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Unread 11-02-2008, 11:37
cjohnson cjohnson is offline
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New Backup Battery Circuit

Hey guys,

I’m posting a new battery backup charging circuit. The one from ifirobotics does not work. This one is much simpler and uses fewer parts. We have been using this one for about 2 weeks.

Cjohnson
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File Type: pdf FIRST BU CHARGER.pdf (30.4 KB, 195 views)
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Unread 11-02-2008, 13:40
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Re: New Backup Battery Circuit

Thank you for contributing this. I have a few questions, however.

First, you say "The one from ifirobotics does not work." We used it last year, and we're using it this year (with the addition of a series diode on the input). It's working fine for us. What's not working for you?

Second, you've labeled it "FIRST Backup Charger". Is it from FIRST, or is it your own design? Putting FIRST's name on it implies that it's somehow official.

Third, I'm concerned about your apparent use of a 3A fuse in the main breaker panel. That violates the rules. You have to use one of the 20A KOP breakers for custom circuits. You are permitted to use additional fuses as part of your custom circuit, but you still need the 20A snap-action breaker.

Finally, can you describe the operation of this circuit? For starters, the transistor on the regulator's voltage adjust pin is not self-explanatory.
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Unread 11-02-2008, 14:29
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Re: New Backup Battery Circuit

Cjohnson,
I like some aspects of your design, but I think the battery in question wants to be charged at no higher than 1/10 C or 70 ma without a smart monitor. With different electronics driving the charge rate and monitoring the battery, you can charge at 1/3 C or about 230 ma. Any greater current than this might damage your battery or worse.
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Unread 12-02-2008, 12:49
cjohnson cjohnson is offline
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Re: New Backup Battery Circuit

New Ver 1.1

Changes

R3 = 3 Ohms (.2A peak current)

Title FIRST changed

For more information please go to http://cache.national.com/ds/LM/LM117.pdf Page 21
Application 906329
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File Type: pdf BU CHARGER v1_1.pdf (18.0 KB, 79 views)
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Unread 12-02-2008, 13:36
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Re: New Backup Battery Circuit

CJohnson,

One thing I noticed is that the schematic in the National data sheet which you reference shows the battery as the only load in the charging circuit. If you plan on connecting the output of your circuit to the RC backup battery input, which you must do, then you'll provide a short circuit around R3 through the RC's ground connection back to the circuit breaker panel. I think that'll eliminate any current limiting ability of R3.

Steve
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Unread 12-02-2008, 15:33
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Re: New Backup Battery Circuit

Cjohnson,
I thought I had seen that circuit before. This charger is meant for a lead acid cell which requires a constant voltage but current limited source. In operation, excessive current (through the battery and R3) is prevented from flowing as the transistor reduces the value of R2 and thereby reduces the output voltage until the current is stable at the design peak.

NiCad cells require a constant current for charging. As Steve has pointed out above, R3 is shorted by the sneak around ground through the RC, the backup battery connector and the robot common wiring. With R3 shorted this circuit produces an output voltage of 7.61 volts up to the cutout current of the LM317. C, sorry I didn't catch this before. I feel I mislead you with my earlier post.
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Unread 12-02-2008, 18:02
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Re: New Backup Battery Circuit

Quote:
Originally Posted by cjohnson View Post
Hey guys,

I’m posting a new battery backup charging circuit. The one from ifirobotics does not work. This one is much simpler and uses fewer parts. We have been using this one for about 2 weeks.

Cjohnson
Team 2447
I agree, it blew up our backup battery!!!!!
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Unread 15-02-2008, 18:08
cjohnson cjohnson is offline
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Re: New Backup Battery Circuit

Sorry folks, you are right about the ground shorting out R3. On of my team member told me it was isolated. I finally got my hands on the controller and did some measurements and circuit Version 2.0 I have posted. This design I was bit more conservative at 100 ma charge rate. I also have the charge characteristics curve if anyone needs it.
CJ
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File Type: pdf BU CHARGER v2_0.pdf (15.8 KB, 68 views)
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Unread 15-02-2008, 18:42
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Re: New Backup Battery Circuit

Quote:
Originally Posted by cjohnson View Post
On of my team member told me it was isolated. CJ
Is this the same person who told you the IFI circuit didn't work?

Seriously but...

The IFI schematic that was posted until mid-season last year was missing a connector, but once that was ironed out, it really works pretty well.

We used the camera last year and it runs exclusivly off the backup battery. We never had ANY problems with a non-charged battery.
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Unread 16-02-2008, 22:55
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Re: New Backup Battery Circuit

We currently use the IFIRobotics diagram on our robot and it works fine but it keeps on working Even when we disconnect the main breaker it keeps on powering the _WHOLE_ system (RC, Victor speed controlers, Spike Relays and w/ teather even the OI ).
I have a feeling it's either because we've missed a diode somwhere (unlikly :S), or because we connected the inputs via the main power block. should we connect it via the fuse pannel and ground bank?
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Unread 17-02-2008, 01:57
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Re: New Backup Battery Circuit

Quote:
Originally Posted by Barak Shelef View Post
We currently use the IFIRobotics diagram on our robot and it works fine but it keeps on working Even when we disconnect the main breaker it keeps on powering the _WHOLE_ system (RC, Victor speed controlers, Spike Relays and w/ teather even the OI ).
I have a feeling it's either because we've missed a diode somwhere (unlikly :S), or because we connected the inputs via the main power block. should we connect it via the fuse pannel and ground bank?
You're missing a diode in series from the input. That should do it.
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Unread 17-02-2008, 11:01
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Re: New Backup Battery Circuit

Quote:
Originally Posted by DarkJedi613 View Post
You're missing a diode in series from the input. That should do it.
You should also run it to the breaker panel using a 20AMP circuit breaker, and then use a 1.5AMP inline fuse.
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Unread 18-02-2008, 01:27
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Re: New Backup Battery Circuit

thanks for the tips, guys, i already drafted the new circuit and chose the components. gonna go and by them along with some spare potentiometers and control LEDs today.
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