Preferred LiFePO4 USB charger/regulator IC

1 WattWhat?

We are using 800mAHr on 3.3V, so around 2.7 WattHour.

People are leaving essential info on posting, which is really draining my mind reading power, especially over the net.

So, you just want to "regulate" what goes *into* the battery (i.e., nothing between the battery and load -- just between battery and USB)?

Are you sure the design will operate with battery at "full charge" (Vbatt(max))? I.e., just because you only charge it to 3.2V doesn't mean you will never see anything higher on Vbatt (3.6V). And, do you care about how it behaves when the bottom falls out? Anything that disconnects the load so the cell doesn't run into the ground?

He's stating the size of his load. He hasn't expressed any preference for how long the battery needs to last --so, no claims as to how long it will take to *charge*, etc. I.e., your (below) battery mmay give him an hour or two of runtime. It may give *you* TEN hours, depending on

*your* load!

my mind reading power, especially over the net.

Hence, my joke about reading his mind over the net. We need to know the si ze and type of battery involved, as well as load. For example, you can pul l half a watt out of a 3V 200mAHr coin cell at 5mA. But at 50mA (typical 3

2 bitter), it would not last more than one-tenth of a watt. So, it's bette r to spec the battery several time more than your load.

We looked at all kinds of batteries, including LiFePO4, but end up going ba ck to NiMH. We can get 800mAHr (3 x AAA) for $2 and 400mAHr (3 x AAAA) for $1.5. Nothing Li* for less than $10.

No. You can pull 0.5W-Hr out of said coin cell. But, you're only pulling it at a rate of 15mW (5mA * 3V).

His point is that his instantaneous current draw from a 3.2V supply is ~350-600mA -- regardless of the "capacity" (W-Hr) of that "battery". He wants a charging controller that will be able to eek that out of the USB connection -- dumping any "extra" power (not used by his load) into the battery.

I.e., USB port gives you 2.5W (500mA @ 5V) and he wants to (optionally) use ~1-2 of that in his load and the rest into the battery. Depending on the state of his battery (AND his load, at the time), the amount of time to bring the battery back to full charge will vary.

[And, will obviously be longer if he has a larger "battery"] [At least, that's how *I* read his post! :> ]

Don has it right.

I gave exactly the information that you required. The drain is as stated. If I want longer run-time I'll use a bigger cell.

I don't care about the charge time, as long as it shuts itself off safely. The device is mainly used mobile, when it's not on charge.

Don, Thanks for asking about the maximum voltage. I expect that the circuitry will survive 3.6 volts happily, and have to check all the data but possibly even 4.2V. So I don't think that's an issue.

The CPU can monitor the battery and shut down to minimal draw on low voltage limit.

Now, can we get back to the question? What devices do you prefer for this, or can you warn me of any other difficulties I might run into with the ones I mentioned?

Clifford Heath

Do you run it while charging? Our device use around 100mA active, so we de sign a charger that shut it down and draw around 10mA + charge current. So metimes they might leave it charge for days. One good thing about NiMH is that you can just leave it at 4.2V for a long time without much damage. Ho wever, LiFEPO4 charge at 3.6V. No sure about overcharging issues.

Double check on that. My PIC32 (and many other ICs) top out at 4.0V. That is likely a factor of current fab process. So, i would not be surprise to see many chip max out at 4.0V.

We are still evaluating different approaches. We are currently using two w ays: 1. Charge at constant 4.2V and LDO to 3.3V

1. Charge at 4.2V and relay to 3.6 (and slowly drop to 3.0V).
2. is difficult for LiFePO4 with charging at 3.6V. The battery quickly dro p below critical level, unless we get an expensive LDO.

Remember that the characteristics of the port into which your device gets plugged will also have a role in charging time!

I.e., the amount of "power" that you have available to your device (which must be used by charging + operating -- if the user opts to leave the device "on") varies considerably.

If a low power port is feeding your device, you'll have ~100mA to play with -- instead of ~500mA.

There are tolerances on the supply voltage available *from* the port. Plus, potential losses in any interconnecting cable by which THE USER decides to tether you to the port (I think 5m is the legal limit?). So, you'll see IR drops in that cable as well.

E.g., you could have, nominally, 2.5W available (5V@500mA) or even as little as ~400mW! (low Vusb and low power port).

