Mr_O
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Joined: Wed Nov 07, 2012 6:10 pm

Battery Problem For Cool Kids Only!

Wed Nov 07, 2012 6:36 pm

Hi People!

Help us out because we are stupid. We are working on a project where we need to hook 7 Pi Model B Boards together and have them run with screens off of battery power. We have a lot of batteries...buuut...we have no idea what we are doing. Can we hook all of these things to the batteries, and if we can, how do we do it? Thanks for all of your help. This is our parts list.


We need 7

4.3 Inch LCD TFT Rearview Monitor screen for Car Backup Camera
12 Volt DC
2 Watts
0.167 Amps

Raspberry Pi Model B
5 Volt DC
2.5 Watts
0.5 Amps

We Need 1

ASUS 8 port Fast Ethernet switch VIP port GX1008B
9 Volt DC
0.5 Watts
0.056 Amps

We Have A Large Number Of

K2 Energy K2B12V10E Battery Pack
12.8 Volts DC
9.6 Ah
122 Whr


Once again, we have no idea what we are doing and would love some assistance on how to hook all of this stuff together if possible.

Thanks Again, Mr. O and Friends

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Burngate
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Re: Battery Problem For Cool Kids Only!

Thu Nov 08, 2012 9:50 am

You're going to need some way of converting the 12v from the batteries to the 5v for the Pis and the 9v for the switch
Also since batteries don't keep their voltage constant, you may (or may not) need some method of obtaining regulated 12v for the screen

For the 9v @ 56mA you could use a simple linear regulator
However for the Pis, a switching regulator would be better

There have been many posts about switching regulators. Try searching the forum

bredman
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Re: Battery Problem For Cool Kids Only!

Thu Nov 08, 2012 9:53 am

The MagPi magazine recently had an article about running a RPi from batteries.
www.themagpi.com

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tedhale
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Re: Battery Problem For Cool Kids Only!

Thu Nov 08, 2012 1:38 pm

The article in the MagPi points out an important issue when using batteries - the efficiency of the voltage converter.
If you just need to show that this will work, then it's not too much of an issue.
However, if you really need to run this on batteries for an extended period of time, the efficiency of the converter becomes very important.
The difference between good and bad converters may be as much as a third to half of the battery life.
- Ted B. Hale
http://raspberrypihobbyist.blogspot.com

Mr_O
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Joined: Wed Nov 07, 2012 6:10 pm

Re: Battery Problem For Cool Kids Only!

Mon Nov 12, 2012 6:26 pm

Thanks for your help. Its great to be appreciated. What would we use to power the LAN switch? Keep in mind that we are poor and we are in high school.

Thanks Again, Mr. O and Friends

bgirardot
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Location: Switzerland

Re: Battery Problem For Cool Kids Only!

Mon Nov 12, 2012 10:16 pm

It would help to have just a little more information if you are still looking for advice on this.

How long do you need to run these for?
Are they all going to be in one location or spread around?
How many approx is "a large number of batteries" ? (nice batteries by the way)

Any more info you could provide would help, as well as an update on what you might have gotten figured out and do not need any more advice on.

Dilligaf
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Re: Battery Problem For Cool Kids Only!

Mon Nov 12, 2012 10:38 pm

You MAY be able to run the lan switch on lower voltage (7.8V) by wiring it from the 5V output to the 12.8 volt battery providing a voltage of 7.8V depending on the voltage drop at the battery under load

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Mortimer
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Re: Battery Problem For Cool Kids Only!

Mon Nov 12, 2012 11:03 pm

Dilligaf wrote:You MAY be able to run the lan switch on lower voltage (7.8V) by wiring it from the 5V output to the 12.8 volt battery providing a voltage of 7.8V depending on the voltage drop at the battery under load
No a good idea, the ground reference for the signals going to and from the switch wouldn't be at the right level for the switch and vice versa.
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bgirardot
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Re: Battery Problem For Cool Kids Only!

Mon Nov 12, 2012 11:18 pm

I would start with opening up the hub and locating the voltage regulator on the board then look up its specs, it might take nominal 12v all by itself.

