltage stays at a high enough value that the charger no longer turns on.

Those of us who have been doing this since the days of SLA batteries can well remember the red/green light dance on our "smart" chargers.

At what point should that start? My cells were around 3.7 volts and I have been trying to charge them for 3 days. They are now just under 4 volts.
 
That sounds like they're balancing, but not charging per se. Maybe do that once in a while, but bulk charge on a routine basis?
 
I take that you are using a r/c balance charger with no BMS. If so I would say there is a problem with the charger or a bad weld/connection. Has the battery been charged before and did it work?
My Ichargers all taper way down when the cells are close to full but never on and off. If you are using a smart charger with a BMS on the battery, yes it will cycle on and off.

Dan
 
I think we may need to know what charger you are using, on how big a pack.

Sounds like something is not right, you may just need to learn to set the max amps, or battery type.

Right now, its not doing shit, that's for sure. Once it is balancing, which should be at about 4.2v per cell, you should see very low amps, and that on and off happening. It will fill up, then shut off while the discharger bleeds down the higher charged cells, then turn back on. Less than one amp while balancing is normal.

My best guess? You have the charger set to lilo, instead of lipo, and its balancing at 4.0v. Set to lipo to charge to 4.2v. If this is the case, its never going higher than 4v.
 
This may be normal. Or it may be a sign of a problem.

You said this is a new pack. So were the cells all cycled and capacity matched before the build? Or are things way out of balance and the chargers have a huge balancing job to do? Depending on the states of the cells, and the algorithm that the chargers use, it could take a long time to get balance. The charger must stop charging when any cell group hits max voltage, and slowly drain that cell group down, then repeat, inching toward balance. Some chargers are much better than others at doing this because they can charge individual cell groups while others can only charge the whole pack and selectively drain the over charged groups, and often they can only do the draining phase slowly even if they can charge fast.

If the cells are not good quality, or the connections in the pack have problems, it might never finish. It sounds like it is making progress. You might watch the individual cell group voltages so you can see which are getting too full first and which are catching up. This is progress. This pack is far larger than the RC chargers were designed to charge, so it might take a very long time to complete. Once balanced future charging should remain balanced if the cells are good quality and the discharge cycles are modest.

Summary
The initial charging of a battery can take a long time to achieve the full balance condition if the cells were not all matched for capacity and charge state before assembly. Some chargers are much faster at achieving this rapidly. The BC168 is one example of a charger that can balance much more quickly by independently charging each cell group (since it consists of coordinated individual cell chargers rather than one large bulk charger and a set of individual cell drainers which is what the majority of RC chargers contain).
 
Are you running the charger in "balance" mode? Or in "charge" mode? At least in the RC charger I have here, the former does exactly that--maintains balance of the cells while charging. THe latter just bulk charges and stop charge if any cell exceeds the limit for the cell type I've chosen.

Also, have you chosen the right cell type (chemistry) in the charger for the cells you're using?


All that said, if your cells drop out of balance enough to cause the charger to have to stop and start and balance them like this, you might want to look into replacing the cells in the parallel groups that are lowest (and/or that shoot up in voltage quickly once charging current resumes).
 
I'm still betting on he's got the thing set to lilo, and its working exactly as you'd expect. At some point just short of 4v per cell, its stopping and going into balancing to 4v mode. Taking forever, if they are not very balanced.

If the cells are not brand new, then whatever the RC charger balances to, the cells will drop the surface charge and level out at their true capacity. My experience with hobby king lipo is that good cells end up at around 4.18v, and when they get old, more like 4.12v as the max, after a charge and balance to 4.2v. Even shit 3 year old cells ( but not severely puffed) with about 50% capacity left at 3c discharge, would still balance out at 4.12v or so.

If you got a really puffy cell, then yeah, theres your problem.
 
26p could be 50ah or more. 5s you would charge to 21volts if all are balanced at 4.2v 26p are these an odd lot ot used cells ? How is this pack solder or spot welded ?
Plus what is the state of charge ? Just one block at a time. Like
1. 4.02v
2. 3.95v
3. 3.99v

5. Xxx volt
It helps alot to write it down on paper and give each pack a name or number.
 
Alan B said:
This may be normal. Or it may be a sign of a problem.

You said this is a new pack. So were the cells all cycled and capacity matched before the build? Or are things way out of balance and the chargers have a huge balancing job to do? Depending on the states of the cells, and the algorithm that the chargers use, it could take a long time to get balance. The charger must stop charging when any cell group hits max voltage, and slowly drain that cell group down, then repeat, inching toward balance. Some chargers are much better than others at doing this because they can charge individual cell groups while others can only charge the whole pack and selectively drain the over charged groups, and often they can only do the draining phase slowly even if they can charge fast.

If the cells are not good quality, or the connections in the pack have problems, it might never finish. It sounds like it is making progress. You might watch the individual cell group voltages so you can see which are getting too full first and which are catching up. This is progress. This pack is far larger than the RC chargers were designed to charge, so it might take a very long time to complete. Once balanced future charging should remain balanced if the cells are good quality and the discharge cycles are modest.

Summary
The initial charging of a battery can take a long time to achieve the full balance condition if the cells were not all matched for capacity and charge state before assembly. Some chargers are much faster at achieving this rapidly. The BC168 is one example of a charger that can balance much more quickly by independently charging each cell group (since it consists of coordinated individual cell chargers rather than one large bulk charger and a set of individual cell drainers which is what the majority of RC chargers contain).


So I think this is the issue. I had the charger set correctly. It was just that 2 of the series where the only ones that needed charging (balanced). I haven't completely balanced my packs yet. This pack just finished today. Hopefully, I will not have this issue again.
 
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