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Trickle charging Li-ion...

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Externet

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Hi.
A lithium cell supposed to be limited in charging voltage to 4.2000 volts.

If 4.5V is applied to a charged cell, but the charging current limited to 1mA and for only a few hours; should it harm the cell ? Would the cell rise past its 4.2000V limit anyway ?
 

dknguyen

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Yes. Lithium cells are not supposed to be trickle charged. Don't take shortcuts or half-measures charging lithium batteries.
 

Nigel Goodwin

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If in doubt use a protected battery, or add a protection board.

In my recent Li-Ion powered project I carefully control the battery charging and discharging in the software, but also added a protection board as well.
 

MrAl

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Hi.
A lithium cell supposed to be limited in charging voltage to 4.2000 volts.

If 4.5V is applied to a charged cell, but the charging current limited to 1mA and for only a few hours; should it harm the cell ? Would the cell rise past its 4.2000V limit anyway ?
Hi,

The correct way to charge is to use a limited current then switch to constant voltage once the cell gets near 4.200 volts. The charge is then completely terminated when the current drops to a certain level.
The way an analog circuit is set up is such that the voltage is limited to 4.200v and the current is limited to whatever the recommendation for that cell is, and as the voltage gets closer and closer to 4.2v the current starts to drop off because the voltage difference between 4.200v and the actual cell voltage at the time is very very small and that in turn with the cell internal resistance makes the current go down. It's interesting to look at this process in detail.

However, there is also another spec sometimes called the termination current. This current is typically 30 to 50ma but could be as high as 100ma. The reason for this current value is so that there is little or no electroplating of metal which causes some safety issues.
Recall that an electroplating process is caused by movement of electrons through a medium. The electrons carry atoms of metal with them. Note now that charging also involves the movement of electrons, and thus there could be a point where the electrons start to cause plating of one of the electrodes of the battery with the medium being the electrolyte. To stop this from happening or keep it at a minimum, the termination current cutoff point is used to stop the flow of current once a certain relatively low current level is reached.
It's hard to say just how long the low current can exist before too much damage occurs, so it's best to stop after the specified current is reached or soon after.

Note also the charge specification usually states constant current and constant voltage with a specific charge termination, but it's really limited current with constant voltage with a specific charge termination. That means it does not have to be constant current it just has to be limited, which in turn means that the accuracy of the charge current itself during the more or less constant current phase can be way off, even 20 percent is ok. The max charge voltage however is 4.200 with a 1 percent spec, but there too you can stop at 4.150v for example and extend the life of the cell to some degree.
 

Nigel Goodwin

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which in turn means that the accuracy of the charge current itself during the more or less constant current phase can be way off, even 20 percent is ok.
It can be a LOT further off than 20%, the constant current during charging is one of the things that determines how fast you charge it, with lower currents been 'nicer' to the battery - likewise the end cut-off current, for maximum charge you can take it down to 130MA for the 18650's I was using, but if you cut off earlier the batteries will last longer, but won't be as highly charged. Interestingly the protection board I've now fitted cuts off well above that.
 

MrAl

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It can be a LOT further off than 20%, the constant current during charging is one of the things that determines how fast you charge it, with lower currents been 'nicer' to the battery - likewise the end cut-off current, for maximum charge you can take it down to 130MA for the 18650's I was using, but if you cut off earlier the batteries will last longer, but won't be as highly charged. Interestingly the protection board I've now fitted cuts off well above that.
Hi there,

Well i would not go any higher than maybe 20 percent, but yes you can go lower than 20 percent as long as you stay above the min charge acceptance level.

I do have to wonder what the tolerance on the various protection boards is.
 

audioguru

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The Battery University says that a Lithium-ion battery must not be overcharged or trickle charged because it causes metallic lithium to replace the ions and it shorts the cell. What happens when you short a fully charged lithium-ion cell? A nasty explosion and fire. Lithium burns very hot like titanium and magnesium metals. See the "ium"?

Why do you want to trickle charge a Lithium-ion battery? They hold a charge for many years. I have the Lithium-ion battery from my daughter's first cell phone. It still has the same charge it had about 20 years ago.
 

JonSea

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The advice to have a small fire-proof safe close at hand when experimenting with lithium batteries is probably sound.

If the battery starts to cook off, dump it in the safe and lock the door shut.

