What battery for uC circuits

[edeca]

I am looking to build a fairly small uC circuit, with a number of sensors (temperature, humidity etc.) and either an SD card or RFM radio module. All the parts will be 3.3v compatible, most are low current and can sleep for 99% of the time when readings aren't needed.

What batteries have people used for this sort of thing before? Obviously I could use a standard PP9 or AA battery, but I'm looking for alternatives. If people have reused other batteries (remote control, DECT phone, Nokia batteries, whatever!) I'd love to hear about it.

Rechargeable is an obvious bonus, small would be great. Obviously can't have the moon-on-a-stick, but would appreciate any suggestions.

 

[nickelflippr]

I am a fan of LifePo4 because of the safe battery chemistry aspect, low voltage discharge, and higher nominal voltage than nimh. They can be abused, overvoltage, undervoltage, without catching on fire like LiCo (Li-Ion) Here is a link to a discharge curve from the cpf forums.

Charging solutions seem very limited right now, so a hobby charger, chinese import, or home grown solar solution could be the main hurdle of LifePo4 at this time.

 

[LTX71CM]

nickelflippr, what ever do you mean catching on fire... YouTube - World's Most Dangerous Battery!

 

[crutschow]

I would also suggest just using 3 NiMH AA or AAA low self-discharge (pre-charged) batteries. They have high capacity, are readily available at relatively low cost, and are not difficult to charge. The low self-discharge types also will retain most of their charge for over a year, unlike standard NiMH which lose most of their charge in that time, so are good for low power applications where required re-charge is infrequent.

 

[nickelflippr]

LTX71CM:
Forgot about LiPo batteries, coming to a cell phone near you, don't forget the hot dogs, and marshmallows!

 

[DemonicSandwich]

How much current does the device draw?

If it's only going to wake up to read a sensor, then sleep if there is nothing more to do, then use a pair of alkaline cells.

A fresh alkaline cell is about 1.6v and has a self discharge rate of about 2-5% per year at 25*C.

 

[DirtyLude]

I've been using CR123A's from DealeExtreme.com A charger is only about $5.
**broken link removed**

Jee Labs has a lot of stuff on battery power remote with boost converters. You'd have to hunt through the archives for it.
Jee Labs

 

[MrAl]

Hello there,

I have been using AA cells to power my refrigerator monitor, which sleeps most of the time and wakes up once in a while to measure and display. Keeping display time to a minimum, the projected life span of the two AA's is about 2 years. I hate buying batteries but that's good enough even for me.

 

[edeca]

I've been using CR123A's from DealeExtreme.com A charger is only about $5.
**broken link removed**

Nice, do you just use a single cell and a boost converter? I notice that is 3v, but many things (including the ADC) will work better at 3.3v. I have never looked at boost convertors, so would have to go read.

What do you use as a holder for the batteries?

Also, you mention charging them but I can only seem to find lithium non-rechargable versions. Perhaps I need to look further..

 

[edeca]

OK all. AA and AAA batteries are great and easy to charge. Size is the only issue, compare them to a Nokia battery for example.

I will try and use AAA if practical (my **broken link removed** should just about accept 2 of them) but would like experience of other solutions if anybody has tried them.

Cheers!

 

[blueroomelectronics]

How frequently does the sensor wake up and transmit?

 

[DirtyLude]

Nice, do you just use a single cell and a boost converter? I notice that is 3v, but many things (including the ADC) will work better at 3.3v. I have never looked at boost convertors, so would have to go read.

What do you use as a holder for the batteries?

Also, you mention charging them but I can only seem to find lithium non-rechargable versions. Perhaps I need to look further..

The ones I have are 3.6V, so a single reverse voltage protection diode drops it at or below the 3.6V max for most 3.3V devices and it will drop from there unregulated.

Battery Holder I use:
Digi-Key - 708-1412-ND (Manufacturer - BX0123)

Rechargeable CR123a 3.6V:
**broken link removed**
**broken link removed**
**broken link removed**

This is the charger I have: (You need separate chargers for 3V or 3.6V batteries)
**broken link removed**

The application I have running is not run for long periods, so I do not have a detailed breakdown of their life, but I'm using an MSP430 and an RFM module.

 

[Mosaic]

Have u considered N cels?

 

[edeca]

How frequently does the sensor wake up and transmit?

In this instance every 15 minutes, but I was also asking the question generally.

 

[edeca]

The ones I have are 3.6V, so a single reverse voltage protection diode drops it at or below the 3.6V max for most 3.3V devices and it will drop from there unregulated.

Thanks for the links, that is excellent! I have found a holder at a UK shop now, just need to Google for batteries later. Do you just use a single 3.3v zener to "regulate" the power, or do you also use an LDO regulator?

These seem like really nice little batteries that I wouldn't have considered otherwise. Cheers!

 

[edeca]

Have u considered N cels?

These look to be similar to the CR123A that DirtyLude is recommending. There are also a lot of pages on biology when searching for information about them! I'll take a further look later, thanks.

 

[DemonicSandwich]

Thanks for the links, that is excellent! I have found a holder at a UK shop now, just need to Google for batteries later. Do you just use a single 3.3v zener to "regulate" the power, or do you also use an LDO regulator?

These seem like really nice little batteries that I wouldn't have considered otherwise. Cheers!

He is referring to using an ordinary rectifier diode or small-signal diode in series with the battery since the diode will drop ~0.6v.

A zener or linear regulator will use current regardless of the load they are regulating, nullifying the savings of the PIC's sleep mode.

 

[spuffock]

I favour the cr2032 lithium button cells. OK, theyre primary cells,but if you qualify anything that takes more than ten microamps average as a current hog, they do fine.

 

[edeca]

A zener or linear regulator will use current regardless of the load they are regulating, nullifying the savings of the PIC's sleep mode.

So I wondered how much it really ruins the sleep savings? I just conducted a quick survey of LDO regulators. I picked the first three on Farnell that matched these characteristics: max 50mV dropout, min 50mA output current, 3.3v fixed output (I also excluded any items out of stock or not available in the UK, in the interests of complete openness!). All three of the results below were £1 or less.

The typical quiescent currents were 400uA (Texas TPS73133), 170uA (Texas TPS79133) and 1.1uA (ON Semi MC78LC00).

So with a 1000mAh battery, at 0.4mA or 0.17mA you aren't going to get a full year from the LDO alone even if the PIC is in sleep mode. But from the 1.1uA you are going to get more years than I will probably be alive!

I'll try and use sensors that don't require the internal ADC or a highly accurate Vdd/Vref, that could work nicely with a single diode drop. But given the calculations above, is an LDO really the spawn of the devil?

Edit: As 3v0 just pointed out on the chat, the LF PICs will happily handle up to 5.5v Vdd. So no real need to voltage regulation as long as the sensors support it too. Could always have a switched Vdd for peripherals using a small transistor or FET.

 

[DirtyLude]

Not so much for your application, but if you are running an MSP430 off of a coin cell battery and just waking up for external triggers like button presses, then yes, they can be an unnecessary drag.