gsm module connected to a Li-ion battery - Page 1
Forum section: electronics / projects

author: trevwhite
author_rank: Frequent Contributor
timestamp: August 11, 2012, 11:18:40 am
content: Hi all.I am wanting to produce a board with a GSM module on it. I have been looking at the sim900. It states that when transmitting it may require loads of up to 2A. These are only pulses of high current drain and its normal current drain is around 30mA is so.My idea for the power supply is to connect the backup battery - a 16850 Li-ion cell across it. In normal use the charger for the battery will provide the current for the gsm and the battery will handle the large current pulses.I do not know how this will degrade the battery over time. The datasheets state they are good for 300 charges but this is full charges. When its just dealing with small current pulses and gets charged up between them what affect do people think this will have on the battery?Thanks for any helpTrev

author: tnt
author_rank: Regular Contributor
timestamp: August 11, 2012, 11:25:45 am
content: I would use a large capacitor near the module to handle the pulse.Expect 577 us pulses every 4.615 ms most of the time during active call / active transfer.

author: trevwhite
author_rank: Frequent Contributor
timestamp: August 11, 2012, 11:46:08 am
content: Hi, thanks for the reply.What is large? 200uF? 1000uF?ThanksTrev

author: T4P
author_rank: Super Contributor
timestamp: August 11, 2012, 12:00:54 pm
content: i guess about 470-2200uf

author: 8086
author_rank: Super Contributor
timestamp: August 11, 2012, 12:02:53 pm
content: The sparkfun GM862 eval board uses a 100uF tantalum for this purpose, so that should be okay. If in doubt you can always use 200uF.

author: trevwhite
author_rank: Frequent Contributor
timestamp: August 11, 2012, 12:07:37 pm
content: Just checked out the sparkfun board. That has a regulator that can handle 2A loads. I am trying to avoid that by having the battery take care of them. But I do not know how this will degrade the battery over time.

author: SeanB
author_rank: Super Contributor
timestamp: August 11, 2012, 12:09:18 pm
content: 100uF 6.3V electrolytic surface mount right next to the GSM module, and a 1000uF 10V somewhere near the power inlet will do. You might want to use a ferrite bead on the supply to the module before the capacitor as well. This will help with the pulses, and then you can connect the backup battery across the supply.

author: Rerouter
author_rank: Super Contributor
timestamp: August 11, 2012, 12:37:21 pm
content: to me the gsm module effectivly has a pwm of 1/8th of the time, with the 30mA standard drain your looking at an average draw of 0.46Anow that is a rather slow pulse, and if your aiming for under 1A load on your regulator, then you will need atleast 5,000uF,if under 1.5A then 2,500uF

author: 8086
author_rank: Super Contributor
timestamp: August 11, 2012, 12:40:33 pm
content: Quote from: trevwhite on August 11, 2012, 12:07:37 pmWhen a battery is connected to the board, the regulator is out of the picture. The tantalum they have on there is specifically to handle the 2A current bursts during TX.

author: trevwhite
author_rank: Frequent Contributor
timestamp: August 11, 2012, 12:49:43 pm
content: Well my situation is that there is no regulator. Just a battery. I was going to hook up a charger for the battery that has a charge current of 100mA.I am more curious to know what affect taking these pulses once in a while will have on the battery over time.

author: cwalex
author_rank: Frequent Contributor
timestamp: August 12, 2012, 11:18:40 am
content: Quote from: trevwhite on August 11, 2012, 12:49:43 pmFrom what I have read the 300 charge cycles are full charge / discharge cycles and if you discharge it partially it will cope with that kind of use much more times than the 300 charge / discharge cycle rating. I have used notebooks for many years and rarely use them on battery, most of the time they are plugged in at the mains and the batteries still go bad after a couple of years or so.

author: PChi
author_rank: Frequent Contributor
timestamp: August 12, 2012, 05:54:15 pm
content: Typically GSM phones have relied on the battery to supply most of the transmit pulse and batteries have usually survived. The capacitors fitted aren't large enough to supply the majority of the peak current without excessive voltage droop.The TX current pulses are only 2 A at maximum transmit power so hopefully will be less most of the time when the RF link allows it. Also unless you are a millionaire you aren't going to be transmitting all the time.