Davidpearl
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Portable Lithium Ion for HDMIPI

Thu Aug 07, 2014 7:44 pm

Hey guys,

Hope you are well,

I need some assistance as I am not too technical emended when it comes to power (Voltage, Wattage, Amperage etc....)

I have purchased, albeit a long time ago and still waiting for, the Kickstarter HDMIPI which will be being delivered relatively shortly. I have already started plans on making a small portable media centre (the main reason I purchased the screen) and trying to work out how to make a portable power source.

https://www.kickstarter.com/projects/69 ... the-raspbe

I have done some investigation and prelim work in what is required and started sourcing materials to mess around with to get something sorted before I receive the screen.

The information I have had around the power from Alex Eames of the HDMIPI team is as follows:

"The micro-USB input takes 5.2V, the 1.6mm barrel jack takes 12V. The screen needs 5 Watts and the Pi needs about 2-3 Watts. So at 5.2V you need an Amp for the screen and about 1/2 Amp for the Pi."

So the Driver board will power the screen and the PI via the 1.6mm Barrel Jack at 12V - I have a 64GB SSD drive I will be using for storage of media so this will need to be powered through the PI as well.

My thoughts on the Flow chart I imagine would be:

Batteries -> Buck Converter -> Driver Board -> HMDIPI Screen and Raspberry PI powering IR sensor for remote and 64GB SSD

With this information I have bought the following:

10 x 3.7V 6000 mAh Lithium Ion batteries (3.7V x 9 = 33.3V // 6000 x 9 = 54000 mAh)
http://www.ebay.co.uk/itm/261418209606? ... EBIDX%3AIT

1 x LM2596 DC-DC Switching Adjustable Voltage regulator Buck Converter to step down the 33.3V to 12V (Buck Converter operates up to 35V)
http://www.ebay.co.uk/itm/281218773432? ... EBIDX%3AIT

I need help in a few areas due to my lack of knowledge -

- Are my above calculations correct and will they theoretically supply the 12V to the driver board?
- Do I need to add a PCB for protection for the Lithium Ion?
- Is there a way to create a toggle to switch between a Barrel Jack with a 12V power supply to the internal batteries?
- Which way is best to wire the batteries? Parallel or series?
- Will this provide enough Amps?

Sorry if some of this may seem stupid to people but I have ideas and don't always have know how to execute, bit of an ambitious project for someone with limited knowledge in this area

Anyways if anyone has anything that can help, valid input, criticisms, ideas then please feel free to share them.

Hope to hear from you guys soon and many thanks in advance.

Kind Regards,

David

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alexeames
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Re: Portable Lithium Ion for HDMIPI

Fri Aug 08, 2014 9:11 pm

The LM2596S is a good choice. I have lots of them (I buy them in 10s, dirt cheap on ebay) and have never had one fail. I have one powering a home-made 'UPS' for a temperature sensor that transmits at 868 MHz. In the event of a power cut it loses handshake with the receiver, which is annoying. That one has been in constant use for 2 years.

One thing I will say is that the higher voltage you choose, the less efficient your conversion is likely to be, but I don't know by how much. I regularly power HDMIPi + Pi with a 3s lipo (3 cells in series) regulated down to 5.2V and passing into the driver board via the micro-usb socket.

I can really see no reason to use more than 3 cells in series if using the 5.2V port or 4 if using the barrel jack port (at 12V).

If you've bought 10 cells, personally I would set the regulator to deliver 5.2V, use the micro-USB input and make up a 2s5p pack. That's 2 series and 5 parallel.

Lipos are nominal 3.7V/cell (4.2V when fully charged) so you should have plenty of headroom above 5.2V.

You'll then end up with a whopping 30Ah pack at 7.4V (222Wh). Assuming a 90% efficient energy transfer reduces this to 200 Wh.

The screen takes 5 Watts and the Pi takes about 2-3 Watts (depending on model and what you're doing). So if we say 8 Watts, 200Wh/8W = 25 hours.

Lipos hardly ever have their full rated capacity, so I'd knock this down to 80%. That gives you a 20 hour duration, hammering a model B with the screen on.
Alex Eames RasPi.TV, RasP.iO

Davidpearl
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Re: Portable Lithium Ion for HDMIPI

Fri Aug 08, 2014 9:49 pm

alexeames wrote:The LM2596S is a good choice. I have lots of them (I buy them in 10s, dirt cheap on ebay) and have never had one fail. I have one powering a home-made 'UPS' for a temperature sensor that transmits at 868 MHz. In the event of a power cut it loses handshake with the receiver, which is annoying. That one has been in constant use for 2 years.

One thing I will say is that the higher voltage you choose, the less efficient your conversion is likely to be, but I don't know by how much. I regularly power HDMIPi + Pi with a 3s lipo (3 cells in series) regulated down to 5.2V and passing into the driver board via the micro-usb socket.

I can really see no reason to use more than 3 cells in series if using the 5.2V port or 4 if using the barrel jack port (at 12V).

If you've bought 10 cells, personally I would set the regulator to deliver 5.2V, use the micro-USB input and make up a 2s5p pack. That's 2 series and 5 parallel.

Lipos are nominal 3.7V/cell (4.2V when fully charged) so you should have plenty of headroom above 5.2V.

You'll then end up with a whopping 30Ah pack at 7.4V (222Wh). Assuming a 90% efficient energy transfer reduces this to 200 Wh.

The screen takes 5 Watts and the Pi takes about 2-3 Watts (depending on model and what you're doing). So if we say 8 Watts, 200Wh/8W = 25 hours.

Lipos hardly ever have their full rated capacity, so I'd knock this down to 80%. That gives you a 20 hour duration, hammering a model B with the screen on.
Alex - Thanks so much!!!! really helpful and helped add a bit more context and clarity (With facts).

