Li-Ion battery stack

It has been discussed at various companies I've been associated with. The problem is the manufacturers don't like the idea since it makes packaging complicated. Of course it would be good for the semis since it means more chips or at least more complicated chips.

I'm not a motors and rotors person, but aren't motors really current driven, not voltage.

The motors I am thinking of are usually BLDC types, ie permanent magnet brushless. So they have a no-load speed related directly to the DC supply voltage. At that speed the back EMF from the motor is ( nearly ) the same as the applied voltage, so little current flows. As the motor motor load increases, it slows down, and the resultant voltage difference draws more current from the supply.

I guess 'more complicated' depends on your viewpoint. The motor drive circuit is more complex, but the charging circuit becomes simpler. By the time you need a speed/power control system for the drive, the drive circuit is already pretty complex.

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Adrian Jansen           adrianjansen at internode dot on dot net 
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On a sunny day (Thu, 04 Sep 2014 09:49:41 +1000) it happened Adrian Jansen wrote in :

Howso, you still need to monitor current and voltage for each cell? When in charging series you only need to monitor each cell voltage.

I use 11.1V lipos in a model airplane, driving directly the propellor motor via PWM unit plus electronics (receiver etc) via its own regulator. How would adding 3 extra very high power converters help? Efficiency would go way down.

4 of those cells in series can power a 24 V motor.

Multiple cell charge controllers are very cheap on ebay.

On a sunny day (Thu, 04 Sep 2014 05:02:55 GMT) it happened Jan Panteltje wrote in : Errata:

2 duh..

I think there are probably some devices that do this - they have two battery holders, and you can populate just one for light weight or lower cost, or both for longer runtime. Since the two batteries might be at different states of charge, they probably don't just parallel the batteries, but instead, have two separate DC-DC converters.

Strikes against it, off the top of my head...

Efficiency? Each of those boost converters will lose a little, just like any converter, but perhaps you're a little ahead to have just one converter on a string of seven 3.7 V cells, rather than seven converters, one on each cell.

I-squared-R? Each of those boost converters will hit its cell fairly hard, heating up the connections.

Peukert? With most batteries, the easier you go on them, the longer they last. If you can get an amp for ten hours, that doesn't mean you can get ten amps for an hour.

If you care about regeneration, all of those boost converters have to "work both ways". Which is not impossible, but it adds complexity.

It might be entertaining to get all those boost converters to share the load equally, or at least mostly equally. I know it can be done - I've seen commerical converters that can be paralleled - but again, more complex.

As far as I know, the people that need RICH, CHUNKY VOLTS tend to string enough cells together to get something in the range of 100 to 600 V DC, and then discharge and discharge the whole string at once. There might be "taps" at each cell or every couple of cells, so the brains can check on the balance of the string, but these are just for monitoring. I am talking about stuff like the phone company, PV solar backup, and traction. Most all of these have a fancy DC-DC converter/charger box for the whole string, and it seems like the preferred trade-off is to put all the eggs in that basket, rather than at each cell.

Matt Roberds

But you need multiple chargers. I suppose you aren't going to fuel gage this.

If I understand your scheme, you are using all the cells at once to drive the motor. How about a scheme where you drain each cell individually. Then the controller is simple again.

** I hope not.

The sensible engineering solution is to use ONE Li-Ion cell that has the needed energy capacity and ONE DC to DC converter.

That simultaneously maximises efficiency, minimises component count and weight and so maximises reliability.

It is also the cheapest solution.

Charging is a doddle - cos only one cell to charge.

Your idea sounds bonkers.

.... Phil

Yeah, I mean everyone knows that is what they do in autos, use one

*HUGE* cell. That's why electric cars are so inexpensive not to mention light.

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Rick

Phil says a lot of moronic stuff, but that one was uber-stupid.

Some notbooks batter packs parallel cells. Supposedly OK if the cells are from the same lot, yada yada yada. But you can't get a single cell with arbitrarily capacity.

Regarding efficiency, if 24VDC output is required, going from a single cell LIon to 24VDC will probably require a boost converter just to power the SMPS. Well for some topologies. So it is usually better to have the high stack so you are boosting less. Personally I'd go for a dozen cells. Figuring on 2.5V at the discharge point, the source voltage will always be over the 24V requirement. Then again, I'd try to find motors with a lower operating voltage.

In someways the driver could be come simpler, I've seen one brush-less driver where the FETs only did simple commutation i.e. on/off like if it was brushes, no PWM. The speed control was a boost converter on the input voltage

-Lasse

Oh really?

Jamie

** Neither the OP nor I were speaking about electric cars.

The OP's app requires a maximum of about 400W of DC power and single Li-ion cells with ratings of 200Ah to 400Ah are available that will deliver that for several hours.

** Nobody said you could - pisshead.

** A simple square wave inverter would do.

My point was that using a single cell is a *desirable goal* cos it eliminates all the nasty issues with series charging and discharging while operating the cell at only a fraction of its Ah rating.

.... Phil

This is something like English, only not.

Jansen

:

voltage

motor via PWM unit

You have something rather strange for propeller control then. Here they are VFDs driving polyphase motors. Search "engine speed controller / (ESC)" for more info.

?-)

I read it as sarcasm. you have to make an effort to be that wrong.

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I consider the source.

Jamie

Yes, thats about what I had in mind. Do you have a link to a design, or product ?

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Adrian Jansen           adrianjansen at internode dot on dot net 
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It was the controller for a 12-24V Danfoss compressor

google found a picture,

-Lasse