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04-27-2010, 12:40 PM   #1
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Join Date: Mar 2009
Location: Berkshaire, UK
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Unusual battery behaviour

I have a Pentax k-M
I use Ni-MH batteries (and I know about the setting for this in the menu)

I bought some 2850 mAh batteries from 7-day shop.

They worked great! ... ran for ages showed green for the longest time

...so I bought 3 more sets (12 batteries) these are terrible, I change them, put them in the Camera they show green, 1 hour later yellow, then red ...then the camera won't turn on.

I fully charge them store them for a few days ... the camera won't turn on.

OK easy call, I've got some duff batteries internal leakage.

HOWEVER I have a ANSMANN battery tester .

The Good batteries are 100% & 1.25V when fully charged. They drop to 30% and 1.??
(1.16 from memory) the camera still sees these as red.

A fully charged set of 'bad batteries' also shows as 100% 1.36v and the Camera sees as green ... but when they show as 80% & 1.26v the camera stops working

I just unloaded a set that showed yellow .. they were 3X100 1.35 v + 1 X 90% 1.29v


In case this were a problem with the ansmann tester I plugged them in into a filament bulb , then put them into a battery discharger ... it took many hours to empty them

So how the camera deciding what is 'full/empty'? The nominal voltage for Ni-MH batteries is only 1.2v so all are well above spec


Just put in a stored set that showed 4 X 100% 1.36v (camera shows green) ...I wonder what they will do :-)

04-27-2010, 12:53 PM   #2
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Join Date: Sep 2007
Location: USA
Posts: 1,812
QuoteOriginally posted by graemev2 Quote
I have a Pentax k-M
...so I bought 3 more sets (12 batteries) these are terrible, I change them, put them in the Camera they show green, 1 hour later yellow, then red ...then the camera won't turn on.
There is a sticky thread on the top of this forum:
Sticky: Battery for K-x (and other cameras that take AA's)

For the more technically minded - this is how to test the viability of any AA battery to be used in a Pentax dSLR.

There is a big difference between load and no-load voltages -
when the batteries are in use in the camera they are under load, and the voltage can be quite a bit lower than the open-circuit voltage when the batteries are under no-load.

Pentax dSLR can be quite demanding on current -
ie: the load can be pretty high.

Check out Post #23 in K200D Battery Meter Problem (mentioned above)
where dmessing measured actual voltage and current draw of his K200D in operation.

Basically the K200D draws 1.296 Amps at start up, and the peak current demand is 1.480A.

The threshold voltage for the K200D was 4.59V total (~=1.15V per cell if they are exactly matched)

I would think other Pentax dSLRs are going to be very similar, if not the same.

So to test the viability of your cells - use a resistor load of 1 ohm (and at least 3watt rating) - which would draw in theory 1.5A for alkalines and 1.2A for NiMH, and approx 1.65A for lithium AA - this is close enough to the start-up and perhaps the max current draws.

Under this load measure the battery voltage, if the cell is below 1.15V then it probably is not capable of powering up the K200D (or if in series the other cells will have to compensate/support it).

Use only cells that pass this criteria with a good margin, and see if the camera behaves any better.

Or with a battery analyzer like the Maha C9000 charger -
a much easier way to check the viability of any NiMH battery -
is to use the discharge function on a recently and fully charged battery -
set the discharge rate to 1300mA and look at the voltage reported -
it should be quite a bit above 1.15V -
if not the battery probably won't even power up the camera.

If that passes - remove the battery, wait a few moments and re-insert it
now set a discharge current of 1500 mA - and check the voltage -
again it should be a good margin above 1.15V.

To check how long the battery is likely to last compared to another is to leave that battery discharging at the 1500mA rate and see how long it takes before the voltage drops below 1.15V - this time is not how long the battery will last in the Pentax dSLR - but it is useful to compare with another NiMH battery - eg: eneloop vs. any other NiMH - one will be quite surprised how much more quickly a typical older regular NiMH will drop below 1.15V than an eneloop -
that's the reason why eneloops last longer in the Pentax dSLR than even regular NiMH that have much higher capacities >= 2500mAh....
because eneloops have better voltage maintenance under load.
05-03-2010, 03:57 AM   #3
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Join Date: Mar 2009
Location: Berkshaire, UK
Posts: 12
Original Poster
I had seen the sticky , started to read it but it mostly seemed to be a discussion on what
are good batteries to buy.
BTW. The URL you quote, does no work, you quote:
https://www.pentaxforums.com/forums/92821-battery-k-x-other-cameras-take-aas.html
The sticky is at
https://www.pentaxforums.com/forums/pentax-dslr-discussion/92821-battery-k-x-...-take-aas.html
... so I wonder if it's been reorganised since you posted ?
Assuming the same applies to your other link (I'll try adjusting it) ... yep that works
... so ...this might work (I guess the forum S/W might barf on this)
<a href="45346-k200d-battery-meter-problem.htm">
Thanks , that's more what I'm looking for. Even so (from my original post) I think I'm within those parameters by a long way . I wasn't simply measuring voltage under no-load conditions the ansmann tester is (I'd lead to believe) a pretty good tester
(Ansmann Energy Check LCD - Professional Battery Tester)
...I think it puts a load on the battery, then measures how long it takes to recover the original voltage [no evidence for this]
...I could live with a false 'yellow indicator' it was "the unable to power up" with full/1 day charged set that cause me the problem. I carry 2-3 spare sets and at least one set (charged 100%) could not power the camera up.
BTW. It seems (may be fooling myself) that this effect is going away as the batteries age ... their capacity is not going up (I assume it's going down) but I think the camera seems to think they are more full .
...I also chose the Pentax BECAUSE of the 4XAA battery option. Not because I have professional chargers (which I possibly do now :-( ) ... but because even up a mountain far from mains power you can probably beg borrow or steal (or a a pinch , buy from the gift shop) as set of plain old AA cells .... and so NOT LOSE THE SHOT!
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