Meg Battery Voltage

mempilot

I had a primary handset battery drop below nominal voltage on an easy 4th of July twilight dive. The predive no-load voltage check showed 6.0 volts, while the load voltage was 5.9 volts. This battery pack had the following dive day history since my purchase of the unit in May:

5/29 7.2v
5/31 6.8v
6/10 6.8v
6/12 6.5v
6/?? ?.?
6/?? ?.?
7/4 6.0

Each dive day in this short history comprised of the handset being energized for about 4 hours with the solenoid being active for @ 2 hours of diving. It looks like the battery pack was losing an average of about .3 volts per dive day or 4 hours of being on.

However, on July 4th, the unit was only energized for about 2.0 hours with the solenoid active for about 30 minutes prior to dropping below nominal voltage of 5.2 volts. In other words, it didn't take long to drop from 6 to 5 volts.

Being taught that 5.5 volts is a good time to trash a battery pack, I actually had planned to use 6 volts as an indicator to do so. The only reason I didn't do it at 6 volts prior to this dive is that I didn't have time and was only doing a single shallow dive. "Murphy, you suck!"

Well, as would be expected from the above data, the solenoid stopped firing. I recognized the dropping PO2 via the HUD immediately. I was level at 65 fsw and I could still read the handset. There was enough ambient light, and I failed to realize that the backlighting wasn't on and that the handset was in energy conservation mode. So, I failed to understand the symptom, but I recognized the end result. I started flying the unit manually with the O2 bypass. I ascended to the deck of the shipwreck and decided I didn't feel like dealing with the loop PO2 until I was at a safety stop (I was no where near the NDL for OC air). I bailed out to OC, (good practice), and I ascended to 20 fsw where I flushed the loop with O2 and went back on the loop for the remaining few minutes of the dive. On a deco dive, I would have stayed on the loop to maintain the higher PO2 and deco schedule.

The question I have after this long disertation is this:

Do these AA batteries tend to drop off dramatically at the end of their life?

Are they unpredictable as to at what voltage this exponetial voltage drop happens?

Is 5.5, or 6.0 IMHO, a decent margin for disposal?

It was a minor inconvenience on this dive, but on a longer deeper dive, it would have been more of an inconvenience. I have been switching back and forth between flying it auto and manual.

Thanks for any insight. BTW, the battery pack was assembled in 2004 with new batteries. The batteries were all marked with a shelf life of 2011 for disposal. Assuming the packs start out their life at 7.5 volts nominally and should last @ 50 hours, this pack seemed to be fairly unused at the time of me purchasing the unit.

__________________
MEM "Da Pilot"

Black holes are where God divided by zero.

"If at first you don't succeed, don't dive silent."

"Would you mind not shooting at the thermo-nuclear weapons." ~ Vic Deakins

"Donkey's kill more people annually than plane crashes."

Skipbreather

The 50 hour life is for the SECONDARY. The primary was advertised at 25 hours. Both per the 5 AA cell pack. Sounds like your pack gave it up right on schedule.

2 x C lithium will fix all that

__________________
"Entropy RULES! Enjoy the interim."

mempilot

Ah... 25 hours makes more sense.

Should it die off like that from 6 volts though?

Do you have the details for the Lith C's?

__________________
MEM "Da Pilot"

Black holes are where God divided by zero.

"If at first you don't succeed, don't dive silent."

"Would you mind not shooting at the thermo-nuclear weapons." ~ Vic Deakins

"Donkey's kill more people annually than plane crashes."

tibby

What's this trick about 2xC lithium cells? Can you please explain? I won't get my Meg for a couple of months but this is interesting.
Thanks,
Tibby

divingabe

Hi,
I also burned through the AA batterie pack quite quickly when I first got my MEG last year. It was very easy to make new packs but I found it inconvenient to keep changing. I go out on charters on the weekend and my MEG is often turned on for 7 to 8 hours at the time. You can see how often the packs got used. I purchased some lithium c cells and made a new pack and I have found that they last and last and last ( just like the bunny!!!). Here is a link to the particular cells Primary Lithium thionyl chloride Battery C Size 3.6V 9.0 Ah , same as LS26500 and ER26500 etc - LM-ER26500. I cannot give you the time length for a pack. The first pack I installed out lived 2 secondary packs of AA's.
I used it for the last four weeks of the dive season 06, then went to Florida to do a cave course which was 7 days of diving which meant the MEG was on from 9:00 am to 4:00pm app. plus it was good for the first 5 weeks of the 07 season.
The convenience of not having to worry about having sufficient power for long dives made it worth while for me. Dollar wise it probably boils out to be very similar especially if you make up your own packs. Should you decide to go ahead with the C cells make sure you get the tabs with the batteries so you don not damage the cells when making your solder connection.

Gabe

tibby

Hey Gabe, thanks for the explanation. That makes sense and no wonder, these babies are 9 Ahr each (which I find almost incredible BTW) so they should outlast the AA packs by a factor of at least 3.
Yes, I have learned the tabless battery soldering lesson the hard way, thanks.
Cheers,
Tibby

mempilot

Divingabe,

Thanks for the Lith C explanation. For now, I've got a box of AA packs to go through (so many I haven't counted them). I got all the packs from "The Cave" movie stock. They all test out to @ 7.5 volts and their shelf life is to 2011. I guess with all those batteries, I can just change them out every 10 to 12 hours or so for some time to come. I will; however, be building some Lith C packs in the future.

Eric

__________________
MEM "Da Pilot"

Black holes are where God divided by zero.

"If at first you don't succeed, don't dive silent."

"Would you mind not shooting at the thermo-nuclear weapons." ~ Vic Deakins

"Donkey's kill more people annually than plane crashes."

scubadan

Who is building/selling Lith C packs these days?

wedivebc

Ask Ron at CCR Dolphin He sells them I think.

__________________
Cheers,

Dave....

Man is the only animal burdened with the knowledge he will eventually die

jan

Maybe a bit of topic but could have your interest in the "nice to know" category.
I made a test to se how low the Meg actually would alow the battery to go. I stopped diving the unit at 4,2V. The solonoid still worked but the display faded away each time it fired and slowly activated again. Water temperatur at around 6-8 degrees.