LiFePO4 (3.2V) Project, Within a 3 X AA Battery Holder!

Thinking "inside the box" ? Check this novel electronic project case, which is formed from a partially gutted 3 x AA cell switched battery holder !  Circuitry is mounted on a sliced solderless breadboard, while a "Kiwi Board" layout suits a final soldered version.  Power is via a single 3.2 V  LiFePO4 (Lithium Iron Phospate)  AA sized rechargeable cell.  Yes - just one cell- do away with multiple cells and their associated matching,aging,charging, housing, connection & terminal contact woes !

This 24 step Instructable covers -

* LiFePO4 cell features & cautions
* Battery box basics
* Partial gutting of a 3 x AA battery box
* Solderless breadboard etc options
* Railed breadboard slicing.
* Circuitry to suit

 

 

Step 1: LiFePO4 ("LFP") Rechargeable Cell Features

LFP batteries are still quite new & their long term features have yet to be fully verified, but their claimed characteristics include –

* An output of ~3.2 V, which remains quite steady under load, only falling in the last 5% of capacity (Li-Ion starts near 4.2V but falls progressively to ~2.7V, while Lead acid is nominally 2V, and NiCd/NiMH is only 1.2V)

* Lightweight and compact – extremely good power to weight ratio (appealing for motorcycles etc)

* Require constant current (CC) charging, which then tapers off as 3.6V constant voltage (CV) is reached. Note –LiFePO4 cell voltage settles back after full charging to ~3.3V, with 3.2V being the usual quoted value

* A specialized (but cheap) charger should be used, although simpler approaches may suffice in a pinch (providing LFP charging needs are respected !).

* No memory effect – cells can be charged/discharged at any state.

* Extremely low standby losses.

* Modest but appealing Ah (Amp Hour) capacity (but lower than comparable Li-Ions)

* A cycle life of several 1000 times (and far greater than Li-Ion’s annoying  and costly 100s)

* Can be near fully discharged (although 2.5V is the recommend cutoff), but will probably be ruined if totally flattened.

* High charge (~1C) and discharge (~10C)  –both rates however lower than comparable Li-Ions. ( “C” refers to the capacity in Ah, with 700mAh being 1C for that AA cell type)

* Quite safe for all discharge applications, as the cathode is non flammable and stable. No lithium remains in the cathode of a fully charged LFP cell.

* Excellent sub-zero and elevated temperature performance.

* Environmentally benign (“green”) in manufacture, usage and disposal -no hazardous internal contents.

* Capable of even further performance enhancement when doped with Yttrium (Y -pronounced “it-tree-um” and a common element- found apparently in cabbages!). Such cells are titled LiFeYPO4 (LFYP).

Step 2: Charge/Discharge Curves

Note the flat discharge curve!  Most of the cells energy has been used by 2.8V, which is a common LFP low voltage cut off point. However 2.5V is acceptable, but cells should never be let to run down below 2V or they'll be ruined....

LFP cells should be initially charged under CC (constant current) to 3.6V, then held at CV  (constant voltage) when this is reached.  After such charging they'll settle back to their 3.2V supply level

Step 3: Sourcing Cells

At the time of writing (April 2013) LFP cells and batteries are still elusive at traditional outlets. Specialists are beginning to stock them, especially as 12 V LFP batteries for performance motorcycles or demanding standby solar power applications. (Usefully 4 cells x 3.2V gives 12.6V, and smart LFP charging at 14.4V (4 x 3.6V) is comparable to traditional 12 V lead acid systems).  

My selection of cells and dedicated LFP charger were initially obtained from a specialist NZ firm, but prices were noted far cheaper via direct imports from Hong Kong outlets which focus on global battery sales. Although concerning for international air freighting, feedback from radio controlled plane enthusiasts indicates such direct lithium battery orders thankfully arrive in very rugged protective packaging.  Here's the order - post free- for 6 cells, 2 placeholding dummies and a smart LFP charger!  It arrived trouble free about a week later here in New Zealand.  (Note however that China & NZ beneficially have a free trade agreement)

Step 4: Skinflint LFP Care - Approaches

With such reasonable prices for LiFePO4 cells,chargers & accessories it's probably hardly worth mucking around! Versatile smart combo LFP/Li-Ion/NiMH chargers are available as are also single LFP cell USB chargers.

In a pinch however simple DIY approaches may suffice - LFP cells are quite tolerant!.

* Dummy cells can be made by a nail or screw trimmed to length & housed in a piece of slim bamboo,dowell or plastic etc

* Charging could be via a current limited 3.6V source - bench top power supply or even 3 series NiCd/NiMH cells?

* Battery state could be monitored via a DMM (Digital multimeter) or even a white LED (which  is bright at 3.6V, dims <3V & goes out by 2.5V).   More sophisticated approaches using an adjustable regulator diode (TL431 etc) may have mileage too.

Step 5: Alerts !

