Question about Lithium cells

[Rossrosh]

I have 2 questions:

If a battery's charging current is rated at 1A, and if I have a pair set in parallel, would I be able to charge it at 2A? If I understand correctly, current is divided across cells in parallel. Whereas voltage remains the same. So then the pair will get 1A each?

The other question: what would happen if I charge bellow the rated current, say at 300mA? (which my charger has setting)

 

[Wolf]

Whereas voltage remains the same. So then the pair will get 1A each?

Theoretically yes if both cells are the same and the IR of both cell is the same or pretty close.

what would happen if I charge bellow the rated current, say at 300mA?

It would take longer to charge the cells but perfectly all right.

Wolf

 

[Rossrosh]

Follow up questions... Naturally

If I were to charge higher than the rated current... Would that be bad (say at 4A, when rated for max 2.5A)?

And if I were to discharge (not short bursts, but continuously roughly for 15 to 20 mins) at a higher current than rated (say 7A, when rated for max 5A), would that be bad?

How would I go about charging: 2 cells in parallel, connected to 1 single cell in series? (I have no idea what this configuration is labeled)
This setup is because of the device's dimensional limits)

 

[Rossrosh]

Theoretically yes if both cells are the same and the IR of both cell is the same or pretty close.

It would take longer to charge the cells but perfectly all right.

Wolf

Hope you could help me with these follow ups:

Q1:
If I were to charge higher than the rated current... Would that be bad (say at 4A, when rated for max 2.5A)?
And if I were to discharge (not short bursts, but continuously roughly for 15 to 20 mins) at a higher current than rated (say 7A, when rated for max 5A), would that be bad?

Q2:
How would I go about charging: 2 cells in parallel, connected to 1 single cell in series? (I have no idea what this configuration is labeled)
This setup is because of the device's dimensional limits)

 

[Wolf]

Q1:
If I were to charge higher than the rated current... Would that be bad (say at 4A, when rated for max 2.5A)?
And if I were to discharge (not short bursts, but continuously roughly for 15 to 20 mins) at a higher current than rated (say 7A, when rated for max 5A), would that be bad?

Short answer, yes and yes.
Long answer. There is a reason the manufacturer has spec sheets for their batteries. Charging at a higher current will create heat and of course wear the battery out faster. Also discharging at a higher current does the same. The solution of course is get a battery that has those specs. Plenty of them out there. As an example the Sony US18650VTC6 has a 5 Amp charge and 6A pulse charge capability with a continuous 15 A discharge.
Why try to make a cell do what it wasn't designed for?

Q2:
How would I go about charging: 2 cells in parallel, connected to 1 single cell in series? (I have no idea what this configuration is labeled)
This setup is because of the device's dimensional limits)

Kind of a goofy design but the only way that I can think of charging that would be with separate chargers. Problem is you wont get the full mAh out of the batteries
I assume this is what you had in mind.
Wolf

 

[Rossrosh]

Short answer, yes and yes.
Long answer. There is a reason the manufacturer has spec sheets for their batteries. Charging at a higher current will create heat and of course wear the battery out faster. Also discharging at a higher current does the same. The solution of course is get a battery that has those specs. Plenty of them out there. As an example the Sony US18650VTC6 has a 5 Amp charge and 6A pulse charge capability with a continuous 15 A discharge.
Why try to make a cell do what it wasn't designed for?



Kind of a goofy design but the only way that I can think of charging that would be with separate chargers. Problem is you wont get the full mAh out of the batteries
I assume this is what you had in mind.
Wolf
View attachment 25663

No, definitely not going to use it beyond it's design limits. Just educating myself/curiosity.
What if the battery is charged just 300 or 400mA more than it is rated? Would that be as dangerous/bad as the scenario I mentioned earlier?

Goofy is the least, can't even find a configuration label for it.
If that config is connected to a single 2S BMS, as you said it'd likely leave one of the cell in parallel not fully charged.

