Battery Basics 101

[BabyKiller]

lol

[bluemax_1]

hogjowlz and babykiller, you're treading on some dangerous ground when trying to figure out what an AA NiMH batt puts out. If you look up the differences between alkaline batts and NiMH or NiCad batts, there is a crucial difference in the way they put out power.

babykiller, your question about how much mAH an AA batt puts out, if it's a NiMH or NiCad, it will be listed on the side of the battery. The highest rating at the moment for a AA NiMH is the Sanyo 2100 mAH batt. Meaning it can supposedly sustain a 2100mA output for one hour.

However, and this is very important, hogjowlz, your statement about the 1250mAH batt being the capacity and that it doesn't output that all at once is incorrect. The actual output of the batt is dependent on current draw. Most circuits regulate the draw, but if you ever short circuit a battery, i.e. just take a wire and connect the 2 terminals, then current draw is dependent on the battery's internal resistance. An alkaline batt has relatively high internal resistance so even if you short it out, it has a limited current output. Also, because of an alkaline batts chemistry and construction, these batts don't perform well under load. The performance drops quite significantly in fact, the greater the draw.

NiMH and NiCad batts though have very low internal resistance which makes it VERY dangerous to short these batteries out. They can put out tremendously high currents if you short them out. Yes, even AA size batts. If you link 4 AA-size NiMH or NiCad batts in series and short out the +ve and -ve terminal of this pack with a screwdriver for instance. It won't take very long, just a few seconds to get the screwdriver really hot, and if you leave it connected, the current draw is high enough to MELT the screwdriver. Be careful with these batteries!

If you don't believe me, try it, but try not to burn the house down in the process, and I take no responsibility for anyone crazy enough to try this.

Also, Namuna, a small point about the rating of the output on that radio shack adapter. It is actually correct to state that the output is 800mA and NOT 800mAH. It means that there is an output of 800mA constantly. Thus an 800mAH battery would take one hour to charge, then theoretically be able to output 800mA for one hour. If current draw is higher or lower, than the battery would be able to sustain output for a shorter or longer duration. In practice of course, the inefficiencies of a battery never let it maintain the constant rated output, so if you had a device which draws 1600mA, the 800mAH battery would not run for 30 minutes. It would probably run less than 30 minutes.

This is why NiMH and NiCad batteries are so dangerous. shorting it out means near 0 resistance across the terminals. Because the battery itself also has near zero internal resistance, in theory, I believe I've read, NiMH and NiCad batts can put out near infinite current when shorted out. This massive flow of current heats up whatever is shorting it out and the battery itself. Keeping a fully charged NiMH battery in your pocket with a whole lot of loose change can short the battery and start a fire in your pocket.

[bluemax_1]

BTW here's a link another forum member put up to a site with NiMH and general battery info
http://www.extremetech.com/article2/0,3973,15710,00.asp

P.S. sorry, can't remember which member posted the link.

[Buzzbait]

I just ran through this thread, and now I'm a little confused. Hog says that 3 volts won't charge a 2.4 volt battery (dual cell mod). You actually need something closer to 3.6 volts. Yet Namuna recommends getting as close in voltage, between the charger and the batteries, as possible. Who is right, assuming that you're charging NiMH batteries?

I ask this because I'm thinking about adding a regulated AC adapter to my Motorworks charging base, but I'm not sure which adapter to buy. I was all set to use a 3V, 700mA Radio Shack adapter, and using the timed (50 second) charger to make 13 consecutive charges to my dual cell car, modded with two 110mAH NiMH batteries.

But if a 3V adapter won't do the job..... Do I step up to a 4V adapter?

And while we’re on the subject, what would be the highest safe current to use? At 4V, Radio Shack can provide adapters rated at 300mA, 800mA and 1000mA. At 300mA, I’d be charging for almost a half hour, which is a long time to wait. Stepping up to the 800mA adapter would cut things down to 10 minutes. It’s also a lot easier to count 12 charges, as opposed to 31 charges.

[hogjowlz]

when we charge of my ee buddie's power supply its usually 4v @ 3-500ma.

as far as shorts go ive never seen my stuff get that hot. i have shorted a car once by hitting a door(really hard) and the shock jabbed a solder connection through the insulator on a battery and all kinds of strange stuff happened but it didnt get super hot right away.

you wana see a lota heat play with photography power packs.
my friend repairs them and a switch shorted one night and vaporized part of a screw and it arked allover the place and well "blam" it sounded like an m-80. i was about 5 feet from it and he was about 1foot from it. you can always run an auto fuze in your car if you are worried about shorts.

and well namunas statments are all on theory and the most efficient way to charge a cell. if you are charging an expensive industrial battery or even a cell phone id listen to him.

in theory you want just enough voltage to overcome internal resistance and trickle the current into the battery. and yes thats the proper way to charge a cell.

but these things cost about 2bucks a piece and if you charge the "proper way" you will be waiting a long time to play with your car. i cram the juice in so i dont wait more than a few mins to run my car at top speed. i have a few dual cells that still run hard after several months of use and they hold just as well as when they were new. when they do go bad it will cost me 4bucks to replace.

you guys should not get ulcers over 2 dollar batteries.

[Gr-Dragon]

When it was mentioned that you need a voltage roughly 1.5x that of what you're charging to overcome internal resistance it was said based on the assumption that what you're charging has a high internal resistance. Now that it has been established that NiMH and NiCad batteries have a miniscule internal resistance it should be more understandable as to why it is fine to charge them from a 1.5v source.

I do have a question for the more technically inclined people here. What if the charging source were to be a rechargeable (NiMH or NiCad) AA? It is my understanding that we're able to charge NiMH and NiCads w/ alkaline batteries due to the high internal resistance of alkaline batteries causing only a partial short allowing current to dump into the battery you intend to charge without all cycling back to the battery you're charging from. If I were to use a NiMH AA to recharge a NiMH 1/3AAA directly wouldn't that cause a full short resulting in bubbly batts due to the miniscule internal resistance on both ends (< 1ohm)?

Oh, and don't alkalines have an approximate internal resistance of about 1.2ohms? (this would make sense if they're putting out 1.25amps on a full charge, while their voltage is still at 1.5v ... I = V/R .... I = 1.5/1.2 ... 1.25 = 1.5/1.2).