Choosing resistors for voltage divider

Thread Starter

Soulgriever

Joined Apr 26, 2018
3
Hello,
Currently I'm trying to design a 4s 18650 charger. For my design I have chosen the LT1513 SEPIC charging IC due to its simplicity. In the manual it has a weird formula for choosing the value of R1 and when I try to use their formula my R1 value isn't even close to what they specify.

If I'm understanding the way the charger works properly I need the following values to program the correct charging voltage.

R1=155k
R2=12.4k

Thus giving me a charging voltage of 16.8v and a reference voltage of 1.245v

I have attached the manual for reference

More about my project:
12v-16.8v battery operating range
Desired charging current 1-1.5A

Using 4 18650 ~3000mah cells in series with a balance board at the battery.
I have not picked a transformer or rectifier so I'm not sure what my input voltage/max current will be. Probably ~25v to get the full potential charging current.
 

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AlbertHall

Joined Jun 4, 2014
12,023
From your attached datasheet:
upload_2018-4-26_22-21-9.png
Note that this list does not include Li-ion cells because it is not a suitable way of charging these cells.
You must use a proper charging circuit to avoid the cells catching fire or exploding.
 

Thread Starter

Soulgriever

Joined Apr 26, 2018
3
From your attached datasheet:
View attachment 151387
Note that this list does not include Li-ion cells because it is not a suitable way of charging these cells.
You must use a proper charging circuit to avoid the cells catching fire or exploding.
The charger I picked is a multi-chemistry charger. For a single cell or multiple cells in parallel this charger will work just fine as-is as long as I choose the correct max voltage and correct max current. Now since my cells are in series I have to put a balancing circuit on them. Once that balance circuit is in place as far as the charger is concerned my battery is a 16.8v 3000mah single cell.

As far as charging current is concerned it is a general rule of thumb for 1/2mah so since my cells are ~3000mah I can safely charge up to 1.5A to my understanding.

To properly charge a lithium cell their is 2 charging stages, constant current does most of the charging, once the cell reaches a set voltage it charges at a constant voltage to top the cell off, lots of people out their use a cheap bms board to charge their lithium cells which on settles I believe the constant current part (can't remember) but it doesn't finish charging the cell and can damage the cell over time.
 
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