Tekna left the DPV business years back after an unfortunate explosion. Water flood detectors are common place in more expensive camera housings although they don't cost that much themselves. I am surprised DPV manufacturers using Lithium batteries aren't universally providing such detectors with an effective alarm in their units. Overtime, housings will flood, sooner with operator error. I go through a lot of SLA batteries on my Apollo, usually one set each year for better performance and with lessor use. How is the battery usage on the newer Lithium units?
Flood detectors are nice in theory, but it also means another hole in the pressure hull, and through holes are one of the major sources of flooding. And, in some cases it is redundant. If your DPV is weighted to be an ounce or two positive, and it suddenly goes neutral or negative, then you know you have a leak. On our Makos that was never really possible, as weight and trim on a Mako in fresh water is a series of hacks and compromises to sorta get there. But we also had clear Lexan nose plates, so any water in the scooter was obvious during the dive.
The shaft seal is the one unavoidable hole in the hull but a good hub design with a line trap to prevent line wrapping around the shaft and eating the seal helps a lot. So does ensuring your scooter never gets set upright on un even ground where the weight of the DPV is placed on the hub, as that can cause the seal to unseat and leak.
Otherwise, the fewer thru hull connections the better, and the better protected an actual o-ring seal is the better. If you have a seal that is well protected, keep the o-rings and sealing surfaces clean and lubricated and inspect them on a regular basis leaks should be a non event. That's probably the thing I like most about our current DPVs. The Makos had fragile hulls and even more fragile o-ring seals that were a pain to maintain.
Batteries:
We normally got a couple years out of an SLA battery before a cell went bad, but that included re-charging reasonably shortly after the dive and using a smart charger. Eventually however, a cell would go bad, which would be noticed by the smart charger not kicking down to a maintenance charge and instead cooking the whole battery trying to reach it's set cut off voltage.
We skipped the whole NiMh era due to their high self discharge rates, related problems with cell reversals, and potential issues getting them back up to capacity just did not fit with our diving schedules.
Lithium ion batteries will normally last around 3 years but they do lose capacity over time. For example, a two year old lithium ion DPV battery will probably only be giving you 80-85% of it's rated capacity. And that's assuming you've taken really good care of it. You don't want to store them near empty as while their self discharge rate is very low, over several months the battery will end up getting so low that it won't accept a charge. They have protection circuits that power down or completely shut down the battery for a reason. Storing them 40%-60% charged seems to be the preference I've heard from most folks who seem to know what they are talking about with lithium ion batteries.
The cycles also matter not just number, but also magnitude. You should be able to get 300 to 500 cycles out of a lithium ion battery, but a very deep discharge probably has a greater effect on battery life than a more moderate discharge. For example, if you have a DPV rated for 60 minutes at maximum speed and you routinely run it to the point that the low voltage protection circuit cuts in and slows down the DPV or shuts off the battery, you'll probably get less battery life than if you had a DPV with a larger battery with a 120 minute run time at max speed that you only deplete to about half it's capacity on each cycle. On the other hand that larger battery will cost twice as much so unless you need the longer run time now and then, or get more than twice the life out of the battery, it still might not be any more cost effective.
The primary disadvantage of Lithium ion DPV batteries (other than the risks associated with water leaking into the DPV) is the high cost. An inexpensive, small DPV battery is still going to be $850-$1000, and a larger battery will run you $1500 to $3000.
That's one of the reasons we opted for Piranhas with power tool batteries. The DPV is modular so I can add or subtract a battery section based on what I need for burn time. Worst case, I can replace the batteries annually if needed, and still spend a lot less money over a three year period than replacing a comparable capacity DPV battery every three years. Plus they are also airline travel friendly (unless TSA, the FAA or the airlines change the rules).