Failing that, depending on what he says he wants to do with them, as someone mentioned above, a simple 9V linear regulator would do the job.

If he needs to do more than proof of concept or one time use or they don't feel comfortable soldering or using a breadboard, I'd go with an RC hobby UBEC that he can probably get at any local hobby shop or from an online hobby shop for 5 bucks. Actually, depending on their skills I'd do that for the RPis as well.

Edit: now that I look at it again, for 56mA, the 9v linear regulator is the way to go if at all possible I think, the ubec is over kill for the hub.
Edit2: The hub's power requirements are actually 9v @ 0.5A so 500 mA

Mr_O
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 5:40 pm

Hi Guys,

In response to bgirardot:
We need to have them run for 8 hours with a 30 minute break, a full night of charging time, and another
4 hours of use.
They will all be in one place.
We have 26 batteries and 15 chargers (a donation), however, in order to charge a battery, we would
need to unplug it, and we can't have all of the batteries plugged in at the same time.(we would prefer
to use 8 or less batteries at one time)
We would need to be able to keep all of the Pis on when we switch the batteries.

We still are not sure how to turn one source of current into three currents and have two of those to split into seven currents each. Our parts list isn't set and stone so if there is a better part than what we are currently using, please tell us. Thanks for your help.

Thanks Again, Mr. O and Friends

ghans
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 6:51 pm

Couldn't you discharge one set of batteries while charging the others ?
With 12V@9.6Ah batteries , one of them could last 8 hours on a Pi easily.
To connect the Pis , get a low-drop regulator for 5V. One regulator+battery per Pi.
(I believe that should be 35-40 dollars for 7 regulators).

When those seven batteries have discharged a little ( 8 hours should not discharge them really ) , swap
them for the other seven which are charge-maintaining (if your chargers do that ), so that the next day you can
use them again.
With the remaining batteries you could power USB Hubs.
Note that my solution involves some soldering and cabling (get enough good-quality Micro USB cables ,
max. 18 dollars or so) , but should be easy to manage.


ghans
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Dilligaf
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 7:14 pm

First find a switch that runs on 5V rather than 9V then get several of these http://www.hobbyking.com/hobbyking/stor ... ction.html one for each Pi and one for each switch. You can then power the Pis and switch with the UBECs and power the screens from direct battery. The UBECs come with a connector that can be adapted to power the Pi from the gpio pins by simply relocating a terminal in the connector housing. They are about 90% efficient, you really aren't going to get much better than that, and in my experience provide a very steady 5V supply to the Pi. Each Pi/screen combo should be able to run off of one 9.6 Ah battery for 9 hours or more if your specs are correct (they seem low for the screen assuming they are lighted somehow). A 5V 8 port hub should run even longer on its own battery.

ghans
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 7:43 pm

So many batteries .. so much fun you could have ! Ah yes , i forgot the screens ,
i believe you'd just wire the screens in parallel with the UBECs ?
Really , if the Pi (say 0.7 A) + Screen consume only 5.5 Watt , then 5.5W x 8h are 44 Wh needed.
The batteries boast 122 Wh ! You are really lucky !
As for the router : If it comes to that , wire two batteries in parallel , and get a 12 V regulator.
Protect the batteries from discharging into each other though !!

ghans
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Mr_O
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 9:43 pm

Hi Guys,

Just made a 'professional' diagram of our Pi project.
Pi2.png
(Not To Scale)
Pi2.png (36.11 KiB) Viewed 6095 times
Thanks For Your Help, Mr. O and Friends

Dilligaf
Posts: 283
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 10:28 pm

Sorry, I was thinking you wanted independent units which may be easier to manage. 2 UBECS one feeding 4 PI and the other feeding 3 PI, the monitors can be wired direct to the batterys, 7-8 batterys hooked positive to positive, negative to negative. Best to find a 5 volt or 12 volt switch, if that isn't possible then you will need a 9 volt regulator that drops 12V to 9V (don't have a part number handy, sorry). The batterys hooked pos to pos and neg to neg will output 12.8 volts with the combined capacity of the number of batterys in the battery bank. The monitors being automotive will accept up to at least 15 volts. The 12.8 volt "open circuit" voltage will drop when the batterys are in use. The reality of it all is that you can do the calculations but you will need to do real world testing to determine the minimum number of batteries that will last for your desired run time depending on age/condition of the batteries.

bgirardot
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 10:39 pm

I think the individual batteries are a better idea than the batteries in parallel. You are going to need higher priced UBECs if you want to use the batteries as a group. Wiring the batteries in parallel is also slightly more complicated than just using them individually.

