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mempilot · 6th July 2007, 16:44

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.

6th July 2007, 16:44	#1 (permalink)
mempilot Eric Stadtmueller Current Rebreather/s: Megalodon Other Rebreather/s: Megalodon Join Date: Aug 2006 Location: Ft. Lauderdale Posts: 1,442	Meg Battery Voltage 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. __________________ Eric Stadtmueller, otherwise known as, MEM "Da Pilot"
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