*ist-D battery grip: D-BG1 and battery issues

Hi all,
I decided to post a new topic on the "*ist-D-battery-grip-battery-issue" problem. I think I have some more information that pins down the problem, or at least some of it

Previous threads on this subject and the D-BG1 in general:
https://www.pentaxuser.com/forum/viewtopic.php?t=2258
https://www.pentaxuser.com/forum/viewtopic.php?t=1787
https://www.pentaxuser.com/forum/viewtopic.php?t=2257


Out shooting on Saturday and the same thing happened as previously. I tried a few things out to see what was going on. *ist-D (with 1.12 firmware), D-BG1, and 8 x 2500mAhr NiMH batteries (4 in body, 4 in grip).

Took a shot; the shutter made a feeble "dying" sound; started writing then died - lost the shot.
Yet the battery meter had said "full"

Here's what I found: took out the body batteries, put the grip back on - and the battery meter was flashing empty. Next, I put back the body batteries and took out one of the grip batteries - fine.

So it seems the body is making a decision about where to take the power from, and in this case it made the wrong decision...

Back home I measured all the cells:
Grip: 0.977, 1.296, 1.326, 1.224
Body: 1.287, 1.283, 1.284, 1.284

One cell in the grip was flat (0.977). The problems that I see are:
1) Most fast chargers do two AA cells at a time and can't detect an imbalance in the charge between the two cells.
2) The discharge graph of NiMH is unbelievably sharp. Just above 1.25V they are pretty much full, around 1.1V and they're as good as dead. As they go flat their internal resistance increases, so one dud cell can bring the whole lot down (as a pack they measure ok when taking no current, but as soon as you load the pack the dead cell gives out)

I put the offending AA in a slow charger (that charges individual cells), and it was then fine.

So, now, I'm not so sure what's the best approach. But here are some suggestions:
A) Only fit batteries in the grip. That way you get a clearer idea of how much charge you have and avoids this confusion.
B) Use CRV3-R. I'm investigating using the rechargable lithium-ion cells. They have a higher voltage and perform longer (e.g. 1200mAh LiON out-performs a 2300mAhr NiMH). But the price is high, also this quote from Steve's Digicams:
Though I did find a web-page where someone showed how to modify the grip to take the CRV3-R cells (by adding a diode in series with the cells), but I just can't find the page again...
C) Individually charge my AA cells. Well, at least from time to time - just to make sure I don't have any duds in the pack.

Excuse my ramblings... but it's still Monday morning after all

Matt

Brilliant deductive work. for what it's worth I have an energizer battery charger that was inexpensive, and charges each cell independantlty. I think it was like $30.00 can at Walmart. and $14.00/4 2500mah baterries.
one led for each cell and I always mark my batterries with a marker to keep them as "sets".

Thanks for this, Matt! Used to be an engineer, but stopped before NiMhs appeared! Was slightly more acquainted with the quirks of Ni-Cads!

Also, its a shame that the 'natural' potential of some of these re-chargeables isn't 1.5 v like the CRV-3/ AA alkaline!

Can't try out any suggestions - camera and grip still at pentax sorting out banding issue on some pictures. Have asked them to look at this issue with the grip at same time, tho'!

Andy

Hi Matt

If the 2 battery packs are in paralell with each other, which I assume they are, that would be an extreemly bad, if not dangerous, practice!!

As I see it, you have a battery comprised in a set of 4 cells, if one cell is more discharged than the other cells then there would come a point where it will 'see' the opposite polarity and would charge up reversed (i.e. it's +ve pole would be negative and it's -ve pole would be positive). This means that the battery as a whole (4 cells) would be minus the 1.2 Volt of that cell, on top of that the other cells would have to overcome the reversed charge in the 'faulty' cell, so the total battery voltage would be alot less than the nominal 4.8 Volt it should be.

This used to happen in my other incarnation as a fast electric model boater, and is alot more critical with the Amperage we were using, but still applies.

To avoid this problem we used to match cells for internal resistance (using a Wheatstone Bridge) and always kept the cells together as a battery (charge them up together, and discharge them together). With the current required for a camera I don't think we need to match the cells, but never put different A/H rated cells in the same battery pack, because the lower rated one will discharge first and the above phenomana will occur.

Then we come to the point about having 2 battery packs in parallel.

The 'correct' way to do it is to have a voltage sensor circuit which will switch from one set of cells to the other when it detects a lower than required voltage, which is difficult to acomplish due to the 'switching' component having a voltage drop so neither battery pack will produce 4.8 Volt, hence my reason for thinking that they just put the 2 battery packs in parallel.

Why is this so bad?

Most chargers limit the available current in some way, cheaper ones just use timers then limit the current to a 'safe' level, more expensive charger detect the Alpha point (a slight drop in terminal voltage when a cell is 95% charged, but long before it starts to vent) then limit to a safe level.

Imagine a situation where one battery pack is fully charged and the other is discharged, current will flow from the 'good' battery pack to the other due to there being a Voltage difference between the two and there is nothing to limit the current available!!! Further imagine that one of the cells in the discharged pack is totally discharged and begins to charge up 'reversed' this will lower the discharged battery pack voltage still further say to 3 Volts or so, then the Voltage difference is greater so even more current will flow from the charged battery pack to the point where another cell fails exaserbating the problem in a (possibly) dangerous way.

Suggestions,

Always keep cells together as a pack, never mix & match.

Never mix different A/H ratings in a battery pack.

If indeed the 2 packs are wired in paralell then treat both packs as one battery (i.e. charge all 8 cells up together and discharge together) and never have different A/H ratings in any of the 8 cells. In other words use both sets of batteries as if they were one.

Hope this helps Chris

Very interesting, and totally logical. Thank you.

G

Thanks everyone for your help in getting to the bottom of this.
Great Stuff, I'm pleased someone knows what they're talking about!

Chris,

I absolutely agree with you - except I can't believe that Pentax just parallel wired the battery grip with the body (as you say, it's a real "no-no"!!) I'm in the electronics design industry and would never dream of doing that. In fact, the shot count specified for the grip is lower than the body, and I suspect that the grip is diode coupled.

However, the very strange goings on that happen may indicate they haven't! So, for me I'm just using one set of batteries - in the grip.

Matt

Hi Matt
Just noticed your reply to this thread, like you I can't believe that Pentax just put them in paralell, but I don't see them using diode coupling either due to the 0.6Voltage drop, unless they use 'switched mode' PSU's which I'm pretty sure they wont. Thinking further on this it could be they use power MOSFET's these can have an internal resistance of only a few micro-ohms across the scource and drain or can be switched to almost infinite resistance (many hundreds of Megohm) But then you would need to know which set of batteries is the 'Master' (i.e. those which are being monitored, I presume those in the camera body) and how would you design the circuit to switch back when the cameras batteries was the higher voltage? I wonder if this is what they've done and your problems are caused by this?

If this is right (and I have no way of knowing this) then the sequence would be;-

1, Camera batteries fall below threshold - switches to hand grip batteries. Everything OK.

2, If hand grip batteries fall below threshold it can't switch back because it's only monitoring the camera batteries. Camera annoyingly turns off.

3, Putting recharged cells back in camera should result in hand grip batteries being turned off. Everything OK again.

4, Putting recharged cells in hand grip with no or low batteries in camera would mean you've reverted to a single battery pack system. Which is probably how it should have been designed in the first place!

Hope this makes sense, and just to reiterate I have no way in knowing if this is true or not, just my meandering mind.

Chris