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looking for DC battery charging circuit

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aliyesami

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hi !
i want to use a rechargeable 9v DC battery in my project, how can i build a charging circuit for it so i can charge it with an external adapter n also be able to use it when adapter is plugged?

thanks
 

Hi,

first decide what battery you want to use. Current rating, capacity and chemistry.
Then - with the knowledge of the chemistry - you may search the internet for charging techniques and their drawbacks and benefits. Decide what to use.
Then - with the knowledge of charging technology - decide what charging current and input voltage you need. Find a suitable adapter.

Then decide how you want to build your circuit. All by your own with comparators, OPAMPs and transistors, or using a dedicated charging IC.

There are many IC manufacturers for charging circuits. Usually they have interactive selection guides to find the best IC for your specifications.
Read datasheets of the IC and related application notes.

Klaus
 

You are probably talking about charging a little Ni-MH 9V battery. Energizer make one with a capacity of 175mAh. It is recommended to charge it at 17.5mA for 12 hours if it was completely discharged. If continuously trickle-charged then the current should be 175mA/40= 4.4mA or less. A smart charger IC detects a full charge then disconnects.
Energizer has a Ni-MH battery manual: https://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf
 

i want to use these phone type batteries and get 3.3v 300mA , any recommendations for battery that has higher voltage than 3.3v since i will be using a LDO to get 3.3v regulated output.

thanks
 

Hi,

what are "these phone type batteries"? Do you have technical data?

i want to use these phone type batteries and get 3.3v 300mA , any recommendations for battery that has higher voltage than 3.3v since i will be using a LDO to get 3.3v regulated output.
Click to expand...
Using 9V batteries to generate 3.3V / 300mA with a LDO.... is wasting about 60% of battery power
and generates 2.7W of heating power (insted of 1W).

This is good for keeping hands warm in winter ;-)
Oh, i just discovered a new feature for your device...

If you don´t need this oven, then I recommend to use a buck switching regulator.

Klaus
 

can you give me a part number for this buck switching regulator please that will work with 9v ?
I wanted to use these flatter batteries used in cell phones, i dont know any name /type thats what i was asking if someone knew.
 

Hi,

google for "DC DC buck converter".

Then check for:
* input voltage range
* output voltage
* output current.

Klaus
 

The flat battery used in cell phones is a Lithium type that is 4.2V when fully charged and is about 3.2V when it should have its load disconnected. Then it averages 3.7V during a discharge. They use a charging circuit that is powered from 5VDC.
Since you want 3.3V then you need a boost/buck switching regulator that will work when its input is as low as 3.2V.
 

ok i found the battery that i will use " Li-ion 18650 Cylindrical 3.7V 2600mAh Rechargeable Batteries "
now how can i build a charger for this ? i want to use a chip solution if available.
please advise / recommend.
thanks

- - - Updated - - -

here is what i found on texas instrument site , would this work for my Liion 18650 3.7v batteries?



and in the above circuit I would just extend the output of the LP2951 chip to the DC-DC buck converter to step it down to 3.3v feeding the circuit?
or I would disconnect the battery (mechanically via switches) when i plug the adapter so only the adapter is supplying the DC-Buck converter and not the battery?
but then the adapter should also be charging the battery at the same time.
this sounds too complicated :)
 
Last edited:

I hope you didn't buy a fake Chinese 18650 battery cell from ebay:
https://www.youtube.com/watch?v=eOshOXcSkDA

The LP2951 is a voltage regulator not a Li-Ion battery charger. It will overcharge a Li-Ion battery cell and possibly cause it to catch on fire. Please post the link of the article you found.
 

Hi,

In the given document the LP2951 is used as charging IC. Have you noticed the dedicated LiIon charger IC LM3420?

You came to TI.
Did you use their selection guide?
**broken link removed**

Klaus
 

KlausST said:
Hi,

In the given document the LP2951 is used as charging IC. Have you noticed the dedicated LiIon charger IC LM3420?

You came to TI.
Did you use their selection guide?
**broken link removed**

Klaus
Click to expand...

hi Klaus!

two questions :

1- you recommending to use LM3420 instead of LP2951 but LP2951 will still work as per TI design?

2- I looked at the datasheet of LM3420 , does the board layout of 11.2 correspond to the circuit diagram of fig 7? and if yes which one is Q1 Q2Q3 ?
 


Hi,

1- you recommending to use LM3420 instead of LP2951 but LP2951 will still work as per TI design?
Click to expand...
No, I don't know your charging current and input voltage...
And before you talked about 9V. So maybe you need 2 or 3 pieces of 18650cells in series..
From my post#2:
Then - with the knowledge of charging technology - decide what charging current and input voltage you need. Find a suitable adapter.

......

There are many IC manufacturers for charging circuits. Usually they have interactive selection guides to find the best IC for your specifications.
Read datasheets of the IC and related application notes.
Click to expand...

2) i didn't read the complete datasheet

Klaus
 
my board requires 3.3v and 500mA , so i am planning on using a DC-DC buck MAX748A to get it. the Input of MAX748A will be 18650 which delivers around 3.7v I think i am sure it can deliver 500mA righ?.
then i want to build a charger based on your recommended chip LM3420 to charge this battery .
is it a working architecture?

yes before i was looking for a 9v battery but then when i found this DC buck which can handle as low input voltage as 3.67 I dont want a 9v battery.
 

if you use a real 18650 cell not a fake one and it has a true capacity of 2600mAh then it will produce 3.2V to 4.2V for about 5.2 hours when its load is 500mA. But the buck regulator will not provide 500mA when its input voltage drops below 4.3V. A graph on the regulator datasheet shows an output of only about 250mA when the battery is half discharged to 3.7V and almost no output current when the battery voltage is near 3.3V.

Two 18650 cells in series produce a minimum of 6.4V which will be good to feed the buck regulator.
 

Audioguru said:
if you use a real 18650 cell not a fake one and it has a true capacity of 2600mAh then it will produce 3.2V to 4.2V for about 5.2 hours when its load is 500mA. But the buck regulator will not provide 500mA when its input voltage drops below 4.3V. A graph on the regulator datasheet shows an output of only about 250mA when the battery is half discharged to 3.7V and almost no output current when the battery voltage is near 3.3V.

Two 18650 cells in series produce a minimum of 6.4V which will be good to feed the buck regulator.
Click to expand...

ok I will use two 18650 in series then.
can you or someone recommend me which circuit i should use on this data sheet ? there are so many options .
https://www.ti.com/lit/ds/symlink/lm3420.pdf

11.2 Layout Example is for which circuit?
 

KlausST said:
Hi,


I think it's the schematic of page1

Klaus
Click to expand...

so can I use this circuit for charging these two 18650 batteries in series?
it says "2-cell Li-Ion " what does it mean ?
also the input voltage is 13-20v , can I use 9-12v adapter?
LM3420_circuit.jpg
 

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