Gm862 battery circuit
To run the Telit’s GM862 cellular module/modem from a battery you must read the Telit documentation very carefully. It states that “A Bypass low ESR capacitor of adequate capacity must be provided in order to cut the current absorption peaks, a 100μF tantalum capacitor is usually suited.”
Well I read this point about a low ESR capacitor in the Telit’s hardware guide and didn’t think much about it. I used a 100μF capacitor, but it was not tantalum nor was it low ESR, and guess what it didn’t work. First I thought that it was my battery, a 19AH 3.6V Lithium Ion battery from Jameco. But after reading this very informative forum entry, I learned that the capacitor was the problem.
The only adequate capacitor that I had was a 1F (yes a monster 1F) capacitor from Sparkfun. I used this capcitor and after giving a few minutes for the capacitor to charge, the circuit worked and GM862 was able to register with the network.
Well I thought it worked with the 1F capacitor, but the reason was that GM862 was connected via the USB in addition to the battery, and after it registered with the network, I disconnected from the USB and it was able to continue with the operation using the battery. But I’m unable to start GM862 with the battery.
I’ve ordered a low ESR 100 mF Tantalum (with a 600 m Ohm ESR) from digikey – $7 each. I suspect that with this capacitor, I’ll be able to start the module with the battery.
I wish the Telit site and documentation included more information on how to connect a micro-controller to the modem and the list of alternative parts to use. I’ve not been able to find any of the transistor, voltage level translators, etc. that Telit recommends on Jameco, DigiKey or Sparkfun. More detailed information on how to use the Telit modems is badly needed.
The problem that I was having was the battery and its inability to maintain the minimum voltage (3.4V) at 150mA current peaks that GM862 required during network registration. I connected two 3.6 V batteries in parallel and that fixed the problem – lowered the internal resistance of the batteries and allowed the batteries to maintain a higher than the minimum 3.4V voltage.
July 8, 2009 at 5:16 am |
Someone asked if they could use multi-year battery powered telit. Here is my response:
1. Tadiran or SAFT batteries will work fine. You’ll need to make sure that you pick the models that have sufficient current capacity to start the modem. Keep in mind that Telit could need as much as 2A to get started. Under good conditions the usage is more like 140mA to start and register with the network, but it all depends on the local cell coverage.
2. I don’t use GPS so I’m sure of the power requirements. But if all you’re doing is communicate once a day, then with 19AH battery you can look forward to about 5 years of usage. But again it depends on cell coverage and your antenna design. Your choice of antenna will have a huge impact on power consumption. Experiment with AT#MONI and different antennas and placements and see what PWR/RxQual do you end up with.
3. You’ll turn off the module completely after communication. You’ll need to do that to get multi-year usage. Don’t just put it in low-power mode (unless 2 way communication is a must requirement). Completely turn off the modem, and then turn it on just before you’ll need to communicate, communicate, and then turn it off again.
4. You can also run two or more 19AH batteries in parallel and thus get 38AH or more power for a lot longer operation.
July 31, 2009 at 8:31 pm |
Hi, interesting stuff !
One remark, you say to the previous replier that the battery needs a 2 A rating. I don’t think such batteries even exist? I thought that was the reason the low-ESR tantalum was needed?
Grtz,
Tom
August 2, 2009 at 5:32 pm |
Hi Tom, thank you for your note. I use 2 lithium-thionyl chloride batteries in parallel and use two super caps (10 F each). Without the caps, I’m unable to start the Telit module with batteries. The caps provide the ability to service the spike in current used by Telit (1A with 1900 MHz – aka PCS – and 2A with 850 MHz – aka GSM). Normally under reasonable receptions, the current draw at startup is more like 140 mA. But once you start to have poor reception, the cell tower will inform the modem to transmit at maximum power 1 Watt for PCS and 2 Watt for GSM. With the super caps I’m able to start the modem with less capable batteries, e.g., SAFT LS 33600 at 400ma/0.1 sec 230 ma, but you can always go with more capable batteries e.g. SAFT LM 33600 at 4000 ma / 0.1 sec 2500 ma.
Telit documentation does not provide sufficient guidance on how to run with a battery. The low ESR caps that they recommend, is really hard to come by. I did find something similar, and it still didn’t work with the SAFT batteries that I was using. The two 10 F super caps in series (you can them from sparkfun), is the way to go. Without the super caps, the voltage will drop below 3.55 V and at this level, the modem will shutdown.
You can also go with the rechargeable batteries that normally operate at 4.2 V. But then again, you’ll need lot of rechargeable batteries to get the AH you get out of the non-rechargeable ones. This will depend on your application.
August 2, 2009 at 6:59 pm |
I have always been happy running it from a regular power source
The peak currents seem to be challenging.
While reading the Telit documentations I noticed the 100 uF low-esr cap is mentioned often. I have looked up pricing of such a thing and its cost is equal to the 2 supercaps you’re using. Interesting. I think I will order one such low-esr cap and keep you posted. (wont happen too soon though). (If you live in the USA you might be able to get it from Newark). Another reason why they didn’t work could be because they’re not close enough to vbatt (eg when placing them on a breadboard). (I’d go for a 220 uF tantalum with 60mOhm esr. The ESR value you bought might also have been too great).
Something else that comes to mind is, starting is one thing. Should network signal reception get a problem, as you mention it will turn to max power. (As I live in a densely populated region in Europe, it’s hard to test here, even in basements I get OK reception). The question is if the caps will still be OK then. You may also be wanting to look at batteries like LSH 20 (not tested though, just judging by the datasheet). I guess you would need a GSM base station emulator device to be 100% sure.
T.
August 22, 2009 at 11:46 pm |
Regarding the Tantalum cap, I have requested some samples from Kemet and Vishay for the two follow items at Digikey. We’ll see how they work.
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=399-3879-1-ND
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=718-1464-1-ND
January 15, 2010 at 10:03 pm |
Hello Shahram,
I made a blog called embed21.com
This blog I will be using to post all of my projects online. As I saw your blog was so helpful, I added a link to it at the bottom of the page.
Talking about powering the module, I think this article will be very helpful:
http://www.embed21.com/?p=35