(Picking your brain here
I thought of a few options... I could include 2 BMS, 1 each for each set. The BMS of the single cell in series would stop charging once full; while the pair in parallel continues charging. (?)
Option 1, each BMS connected to a single charger in parallel, Same voltage, but split Amps.
Option 2, the two BMS connected in series, requires 8.4V, but maintains same Amps across the two sets of cells.

In your diagram, would it be possible to utilize the two chargers whilst simultaneously discharging the batteries? Shorts, etc?

 

[floydR]

Cells come in different sizes, both physical size and mAh perhaps you could use 2s1p 18650's that would supply what you need in terms of voltage and current. leaving the third cell off. it would make the battery simpler.
later floyd
edit if 2s supplies enough voltage, then you should be able to find a (2)cell that meets both the current and storage capacity

 

[ajw22]

The "goofy" design is a bit pointless, because it'll have pretty much the same capacity/capability as a simple 2S arrangement.
If you only have space for 3 cells, consider either 3S or 3P arrangement.

 

[Korishan]

Kind of a goofy design but the only way that I can think of charging that would be with separate chargers. Problem is you wont get the full mAh out of the batteries
I assume this is what you had in mind.
Wolf
View attachment 25663

This wouldn't work unless using two separate power supplies to power the chargers. Remember, they can't have common ground across both chargers. Isolation is required.

A 2s BMS would handle the 2s1.5P (is the configuration I'll call it for now). As long as the BMS had built in balancing, ie it would bleed off the excess power from the 1p side until the 2p side catches up. This is a huuuuge waste of power.

I thought of a few options... I could include 2 BMS, 1 each for each set. The BMS of the single cell in series would stop charging once full; while the pair in parallel continues charging. (?)

You could use 2 BMS's, sure. But you will then run into the problem of the 1p cell getting charged before the 2p side, and therefore the BMS on the 1p cell will cut power, shutting down the whole circuit. So this option is not viable.

Option 1, each BMS connected to a single charger in parallel, Same voltage, but split Amps.

This won't work because you have to have isolation. They can't share grounds, otherwise you'll have a dead-short across the first cell. You would need two power supplies.

I agree with Floyd. If you have a certain space limitation, you should look for cells that would better fit that space. There are plenty of LiPo's that come in various sizes and dimensions. These are also called "pouch cells". They generally cost about the same as 18650's or other cylindrical cells for a given capacity & discharge/charge rate.

Or as ajw22 mentions, go with a 3S configuration and then use a buck converter to run at the lower voltage of your device, and have a charge circuit that can charge at the 3S rating.

Is this a particular application that is real world, or are you just coming up with a scenario to test our knowledge? If it is real world, it would be nice to know what the actual application is as it could help us give more directed and applicable information specific to the purpose.

 

[Wolf]

What if the battery is charged just 300 or 400mA more

As I said before the manufacturer has specs. Same as rev-limiter on a car can you disable it and rev more? Sure. Will it damage your motor? Most certainly in the long run.
In my automotive days customers used to come in and say "I just overheated it a little bit" Pop the hood and the timing cover is melted to the engine block. Yep just a little bit.
Go by the spec and you will be safe. No exceptions. Li-Ion can be dangerous treat it with care.

Wolf

 

[Rossrosh]

Cells come in different sizes, both physical size and mAh perhaps you could use 2s1p 18650's that would supply what you need in terms of voltage and current. leaving the third cell off. it would make the battery simpler.
later floyd
edit if 2s supplies enough voltage, then you should be able to find a (2)cell that meets both the current and storage capacity

I guess if I set it in 3p... Then I'd have a higher capacity, and then step/boost it up to the desired voltage. I guess I'll still have a higher capacity compared to just only 2 cells in series?

 

[Rossrosh]

This wouldn't work unless using two separate power supplies to power the chargers. Remember, they can't have common ground across both chargers. Isolation is required.

I had a feeling about that. Also I've been searching for a higher Watt isolator (8 to 10W) for another project, but haven't come across one so far (except for only 5W).

A 2s BMS would handle the 2s1.5P (is the configuration I'll call it for now). As long as the BMS had built in balancing, ie it would bleed off the excess power from the 1p side until the 2p side catches up. This is a huuuuge waste of power.