Don't underestimate the difference (factor of 5 or 6) in terms of charge times! *And*, the impact having the device "powered on" while being charged (i.e., user's may complain that the device

*never* charges -- if you are consuming 2W and they are on a low power port, etc.).

Look at the battery spec. Even if *you* can't charge it to an "unacceptable potential", a "new" battery may be "pre-charged" to a level that causes you grief!

[I'm not going to worry about my misuse of the term "battery" here :> ]

You just want to make sure a device sitting on a shelf for "some period of time" doesn't KILL the battery (assuming replacement batteries are expensive, annoying or "unsupported".

I've been kicking the can down the road, so far, regarding my needs (wait for all the software to be done before designing the hardware). So, can't point to a "preferred device".

And, not sure how detailed warnings you need... :<

E.g., hot plugging/unplugging always has design consequences. If you are looking for hobbyist quantities, some "SMT" devices can be carefully soldered "by hand". Others (e.g., those with thermal connections under the device -- common for power handling devices esp in small packages! -- which complicate "hand assembly".

If you can afford it (space), make it easy to break the foil to the battery so you can insert a current probe, etc. You can usually work-around this but remembering that you are

*likely* doing to want to do it can save you an OhShit moment later.

From a consumer's point of view, any *holder* you can provide into which a COTS *tabless* cell can be inserted will get lots of first-born children named "Clifford" in your honor! :> OTOH, with a single cell, maybe they won't be *as* ecstatic (as they would with a multi-cell BATTERY) :<

Consider whther or not you want to also allow a "wall wart" to power (or charge!) your device. E.g., a GPS tends to see much more use tethered to a power source than a USB source *or* completely portable. There are consequences inthe power handling there, as well.

AND, clearly/unambiguously indicate connector polarity and

*dimensions*, if possible! The morons who decided for the typical barrel connector legend:

• ---- . )------ -

should be skinned alive! The ")" representing the sleeve of the barrel often appears as a CLOSED CIRCLE -- because the entire symbol (including the + and - "raining wheels") has to be reproduced at such a small scale! A smarter approach would have been a + or - INSIDE a circle (indicating the polarity of the center conductor wrt the outer sleeve).

As a result of these nitwits, we'll be stuck with this stupidity for another generation! Sheesh!!

I'll try to move to a machine with a PDF reader so I can see what, specifically, the devices you've mentioned are/do.

What he said...plus. Is this a REAL USB device that can negotiate power levels with the host? Or are you using the host as a dumb power supply? Is this a production device that is required to meet, and be tested to international safety standards? There's lots of stuff in those standards that many people would consider redundant, irrelevant or not applicable to your particular situation. Those people would be the future unemployed.

I have been doing the charger "myself" on NiMH, NiCd earlier and intend to do so for Li-whatever when I need to. "Myself" meaning that I have written the charge/maintenance code, run it on the smallest MCU in the system (well last time it was the only one, an MCF52211). Takes feedback for voltage at the charger input, voltage at the battery, current flowing into the device from the charger and current flowing into the system (i.e. from the battery). Precision is considered as needed in the particular case, charge current is PWM regulated (stepdown convertor). So far has served me well - since I do not have large quantities to build I want to be able to trim things without soldering etc.

Don's max. voltage reminder may be less easy to shrug away than it seems though. I remember some years ago I had a tiny MP3 player which worked for a while until on a very hot day it died (while not being in use). I opened it, the MCU clock was not starting. My main suspect was overvoltage kill (I did not go much further to verify that though). It had a Li-ion battery which powered it unregulated.

Dimiter

------------------------------------------------------ Dimiter Popoff, TGI

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Actually, on a related note... monitoring the temperature of a battery (esp under charge) is usually worth the extra component cost. As batteries fail (lose the ability to take a charge), they often get a lot warmer during charging.

Many charge controllers have provisions for a temperature sense input. And/or, safeguards (often simple timers) to ensure a battery doesn't keep calling for charge indefinitely!

Finally, it's worth the effort to "qualify" your battery provider. Impurities in cells can lead to spectacular results! :> (not the sort your lawyer/accountant will enjoy)

Of course a temperature sensor is good. Usually the problem that comes with it is the necessity for as many sensors as battery cells and ensuring a good temperature contact mechanically. On the Nukeman I had put a single sensor (it had 18 cells, mind you) which was just to detect overheating. But NiMH being full can still be detected by the dropping voltage while being charged (because of heating). Worse than NiCd but still usable. Then I also put a max. battery voltage limit to the regulator (which regulates for current) - this also allows the thing to run with no battery connected - and some sane time limit to the charging in case slope detection fails or if I want to set the charging to too low a current for slope change to take place.

