Nov 18, 2012, 06:49 AM Registered User Joined Mar 2011 28 Posts Discussion 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?
 Nov 18, 2012, 10:47 AM Dave the Rave Joined Jun 2007 935 Posts 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.
Nov 18, 2012, 06:49 PM
Registered User
Joined Mar 2011
28 Posts
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 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.
Nov 18, 2012, 07:03 PM
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Joined Oct 2004
1,479 Posts
Quote:
 Originally Posted by Ramidan 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.
Nov 18, 2012, 07:39 PM
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Joined Mar 2011
28 Posts
yeah,

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

Quote:
 Originally Posted by rmteo 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.
Nov 19, 2012, 09:04 AM
Illinois
Joined Sep 2001
35,623 Posts
Quote:
 Originally Posted by Ramidan 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
 Nov 19, 2012, 03:21 PM Stuart UK, Cardiff Joined Dec 2008 3,819 Posts 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.
 Nov 19, 2012, 07:29 PM Registered User Joined Mar 2011 28 Posts 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)
Nov 19, 2012, 08:13 PM
Wake up, feel pulse, be happy!
United States, MN, Minneapolis
Joined Aug 2009
13,374 Posts
Quote:
 LIPOs are really explosive devices in battery packages, treat them wrong and they will explode in a large fireball.
Errrmmm.... Not exactly.
 Dec 01, 2012, 06:49 AM Registered User Joined Mar 2011 28 Posts 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. Last edited by Ramidan; Dec 01, 2012 at 06:59 AM.
 Dec 01, 2012, 08:17 AM User Colorado Joined Oct 2004 1,479 Posts
 Dec 02, 2012, 08:10 PM Registered User Joined Mar 2011 28 Posts Oh man, thank you so much. I just ordered 5 of them.
 Dec 02, 2012, 08:15 PM User Colorado Joined Oct 2004 1,479 Posts Pretty amazing isn't it - \$12.50 for 5 with free shipping. Should arrive in the USA in about 14-20 days or so.
Dec 03, 2012, 08:06 AM
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Joined Mar 2011
28 Posts
Quote:
 Originally Posted by rmteo 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.