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Q & A: Battery clarification

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Most recent answer: 10/22/2007
Q:
Your answers to "What battery is better, C or D?" and "Using bigger batteries" raises the Q: Do batteries automatically discharge their maximum current output on a load, or do the current requirements of a load "control" the batteries’ current output, that is, "draw" only the amount of current needed? Your answers to the two questions seem to answer both of my Qs "Yes", which seems a contradiction. Unless a device designed for 2 series AAAs (in series), for instance, has overcurrent protection, why wouldn’t 2 Ds to power it damage the circuit? And, if the Ds would work, would they work at their peak efficiency as if in a circuit designed for 2 Ds. Also, how does the functioning of an AC adapter (e.g., with a 3v, 500ma output) differ from 2 1.5v batteries in series? Thanks.
- Eddie
Houston, Texas
A:
Hi Eddie,

We stand by our answers -- yes, you may use AAA, AA, C, or D cells interchangeably, as long as they are wired up the same way and that you can make them fit.

The big difference between these batteries, apart from size, is really the total energy stored in them. Adam said the current was different, but as you correctly note, it is not the battery (usually) which determines the current which flows, it is the load. A battery pushes electrons in wires with a voltage that is fixed at 1.5 volts (if you're using one of the batteries mentioned above, and it's not dead or shorted out). Then the load determines how much current will flow, often obeying Ohm's Law, V=IR, or whatever the load requires at the voltage it's being driven at. Only if the load requires lots and lots of current will the voltage on the battery sag below its rated voltage (or if the battery is wearing out). (I believe that the maximum current that can be supplied before the voltage starts to sag does depend on battery size. So a small battery might fail to work in some applications even in the short run./mbw)

So then the difference between the battery sizes isn't voltage, and it isn't current (except under extreme circumstances where the battery is shorted out). It's the product of current*time, which is proportional to the total energy stored. Run a piece of equipment with bigger batteries which have the same voltage, and you'll have to replace them less frequently.

I'm not sure what you mean by "efficiency". All sorts of batteries are very efficent at low current draws. The only loss I can think of when oversizing a battery for a particular application is that its "shelf life" will expire while it is sitting in a device which uses up much less energy than is available. Then you'd still have to replace the battery after its lifetime, regardless of whether a smaller battery could have done the same job in the same time. This is normally of minor concern, as you'd have to change the batteries at a minimum of the shelf-life of the battery anyway, and then it's just a case of spending more money on big batteries versus small ones (battery price doesn't always scale with battery size, though). What is perhaps more important is the inconveniece of lugging around those big, heavy batteries if they're not going to be fully used by the time they get so old they stop working anyway.

Tom

(published on 10/22/2007)

Follow-Up #1: More battery clarification

Q:
TO: Tom RE: Battery Clarification Thanks for the clarification. I take your answer on batteries to have implicitly answered the Q on the AC to DC converter. Because others may read your answer the same way, I thought that you might want to also tell readers what I just learned upon purchasing a radio that runs on a rechargeable battery. The adapter available for the radio is rated 9V, 100mA, which can remain plugged-in interminably, but mfg advises owners that while radio can be run/recharged with 9V adapter with higher current output, the user must limit that recharge to a couple of hours to avoid damage to battery or circuitry. I don’t know if that limitation is applicable to all rechargeables, but it is a caveat worth disclosing. Thanks again, Eddie
- Eddie
Houston, TX
A:
Yup, I just forgot to answer the question on the AC adapter. As long as the voltage is rated the same as the batteries it's replacing and it can supply enough current (usually not a problem), it should be fine to substitute it for the batteries. This hasn't always been the case, though. Cheap, old AC adapters sometimes were plagued by "ripple". If the DC voltage output of the adapter isn't controlled well enough, it can fluctuate, usually at the frequency of the incoming AC (or often at twice the frequency, because of the rectifier). While the average DC voltage output may be the desired voltage, the peak voltage could be significantly higher. Sensitive electronics may stop working when connected to an AC adapter which has lots of ripple on it. I once fried a small, handheld game from the late 1970's with a cheap AC adapter in exactly this way. Technology has marched along, and AC adapters are perhaps better than the one I had.

As you mention, some battery chargers do have the warning not to overcharge them as damage may result. I've seen some rechargeable batteries which in fact recommend the opposite -- that you can leave them charging indefinitely (I have a battery-powered lawn trimmer which says this -- I believe it has a lead-acid battery in it). Given the ease with which Americans sue manufacturers over product safety these days, I am surprised that any battery which can leak when overcharged, even with adequate warnings pasted all over it, wouldn't be an invitation for lawsuits. I guess we routinely use hazardous equipment with clear warnings that negligent use could cause damage (cars, for example!).

Tom

(published on 10/22/2007)

Follow-up on this answer.