Do not use rechargeable batteries -- but why?
September 26, 2011 4:37 PM   Subscribe

Why am I not supposed to use rechargeable batteries in my vibrator? (NSFW)

The manual for my new vibrator states, "Do not use rechargeable batteries." I've been using rechargeables in another vibrator with no problem, other than needing to charge before each use. So, is it just the inefficiency? Or is there some sort of quality-of-vibrator-life issue?
posted by anonymous to Technology (15 answers total) 2 users marked this as a favorite

Rechargable batteries have a lower nominal voltage, so your vibrator won't run as fast.
posted by Confess, Fletch at 4:48 PM on September 26, 2011 [5 favorites]

Alkaline batteries, which most rechargeable batteries are, are more prone than other battery types to leaking potassium hydroxide, which you don't want up in your parts.
posted by tumid dahlia at 5:32 PM on September 26, 2011

tumid dahlia regular batteries are alkalaine. The ones the manual says to use. Rechargeables are nizn,or nimh
posted by majortom1981 at 5:50 PM on September 26, 2011 [1 favorite]

This isn't a technical answer, but I found that use of rechargables in some of the objects in our toybox drains them both really, really fast and to the point that I can no longer recharge some that were just purchased this year.

(The particular item in question is from a German company I can't recall the name of, and may have some crappy wiring, since it drains while the device is off and closed and we have to remember to unpack it entirely after use.)
posted by Weighted Companion Cube at 6:24 PM on September 26, 2011

The full charge in rechargeables doesn't last very long, in days, compared to alkaline. If you have a device that draws a small amount of current on an irregular basis, alkaline is the way to go. Rechargeables are much better for "high current" applications like the flash on your camera, or a piece of electronics that might get depleted in a couple hours. My guess is that it's nothing to do with safety but rather with "user experience."
posted by wnissen at 6:40 PM on September 26, 2011

Not sure if this is the reason for external devices, but a factor for internal ones may be that alkaline batteries can't release their charge as rapidly as rechargeable ones, so if there is an electrical short (and the electronics in these toys is generally shockingly cheap and crappy), an alkaline battery will get uncomfortably hot over a minute, while a rechargeable could cause serious burns very quickly.
posted by -harlequin- at 7:31 PM on September 26, 2011 [1 favorite]

Besides the lower nominal voltage causing regular DC motors to run slower some devices can be harmed by the increased current draw caused by the lower voltage.
posted by Mitheral at 8:25 PM on September 26, 2011

Excuse me? I don't know what the answer to this question is, but I must correct Mitheral's answer. Decreased voltage causes lower current, not higher.
posted by fearnothing at 10:39 PM on September 26, 2011

fearnothing -- Ohm's law applies in the case of constant resistance, which is not a good way to describe a motor. If instead a motor draws constant power, then a lower voltage does indeed correspond to an increased current (P = IV).
posted by zxcv at 1:36 AM on September 27, 2011

tumid dahlia regular batteries are alkalaine.

Well slap me down with a big buzzing pink thing, you are of course correct. Dunno where I got that notion from.
posted by tumid dahlia at 2:30 AM on September 27, 2011

zxcv is correct: Motor current is inversely proportional to speed, due to the counter-EMF produced by the armature as it rotates. See answer to Question 2.

Rechargeable NiMH/NiCd batteries are 1.25V vs 1.5V for a standard AA/AAA/C/D battery, hence the device could draw more current using rechargeables. That said, if it were my vibrator I would just use rechargeable batteries unless the device was exceptionally expensive or very difficult to replace (in the unlikely event that rechargeables caused a failure of the device).
posted by 6550 at 4:34 AM on September 27, 2011

fearnothing -- Ohm's law applies in the case of constant resistance, which is not a good way to describe a motor. If instead a motor draws constant power, then a lower voltage does indeed correspond to an increased current (P = IV).
posted by zxcv at 1:36 AM on September 27 [+] [!]

Agree, in my experience, motors are a sort of constant wattage device. They will try to pull as much current as they can until they reach some kind of internal limit. Perhaps the machinery in question was designed to assume that it will be getting limited by the current capacity of an alkaline battery, and when presented with the excess current that a rechargable battery can provide, will burn itself out.

I would not be concerned about trying those batteries in your new purchase, however. It is most likely that the thing just won't work as well.

(For reference, lithium batteries provide even more current. Like the super-expensive blue Energizer Lithium batteries. I had a camera that would kill a heavy-duty battery in 4-6 shots. It would kill an alkaline in maybe 10. It would last hundreds of shots with one of those little blue guys.)
posted by gjc at 4:43 AM on September 27, 2011

Complicating this issue is that fact that single-use batteries generally have more internal resistance than rechargeables. If you measure the terminal voltage of a single-use battery while it's actually powering a heavy-draining appliance, it's pretty common to see the nominal 1.5V/cell drop down to around 1.3V even when the cell is fresh, which is pretty close to the nominal 1.2V a rechargeable is good for. The terminal voltage of a single-use battery will also droop as it discharges; a battery is generally considered "flat" when its terminal voltage has dropped to 1.1V or so. Rechargeables tend to maintain a solid 1.2V for most of a discharge cycle and drop out very quickly at the end. On the other hand, they do self-discharge a lot faster than single-use batteries. This might be important for toys that don't get used frequently.

Bottom line is that you're highly unlikely to damage anything by using rechargeables in an appliance designed for single-use cells, but you might notice performance differences that might be important to you. Only way to find out is to try them, but don't go an buy a set of rechargeables specifically for your toys unless you've made sure they work OK for you.
posted by flabdablet at 6:39 AM on September 27, 2011

I also assume it's related to the user experience of a flat device after a period of inactivity.

If the total use time of an AA(?) battery device is measured in minutes or hours (heating, motors, camera+flash) it's better to use rechargeable (NiMH) batteries. They have a slightly lower total capacity than non-rechargeable alkalines but can better handle a high current draw. Alkalines are the best choice for devices like clocks, remotes or car flashlights; they have low self-discharge and can last many years in current sipping applications.

In photography, rechargeable NiCd batteries were known to be able provide even more current than NiMH batteries, so they were the preferred choice for ultra fast flash recharge times. However, I think it will be very difficult to find new NiCd batteries and a suitable charger. The advantages for a motor application might be negligible and the disadvantages of NiCd are many (lower total capacity, slow charge times, high self-discharge).

So the 2 most common rechargeable AA(?) battery types: regular NiMH and NiCd both suffer from high self-discharge.

If there are periods of inactivity between you charging the batteries and the use of the device, you could use low self-discharge (aka 'pre-charged') NiMH batteries (i.e. Sanyo Eneloops) instead of alkaline non-rechargeables. Regular NiMH rechargeable batteries tend to loose a lot of their charge (30% per month?) by the time you actually need them.

For emergency situations, the ultimate user experience will be provided by the above-mentioned non-rechargeable lithium cells, they have amazing shelf-life & very high potential difference (1.7V) but might be harder to find.

Apparently there are no 18650-battery vibrators (yet?). In my opinion, these Li-ion rechargeables would be well suited. They have about the same capacity as 3 AAs, low self-discharge, fast charge times and perform well in high current applications. They are cylinder-shaped, longer and wider than an AA, about 70% of the volume of 3 AAs. Unprotected versions could explode if they get too hot.
posted by Akeem at 7:12 AM on September 27, 2011 [1 favorite]

Here's a data point. My lady friend tried using rechargeable batteries in her Lovecraftian nightmare vibrating device, and it started sparking (!) and burned out while in use.

Could be the reason, could be something else.
posted by His thoughts were red thoughts at 9:46 PM on September 27, 2011

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