User awareness may be the key to LiFePO4 AA cell uptake, as series dummy place holders must be specified with great certainty. With each LFP cell delivering 3.2V at high currents, yet in appearance similar (in AA form) to normal 1.2 -1.5V cells, particular care should be taken to avoid accidentally over supplying devices!  The likes of  6.4V (2 x 3.2V) in a digital camera designed for only 3V ( 2 x 1.5V) will almost certainly ruin it’s electronics…

Step 6: Charging Alert!

And keep a clear head when charging too ! Even a  smart  LFP charger will be fooled/damaged if charging the the placeholding dummy cells is attempted.  The yellow dummy cells are quite easy to identify as they're very lightweight,but they strictly should also be BOLDLY labelled as to their nature. It's easy enough to make your own distinctive dummy cells of course, simply with a filed to length nail/screw inside a suitable piece of bamboo/plastic tube etc.

Step 7: Commercial Solar Sensor Light (via "The Warehouse" Outlets - NZ)

Here's the first dedicated LiFePO4  consumer item noted - a solar powered  PIR sensor light. Although pleasingly very bright the fixed PIR,PV & LED positions rather limited applications, as the preferred position for a night light is not always where the daytime sun is most available for charging!   Quick charge/discharge current measurements made on an opened unit indicated that good sunshine will be needed to ensure the unit operates well (& that the "AA" sized 500 mAh  LiFePO4 has a prolonged life).

Extra: One of these "yard lamps"  was installed at my NZ home in  March 2013 ( NZ autumn), and angled on a home made flexible mount for best illumination & PIR sensing. It's now 6 months later &  our spring, and the lamp has run perfectly all during our winter. Sensing is out to ~10 metres, with triggering by even a night time prowling cat.  The LED is wonderfully bright- folks think it's a mains powered halogen- and the pilot LED alone is bright enough for orientation.

Perhaps best of all it's RELIABLE as the single cell means the classic bugbear of dirt & corrosion at multiple cell connects is not an issue of course!  This is far & away the best value yard light I've run across- it's hard to credit that it uses just a single AA sized cell.  Highly recommended, although some fiddling with positioning may be needed to tradeoff sensing/charging/lighting aspects.

Step 8: A More Appealing Variant

This Bunnings sourced version looks even better, especially since the elements can be individually adjusted.  The single cell is titled & labelled as a Li-Ion but it's 3.2V terminal voltage is a giveaway that it's actually a LiFePO4.

Step 9: Battery Box Basics

Switched AA battery holders are cheap, reliable,neat & convenient and  widely available in 2, 3 & 4 cell sizes. Black is most common, but transparent coloured types may have appeal too- perhaps for "show off" circuitry!?

The 3 x AA type is probably the most popular since it'll provide  3  x 1½V  = 4½ V with C-Zn cells or 3 x 1.2V = 3.6V with NiMH types.   Although a 4 cell type could have been used for this instructable, a partially gutted 3 cell holder was chosen as removal of 2 cell spaces provides efficient use of a sliced breadboard/KiwiBoard.

Of course what makes such an novel approach feasible is that just asingle LiFePO4 AA "14500" (14mm x 50mm) cell delivers a good 3V - 3.2V, which is quite enough to power a wide range of today's energy sipping circuitry!

Step 10: Battery Box Conversion

Battery holder cell contacts are just push fitted and can readily be removed with fine pliers. Just take out the "doubles" initially.  Do not remove the bottom RH corner spring contact, as this is connected to the switch and must be retained!

Step 11: New Positive Contact.

The red wire positive needs to be moved over to a new position to suit the single AA cell power source.

Step 12: Rib Removal

The rib ends between 2 cells can now be snipped- pushing with thumbs will readily snap it off at the base for removal.  Some residues may remain & influence breadboard placement unless flattened -the end of a file was found suitable for this.

Step 13: Circuit Layout Considerations

Now comes the question of what can be put in this space!  The volume revealed is 30mm across x ~50mm long x ~ 12-15mm tall, which is quite generous for modern circuitry needs, although somewhat limiting for tall components.

As solderless breadboards have long been the recommended way to initially evaluate circuitry their use was considered here- soldering of course should be the LAST thing to do.

The small 170 tie point (10 x17) colourful breadbreadbords shown above may tempt, but their sourcing can be an issue and ( even when turned sideways) they're  too wide (by ~4mm) for the slot.  Furthermore they annoyingly lack valuable side supply/ground rails. 

Step 14: Breadboard Basics

The more widely available larger breadboard shown is usually preferable,as it's top and bottom rails allow convenient supply and ground taps. Double rail types can also be used (& they'll fit when sliced into the holder OK), but it's recommended that a top/bottom rail supply system is still followed. Marking with a red (+ve) and black (GND) spirit based felt tip pen helps clarify these rails.

Step 15: Kiwi Board

Although veroboard/stripboard or a dedicated PCB could be used, so called "Kiwi Patch Board" suits the final soldered version. It matches solderless breadboards & it's silk screening makes it easy to "paint by number" when lifting components & links over from the tamed breadboard. Naturally it's lower profile will allow taller components to be housed too, and the extra rails in the centre "channel"  give convenience.

