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.

Team 2447



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.

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.

New Ver 1.1


R3 = 3 Ohms (.2A peak current)

Title FIRST changed

For more information please go to Page 21
Application 906329

BU CHARGER v1_1.pdf (18 KB)

BU CHARGER v1_1.pdf (18 KB)


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.


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.

I agree, it blew up our backup battery!!!

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.

BU CHARGER v2_0.pdf (15.8 KB)

BU CHARGER v2_0.pdf (15.8 KB)

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.

We currently use the IFIRobotics diagram on our robot and it works fine but it keeps on working :stuck_out_tongue: 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 :P).
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. :slight_smile:

You should also run it to the breaker panel using a 20AMP circuit breaker, and then use a 1.5AMP inline fuse.

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.