Li-Ion battery current limitation??

Thread Starter

Aniket_Jangam

Joined Apr 6, 2021
4
Hello all,
I am using TPS61253 Boost IC to boost 3.8V from 375mAH Li-Ion battery to operate a load of 2A and also charge the backup Li-Ion battery of 115mAH using BQ25895 chip. Looks like battery is not able to deliver the necessary current and the load circuit does not boot up correctly. Is there a way to fix this issue?
If I use an external supply at 4.7V and 3A. the complete circuit works, charges the backup battery and is stable. So I am guessing the issue is the current limitation from the 375mAH battery.
Appreciate if anyone could guide me to the correct way of doing this.

Regards,
AJ
 

crutschow

Joined Mar 14, 2008
27,751
Yes, that battery is too small to power the load you have.
The correct way is to get a larger battery.
How long do you need to provide 4.7V @ 3A?
That will determine how large a battery you need.

And why are you charging one battery from another?
You gain nothing since you are limited by the capacity of the first battery.
 

BobaMosfet

Joined Jul 1, 2009
1,850
The way Boost converters work is they draw massive amps in short bursts- your 375mAH Li-On is rated to deliver up to 375mA over 1 hour. It can only deliver far less instantaneously- a datasheet for the battery will reveal this. proper calculations for boosting (something you should have done in your design) will reveal how much current is actually drawn by the boost circuit to achieve the rated voltage and current output.

For example, if I were using the ubiquitous MC34063A to do what you need, it would draw up to 6.357A (Ipeak) to satisfy this requirement, or just over 24W.
 

Thread Starter

Aniket_Jangam

Joined Apr 6, 2021
4
Yes, that battery is too small to power the load you have.
The correct way is to get a larger battery.
How long do you need to provide 4.7V @ 3A?
That will determine how large a battery you need.

And why are you charging one battery from another?
You gain nothing since you are limited by the capacity of the first battery.
Thank you for the input.

The smaller battery is supposed to be the backup battery, which would be utilized when the bigger battery runs out of juice. Since there is the BQ25895 charger IC in between, the larger battery becomes the power source and the smaller battery is the one connected on VBATT pad.

I was wondering that there are bunch of wearable devices with limited amount of battery capacity around 400mAH. How do they achieve all the features?!
 

Thread Starter

Aniket_Jangam

Joined Apr 6, 2021
4
They don’t need 2A. What is the devce you are running?

Bob
I have Android SoM which needs 1A max and a cellular modem from Telit which needs 1A max, as per their datasheets. The cellular module is the one which cannot boot up due to the current limitation.
 

crutschow

Joined Mar 14, 2008
27,751
To determine battery life you divide the mAH rating by the current draw.
From that you can see that 0.376AH battery won't last long with a 2A load.
You also need to look at the maximum ampere draw rating from the battery (which is only indirectly related to the AH rating.
 

BobTPH

Joined Jun 5, 2013
3,686
So, basically a phone. Typical cell phones have batteries of about 2000maH and can run for hours, so they do not average near 2A. They probably peak at higher than that though.

Bob
 
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Thread Starter

Aniket_Jangam

Joined Apr 6, 2021
4
Yes it is like a phone but in a wearable format. The Android and Cellular modules are stripped down versions so they can operate at 2A.

I just observed that the device was working with fully charged batteries and the WiFi was not being used extensively.
I think I would need to have better understanding of the current usage to be sure about the circuit operation.

Thank you Crutschow, Bob and BobaMOSFET for your responses and guiding me in right direction.
 

Audioguru again

Joined Oct 21, 2019
3,544
The Name-Brand lithium batteries for my radio controlled model airplanes is 280mAh but it powers the main motor at 6A for 5 seconds at a time (longer and the airplane cannot be seen). I rarely fly at full power then each charge flies an airplane for 10 minutes.
Ebay sells Lithium batteries that are mostly full of flour (for the weight). But inside is an actual tiny battery.
 
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