What is "modified sine-wave" power?

June 16, 2005 3:42 PM

What is "modified sine-wave" power, and what will it do to my electronic devices? I have a portable Xantrex battery, and it came with a warning that certain rechargeable devices may not be compatible with its modified sine wave output.

Contacting the manufacturers of said devices resulted in the moral equivalent of "Whuuu?".

Some rechargeable batteries seem to get a weird charge off of it - they register as full after an hour or so, but then very quickly discharge.

How bad is this? Am I doing permanent damage to my batteries? Do I just need to let them charge longer?

posted by Caviar to computers & internet (9 answers total)

Mains power is AC - alternating currant, which means that it switches from negative to positive 60 times a second (or 50, depending where in the world you are). If you graph that oscillating voltage of positive falling to zero, going negative, rising to zero, going positive, and repeating, you'd see a sine wave.

Most devices just use the electricity, but some devices also use the oscillation. And example is some older clock radios, which counted the oscillations for their timing signal.

What the warning is saying is that the AC that it generates is not a perfect sine wave, (it might be a square wave for example). It does this because batteries are DC (direct currant - no oscillations), so it has to generate oscillations, and the circuitry which does this does not generate a sine wave, but an approximation.

As to which modern devices you would expect to fare badly with the modified sinewave AC, I wouldn't really know, but my guess would be you're probably not damaging the batteries, just undercharging them, perhaps because the charger's AC-to-DC circuitry might be based on a transformer, which works less efficiently on the modified sinewave (I'm speculating).
posted by -harlequin- at 3:59 PM on June 16, 2005

The normal AC power you get out of a power socket in your house is not actually a simple voltage; it oscilates, in the USA, at 60Hz. These oscillations take the form of a sine wave (i.e. smooth curves).

Cheap power inverters produce a square wave, which has the same frequency but sharp edges instead of smooth curves. Square wave power is fine for light bulbs, but things like variable speed motors (electric drills, for example) and sensitive electronics (computer power supplies) often crap out, sometimes with a release of magic smoke, and not always immediately when connected to such a nasty, harsh power supply.

Modified sine wave is halfway between the two. It can still cause problems with some computer power supplies, dimmer switches, and "wall wart" transformers, but more devices are happy with it.

Most likely the charger has a similar kind of transformer inside it, and it's getting confused. Depending on the type of batteries, you may be harming them. NiCad, for example, have a "memory effect" if you don't charge them fully -- you end up reducing their capacity. NiMH and LiIon don't have this problem.

On preview, I think harlequin explained the power side better.
posted by 5MeoCMP at 4:07 PM on June 16, 2005

To flesh out harlequin's excellent response, a modified sine-wave inverter -- the thing that converts DC to AC -- creates a stream of power with fluctuating voltage. Most modern electrical devices can deal with fluctuating voltage just fine. Some devices, such as laser printers, digital clocks, and computers work much better on pure sine-wave power. These kinds of inverters are more expensive.
posted by pmbuko at 4:07 PM on June 16, 2005

If you're plugging battery chargers into the output of this thing, you're probably charging your batteries with the wrong voltage, which is most likely damaging the batteries and/or the charger.
To elaborate on -harlequin-'s answer...the 'effective' voltage of AC sources is the RMS value (Root Mean Square). For sine waves the RMS voltage is 70.7% of the peak voltage. For square waves, it's 100%. For a "modified sine wave", it could be anything. Battery chargers that derive their charge voltage from this RMS input voltage will only work properly with a pure sine wave AC input.
posted by rocket88 at 4:38 PM on June 16, 2005

Computer power supplies should not have any problem with modified sine wave.

The power input to a computer power supply will have a fuse, some EMI filtering, some surge suppression parts, an inrush current limiter, a rectifier and a big capacitor. From there on it is all DC. All of those components should be okay (hmmm, the inrush limiter might not behave exactly correctly, but it isn't likely to be anything worth worrying about...). Infact, the rectifier and bulk capacitor will behave better in some respects.
posted by Chuckles at 4:50 PM on June 16, 2005

To clarify, I'm looking at using laptops, digital camera batteries, and cell phones.

The theory is interesting. Is there any way to test for a specific device whether or not this is okay?

Is there something in particular I should be asking the manufacturers or some specification I can look for?

As I found out, if I call up random customer support person X, they're not going to know what I'm talking about. Is there a person/role at the company I can ask them to contact to find out? Who is likely to know this?
posted by Caviar at 5:17 PM on June 16, 2005

I think rocket88 is close to the issue.

Many battery chargers, especially ones that are more than a couple of years old, are timed - they charge for a certain number of hours after you connect them. If you give that kind of circuit a square wave instead of a sine wave but at the same peak voltage you can get a 1/.707 = 41% over charge - depending on the exact circuit details. That would be very bad for batteries.

If your devices have electronic chargers with temperature and current control you probably don't have anything to worry about. My guess is that all modern laptops have electronic chargers, the cell phone I'm not so sure about, cheap stand alone chargers typically don't. How you get real information instead of guesses without doing a lot of irritating research is beyond me...
posted by Chuckles at 6:41 PM on June 16, 2005

For "close to the issue" read "pretty much exactly correct"... :)
posted by Chuckles at 6:43 PM on June 16, 2005

I second 5MeoCMP and rocket88. I'll bet you can find higher quality battery chargers that work differently than Chuckles describes. Surely some exist that regulate their dc supply and/or use a precision voltage reference to insure proper charging. I'll wager that most laptop supplies carefully regulate their dc output, so they're probably fine. Respect the rated output power limit, though. Otherwise, I'd expect its ac characteristics to be pretty evil.
posted by fatllama at 8:16 PM on June 16, 2005