There are some impressive videos on YouTube showing what can go wrong.
 

MrAl

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The Battery University says that a Lithium-ion battery must not be overcharged or trickle charged because it causes metallic lithium to replace the ions and it shorts the cell. What happens when you short a fully charged lithium-ion cell? A nasty explosion and fire. Lithium burns very hot like titanium and magnesium metals. See the "ium"?

Why do you want to trickle charge a Lithium-ion battery? They hold a charge for many years. I have the Lithium-ion battery from my daughter's first cell phone. It still has the same charge it had about 20 years ago.

Hi,

The temptation is to charge the cell with a voltage source and resistor, so that it stays charged even if used a little. This is not a good idea for the reasons specified in various posts here.
 

Diver300

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I've been testing a GPS / GSM tracking unit with a solar cell to charge the Li-Ion battery, instead of from the usual 12/24 V supply.

The solar cell voltage is only a bit more than the battery voltage, so I haven't bothered with MPPT, and all I have used is the normal charging IC, the L6924D (https://www.st.com/resource/en/datasheet/l6924d.pdf). I've increased the maximum charging time to around 12 hours.

Obviously the charge rate will often be very low. From the voltage graphs, there are no clear points where charging starts or stops each day as the sun gets round to the solar cell. The fastest charge is approximately C/10 but it could be anywhere from zero up to that.

The charging IC is happy to work with a current-limited supply which is what the solar cell is, and to limit the voltage. The IC has a constant voltage phase, at the maximum voltage, and it stops charging when the current taken by the battery falls below a preset level. When run off the solar cell, the charging doesn't seem to stop and the voltage is held at the maximum voltage for a couple of hours until the sunlight leaves the solar cell.

Will that harm the battery? Does limiting at 4.1 V rather than 4.2 V make it easier on the battery?
 

Nigel Goodwin

Super Moderator
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Hi there,

Well i would not go any higher than maybe 20 percent, but yes you can go lower than 20 percent as long as you stay above the min charge acceptance level.
20% of what?.

Charging currents can (and do) quite happily vary massively, 0.2c is the 'nice' charge, and what I aimed for as it gives maximum charge capacity, but as much as 2c is perfectly normal for a fast charge, but only gives 80% charge capacity. The minimum cut off current is usually 0.05c for the same maximum charge.
 

MrAl

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20% of what?.

Charging currents can (and do) quite happily vary massively, 0.2c is the 'nice' charge, and what I aimed for as it gives maximum charge capacity, but as much as 2c is perfectly normal for a fast charge, but only gives 80% charge capacity. The minimum cut off current is usually 0.05c for the same maximum charge.
Hi,

I thought it was obvious that it means 20 percent of max charge current.

In my experience the max charge rate is not a function of C, but is a function of the manufacturer's data sheet. It states what the max charge current is and sometimes other interesting info.
 

MrAl

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Most Helpful Member
I've been testing a GPS / GSM tracking unit with a solar cell to charge the Li-Ion battery, instead of from the usual 12/24 V supply.

The solar cell voltage is only a bit more than the battery voltage, so I haven't bothered with MPPT, and all I have used is the normal charging IC, the L6924D (https://www.st.com/resource/en/datasheet/l6924d.pdf). I've increased the maximum charging time to around 12 hours.

Obviously the charge rate will often be very low. From the voltage graphs, there are no clear points where charging starts or stops each day as the sun gets round to the solar cell. The fastest charge is approximately C/10 but it could be anywhere from zero up to that.

The charging IC is happy to work with a current-limited supply which is what the solar cell is, and to limit the voltage. The IC has a constant voltage phase, at the maximum voltage, and it stops charging when the current taken by the battery falls below a preset level. When run off the solar cell, the charging doesn't seem to stop and the voltage is held at the maximum voltage for a couple of hours until the sunlight leaves the solar cell.

