$2 diversion controller idea ?!

[elt]

I saw this nifty battery checker yellow tagged (on closeout) at my local Radio Shack -

I got to thinking though that if it was switching on leds at different voltages that maybe it could switch on relays at different voltages...

So I opened it up. (The case screw is under the magnet on the back.) Looks like a voltage divider network with a "upa2003c" driving the LEDs. Wasn't familiar with the the upa2003c chip so I looked it up and low and behold it's a lamp/relay driver chip! Power dissapation limits appear to make it good for about 75 to 100 mA ... perhaps a 15 amp relay. It may take a little more current than that to drive bigger relays but (I suppose) that if you tweaked the voltage divider network you could have up to six 15 amp relays turning on at whatever voltages you'd like.

Hey, it's just an idea... If not, it is a $2 voltage readout ; does draw 20ma though

- Ed.

[SamoaPower]

I like bargains too. But we have to remember the the main purpose of a charge controller is to establish a charge algorithm for the batteries. A diversion type controller has a secondary benefit in keeping a load on our machines.

Any step type load diversion won't give you good control of the battery charge, which is the main objective.

[ghurd]

Problems I know of with the thing.

They have a little bit of 'flutter' sometimes. Super fast blinking of an LED. Probably not good for a relay.

There is a point where the LED may glow. Maybe it's blinking at 500Hz, but it looks like it is glowing.

It seems indecisive. Its kind of handy anyway.

What do you want for $2?  

G-

[elt]

Any step type load diversion won't give you good control of the battery charge,

How about "good enough" ?

I know that many commercial PV controllers use variable charge rates but most homebrew wind controllers that I've seen are just a voltage comparator or two driving a relay. With this little guy having six outputs, seems like a curve could be reasonably approximated?

[Ungrounded Lightning Rod]

If it doesn't have a bit of histeresis already, add a large-value resistor from the lamp output back to the tap on the voltage divider to proved a tiny bit of positive feedback.

[Ungrounded Lightning Rod]

(This assumes the device drives the output in the correct direction - for instance, it switches the + end of the diodes and the input is the + side of the comparator.  If it's the other way adding the resistor gives you negative feedback and tends to encourage flicker.)

[SamoaPower]

Good enough?  Heck no, not even close. That is, if you care about battery longevity. If you don't mind replacing them every few years, sure, go ahead.

Most controllers use a multi-phase charging algorithm, which means bringing the terminal voltage up to a point, holding it there for a while and then, bringing it back down to a float level. Even this is just an approximation of what should be done for best lifetime. None of the available controllers, even the $500 ones, do it the way it should be done, in my opinion.

Good enough? Hardly.

[elt]

None of the available controllers, even the $500 ones, do it the way it should be done, in my opinion.

Okay ... I found your gospel http://www.fieldlines.com/story/2006/1/23/8428/49319

and your 6th commandment: "No one makes a charge controller that does it right."

Its seems to me that you've hijacked the notion of building a low cost controller for the opportunity to get on your soapbox again. The info was interesting in the context of your story; it's not helpful and doesn't contribute to this thread.

Peace,

- Ed.

[electrondady1]

i have no understanding of electronics but could this devise be modified to control six sequentialy activated resistance heating loads?

[ghurd]

It would be easier to start from scratch.

G-

[elt]

electrondady1,

I think that it would be very easy to modify the device to sequentially activate loads. The capacity, though, would be limited to 5 loads. The sixth (lowest voltage reading) LED doesn't go through the coil driver chip; it just turns on if there's enough voltage.

There's a dilemma for you though. If you don't have an understanding of electronics, you might not be able to do it without instructions. I or someone else might post detailed instructions at some time but the units on are close out now. They might not be available later. If you can use a $2 battery voltage checker, why not buy one (or two.) Then, if it's successfully turned into a controller and documented, you'll have the device in hand.

Here's a picture showing both sides of the circut board:

(The back side is mirrored so that the traces and vias line up with the components seen on the front side.)

Other than the coil driver chip, everything is made of nice big discrete parts. It's a one sided board so it's easy to trace the circuit.

I haven't done it yet, but here's the plan of attack:

The five resistors on the lower part of the board are used to sense the voltage. They can be replaced to set whatever trip voltages are desired.

The resistors at the top are current limiting resistors for the LEDs. The are connected directly to the plus voltage. The other side of the LEDs are connected to driver chip.  The driver chip is a "sink" type switch; it works by providing a "ground" to the LED.

A very neat looking installation could be made by removing the current limiting resistors and LEDS and using their holes to attach the relay coil leads. Personally, I think it'd be nice to keep the LEDS as indicators. In that case, you'd solder the coil negative lead to the negative side of the LED or the the pin on the driver chip.

Ghurd mentions that there may be a "flutter" at about 500 Hz. Without the benefit of actually having made the mod, I'm not going to be scared of that right now. That's much faster then the typical coil will switch at and I've seen a designer flutter the voltage on purpose (actually, use a PWM signal) to lower the holding current of a relay once it's latched... my gut feel is that that's going to be okay.

I would like to see a little hysteresis in there though. I see a case where the voltage is just a little bit over the trip voltage and switching in the load draws it down to lower than the trip voltage so the coil gets released then the voltage rises again... I don't think that this hurts anything but the relay; they are only rated for so many open and close operations.

Using FETs to do the switching might make the lack of hysteresis a non-issue; however, I'm an utter novice when it comes to designing with FETs so I'm not comfortable proposing that right now.

 - Ed.

[SamoaPower]

I'm sorry Ed if my comments offended you and I didn't tell you what you wanted to hear.

The soapbox notwithstanding, I thought my comments were right on target for this post. There was no hijack.

As a proper battery charge controller, this approach simply won't work, period. Even as a battery meter, it's pretty worthless, no matter the cost. Measuring battery voltage in 0.5 and 1.0 volt steps is ludicrous. It's no wonder that Radio Shack wants to dump them.

If you want to discuss the issues, I'd be happy to, but I won't trade insults with you.

[elt]

No, I think that I was right on target...

[elt]

Here are nice inexpensive automotive relays that can be directly driven by the chip

http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=255&M=JSM1-12V-5

Rated at 160 watts; $1.50 a piece new.

(Many automotive relays have a coil current draw higher than the 75 mA spec'ed for the chip so if you're picking some other relay, do take a look at its coil current.)

 - Ed.

[ghurd]

They are very useful. And handy

The alternator tester end can tell if a car alternator is working.

The battery tester end can tell if the battery is dead.

I know of a few hard wired into small solar systems, where the battery is charged and used in different locations. One has a mom switch so it isn't using power all the time. Another is wired on the PV side of the controller so it doesn't need a switch, and it also shows the clamps have a connection to the battery.

It can be used to test fuses, circuits at a distance (like messing with wires at one end of a car to get a connection at the other end), etc.  I could go on and on, but I won't.

I'd rather not use a $100 meter in a running engine compartment, on a nasty old battery, under the car,...  Or a $10 meter for that matter.

BTW, the last one I opened didn't look like that.

G-