Dimiter

------------------------------------------------------ Dimiter Popoff, TGI

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I think many OEMs get away with a single sensor by expecting the cells to be from the same manufacture batch -- and expecting their behaviors to largely "track".

Likewise, expecting this to continue to be the case thereafter (replace the BATTERY and not just individual cells out of it).

Clifford is apparently using a single cell design so one sensor covers "all cells" :>

Yikes! Because you needed ampacity (wide) or potential (stacked)? (as well as CAPACITY)

Batteries are a royal PITA! They make a device infinitely more useful (depends on device) but bring so many "side effects" to the design (and use). Esp for non-replaceable designs!

I can't count the number of things, here, that have lost their utility simply because they weren't used *often* enough to merit keeping them charged. And, as a result, the "batteries" have died inside. Cheaper/easier to replace the entire device than to hope to find a battery replacement! :<

[Hence my comment to Clifford re: "battery holder" to endear him to DIY customers!]

Both, generally I needed many hours (was it 6 or 8...) autonomous operation (here it is:

) . I had two groups of 9 batteries in series each tied in parallel. I was wise enough to have them discharged before making the parallel connection, obviously, and was careful enough to measure the voltage before that, but generally things worked quite OK. I had mounted each cell to be pressed against its parallel counterpart and that was all, the battery lasted for years. Used those 76mm long 17mm diameter cells, the whole pack was 75Wh. Not that much more compared to what todays laptops have, nor was it much larger. Somewhat heavier than equivalent Li-ion of course.

Dimiter

------------------------------------------------------ Dimiter Popoff, TGI

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Hi Dimiter,

[still 30C, here! Ice >> Yikes! Because you needed ampacity (wide) or potential (stacked)?

Hmmm... I guess I would have tried to treat them as separate packs charged independently with their outputs merged through sharing "diodes" (or actively)

I'm treading carefully on my battery requirements. Trying hard to find ways to "eliminate load" so I can reduce the size and complexity of the "battery". But, I'm more constrained on space/volume than you are so I don't have much choice to "add capacity".

I guess I'm hoping for a "breakthrough" before I *need* it! :>

My current strategy is to make devices inexpensive enough that you can buy several and just swap them out as their batteries need recharging. Even if that means carrying a "spare" with you...

No, summer was half as long as it should have been but I guess that paid for a delayed cold. Still around +15C max/5C min, fairly merciful for mid November.

These were (well, must have been) my initial thoughts back then (20 years ago when I first did it this way). But things are not that bad, really. Given that I sell a battery pack (cells soldered to a PCB) and I buy cells of the same batch, have parallel cells stuck to each other, it makes little difference to whatever they have wound inside the batteries whether it is in one or in two packages. Once you charge the whole pack and keep for a day or two on trickle charge things level out quite OK.

Hah, aren't we all (for this or some other breakthrough) :D .

That's of course a good strategy but not applicable to my low volume (sometimes one-off) designs, hence the different approach. I have yet to make a Li-ion charger though, hopefully my bold approaches don't put me in the news :D :D :D . (I think I am wise enough not to be too bold with these things but then who knows.... :D ).

Dimiter

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

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Ah, OK.

I suspect for an instrument with the level of "investment" as yours, you probably don't encounter folks trying to replace batteries "on the cheap".

Sometimes we get lucky! :>

Some of my devices are in the "coin cell" ballpark. If you're already that small, adding another cell is a big increase in volume, etc. (One I am targeting as a "large wristwatch")

Of course, the advantage of that small size is that it is not unreasonable to stuff a spare in your pocket in case the battery in the first is exhausted and needs to sit on a charger for some time...

Initially, the latter. Using USB connections means chargers are readily available.

Not a concern at this level - it's a ham radio device (fox transmitter and sniffer receivers). The transmitter is programmable (and the RTC is settable) but that's through the iButton port which is also used for event scoring - not through the USB port.

I don't want to roll my own charger, although it's not hard to do a basic-level job (current limit into a 3.4V regulator).

