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        Discussion Battery Concerns on In flight Arm Cluster

#1 Ramidan Nov 18, 2012 06:49 AM

Battery Concerns on In flight Arm Cluster
 
As some of you may or may not know, I bought a rotomast V-22 and have been working deep with autonomy in java. I am now to the point that I want to try to get my on-board computer cluster working using a LiPo battery set. My concern is that the discharge rate is to high to use successfully use in my little cluster.

The cluster is made up of 4 ARM boards and a teeny 4 port switch running Ubuntu, all in all weighing less than 8oz. The power requirements are 7.5v @ 1A for the switch, and each ARM board requires 5V @ 2A. I'm still not 100% clear on the "discharge rates" of a LiPo battery. I'm torn between the added weight of a dedicated battery, or tapping into the current system battery and the BEC, so I guess that is my question here.

I have the craft running on 4 Turnigy 2200mAh 4S 30C Lipo Packs instead of the 2 it initially took. Would it be better to:

A - use a BEC and tap into the 5v output there, build a dc/dc stepper circuit and convert a leg of that to 7.5V for the switch ~or~

B - create a custom shunt and voltage converter circuit and use a separate battery?

#2 dmccormick001 Nov 18, 2012 10:47 AM

Your question is a bit confusing, at least for me it was, because you seem at first to be concerned with the current ratings of your batteries, but then you ask questions about the various voltage requirements of your system. As far as the current ratings of the batteries are concerned, yours look like they are plenty big enough to power the computer "cluster". The computer and switch, as you may or may not understand, will only draw as much current as they need to work, the batteries won't discharge too much into them. The discharge rate of the batteries simply indicates how much current they can safely provide if the load requires it, not what they will force or pump into the load.

As for the voltage requirements, I'd suggest you build a voltage regulator with the 2 outputs you need (5 and 7.5 volts), and power it with the batteries you have. No need for a separate battery, those four 4S 2200mAh packs will never know the computer and switch are even there.

#3 Ramidan Nov 18, 2012 06:49 PM

dmc,

yeah sorry, I'm a little scatterbrained here on this. I have about 50 different spaghetti strands that I am trying to put together. Sorry if it was confusing. This is after I have put in 2250 feet of tile and revamped a back door to my house.

Let me say this. I am concerned that the batteries will push too much current only because I have fried 3 multimeters trying to get readings off of them (directly). I guess I'm gun shy to test on my cluster because it's a $1K piece of equipment, and if I fry it being too cavalier on testing I'm gonna bash my face into the wall.

Thanks for the clarification on the current, I couldn't remember if it force pumps it into it or not. So realistically, I shouldn't rely on the BEC to power the 5v systems? Having to build a voltage regulator to support dual outputs will not be too hard.

Quote:

Originally Posted by dmccormick001 (Post 23299610)
Your question is a bit confusing, at least for me it was, because you seem at first to be concerned with the current ratings of your batteries, but then you ask questions about the various voltage requirements of your system. As far as the current ratings of the batteries are concerned, yours look like they are plenty big enough to power the computer "cluster". The computer and switch, as you may or may not understand, will only draw as much current as they need to work, the batteries won't discharge too much into them. The discharge rate of the batteries simply indicates how much current they can safely provide if the load requires it, not what they will force or pump into the load.

As for the voltage requirements, I'd suggest you build a voltage regulator with the 2 outputs you need (5 and 7.5 volts), and power it with the batteries you have. No need for a separate battery, those four 4S 2200mAh packs will never know the computer and switch are even there.


#4 rmteo Nov 18, 2012 07:03 PM

Quote:

Originally Posted by Ramidan (Post 23303034)
I am concerned that the batteries will push too much current only because I have fried 3 multimeters trying to get readings off of them (directly). I guess I'm gun shy to test on my cluster because it's a $1K piece of equipment, and if I fry it being too cavalier on testing I'm gonna bash my face into the wall.

Yikes!!! Have been connecting the battery packs directly to the multimeter probes while it is set to (high) current measuring mode? That is guaranteed to blow the shunt resister in it.

#5 Ramidan Nov 18, 2012 07:39 PM

yeah,

I figured that out a little too late.... lol

Quote:

Originally Posted by rmteo (Post 23303152)
Yikes!!! Have been connecting the battery packs directly to the multimeter probes while it is set to (high) current measuring mode? That is guaranteed to blow the shunt resister in it.


#6 AndyKunz Nov 19, 2012 09:04 AM

Quote:

Originally Posted by Ramidan (Post 23303034)
Let me say this. I am concerned that the batteries will push too much current

They don't "push" current. They provide the current "drawn" by the circuit.

Ohms Law tells you how it all works together.

Andy

#7 srnet Nov 19, 2012 03:21 PM

I would strongly suggest you get someone who understands the basics to check what you are wiring up.

LIPOs are really explosive devices in battery packages, treat them wrong and they will explode in a large fireball.

#8 Ramidan Nov 19, 2012 07:29 PM

It's been YEARS since I've even remotely built any circuitry (since my college days) - 8 years ago. I am just researching building a LM7805 regulator. (trying my hand at making something simple yet useful)

#9 C₄H₁₀ Nov 19, 2012 08:13 PM

Quote:

LIPOs are really explosive devices in battery packages, treat them wrong and they will explode in a large fireball.
Errrmmm.... Not exactly.

#10 Ramidan Dec 01, 2012 06:49 AM

Well instead of re-inventing the wheel, I ran an internet search for an adjustable DC-DC voltage regulator. Most were rather large and bulky, but I did manage to come across a 10W and 25W adjustable regulator:

http://www.robotshop.com/ca/dimensio...regulator.html or even looked on ebay for a 12-24V to 5v Dc-Dc converter. Found a new small non adjustable ones.

#11 rmteo Dec 01, 2012 08:17 AM

$3 DC 3~40V to DC 1.5~35V Voltage Step Down Transformer Module

http://img.dxcdn.com/productimages/sku_154907_1.jpg

#12 Ramidan Dec 02, 2012 08:10 PM

Oh man, thank you so much. I just ordered 5 of them.

#13 rmteo Dec 02, 2012 08:15 PM

Pretty amazing isn't it - $12.50 for 5 with free shipping. Should arrive in the USA in about 14-20 days or so.

#14 Ramidan Dec 03, 2012 08:06 AM

Quote:

Originally Posted by rmteo (Post 23424492)
Pretty amazing isn't it - $12.50 for 5 with free shipping. Should arrive in the USA in about 14-20 days or so.

I actually paid $16 total. I ordered 5 because they might not be the best quality. They also have a ton of arduino parts and shields on there. I might have to pick up some extra parts.


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