Battery charger for 12V power supply?

May 26, 2007 2:26 PM

Calling electricians: can I use one of those "battery chargers" they sell at Auto Zone to power a small appliance that needs 12V?

The appliance is an RV waste pump that needs to be on just 5 minutes every week. The manufacturer sheet says it needs a 12VDC 20A connection. The only available power is 110 VAC.

It's pretty much impossible to find any kind of 110V to 12V converter, but there's oodles of battery chargers like this at AutoZone, Wal-Mart, etc. Will these work as a basic power supply, or will they act flaky when they don't get connected to a battery? Will those fully automatic chargers refuse to power a motor? Can I just go by the highest amp rating on the charger packaging and make sure it's 20A or above?

Again, I will only be operating the thing 5 minutes a week, so I don't need a certified, by-the-book connection; just something that will get the job done.

posted by hodyoaten to home & garden (13 comments total)

They should work fine (but you'll need a higher amperage than the one you linked to), the automatic type will probably refuse to do it so stick with a cheaper manual type at the 20 amp rating. I've used one for electrolytic rust removal with no problem, btw. Or something like this.
posted by IronLizard at 2:36 PM on May 26, 2007

Chargers usually claim to be able to supply some number of starting amps (briefly) and some smaller number of charging amps (for a longer time). They probably can't supply their starting amps for a full five minutes, so buy something that can supply 20A for actual charging (that's a reasonable number for a "fast charge" setting).

(Unless that 20A rating is the pump motor's starting/stall current, not it's usual current under load, which will be smaller.)

A cheap battery charger might be putting out unfiltered rectified AC (= pulsed DC), which is not what the motor will be expecting. I think the worst that's likely to do is make the motor hum while it runs and maybe get hotter than it would otherwise. For 5-minute supervised operation I don't think it'd be a problem.

It's possible that connecting an inductive load (like a pump motor) to a supply that isn't expecting one (like a battery charger) could damage the charger. But I think this is unlikely enough that it's not worth worrying about too much, because a battery charger is hopefully going to be designed to take a certain amount of abuse.
posted by hattifattener at 3:06 PM on May 26, 2007

Have you considered using a combination of a 12v battery and a trickle charger? That's what I would do in your shoes.
posted by buggzzee23 at 3:23 PM on May 26, 2007

Or you could go this route with a regulated power supply
posted by buggzzee23 at 3:26 PM on May 26, 2007

Ditto what buggzee23 said. Here's a cheaper one from the shack.
posted by SteveInMaine at 4:58 PM on May 26, 2007

I wouldn't use a charger alone. It might work, but chargers aren't designed to have good load regulation, and many 12V chargers don't do much more than rectify the AC line. Of course modding a 12V charger would be pretty easy, but based on the question, I don't think that is a project you really want to tackle.

Using a battery with a trickle charger isn't a terrible idea, but for 5 minutes a week it really isn't ideal You'll be doing an awful lot of trickle charging, for not much use. There could be overcharging issues (bad), or just a lot of wasted electricity.

A regulated 12V power supply with 20A out isn't that hard to come by - many computer power supplies could handle it fine. Here is the pinout, and instructions for getting an ATX supply to turn on without a computer. Computer power supplies are intended to run with some load on the 5V or 3.3V rail as well though, so depending on the model you might have a reliability problem. The up side is that it will only cost $10-20 to find out.

Then there is a regulated supply like the one buggzzee23 linked. That is the correct answer, I suppose, but a bit pricey.

Is this 5 minutes every week an automated or manual process? It will be a pain if the process is automated, because you have to leave the power supply turned on the entire time, which is very wasteful. Perhaps the system could be improved in that case..
posted by Chuckles at 4:58 PM on May 26, 2007

Well I read through the results and went ahead and got this model at AutoZone. It seemed to work just fine, though I noticed the meter was running up near 80A (maybe my connections weren't good, i.e. alligator clips to 12 AWG copper wire, or maybe the meter is junk). I ran it for 5 minutes with no problem, and the motor casing was just slightly warmish, not hot.

(The process is manual, and is conducted by me -- it's the emptying of the RV septic tank)
posted by hodyoaten at 5:09 PM on May 26, 2007

80A? Are you sure it wasn't 8.0A?

Waittasec. If you have an RV, don't you already have a 12V power supply in the RV (engine + battery)?
posted by hattifattener at 6:04 PM on May 26, 2007

If you were really drawing 80A, you'd know it.. The wire and connection points would get plenty hot. I'd guess that the reading is inaccurate because you are using it to drive a motor (which is an inductive load), not a battery, but I can't think of the exact mechanism for that error.

the motor casing was just slightly warmish, not hot.

Well, it means you are unlikely to be damaging the motor, I guess.. 5 minutes isn't very long though, hard to know how easily heat flows from the motor to the casing. I'm also not sure the charger will like driving an inductive load.. Presumably breaking the charger isn't that much of a concern!?!?

At the very least, I'd put a big capacitor across the 12V.. Make sure you get the polarity right though, those caps can be explosive :P

The peak output of the charger will be at least 15V, probably higher, especially when the load isn't turned on, so don't be tempted by capacitors rated at 16V.

I chose 22,000uF as a first guess because at 12V and 20A the load is ~0.6ohms. That gives a time constant of 0.6*0.022 = 0.0132s, which is a little longer than the 0.008s between peaks of full wave rectified 60Hz. There are better ways to calculate the required capacitor, which probably result in a somewhat higher value..

(Hmm.. Computer power supplies may not like driving inductive loads..)
posted by Chuckles at 6:07 PM on May 26, 2007

Computer power supplies may not like driving inductive loads

They don't, but they'll do it. I'm not sure how/why, but my (3) steppers (2.5amp/coil) were giving off some serious static electricity when I tried it. When you put your hand near one, you could feel the hair on your arms start to rise. Perhaps it was the circuit I was using to drive them, but doing this essentially turned my motors into transformers.
posted by IronLizard at 6:51 PM on May 26, 2007

Be careful if you have battery in the loop. I did that with a lead acid car battery and woke up one day to a steaming bubbling fountain of noxious fumes.
posted by StickyCarpet at 10:44 AM on May 27, 2007

Thinking about it some more, the super-cheap circuits I'd expect to find in a battery charger would all be fine with inductive loads.

Switchmode supplies (like computer supplies) can have all sorts of crazy behaviors if they don't like their load, though, oscillating or whatever— maybe that's what happened with IronLizard's setup.
posted by hattifattener at 5:54 PM on May 27, 2007

Glad you found a cheapish way to do this! Just wanted to mention that power supplies of that rating do exist. They're pretty hard to Google for, though.
posted by musicinmybrain at 5:46 AM on June 8, 2007