I was looking at serial - parallel mix but didn't fully understand the benefits of it so just went with what I knew.

In terms of charging the Lipo do I need any special equipment, apart from a charger, or can I build a circuit onto the board that I can plug a wall-wart into directly to charge.

What I want in the end is a portable media centre that I can run on the screen, run the HDMI to a tv/monitor, power via batteries or wall power and charge via port on the back

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alexeames
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Re: Portable Lithium Ion for HDMIPI

Sat Aug 09, 2014 7:39 am

You absolutely need a proper lipo charger and a power source for that. (A lot of the RC modelling crowd hack old PC power supplies and use those for the charger PSU.) Lipos can go bang if handled wrongly. They are much more resilient than they used to be though.

I think the biggest problem you're likely to have, because you're making such a large lipo, is the charge time. Most lipo chargers charge up to about 5 Amps. For your huge pack, that's a 6 hour charge time. At least one of my lipo chargers will time out after 2 hours (for safety reasons).

You might be able to cook up a charging circuit, but for safety reasons, I probably wouldn't elect to do that. There is a whole thread about 'lipo fires' over at rcgroups.com I don't want to overly alarm you, but you do need to be aware of potential dangers. A lot of RC hobbyists refuse to charge lipos in their homes.

Several years ago I wrote a long thread on rcmf.co.uk about lipo usage. I don't know if it's viewable without signing up, but here's a link, just in case. It's good background reading (and might send you to sleep one evening)...
http://www.rcmf.co.uk/4um/index.php/top ... #msg523591

Hope this helps
Alex Eames RasPi.TV, RasP.iO

rluci
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Re: Portable Lithium Ion for HDMIPI

Sat Sep 06, 2014 3:56 pm

Hey guys-
Don't mean to hijack an excellent and informative thread, but I have a related question. I'm intending to power the HDMIpi directly from a 12v power source (Ham radio field work). For the life of me, I cannot locate a 1.6mm barrel connector on DigiKey, Mouser or anywhere else. 2.1mm center diameter is the rule. There are variations of course, but nothing in 1.6mm that I can find.

Can anyone provide more detailed specs or a link to the product? Is the on board connector male or female?

Many thanks-
Rich

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Re: Portable Lithium Ion for HDMIPI

Mon Sep 08, 2014 3:21 pm

rluci wrote:Hey guys-
Don't mean to hijack an excellent and informative thread, but I have a related question. I'm intending to power the HDMIpi directly from a 12v power source (Ham radio field work). For the life of me, I cannot locate a 1.6mm barrel connector on DigiKey, Mouser or anywhere else. 2.1mm center diameter is the rule. There are variations of course, but nothing in 1.6mm that I can find.

Can anyone provide more detailed specs or a link to the product? Is the on board connector male or female?

Many thanks-
Rich

Something with multiple connectors like this?

Image
Doug.
Building Management Systems Engineer.

rluci
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Re: Portable Lithium Ion for HDMIPI

Mon Sep 08, 2014 4:41 pm

Thanks, Doug, but those are notoriously shoddy units for field or mission critical work. I'd prefer to build my own cables; then I know what I have.

Besides, I'm drawing from a 12v source, not 110v, so purchase of a transformer and a half dozen extra (useless) connectors to replace a quality $0.80 barrel connector is a bit overkill on the buy and underkill on the outcome. ;-)

Can you assist with specs for the actual barrel connector only?

Many thanks-
Rich

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Re: Portable Lithium Ion for HDMIPI

Tue Sep 09, 2014 12:27 pm

rluci wrote:Thanks, Doug, but those are notoriously shoddy units for field or mission critical work. I'd prefer to build my own cables; then I know what I have.

Besides, I'm drawing from a 12v source, not 110v, so purchase of a transformer and a half dozen extra (useless) connectors to replace a quality $0.80 barrel connector is a bit overkill on the buy and underkill on the outcome. ;-)

Can you assist with specs for the actual barrel connector only?

Many thanks-
Rich
Hi Rich,

I haven't had any luck at all with identifying the part either, sorry.
Doug.
Building Management Systems Engineer.

rluci
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Re: Portable Lithium Ion for HDMIPI

Tue Sep 09, 2014 3:09 pm

Ok, thanks much. Perhaps someone else to the rescue. Else, I'll just sit back and wait for my HDMIpi delivery!

Rich

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AndrewS
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Re: Portable Lithium Ion for HDMIPI

Tue Sep 09, 2014 6:05 pm

rluci wrote:Can anyone provide more detailed specs or a link to the product? Is the on board connector male or female?
http://hdmipi.com/2014/sanity-check-pas ... ect-to-qc/ is the best I can come up with.

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alexeames
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Re: Portable Lithium Ion for HDMIPI

Wed Sep 24, 2014 3:34 pm

rluci wrote:Hey guys-
Don't mean to hijack an excellent and informative thread, but I have a related question. I'm intending to power the HDMIpi directly from a 12v power source (Ham radio field work). For the life of me, I cannot locate a 1.6mm barrel connector on DigiKey, Mouser or anywhere else. 2.1mm center diameter is the rule. There are variations of course, but nothing in 1.6mm that I can find.

Can anyone provide more detailed specs or a link to the product? Is the on board connector male or female?

Many thanks-
Rich
1.7mm works perfectly well. We're getting some in. The direct digikey link is...

http://www.digikey.co.uk/product-detail ... -ND/568588

These are 4mm external and 1.7mm internal. Perfect fit.
Alex Eames RasPi.TV, RasP.iO

rluci
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Re: Portable Lithium Ion for HDMIPI

Thu Sep 25, 2014 5:39 pm

Many thanks.
Rich

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