Hi all.
I'm currently prototyping a circuit that includes a "mini" solar charger for a 1 (or 2P) Li-On cell (18650 2600 mAh).
Right now the charger part is a slow charging circuit such that :

1. Begins charging when Vbat falls below 3.6 V
2. Applies a charging current from 0.05C to 0.1C (through a simple resistor whenever solar input allows).
3. Stops charging at exactly 4.1 V

The circuit includes temperature monitoring, undervoltage control (3.2 V limit), over-discharge current control etc.

This charging method was briefly mentioned somewhere (maybe at Battery University?), and I've been
testing it for some weeks with many new and not so new cells, with good results.
Of course it has some obvious drawbacks :

1. It's slow (in my case could not be otherwise due to solar cell power)
2. You lose some battery capacity (but not as much as one could think, compared with standard Li-On charging method, due to the small charging current, at 4.1 V the charge is higher than same voltage point in standard cycle.

So, considering this specific application, I would like to hear your comments mainly on :
1. Safety
2. Cell life

Thanks in advance.

 

Hi all.
I'm currently prototyping a circuit that includes a "mini" solar charger for a 1 (or 2P) Li-On cell (18650 2600 mAh).
Right now the charger part is a slow charging circuit such that :

1. Begins charging when Vbat falls below 3.6 V
2. Applies a charging current from 0.05C to 0.1C (through a simple resistor whenever solar input allows).
3. Stops charging at exactly 4.1 V

The circuit includes temperature monitoring, undervoltage control (3.2 V limit), over-discharge current control etc.

This charging method was briefly mentioned somewhere (maybe at Battery University?), and I've been
testing it for some weeks with many new and not so new cells, with good results.
Of course it has some obvious drawbacks :

1. It's slow (in my case could not be otherwise due to solar cell power)
2. You lose some battery capacity (but not as much as one could think, compared with standard Li-On charging method, due to the small charging current, at 4.1 V the charge is higher than same voltage point in standard cycle.

So, considering this specific application, I would like to hear your comments mainly on :
1. Safety
2. Cell life

Thanks in advance.

Hello,

To start, low current charging of an Li-ion cell is not recommended when the charging period is going to be very long. The low current is said to plate the electrodes and deteriorates the cell which could make it dangerous.
Maybe you could use a different battery chemistry if you have to charge at very low currents.
0.05C might be ok though.

For a good trade off of long life and decent battery capacity, charge to only 3.9v per cell. You could get 8 years and tens of thousands of charge cycles that way. The capacity may drop to around 60 percent because of that, so just use a battery with an ampere hour rating that is twice as high as you think you really need, if you cant get by with less capacity of course.
Maybe you could read up on how satellites do it, which is similar to that way.

 

Hello,

To start, low current charging of an Li-ion cell is not recommended when the charging period is going to be very long. The low current is said to plate the electrodes and deteriorates the cell which could make it dangerous.
Maybe you could use a different battery chemistry if you have to charge at very low currents.
0.05C might be ok though.

For a good trade off of long life and decent battery capacity, charge to only 3.9v per cell. You could get 8 years and tens of thousands of charge cycles that way. The capacity may drop to around 60 percent because of that, so just use a battery with an ampere hour rating that is twice as high as you think you really need, if you cant get by with less capacity of course.
Maybe you could read up on how satellites do it, which is similar to that way.

Thanks a lot for your inputs. Yes, as a matter of fact 0.05C will be the usual charge current. No problem in lowering max. voltage too. Good advice.
As for a different chemistry, I'll have a look on that again (3S NiMH was what I started with...)

Again, thanks.