The approach recommended is still  to layout & evaluate on breadboard & -once the circuitry is tamed- finally solder up on suitable piece of such Kiwi Board.

Step 16: Breadboard Trimming

The procedure now involves -gasp!- slicing a "railed" breadboard into a smaller size. It 's becomes 130 tie points, of 11 x 10 (+20 tie supply rails) &  enough for numerous easy circuits,especially if  8 pin  microcontrollers are used.

Begin by revealing the "wings" (only normally used for mounting) under the breadboard ends. Use sturdy side cutters to snip thru' the plastic ribs, and bend them off.

Step 17: Marking Out

Half way marking pre board slicing. Ensure the mid point 12th column is chosen - this allows 11 sets of contacts a side - quite enough for many simple circuits

Step 18: Removal of 12th Row Clips

Use a scapel to cut thru'  the underside backing at the 12th column to reveal the 2 clips. Cutting is much easier & neater with these removed

Step 19: Sliced Breadboard!

A fine blade hacksaw best suits board slicing - use a vice to hold things steady .

Step 20: Matching!

The sliced breadboard will be slightly too wide for the 30mm across holder space. Rather than forcing things (which may make the holder lid difficult to seat) it can be filed or sanded  to suit.

Step 21: Finishing to Size

 Sand or file slightly to allow a smooth fit into the revealed battery holder space.  Coarse sandpaper was found most suitable -naturally work on the breadboard side that has plastic edge to spare!

Step 22: Ready to Populate.

For breadboard work the supply and ground wires can be shortened somewhat & soldered to plug in header pins. For KiwiBoard however solder directly to the supply tracks, or perhaps a couple of header pin sockets. Don't shorted the black & red leads too much - you still want to be able to ease out the board of course.

Note the alerting LFP label in the single AA cell slot. Users may otherwise assume a standard 1.2-1.5 V AA cell type is needed and wonder why  the circuit fails to work!

Step 23: Breadboard Circuitry

Argh-avoid such a messy breadboard layout!   Try to run wires and components low profile and neatly. Use simple colours for wiring function - red supply, black ground, blue signal etc.    Avoid burying any ICs as they'll be then both difficult to identify and remove !

Step 24: PICAXE Example

As a proof of concept trial a single AA LiFePO4 powered PICAXE/ Dorji  433 MHz beacon transmitter circuit was developed and housed in a partially gutted 3 x AA switched battery box. The 3 blue breadboard marks above are the programming points. Assorted low voltage cutoff sensors & PICAXE driven software (especially the PICAXE-08M2 "CALIBADC" command) were considered, but initially just a dumb test LED was used for simplicity. It reminded me of the external "see the power" power-check strips featured on some Duracell alkaline AAs in fact.

I'd only 5mm white LEDs at hand (although naturally a smaller white could be used), but all showed significant dimming below 3V & (MOST usefully for LiFEPO4 !) a total light cutoff by 2.5V -a near perfect matching! Such a simple battery state test could also be included with dumb circuitry (discretes, 555 etc) housed in a similar partially gutted LiFePO4 powered box.

The circuit simply sends an occasional Morse ID beacon tone transmission on the 433 MHz ISM band, and then sleeps at very low currents for an adjustable time. Battery life of the single LFP is estimated as being several weeks due to the low duty cycle. More Dorji transmitter details can be found => www.picaxe.orconhosting.net.nz/dorjiask.pdf (especially P.6).   My "Tape Measure Yagi" Instructable may also be of interest => www.instructables.com/id/433-MHz-tape-measure-antenna-suits-UHF-transmitte/

Step 25: Schematic & Conclusion.

Conclusion: Lithium Iron Phosphate (LiFePO4 /LFP) rechargeable cells look to have a very bright future ahead. Their cheapness, light weight, high cell voltage, steady discharge level and abuse tolerance make them attractive in numerous applications where primary and secondary cells are presently used. On safety grounds alone they may well become preferred to concerning Li-ion/Li-Po types, especially where case damage or overheating may occur.

Although not so much of an issue with cell phones and tablets (where rapid upgrading is the norm) LiFePO4’s claimed 1000s of charge/discharge cycle life may further appeal for demanding electric and hybrid vehicle use, as Li-ion battery packs for electric cars and bikes can be both costly and short lived.

AA "14500" sized LFP cells are cheap,tolerant and energetic, but their higher 3.2V supply voltage makes them potentially damaging to consumer devices if confused with normal 1.5V cells... It's crucial that labelling is read & their nature understood !

Note: This Instructable links to a Lithium Iron Phosphate cell article in the June 2013 Australian "Silicon Chip" electronics monthly. Quite aside from the LiFePO4 insights the layout was motivated by the "potential" of the switched AA battery box, as discrete switches & suitable project cases can otherwise end up costing more than the internal electronics!

LiFePO4 resources, plus details of the circuit above (& driving code) will be held at =>http://www.picaxe.orconhosting.net.nz/LFP.htm