The devices have very low quiescent current (uA) so I'm not worried about the battery going flat on the shelf.

LiPo cells are much less stable (need temp monitoring and cutout) which is why I'm choosing LiFePO4. I should only need low-voltage cutout.

Thanks all for your comments. It appears no-one wants to recommend a chip for this, or perhaps has even used one.

I'll be off USENET for a couple of weeks now. Any further comments would be appreciated, even if I don't respond quickly.

Clifford Heath.

I'd be interested in learning more about the device. Back in the early '60's we had transmitter hunts on 6 meters. I was about 15 years old at the time. Went to my first hunt and watched guys driving their mobile rigs around watching the S-meter trying to find the hidden transmitter. Second hunt, I took a 6-meter converter cobbled onto the front of a transistor radio and a loop antenna. Took one direction reading. Drove 90 degrees away from it and took another reading. Then drove right to the hidden transmitter. Third hunt, nobody else showed up. Good times...

As for your charger...I'd go to Best Buy. They've got a recycle bin for cellphone batteries. Talk the guy into letting you have a pile. Check out what chips they use.

These guys have some battery protection devices and cells with such devices already packaged

I bought two, but never used 'em.

There are protected batteries for flashlights in various sizes with chargers available at the same place.

I don't think your problems are technical. Depending on how long you plan to sell and support the device, finding something that will be available in the future (next week) is a big gamble.

If we have the same concept of a transmitter hunt, I think the rational choice is 3 AAA alkaline cells or thereabouts. Not everything in the world needs to be rechargeable.

Great fun, yeah?

I learnt to hunt with the Melbourne crew (VK3's), who have 30+ years experience and an amazing range of talents. Google for VK3VT (now VK3FOX), VK3MZ, VK3YNG, for some of the most prolific constructors. Almost all the gear is home-built, and these days, the most sophisticated crews have fully integrated in-car systems with motorized rooftop antenna arrays (including in-car control of polarisation in one case), 2m dopplers, Watson-Watt receivers, etc, all feeding into custom-made SDR systems with real-time digital spectrum display (brick-wall filtering), automated bearing extraction from the returned antenna lobe display, drawing vectors on a moving map display, etc. The annual hunt with the VK5's at Mt Gambier in June has a big five-leg night hunt, usually spread across 3 or 4 bands. Regular hunts are on

80m, 10m, 6m, 2m, 70cm and 22cm. Foot events are on 2m mostly. When we run ARDF events (see below) they're 80m or 2m.

6m hunts are interesting in the pine plantations around Mt Gambier. Young pines are planted on a 3m grid, then thinned to a 6m grid. If you watch the vector display on my team's Watson-Watt as the car creeps forward, the bearing does a full 360 swing for every 6 metres you drive forward. Very hard to DF!

Anyhow, I'm in Sydney now, and the barrier to building up such systems is very high, so I'm going back to basics, building 80m gear for ARDF and especially sprint ARDF, events held for runners on foot. ARDF events use five transmitters on the same frequency, transmitting one at a time in a five minute cycle (or for sprint, a one minute cycle, twelve seconds each). My new 80m fox is about the size of a cigarette packet, and pumps out up to a watt. An MSP430 will lock the VCO frequency to its

32KHz clock, while also doing Morse code modulation, managing the ARDF cycles, keeping real time, and running the iButton scoring that I built years ago. that's the plan anyhow.

The receiver front end has been designed to feed an Si4735 digital radio chip, for which we have an SSB software patch. It will use a loop or loopstick and a sense wire, phase shifted and mixed to produce a cardioid - hunting on the null. In future I'm keen to try SDR directly in an ARM7 CortexM4 after down-converting to a 45KHz IF, a broadband DF antenna for HF to build and try, and maybe a hand-held Watson-Watt with a colour cell-phone LCD display.

I'm not concerned with cells, and don't want 3.6 or 3.7 volt Li-ions. I'm concerned with USB charging for LiFePO4, which charge to 3.4V, and don't tend to explode at random. They're used in portable drills etc, which have a higher robustness and safety requirement than your typical phone or MP3 player.

Nice little device. I might have another use for them.

Been there, done that. Alkaline is fine for sniffers, but you burn a lot of cells transmitting one watt or more... and there are six or eleven transmitters for an event (including the homing beacon). They need to be rechargable, and if I do it for the foxen, why not the receivers too?

Clifford Heath.