Will that harm the battery? Does limiting at 4.1 V rather than 4.2 V make it easier on the battery?
Hi,

Now it is getting more apparent what you are trying to do here.
Charging up to 4.1v will extend the life of the cell. Since that is about 2 percent of 4.2v, the capacity will be reduced to about 85 percent of full capacity. Thus a 2 AHr cell will have apparent capacity of about 1.7 AHr.
It's hard to say what the min current should be when the cell is not fully charged yet, but there is a charge acceptance level for every type of battery where there is no charge taking place. Under that level there is no charge taking place even though there is still some charging current flow. The question remains as to if this hurts the battery or not but only when it is not yet fully charged. What i can say about this is that i have not read any studies on this. Without the full knowledge of how this process works i would not do it that way, but would provide a simple cut off circuit that keeps the battery from being charged if there is not enough current available. That will always protect the cell. Keep in mind that if you go too low in current the cell does not charge anyway, so the only side effect on the design here is that you need a cutoff circuit that will not charge below some level like 50ma for example. That should guarantee safety.

Also keep in mind that if you go to NiMH you dont have all these charge problems.
 

Diver300

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As there is a load, the battery current will be changing between charging and discharging a lot of the time. Also the load may exceed the solar cell current, but the solar cell will be taking some of the current drain and extending the time before the battery is discharged, without actually charging the cell.

If charging is stopped, the voltage of the unloaded solar cell won't be much help in estimating the available current. I suppose that I could short out the solar cell when charging isn't required, and measure the current supplied, and use that to decide if enough current is available for charging. The load current would also have to be measured as well.

It would certainly be a lot of work to avoid charging at very low currents. I don't know what benefits there would be in doing that.
 

audioguru

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I think there are some smart ICs designed for charging a Lithium-ion battery and they have a connection for the load that is automatically switched to the battery when the input source power is too low.
 

Nigel Goodwin

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Hi,

I thought it was obvious that it means 20 percent of max charge current.
Which you would define as what? - they don't actually usually give a 'maximum' value, just a range of values getting worse the higher they go.

The 'default' value, which I would imagine you were referring to, as you didn't specify anything else, is only 0.2C.

In my experience the max charge rate is not a function of C, but is a function of the manufacturer's data sheet. It states what the max charge current is and sometimes other interesting info.
They normally specify it in terms of C, as it's then correct for all models.
 

MrAl

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Which you would define as what? - they don't actually usually give a 'maximum' value, just a range of values getting worse the higher they go.

The 'default' value, which I would imagine you were referring to, as you didn't specify anything else, is only 0.2C.



They normally specify it in terms of C, as it's then correct for all models.
Hello again,

Most of the cells i have seen specify a max charge current.
They also specify a max discharge current.

If we are talking about charging, then 20 percent higher than the max charge current is probably ok, but i like to stay at that value or slightly under that value.
If we are talking about discharging, then i dont like to go over that value (so not 20 percent higher).

I dont think there is any such thing as a normal rate, especially these days. The manufacturer specifies all this and that's the only way we can know for sure.

If you would like to link to a specific cell for sale on the web we can take a look.
 

audioguru

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American Li-PO and Lithium-ion 18650 cells tend to have accurate ratings. Some of my Li-PO batteries have a mAh rating of 70C which is the maximum continuous mA for 1 hour. Their allowed peak current is double the 70C which is 140C but for a short duration. Their max charging current is 5C.

<Mod edit: Removed racist comment - Matt>
 
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Nigel Goodwin

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Here's one datasheet - although it's a bit brief.

https://www.batteryspace.com/prod-specs/NCR18650B.pdf

It suggests a maximum charge of 0.5C, but this only allows 95% charge - dropping to 0.2C allows 100% charge, I suppose it depends if you're in a rush :D

Lower charge rates also increase the life of the batteries.

Increasing beyond the 0.5C increases charging speed (fast charge) at the expense of charge percentage, and further shortening of battery life - I would also suggest that you monitor the battery temperatures if fast charging, I built LM35's in my battery packs, but have only used them for logging battery temperatures so far - there doesn't seem much point using it as part of the charge control at my nice low charge rates (they don't get very warm).

I don't thing I'd like to force 5C down them like AG's batteries suggest :D
 

MrAl

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Here's one datasheet - although it's a bit brief.

https://www.batteryspace.com/prod-specs/NCR18650B.pdf

It suggests a maximum charge of 0.5C, but this only allows 95% charge - dropping to 0.2C allows 100% charge, I suppose it depends if you're in a rush :D
Hi,

I think you meant to say: "It suggests a maximum discharge of 0.5C but this only allows 95% charge capacity..."
but i see them going up to 2C on that.
Although it does suggest a max charge rate of 0.5C too, but no data on how that limits the apparent capacity.
 
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