Has anyone had any luck running their emonTx from rechargeable batteries? The advice is to run it off two 1.5v disposable AA batteries. This is too wasteful for me. Running it from the mains seems needless too.
I tried running it off two rechargeable AA batteries initially at approximately 2.55v. This lasted about 24 hours before the voltage dropped too low. Then I tried three AA rechargeables, each about 2500mAh. Voltage was initially 3.7v and lasted almost a week.
Finally, I tried a rechargeable 9v battery. The LED flickered sporadically and then died. It will not power up anymore. I think I've fried the ATMEGA chip and probably some other bits. I wouldn't recommend you try this!
How are you powering yours?
You shouldn't forget to enable Sleep mode.
3 AA rechargeable batteries @ 1.2v each should last a long time.
The ABSOLUTE MAXIMUM VOLTAGE for the RFM12B radio module is 3.8 V, for the Atmel processor is 6 V.
I thought it will fry at a lower voltage, this is good to know
comment not valid after the above post was modified from 6V to 3.8V
I'm pretty sure the emonTx is going into sleep mode. I am using the most up to date sketch for a solar PV setup here https://github.com/openenergymonitor/emonTxFirmware/blob/master/emonTx_C...
There is a line for the RF sleep;
and one for the Atmel sleep;
The Atmel fried at 9v, but the RF module survived! emonTx is working again thanks to a fresh Atmel chip.
So, how do you guys power your emonTxs?
Edited the post above - the RFM12B surface mount version maximum voltage is 3.8 V. (Why do they keep the same type no. for incompatible units? - I'd looked up the old type. Apologies to anyone misled by this).
Further research has provided another more specific data sheet: The RFM12B has an ABSOLUTE MAXIMUM NON-OPERATING VOLTAGE of 6 V. The maximum operating voltage is 3.8 V.
I think that is saying that operation above 3.8 V is liable to cause damage through overheating or emissions outside the permitted limits, but provided it is not operating, it will withstand voltages up to 6 V. So the realistic maximum voltage to use the module is 3.8 V. Three freshly charged rechargeable cells are likely to exceed this voltage.
bpbroom might be interested in reading this post by Glyn Hudson concerning mains operation.
Interesting developments concerning a single plug. So in summary, it is not possible to run the emonTx from rechargeable AA batteries. Two batteries provide too little voltage and from what you are saying Robert, three batteries could be too much. So it's two disposable AA batteries or mains. Seems a shame. Mains it has to be then, at least the power won't cut out so much!
Rechargeable cells typically give 1.2 V, but immediately after being taken off charge they can be up to about 1.6 V, but then fall quite rapidly. (I have this minute measured 2 NiMH hybrid cells that were charged weeks ago and haven't been used since, they still read 1.3 volts each <Edit> and I have now recharged a pair of NiCd cells that read 1.56 V each </Edit>). As you found out, the energy density is not as good as primary cells, but the economy is better! You could go for 4 rechargeable cells and go through a low-dropout 3.3 V regulator like the one fitted on the emonTx board (in which case you'd connect to the "5 V" input). That will typically work down to 3.4 V at the sort of current drawn by the emonTx, which equates to 0.85 V per cell, and that is about where the cell dies rapidly anyway. Of course you are throwing power away in the regulator, but it's fairly cheap power.
DON'T connect the programming connector while the rechargeable cells are connected - you will start to recharge them and possibly damage the programmer.
Interesting insights Robert. I think part of my problem is that I put too much voltage on the 3.3v line. If I'd put the three or four rechargeable AAs on the 5v line it wouldn't have bypassed the voltage regulator and fried the Atmel chip. Perhaps a rechargeable lithium battery would be more effective?
I think all your problem is that you put too much voltage on the 3.3v line - which is pretty much everything.
Yes, The regulator is good to 13.2 V (subject to dissipation limits of course, but these don't come in to effect with the emonTx only as the load. It will need checking if you draw other current from the 3.3 V rail).
I've looked at this data sheet www.sony.com.cn/products/ed/battery/download.pdf (other manufacturers products may differ slightly) that indicates that the cell will be discharging very rapidly by the time the regulator runs into dropout. That's good because you will use the whole capacity of the cell, and bad because you will get little warning of the impending voltage collapse. So it does look viable. I've got to point out that increasing the voltage by putting several cells in series does not help, because they all see the same current so they all discharge at the same rate and the life (in hours) is the same.
But I have to ask, are you monitoring the mains voltage? If so, why are you averse to using mains power?
I also think the emonTX code is not optimal in terms of power savings. I have been reading Nick Gammon's article on power saving techniques, and oh boy is it good.. http://gammon.com.au/forum/?id=11497
I am monitoring real power via a mains adaptor, so it is simple enough to run the emonTx on mains power. It just seems silly not to run a solar PV energy monitor from rechargeable batteries powered by free solar energy. The website gives the impression that the emonTx is designed to run off two AA batteries and the ethos is about reducing ecological impact.
From my experience I wouldn't recommend that newbies try and run the emonTx from rechargeable batteries, just stick with the mains.
The battery input on the emonTx does not go through the voltage regulator. Connecting more than 3.3V to this connection could damage the RFM12B, although in practice you will get away with 5V. If your powering the emonTx with more than 3.3V connect the power between the PWR and GND pins, this will route the power through the MCP1702 voltage regulator, this can take up to 12V.
I would be interested to hear any ideas for optimising the power consumption of the emonTx. I think this could be a topic for another thread..
Look 3 posts above - very interesting.
Open-source tools for energy monitoring and analysis. This project uses the GNU General Public Licence