It would be easier to locate the units if they were somewhat self contained with an individual battery as well.

I fully concur with finding a 5v or a 12v hub/switch. That will simplify matters somewhat. Or as I mentioned, if you are stuck with the hub you have, find the voltage regulator chip on the board and look up its specs.

Dilligaf
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 10:50 pm

The $5 UBECs will work with or without the batteries in parallel, each is rated at 3A nominal 5A peak so should easily support 4 PI each. You are really better off going with individual units, each unit consisting of 1 UBEC, 1 screen, 1 PI and 1 battery, it's much simpler and easier to manage. There is also the option of doubling or even tripling each unit for example 3 batteries in parallel, 3 screens, 3 PI and 1 UBEC

Also with a central battery bank it will be a wiring nightmare if you want any separation of the individual units.

techpaul
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 11:02 pm

First thing I would say with this many batteries be CAREFUL, batteries can cause fires, a friend of mine in USA a few years ago had his offices burn down due to UPS battery failure. Some battery faults create a lot of heat and some like shorting dissipate a lot of energy as sparks and heat very very quickly.

Batteries do not like to be shorted, or stressed. You stress batteries when paralleled up (pos to pos and neg to neg), due to fact that each battery is at slightly different charge level, and one battery is trying charge the lower level batteries. Get a cell fault (all batteries above 1.5 to 3 V are made up of smaller batteries in series) and one battery can pull all the other batteries down.

Condier -

1/ Have separate batteries for separate Pi and screen and associated peripherals, but connect all the negatives of the batteries together with thick wire, to refernce all units together if they communicate or link to each other for data. That way they wont reference their voltages through any other cabling, that cannot take the current. Keep the postives separate.

This method uses less thick cable, is easier to maintain and easier to fault find. Also it is simpler to scale. Worst case work out how many setups can easily be powered by one battery and use that as a multiple. I personally would not go above 2 setups per battery.

If a setup, fails only that part is affected, the rest still work.

2/ If you must parallel up the batteries get some power diode blocks so one bad battery does not bring them all down. Take each battery positive through a diode to a common positive rail, this rail will have to be much thicker wire otherwise it will heat up and maybe melt the insulation. This protects the rest of the batteries from a faulty or lower charge level battery.

This method is a nightmare to maintain and one fault in main power source ALL units go down.

3/ To provide any other voltages get HIGH efficiency DCDC converters, as all circuits consume power and create a loss compared to the original power source (even on the national grid wiring), some power is lost as heat always.

What do I mean by efficiency? Well power comes into a circuit (in this case a DCDC converter) and power comes out, the efficiency is a measure of the ratio of power out to power in, usually measured as percentage, whereby 0% is no power out and 100% is the same power out. You will be looking for DCDC converters at least 70% efficient if not better.

What this means is less heat in the DCDC converters (just like the heat for laptop adapters), so if you had a DCDC converter that delivers 70W to your setup but needs to take 100W from the power source to do this it is 70% efficient and is losing 30W in the DCDC converter mainly as heat.
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techpaul
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 11:09 pm

Personally I would consider doing

Three times this
One battery powers
2 x DCDC converter
2 x Pi
2 x screen

One times
1 x DCDC converter for Pi
1 x DCDC converter for network switch
1 x switch
1 x Pi
1 x display

Take spare batteries and DCDC converters as well.

Get someone to specify the right connectors and wire sizes to use, also consider inline fuses to items connected to battery to reduce risk of fire if a short occurs in connected equipment does not short out battery. Make sure you have spare fuses as well of course.