That doesn't sound healthy for the that 1p

You could use 2 BMS's, sure. But you will then run into the problem of the 1p cell getting charged before the 2p side, and therefore the BMS on the 1p cell will cut power, shutting down the whole circuit. So this option is not viable.

Learning something new (guess now I've a couple of spares). I thought if the battery it's charging gets full... the voltage and current will still flow through the BMS, just without it going to the cells anymore.

This won't work because you have to have isolation. They can't share grounds, otherwise you'll have a dead-short across the first cell. You would need two power supplies.

I agree with Floyd. If you have a certain space limitation, you should look for cells that would better fit that space. There are plenty of LiPo's that come in various sizes and dimensions. These are also called "pouch cells". They generally cost about the same as 18650's or other cylindrical cells for a given capacity & discharge/charge rate.

True. I just started these projects to repurpose cells and parts currently at hand.

Or as ajw22 mentions, go with a 3S configuration and then use a buck converter to run at the lower voltage of your device, and have a charge circuit that can charge at the 3S rating.

Thats true... But not great when the amps are limited below the capable output.

Is this a particular application that is real world, or are you just coming up with a scenario to test our knowledge? If it is real world, it would be nice to know what the actual application is as it could help us give more directed and applicable information specific to the purpose.

It's for real world, but I'm inquiring/learning before applying.

I'll describe, I'm sure you fellas can guess what it is. It's in the shape of a cylinder narrowing towards the end, with a motor above the wide part, that vibrates

Regardless of the application, the idea was to use in series to get the preferred power, but also increase capacity even a little bit, given the physical limitations of the device.
Also to charge the batteries fast at the highest rated charging current of the cells

 

[Rossrosh]

As I said before the manufacturer has specs. Same as rev-limiter on a car can you disable it and rev more? Sure. Will it damage your motor? Most certainly in the long run.
In my automotive days customers used to come in and say "I just overheated it a little bit" Pop the hood and the timing cover is melted to the engine block. Yep just a little bit.
Go by the spec and you will be safe. No exceptions. Li-Ion can be dangerous treat it with care.

Wolf

Agreed. Hence I appreciate all the input from the members in my preliminary inquiries before I put it into practice.

 

[Wolf]

I guess if I set it in 3p... Then I'd have a higher capacity

That would definitely be you best option. You would no longer require a BMS as you are charging 1 Battery. You can then charge the 1s3p at max cell charge rate x 3 and depending on your power requirements a small buck/boost converter. Brilliant. There will be some small losses due to the boost but should be minimal.
Wolf

 

[Korishan]

Regardless of the application, the idea was to use in series to get the preferred power, but also increase capacity even a little bit, given the physical limitations of the device.

Using a buck convertor on the 3s would work just fine. The 3s would operate at a lower amp load on the cells than the 2s would. This alone increases run time. The higher the amp load on a cell, the lower the capacity it can give. This is generally true with all batteries. The amount of difference is dependent on the amount of current load the cell can handle. If it's designed for 1C and you only pull at .25C vs .5C, there might not be a big difference. But if you pull compared to 1.25C, then you'd see a difference.

and as wolf says, you can charge at x3 the rated input. This also equates to x3 the rated output as well. So as the batteries decrease in voltage, and the current increases (to maintain wattage consumption in the motor) the 3p will be able to cope with it, for a little longer than a 1p would anyways.

 

[Rossrosh]

Using a buck convertor on the 3s would work just fine. The 3s would operate at a lower amp load on the cells than the 2s would. This alone increases run time. The higher the amp load on a cell, the lower the capacity it can give. This is generally true with all batteries. The amount of difference is dependent on the amount of current load the cell can handle. If it's designed for 1C and you only pull at .25C vs .5C, there might not be a big difference. But if you pull compared to 1.25C, then you'd see a difference.

and as wolf says, you can charge at x3 the rated input. This also equates to x3 the rated output as well. So as the batteries decrease in voltage, and the current increases (to maintain wattage consumption in the motor) the 3p will be able to cope with it, for a little longer than a 1p would anyways.

and as wolf says, you can charge at x3 the rated input. This also equates to x3 the rated output as well. So as the batteries decrease in voltage, and the current increases (to maintain wattage consumption in the motor) the 3p will be able to cope with it, for a little longer than a 1p would anyways.

