Gosh, I hadn't known that NiMH batteries can cause problems. I'm using a bunch of Eneloop NiMH cells in my old Pentax 645 without observing any adverse effect --- yet. Well, I suppose if the camera works well now, it is probably alright to use NiMH batteries in the long run. Does anyone have any advice or thought on this?
I use NiMH in most everything including my 645N. Nearly all modern circuit boards contain voltage regulators so the function of the metering and shutter remains perfect, at least right up until the moment the sensing voltmeter switches off the camera at some predetermined voltage level. This has been true since about the 70's and it's why you can swap battery types of similar voltage on most anything that didn't originally require mercury cells. From the 645N owner's manual on pg 11: "Even if the low-battery symbol appears, exposure will be correct as long as the shutter can be released." Whether this is the same for the circa 1984 P645 I can't say with certainty but a VR seems to be the part of the circuitry of the 645N. However, I do find that the shut-down voltage seems biased for the characteristic thresholds of alkaline batteries, which are a little higher than for NiMH. So you may want to keep NiMH cells in your 645 topped off, rather than dragging them all the way down before recharging, especially in cooler weather. (It's just become part of the deal for me using NiMH that I don't use half the charge and put a camera away for two or three weeks with the expectation of there to be enough juice left for another session. If it's an important session, I carry a 12VDC/120VAC fifteen minute fast charger AND spares).
Thank you both for your reassurance. I expose me old Pentax to enough external dangers, and I wanted to be sure I'm not feeding it bad energy. Low self-discharge NiMH batteries are amazing, by the way.
NiMH batteries have lower voltage than alkalines (1.2 versus 1.5) so the voltage shouldn't be a problem.
In some cameras, motor drives operate more slowly due to the lower voltage (that happened with my Nikon F90).
Otherwise you should be fine.
Jim MacKenzie - Regina, Saskatchewan, Canada
A bunch of Nikons; Feds, Zorkis and a Kiev; Pentax 67-II (inherited from my deceased father-in-law); Bronica SQ-A; and a nice Shen Hao 4x5 field camera with 3 decent lenses that needs to be taken outside more. Oh, and as of mid-2012, one of those bodies we don't talk about here.