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
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
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!
Step 7: Commercial Solar Sensor Light (via "The Warehouse" Outlets - NZ)
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
Step 9: Battery Box Basics
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
Step 11: New Positive Contact.
Step 12: Rib Removal
Step 13: Circuit Layout Considerations
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
Step 15: Kiwi Board
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
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
Step 18: Removal of 12th Row Clips
Step 19: Sliced Breadboard!
Step 20: Matching!
Step 21: Finishing to Size
Step 22: Ready to Populate.
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
Step 24: PICAXE Example
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