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I'm building a stationary contraption that, in addition to being plugged into the wall, needs a continually-charged battery backup in case the power goes out. I'll be powering a microcontroller and occasionally (for a few seconds every hour) running a wiper motor.

Assuming I can get my stuff working fine when run off the battery alone, is it alright to permanently hook up a trickle charger to the battery terminals to keep it charged, while keeping my circuit attached in parallel? Will my circuit prevent the charger from properly determining the status of the battery? Do I need to use some diodes to prevent current from back-flowing, or should the charger handle that?

I've searched quite a bit, but am probably using the wrong terminology. I get the impression this is something so stupidly simple that nobody bothers to write it down. :D

I'm currently planning to use a 12v battery like this:

https://www.allelectronics.com/index.php?&page=item&id=GC-125&index=1

and a charger like this:

http://www.harborfreight.com/review/product/list/id/1141/

Any and all advice is welcome!
 

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What you describe sounds like it should work - assuming you are literally talking about 'seconds per hour' of operation. The key is you can't draw power out of the battery faster than the trickle charger can replace it on an 'average' basis.

Most trickle chargers run a few hundred mA or less. I don't see any specs on the exact one you mention (though I have a couple in the garage, so a test might be in order!) So assuming your microcontroller + wiper motor don't draw average power over what the float charger can supply, it should be OK. If there is a small surge when the wiper motor kicks on, the battery should easily make that up in the short term, then the trickle charger would have an hour to make that back up.
 

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Hi,

As your having a microcontroller based project, why not be 100% safe?

by this - assuming you have a spare IO on the micro, use it to drive a relay to control the charger connection - when you trigger the prop, disconnect the charger for that time and re-apply it after operation, that way you're sure of no current sourcing from the charger during prop operation.

A charger is capable of suffering/withstanding surges - the initial connection to a flat car battery can cause a huge current surge, so no great issue there, it's just the risk that after numerous prop operations, the battery may flatten to a point where the charger is sourcing power too - this may not be a desirable mode of operation, so disconnecting it with a relay may be a safer option.

If you're worries about the current rating of the relay for the charger/battery connection, then use the relay to control the mains to the charger.

Si
 

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That is a good thought, though I wonder if the relay might actually introduce problems. The charger/battery is going to be a pretty pure DC system with minimal if any electrical 'noise'. But when you add a relay, every time the relay opens, there will be a big high-voltage spike from the back EMF of the coil. There could also be some high frequency noise from arcing at the contacts - especially since they are opening on a DC current.

The controller should have good filter caps and would likely survive either way, but it seems adding a relay would just add sparks and a high voltage spike to the lines each time it opened/closed plus one more thing in the chain to go wrong?
 

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I have a couple of those exact chargers on my four wheelers. This post got me curious, so I put one on a scope - it's noisy. You'll definitely want good filters on your controller. That said, it does what it's supposed to do very well. Assuming your motor doesn't have a huge load on it, I'd say go for it.
 

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But, did you have it in parallel with a battery and a load?

Though now that you mention it, years ago, I remember some research paper about sending high frequency pulses through batteries to bust up sulfation. Don't know if these little chargers are that advanced or not.
 

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I can't think of the word for it but some chargers can cause issues with electronic devices. Some chargers use a couple volts higher to charge a battery and most don't have a regulated output. Where I work we have portable work station using large batteries and a power inverter to run a computer, barcode scanner and label printer. We charge these batteries with a fully automatic charger from Ctek. We bought these chargers from Interstate Battery. They are labeled safe for use with connected electronics, are just a bit bigger than a float charger. It varies the charge rate as it need instead of poping and off.
 

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But, did you have it in parallel with a battery and a load?

Though now that you mention it, years ago, I remember some research paper about sending high frequency pulses through batteries to bust up sulfation. Don't know if these little chargers are that advanced or not.
Measured it both ways - bare and loaded. It doesn't appear to be very tightly regulated. It was reading 19 - 20 or so volts unloaded, and around 14 with a load (connected to an old ups battery). Not sure what kind of comparator they use to control the charging - maybe I'll get adventurous and tear one apart.
I didn't see any indication of intentional high frequency pulses, just a lot of 60 hertz noise and harmonics. The grid here (Wichita) is pretty dirty - I'm sure that doesn't help - but I don't think there's much in the way of noise filtration in these things.
 
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