If the only link between the Pis is ethernet each setup can be negative isolated as ethernet has isolation in its connectors or after connectors, hence why it can be used in large buildings.
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bgirardot
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 11:37 pm

I guess I was looking at what ghans and dilligaf originally suggested and I still think is the best, most simple, flexible approach that students can easily set up and take down and came up with something like the below diagram. In an effort to give him some practical specifics he can get started on to maybe make a test set up:

ghans UBEC (a high efficiency DC-to-DC converter):
Turnigy 3A UBEC

Suggested connectors for the UBECs:
XT60 Connectors for the UBECs

You may also need connectors for the power to the screens, they are probably a size M or N DC coaxial power connector if they have a typical plug-in type power jack. If the screens came with their own wiring, you could use that or cut off the connectors and just use those if you are trying to save money and it is ok to cannibalize the parts. Otherwise you are probably looking at another $10-15 total for all the DC coaxial power connectors you need, you will also need one for the hub probably.

The batteries look like they use common "push on terminal connectors" that you can buy in any hardware store.

Depending on how far the batteries are from the screen/RPis you should size that wire appropriately for that distance, larger gauge if possible to help reduce resistance.

You will have to cut the micro usb cables, and solder the red and black wires onto the male XT connectors. I am not familier with powering the RPi via the GPIO pins, but micro usb cables are very inexpensive and less prone to potential mistakes or damage like the GPIO pins can be.

The wires out of the UBEC should be soldered on to the female XT connectors.

You will probably have to extend the wires on one side of the UBEC to reach the battery if it us under a table or something, again, determine the correct gauge wire for that.

8 UBECs + connectors = $45.00 delivered
7 USB Cables = $10.00 delivered
Battery connectors + wire ~= $10.00 locally obtained

That is about how I would do it, I am by no means an expert so please anyone here give feedback on my suggestion and Mr O should double check anything I suggested with a source he trusts.
Attachments
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Dilligaf
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Re: Battery Problem For Cool Kids Only!

Tue Nov 13, 2012 11:50 pm

The above pic is what I envisioned as the simplest/safest/most practical means of doing it. 7 individual self contained units. I would have drawn the pic but don't have an artistic bone in my body. That accounts for 7 UBEC, you specified 8, depending on the decided switch the 8th may or may not be needed. On the other hand it may be wise to order a couple spares just in case. Probably want to add some inline fuses off the battery positives for safetys sake.

ghans
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Re: Battery Problem For Cool Kids Only!

Wed Nov 14, 2012 7:32 am

Agree with Dilligaf : Keep it simple.
techpaul suggested 2 UBECs per Battery , but they seem be rated at 3A , so even
one should be enough for two Pis at once ( if you really want two Pis at one battery , instead
of my and Dilligafs sugggestion).

ghans
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Mr_O
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Re: Battery Problem For Cool Kids Only!

Wed Nov 14, 2012 6:37 pm

Hi People!

Love having one battery per Pi and screen. That gave us the idea of this:
Pi Shirt.png
( Not To Scale)
Pi Shirt.png (18.23 KiB) Viewed 6017 times
Our teacher is afraid that the battery will explode or something and will burn a student but we dont think so. Any opinions?

Thanks, Mr. O And Friends

techpaul
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Re: Battery Problem For Cool Kids Only!

Wed Nov 14, 2012 6:46 pm

As they are LithiumIon they have problems when on charge.

Will the kids be able to carry them they are not light.

If lead acid (even sealed as they can vent) I would be worried

With both types make sure the WHOLE of the battery connections are insulated as stray metal is a serious problem.
Just another techie on the net - For GPIO boards see http:///www.facebook.com/pcservicesreading
or http://www.pcserviceselectronics.co.uk/pi/

Dilligaf
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Re: Battery Problem For Cool Kids Only!

Wed Nov 14, 2012 7:47 pm

SO now you're talking mobile? Quite possible with a wireless dongle per PI and a wireless router/access point instead of a switch (these are usually 12V as well). As stated above depending on battery chemistry and weight, SLA (sealed lead acid) batteries emit hydrogen, an explosive gas, probably not a good idea to strap to a kid :) LiOn batteries not so much of a problem, probably don't want to charge them while strapped to a kid.

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