LiFePO4 batteries and chargers

[Pat Tressel]

A question came up over in the Control of Mobile Robots class offered via Coursera about rechargeable batteries (ok, I asked the question :D ).  They have an option to build a small differential drive robot, and suggested LiPo for rechargeable batteries.  Here's what I posted:

A question about the rechargeable batteries:

LiPo batteries make me nervous...
http://techtalk.pcpitstop.com/2013/07/24/laptops-are-still-exploding/
http://en.wikipedia.org/wiki/Lithium_polymer_battery#Risks_and_limitations
...so I'm wondering about LiFePO4 instead. 
http://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery#Safety

(A robot tricycle project I help on uses LiFePO4 -- the batteries are directly under the rider, so we didn't want anything that might burst into flame if punctured.  The project lead is an avid cyclist, and his ebike uses LiFePO4.)

LiFePO4 cells are nominal 3.2-3.3v versus nominal 3.7v for LiPo cells.  Here's 3-cell LiFePO4 pack from the same vendor as the LiPo on the parts list:
https://www.hobbyking.com/hobbyking/store/__14076__ZIPPY_Flightmax_1800mAh_9_9V_5C_LiFePo4_TX_Pack.h...
Nominal voltage is 9.9v and max current is 1.8Ah versus the 11.1v, 1Ah LiPo battery on the parts list.  This seems to be ok for the listed regulator.

What do you think?  Would that be an ok replacement?

I haven't looked for an appropriate charger yet.  I'll ask my friend with the ebike that uses LiFePO4 batteries.  I do know he buys from BatterySpace but their prices are higher than HobbyKing.
http://www.batteryspace.com/96lifepo4batterypacks.aspx

So...  Tyler (et al.) --

What sort of charger was it that we were using for the small LiFePO4 battery pack?  Did it come from BatterySpace?  Was it usable with both 3-cell and 4-cell packs?  (I've forgotten whether the small pack had 3 or 4 cells, but I'm guessing it had 4, to match the large pack's internal wiring.)

The prices I was seeing for 3-cell LiFePO4 packs at BatterySpace were on the order of $25, versus $11 at HobbyKing.  Should I be wary of lower-priced LiFePO4 batteries?  Is BatterySpace just a generally reliable dealer, or are they good for a technical reason?  (Their web site hints that they might be doing some battery innovation, or they might be sourcing from manufacturers that are.)  Or are there just favorite vendors in the ebike community that differ from the favorite vendors in the RC community?  :D

Thanks!

-- Pat

[Danny Dunn]

I have only ordered once from hobbyking,  but it was in fact a small lifepo4 battery and a charger. I never actually tested capacity but I'm happy with it and I've heard others say they are too.

I also bought a turning accucell 6 charger for it which will charge and balance almost any battery up to like 6s in almost any chemistry. It can also run off a 12V battery directly.

I've also heard batteryspace as being the favorite but I don't know much about them.

Lifepo4 is safer and will give you twice the lifespan. Down with lipo!!

If it's within the voltage spec for the regulator it should work just. Just keep in mind the discharge curve profile of lifepo4 and make sure to never over discharge.

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[Pat Tressel]

Danny --

I have only ordered once from hobbyking,  but it was in fact a small lifepo4 battery and a charger. I never actually tested capacity but I'm happy with it and I've heard others say they are too.

That's definitely good to hear.  I'm always a tiny bit hesitant to order from someplace I haven't heard of.

I also bought a turning accucell 6 charger for it which will charge and balance almost any battery up to like 6s in almost any chemistry. It can also run off a 12V battery directly.

That's this guy?
http://www.hobbyking.com/hobbycity/store/uh_viewitem.asp?idproduct=7028

I've also heard batteryspace as being the favorite but I don't know much about them.

Lifepo4 is safer and will give you twice the lifespan. Down with lipo!!

Amen!!!

If it's within the voltage spec for the regulator it should work just.

It's ok for the regulator that was included in the parts list, but there was some griping about that particular regulator...I'll add some info on that below in case you have recommendations.

Just keep in mind the discharge curve profile of lifepo4 and make sure to never over discharge.

Right.  I gather they maintain voltage til right to the end then drop sharply.  So it would be good to have something that shuts down when the voltage begins to drop...hmm...

Ok, the regulator issues...I'm summarizing but hope I'm retaining the main points:

They chose an LD1085 regulator in order to supply the 0.8A that the BeagleBone Black with wifi draws.  But this is a linear regulator, and will get hot, and requires a heat sink.  An alternative that wastes less power and stays cooler is a switched regulator.  Several were suggested that are from DealExtreme, but the long shipping time was a drawback.  The eventual suggestion was OKI-78SR-5/1.5-W36-C which is available from DigiKey and Newark.  Another person suggested getting an LM2596 on eBay.

Whaddaya think?

-- Pat

[Tyler Folsom]

Pat -

Yes, I have used Battery space.  On my e-bike I have a 36V pack and charger. It works well.

On Elcano, we have 4 - 12v packs, with 36V in series and 2- 12V in parallel. All four cells are placed in parallel to charge from a 12V charger. If the charge does not come out right, 12V cells can be charged individually.  Yes, this is a screwy arrangement, and often does not work right.

My blue trike uses BionX lithium cells, which is a bit more exotic chemistry.  It needs to be recharged monthly, and has failed twice when it has been neglected.  The first time it was replaced under warranty; the second time I was out of luck.

The Elcano lead-acids are 3-12v cells in series, with a tap from the 12V cell for the actuators. It has been reliable.

The UWB trike has four 12V lead acids, with a separate one for the actuators.  Someone tried charging a 12V cell from a 36V charger. The battery got hot, and there was permanent swelling, but no fire, and the cell still seems to work.

- Tyler