What battery for GPRS modem application? - Page 1
Forum section: electronics / projects

author: Mad ID
author_rank: Regular Contributor
timestamp: November 22, 2014, 09:29:38 pm
content: Hi,I'm in the process of selecting a battery for a device which has an 2A burst GPRS modem.My current pick is a Lithium Thionyl Chloride 3.6V High Power type like the ULTRALIFE UHR-ER26500. The problem is that even this high power battery cannot provide 2A during bursts. My current calculations show to put cca 10mF of aluminium elec. capacitors in parallel. (voltage must never drop below 3.2V)My other requirements are -40C to 85C operating temperature, 5Ah+ of capacity and low self discharge.Do you have a better suggestion how to run the modem, i.e. battery choice and burst mitigation?Thanks!

author: extide
author_rank: Regular Contributor
timestamp: November 22, 2014, 09:39:55 pm
content: I've done some designs that used satellite data modems (Iridium) -- they typically would use a supercap to handle the burst, in addition to a li-ion battery.

author: Alex
author_rank: Regular Contributor
timestamp: November 22, 2014, 09:44:32 pm
content: What's the burst frequency and duty cycle?Alex

author: Mad ID
author_rank: Regular Contributor
timestamp: November 22, 2014, 10:16:28 pm
content: Quote from: Alex on November 22, 2014, 09:44:32 pmShould be 1/8 duty cycle, i.e. 577us 2A, and than 4ms pause.

author: Wilksey
author_rank: Super Contributor
timestamp: November 22, 2014, 10:21:55 pm
content: A Supercap is your friend, I use the Cap-XX GS206F.

author: Mad ID
author_rank: Regular Contributor
timestamp: November 22, 2014, 11:42:39 pm
content: Quote from: evb149 on November 22, 2014, 10:28:10 pmHi. Thank you for your effort of trying to help. Take a look at the Lithium Thionyl Chloride batteries, they seem very very good. The one I have has an internal impedance of about 0.5-1R (measured during transient load testing on an oscilloscope) and I will surely use a capacitor bank/supercap directly in parallel with it. Sell discharge is <1%/year and energy density very high.What I will also do is investigate the worst case scenario of the modem and see if it can be relaxed.I won't use fuel cells etc, just a primary type battery.

author: Gribo
author_rank: Frequent Contributor
timestamp: November 23, 2014, 11:04:47 am
content: Another one for LiSOCl batteries. Tadiran Batteries have an HLC/battery combo for such applications, which is also rated for extended temperature range. Check theirhttp://www.tadiran.com/index.php/pulses-plus-lithium-cellsline of products.

author: Mad ID
author_rank: Regular Contributor
timestamp: November 23, 2014, 01:43:48 pm
content: Quote from: Wilksey on November 22, 2014, 10:21:55 pmThese are very expensive.I see from digikey that supercaps above 2.7V are couple of times more expensive so I will try to make a circuit with two 2.7V connected in series (maybe some balancing resistor in parallel also). Any suggestion for 3.6V+ supercap below 3$?Thanks.

author: Wilksey
author_rank: Super Contributor
timestamp: November 23, 2014, 11:59:31 pm
content: They can be, I buy them from a distributor in the UK for about £3 a piece in quantity.You can do the same/similar with a cap array, depends on the end product, mine is commercial, so I need to know that I am getting a good supply.

author: Mad ID
author_rank: Regular Contributor
timestamp: November 25, 2014, 12:53:05 am
content: I find this to be pretty difficult, i.e. ensuring 2A burst with a supercapacitor.Here is my current design:Looking at the supercapacitors I find that above 2.7V they start to be very expensive. Putting two in series complicates things and that's why I opted for one.The supercapacitor should have low capacitance (cca 10-20mF), because my circuit operates 1min per day and unused energy will be wasted in leakage.Switch disconnects the system from the battery until next wake-up period.If someone has a batter idea how to design battery operated power supply for a modem feel free to speak outThanks.

author: Wilksey
author_rank: Super Contributor
timestamp: November 25, 2014, 03:15:34 am
content: Have you actually tried with one of these batteries?Instead of a supercap you could use an array or a larger value such as 5000-6000uF.The 26500M says it will sustain a pulse of 1500mA, it doesn't say for how long, the 26500 says 400mA, again doesn't say for how long, there is another which states 100mA for 100ms.It's such a short pulse, albeit frequent, that a 2000-3000uF cap array (i.e. 8 - 10 x 330uF) would probably work out. You could scope the battery voltage to see how low it dips.

author: amahon
author_rank: Contributor
timestamp: November 25, 2014, 09:26:19 am
content: I doubt if you will have success with lithium thionyl chloride because of passivisation on the electrodes will drop the terminal voltage during the current pulse. That gets worse with with age and even more so at high temperatures. A few hours at 85C is likely a killer layer with big pulses. Great for long life and lower currents but not good for pulsing amps. I have seen problems maintaining 3v with less current on AA cells. For inexpensive, The first things I think of are a large bank of electrolytics or maybe a Lipo cell. The Lipo might either replace your primary battery, or if that's not suitable, possibly be charged from it (a small one perhaps?).CheersAndrewOh, I tried once looking for suitable super caps and found that anything remotely affordable was only suited for low currents - the internal resistance was too high.Sent from my iPhone using Tapatalk

author: Mad ID
author_rank: Regular Contributor
timestamp: November 25, 2014, 12:11:20 pm
content: Quote from: Wilksey on November 25, 2014, 03:15:34 amHere is an oscilogram of a Ultralife UHR-ER26500 battery connected directly to active load tester 20mA -> 2A ->20mA etc without any capacitors. The voltage drop is only 0.5V@2A, but I noticed that at first the drop is 1V. Looks like the internal resistance drops after one or two minutes of operation.I will abandon supercap idea and go with aluminium low ESR caps instead. Maybe a SEPIC converter after the caps.

author: Wilksey
author_rank: Super Contributor
timestamp: November 25, 2014, 01:43:11 pm
content: Ok, so you have done some measurements, which is good, I would suggest going for a cap array then, I have found that between 8 - 10 330uF works sufficiently well in place of the SC I use.

author: Mad ID
author_rank: Regular Contributor
timestamp: November 25, 2014, 02:39:30 pm
content: Here is my new circuit which does not use supercapacitors, only Al. electrolitycs. In my circuit I need a boost converter so why not use it to reduce the requirement on capacitor size.Only possible issue I see is with boost driving large empty capacitors at start, I hope a boost with slow start should do the trick.

author: amahon
author_rank: Contributor
timestamp: November 26, 2014, 04:37:56 am
content: The first pulse is breaking down the some of that passivation layer... so has more voltage drop. As the cell ages, and/or as it is discharged, that layer gets thicker and is harder to break through (even at room temp, but gets worse at higher temps very quickly). Try putting your cell in a 50 degC oven overnight, let it cool, and do the same test.... i suspect it will be much worse. Hard to predict and can cause lots of grief... :-(

author: amahon
author_rank: Contributor
timestamp: November 26, 2014, 04:51:39 am
content: Just noticed your circuit... You might be right to watch that power on current. I recently had a similar application with a gsm modem powered by a boost converter charged a bank of caps (6x 10000uF, at 5v, i think). I ended up putting a few ohm resistor in series before the caps in order to tame the initial charge current. Without it the boost converter had a very rough ride trying to settle.

author: Wilksey
author_rank: Super Contributor
timestamp: November 26, 2014, 11:34:52 am
content: Don't forget charge time if you limit current also, I have had circuits where it looked like they were not working but the caps were just taking their sweet ass time to charge up!