Battery-powered Pi?

Many thanks for the suggestions, I guess I'll start with an off-the-shelf charger, I may well get a longer run-time than with my 6V battery.

Now how to change the status here to answered ??

Malcolm,

   At the top of the original post, just under the title, there should be

a link you can click on that says "mark as assumed answered".

Also, at the bottom of each response, there are two boxes you can

click, one for "correct answer" and one for "helpful answer".

Any correct or helpful answer you click on will get automatically mentioned

in the original post.

I'm using this one with Raspberry Pi very successfully: http://www.amazon.co.uk/gp/product/B005JSG7GE/ref=oh_details_o03_s00_i01

Sadly out of stock at Amazon (and all the other suppliers I looked at seem to link back to Amazon!).

I plan on using this: http://www.amazon.com/gp/product/B003ZBZ64Q/ref=oh_details_o00_s00_i00

11000mAh!

I've used it to charge my iPad and a few other gadgets while on the road. No issues at all.

Let us know if it's fine with the RPI, if you would - if it does I predict a surge in sales, even at that price!

@Michael:  Indeed, Li-Po batteries are very fussy in their charging requirements, which is why I shied away from a simple series arrangement with two floating single-cell chargers.  I strongly suspect that there be dragons in that area.  That's why I asked whether anyone knew of a ready commercial module that is specifically designed to keep two Li-Po cells charged up as a unit

I've noticed in 10.8V Li-Ion LED torches (not quite the same as Li-Po, but share some features with them) that the built-in charger has connections to the two inner links between the 3 Li-Ion cells, so there must be something clever going on.

Your suggestion of using a boost converter from a single Li-Po cell up to 5V is an excellent one though.  If the conversion is efficient enough and the component cost less than that of a two-cell charger, it seems the more sensible way to go.

Morgaine.

What commercial RC LIPO chargers seem to do is to discharge the cells that come too close to the 4.2V maximum. This is able to top all of them off to 4.2V without overcharging the cells that reach 4.2V first. This is not efficient. The one I have can only discharge about 100mA, which charging could be done at up to 2A, so when you want to top off the best cell but the worst cell reaches 4.2V, you'll have to keep discharging it at 100mA while only charging the good cell at 100mA (the bad cell cannot handle any more charge).

Floating chargers works slightly better.

Ideal would be a circuit that could selectively charge every separate cell. This is slightly tricky, I don't know of any chips that aid in building this.

The best I've seen is a chip that allows you to charge an inductor using ANY cell, and then using the charged inductor to continue to charge the whole pack. This is energy efficient, and does not waste the energy that needs to be discarded from the cell that is topped off first. So things don't get hot.

Anyway, a boost from a single cell is the easiest configuration. Charge by "buck" converter from 5V, and discharge using a boost converter from the 3.3-4.2V battery voltage.

A boost converter is likely to have a diode in the power-flow part. This causes a 0.4V minimum voltage drop, so in effect the converter has to deliver 5.4V of which only 5V can be used at the output. This is a significant part of the losses of a boost converter.

If my memory serves (I can hunt it down - the article not my memory), Circuit Cellar had a design article in which someone had designed a balanced charging system, the batteries were lithium but high capacity ones such as those used for vehicles etc. The design though could be scaled down, the balancing circuit detected those batteries that were at capacity and disconnected that bank from the charging circuit, which of course is pretty logical, though would require a string/bank of more than just two batteries for that particular design.

I suppose you could have a combination of a constant current source and an active load and switch through a solid state (4066 or similar) the charge so it could be in series up to a certain charge point and then switched over to a single battery at that moment. There are plenty of battery charger/conditioner/fuel guage IC's around that could provide the charging algorithm and a a small microcontroller could be the 'brains' and perform the switching and possibly some other house keeping duties. a FET or IGBT could provide the switch and active load function.

Colin

Colin, Switching batteries between series and parallel is completely nontrivial. If you're going to make a parallel battery pack you can manually ensure they are at the same charge state (e.g. at 4.2V) before connecting them together, but doing as you suggest, when one cell is full, and others aren't then connecting them in parallel is likely to result in bad things happening.