Can anyone answer that for me?

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You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman

They can be any voltage AFAIK.

I believe all rechargeable batteries in the popular cell sizes are 1.2v due to the amount of power created by the chemicals used. Also, the difference in the .3V isn't as important as the "drain" characteristics of the different styles of batteries.

Because that's what a single cell NiMH can output in the way of voltage. A 12V car battery is actually 8 batteries in series. 1.5V x 8 = 12V. A 9 Volt battery is actually 6 cells. Chemical makeup determines how much voltage you can get from a single cell. Usually no more than 2 volts.

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Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320

The current curve is interesting on rechargables vs alkaline. Rechargable batts reach there limit and take a nose dive fast. Alkaline batts current drops off closer to a linear curve.

Wet cells have a voltage of 2.2 volts vs 1.5 in dry cells. Thus a 12V car battery will have 6 cells-not 8. The chemistry was explained to me a while ago, damned if I can remember it.

Has anyone heard the myth about setting car batterys on concrete? Old rumor says the need to be placed on a wood pallet otherwise they will be damaged.
I ask an oldtimer about this once. He said it was true when the battery cases were made of wood and had dovetail joints along the sides. The jolt of setting them on solid concrete would loosen the dovetail joint.

Our family subscribes to this. We have always stored batteries on a block of wood.

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes

You gotta be kidding me?!? Battery cases of wood??? Never heard of such a thing. Figured the sulfuric acid would take care of that real quick.

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1983 M-B 240D-Gone too.
1976 M-B 300D-Departed.

"Good" is the worst enemy of "Great".

Correction: Bad info... a normal 12V car battery actually has 6 cells @ 2.1V each. I'm so used to servicing 16V racing batteries that I said 8 cell.

__________________
Current Stable:
01 ML55 AMG
92 500E (a few mods)
87 300E (lots of mods)
00 Chevy 3500HD Diesel Box Truck
68 18' Donzi Marine
06 GT i-Drive7 1.0 Mountain Bike (with GPS!)

PREVIOUSLY OWNED:83 300SD, 87 420SEL, 88 420SEL, 90 420SEL, 86 560SEL, 86 190E 2.3-16V AMG, 94 E320

Old battery cases were from glass, not wood. Individual cells could be replaced by cutting the loose tar that sealed the top and swapping in a fresh set of plates.

What is the maximum charging voltage with a 16 volt battery. I'm guessing you use 16 volts instead of 12 to start high compression engines?

Turbo start, Racing battery.

I'd rather see an internal resistance spec than voltage. What good is 16 volts at 100 milliamps? P=E times I. Current/amps is what counts when cranking.

And, the early batterys were dovetailed wood boxes lined with tar.

The Willard 6 volt.
http://www.adstocollect.com/servlet/the-663/1917-Ad-Willard-Storage/Detail

http://www.powerstream.com/1922/battery_1922_WITTE/batteryfiles/chapter03.htm

Regarding why NiMH are 1.2volts... the voltage is determined by the elements of the battery.

NiMH, NiCAD 1.2 volts
Carbon zinc, Alkiline 1.5 volts
Lead acid cells are 2 volts
Lithiums 3 volts

I've noticed that the NiMH AA batts are not that much use in my digital camera. It will only use alkaline AA until they get down to about 1.4 volts, then it does the "Oopsie, closing down now" thingy. I then use the same batts in my little French made strap on miners' headlamp thingy. They fade out at around 1.2 volts.

Dang, I like my digital readout voltmeter.

The NiMH AAs last for a handful of shots in the camera before they're over the hill. Perhaps I'll use them in my headlamp only.

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1986 300SDL, 362K
1984 300D, 138K

Something else to consider...

Charging current should be approx 1/10 of the battery's ampere hr rating. Some NiCads had a 600mah rating and are charged at 60ma. The same size NiMH can be as high as 2000mah. If sub'd for the nicads, they will never fully charge at the 60ma rate.