But I assume with a lower voltage, a higher (compensated) current can damage the motor...?

 

[Korishan]

But I assume with a lower voltage, a higher (compensated) current can damage the motor...?

The amount of higher current won't be enough to damage the motor. It'll just get hotter, that's all. And not enough to burn it up.

What really damages the motors is when the voltage is way too high, which causes them to spin much faster than designed. You can normally go 25% above rated (ie, a 12VDC motor can run just fine on 15V, just faster). But if you try to run it close to 2x rated, you'll either burn out the coils or blow up something inside, bearings or tear the rotor apart. That is, unless it's designed to run on higher voltages. Some motors are designed to run on 12VDC and 24VDC, for example. Pump motors are a good example of this.

The motor will try to pull more amps to compensate as the voltage drops, but won't be able to pull enough because the battery can't keep up. The battery will just loose power faster at that point.

 

[Rossrosh]

That would definitely be you best option. You would no longer require a BMS as you are charging 1 Battery. You can then charge the 1s3p at max cell charge rate x 3 and depending on your power requirements a small buck/boost converter. Brilliant. There will be some small losses due to the boost but should be minimal.
Wolf

Please elaborate on x3 charging.
From what I know (so far) :
If charging 3p... It'd still be charged at a constant voltage of 4.2V. The varying factor would be the current. So if I'd want to charge each cell in the 3p at 2.5A, I'd need to supply 4.2V, 7.5A (charger receiver module capable of 5V, 7.5A =around 38W).

This leads to another issue (came to mind) :

Even if I get a 50W (max 10V, 5A) power adapter (I want to use USB charging), the voltage would vary to accommodate the power, whereas the maximum current supply would always top at 5A (5A/3p = 1.66A per cell). So the power adapter will switch to 5V, 5A (25W), below the capable charge rate.

Any ideas how to get around this predicament?
Maybe a buck converter could help? If the converter (was made to/could) request to draw the (maximum) 10V from the power adapter, then buck to 5V and convert the rest to a higher current? (Not entirely sure if that's how it works, shifting the scales of current and voltage, within the 50W)

 

[Rossrosh]

The amount of higher current won't be enough to damage the motor. It'll just get hotter, that's all. And not enough to burn it up.

What really damages the motors is when the voltage is way too high, which causes them to spin much faster than designed. You can normally go 25% above rated (ie, a 12VDC motor can run just fine on 15V, just faster). But if you try to run it close to 2x rated, you'll either burn out the coils or blow up something inside, bearings or tear the rotor apart. That is, unless it's designed to run on higher voltages. Some motors are designed to run on 12VDC and 24VDC, for example. Pump motors are a good example of this.

The motor will try to pull more amps to compensate as the voltage drops, but won't be able to pull enough because the battery can't keep up. The battery will just loose power faster at that point.

Ok that's good then

 

[Korishan]

If a cell is designed to charged at 1C, which is mAh / 1000), then 3 cells in parallel would be x3, or 3C

In other words, if a single cell only gets charged at 500mA, then 3 cells in parallel can be chaged 1500mA and still be fine, because each cell is still only getting 500mA.
I used the "x3" to mean "times 3". I suppose I could of said "3x" to mean "3 times" as well. Just a little wording confusion there as your followup paragraph states the same thing

Even if I get a 50W (max 10V, 5A) power adapter (I want to use USB charging), the voltage would vary to accommodate the power,

Which adapter are you referring to? It would help to better answer the next question.

If the adapter on the "output" side is always 5V, then there's no problem at all as long as the charging circuit can handle the current/voltage, which most should be able to handle. Even a TP4056 works at 5V @ 1A, and 3 in parallel could do 3A.