Written by Oliver Albrecht and reprinted here with his permission
In recent years, Dive Propulsion Vehicles, DPV’s or more commonly called “scooters”, have become increasingly popular in recreational and technical open water diving. Reason enough to take another look at what the market has to offer for cave diving.
Entry level models are appearing at reasonable prices and some older and proven scooters can be purchased used for fairly little money. Undoubtedly there are some very crucial differences among the many different models of DPV’s available on the market. From the user perspective, they vary in thickness and length, the type of motor used, the material they are made of, they may be heavy or light, offer different runtimes, speed and torque. That is just to name a few of the characteristics. But wait, didn’t we forget one main key element? Of course we did – the price. DPV’s may come as cheap as $900 for a used model in good condition, or at around $6000 which gives you an entry ticket into the class of top scooters.
The main factor that determines the price of the DPV is generally the battery it employs. Typical DPV batteries for scooters are priced starting around $200 for an entry level lead acid set, compared to $2000 and more for a top-of-the-line Lithium Polymer battery pack. It is probably fair to say that the battery is the key component in a scooter, and keep in mind that it is a consumable and will need to be replaced in due course. Count on swapping your batteries every 2-5 years, depending on technology, plus of course the amount of hours on the trigger – ouch!
Ultimately, choosing a DPV comes down to deciding on a specific technology and design. Like everything in life, buying a DPV is foremost a matter of personal preference. Some features may appeal to you more, others less so, and there will never be a single choice that is right for everyone. If you are very hesitant spending your hard earned money, you might want to closely evaluate what you really need in a scooter, and specifically in terms of battery technology.
Let’s take a closer look at the different technologies on the market and what sets them apart. As of today, there are three main battery types on the market.
SLA: 30-50 Wh per Kg
NiMH: 70-100 Wh per Kg
Lithium-ion : 120+ Wh per Kg
Lithium-ion batteries are the newest of the bunch and the term “Lithium-ion” is used for a whole family of rechargeable batteries. There are actually quite a few different designs that vary in regard to the materials used and the battery construction. Lithium iron phosphate, Lithium manganese, Lithium polymer are some types that are commonly used in different commercial electronic equipment, including DPV’s.
The major benefit of Lithium-ion based batteries technology is the very high energy density offered. Just so you get a general idea, this is a rough overview of the amount of energy each of the above battery types typically has to offer in relation to their weight:
The amount of energy the battery pack can store and subsequently provide to the DPV pilot during the dive in general determines the range of the unit. There is another term called “power density” which relates to the speed in which the battery can charge and deliver its power. Lithium-ion excels in this regard too. It also recharges much quicker than NiMH and lead acid. While a Lithium-ion pack can be fast-charged in 1-2 hours (depending on the cells), NiMH packs will usually take 4-6 hours for a full charge, Lead Acid batteries may take up to 8 hours to reach full capacity.
So for now, the conclusion would be, highly advanced Lithium-ion based batteries will provide more DPV range than any other battery type, plus they will deliver superior power, if needed.
Now the question is, do you need all of that?
Let’s have a quick look at the exact opposite end of the spectrum, the Lead acid battery. At a fraction of the price of a Lithium-ion battery (starting at $200) Lead Acid batteries provide the lowest energy density of all batteries we looked at. For all practical purposes this results in fairly heavy scooters (70 lb. and more) with a relatively low range of practically approx. 1.5 – 2 miles (for the 70 lb. units).
While Lithium-ion based DPV’s offer a range of up to seven miles and more, an entry lead acid DPV will typically not get you anywhere close to that. In a cave with flow it may even be less than 1.5 miles , round-trip.
At first this sounds as if it could be a serious show-stopper, but for cave diving it really may not be such a hindrance after all. While diving the typical Florida DPV caves like the Devil’s system, Little River, Jackson Blue or Manatee, such a lead acid DPV can usually get you to about 3500’ into the cave with fresh batteries. This range would provide an enormous amount of options to explore the remaining portions of the caves by swimming from where the scooter is dropped off. Considering the depth profile of these caves and the considerable amount of decompression after a 3500’ penetration dive, you might not want to make a second scooter dive anyway. Of course two shorter dives a day would be an option too.
Another technical specification to be aware of is the top speed of the DPV. NiMH and Lithium-ion batteries are capable to deliver energy at a much higher rate than lead acid batteries can, resulting in potentially faster top speeds of the scooter. While there is no doubt that optional high speeds can be very useful and fun, for all practical purposes it might not be too relevant in cave diving. When scootering in a team, the speed is always matched to that of the slowest DPV. So in effect, unless solo diving, chances are that your maximum speed will be by your buddy.
Now if you’re unsure whether SLA technology might be enough for your needs, there is another option for you, the NiMH battery. The power these batteries store are right in between Lithium-ion and lead acid levels, providing some very useful DPV ranges. When it comes to their pricing, NiMH batteries are again in the middle, costing anywhere from roughly $800 to $1500 per battery pack.
So where’s the catch, you may ask?
Well, NiMH batteries do come with some drawbacks. They will lose their charge when not in use. You will need to put them on the charger at least every three months in order for the battery cells not to take damage.
NiMH batteries, many of us may remember this from back in the days they were popular in cell phones, can sometimes provide us with something that is perceived as a “bad charge”. Some call it the memory effect, but in reality there are several reasons a NiMH battery pack may come up with a charge that provides less runtime than expected, resulting in a potential scooter failure during the dive. Very often these effects are caused during previous charge/discharge cycles and the battery will return to full capacity during one of the next charges. But still, it’s probably safe to say that NiMH batteries require a lot more care and insight by the DPV owner in order to minimize the risks of these effects occurring.
Speaking of risks, this would bring us back to the premium batteries, Lithium-ion. As I am sure you know, one of the main drawbacks of Lithium-ion batteries is that they will expand, deform, burn and possibly explode when overheated. Some older types may even, depending on the design, do just that, simply when exposed to water. Short circuits, overcharging, mechanical damage to the batteries, all this may cause severe trouble with very extreme consequences. You’ll likely remember burning laptop and smartphone batteries in the media, plus of course the recent problems Boeing had with some of their Lithium-ion batteries in the Dreamliner jet.
Does that mean these batteries can not be used safely in a scooter, ever?
Battery technology is constantly evolving and the old Lithium-ion batteries that were set on fire
by water are not used in battery packs for scooters. All the leading DPV manufacturers now use advanced cells that are very carefully designed and constructed with safety in mind. Electronics constantly monitor the cell temperatures and other critical values during charging and use, preventing, as best as possible, any overheating and damage to the batteries. And by now you probably guessed it already, do-it-yourself battery packs are pretty risky business when it comes to Lithium-ion batteries.
Do you plan to travel with your DPV?
While Lithium-ion models come in travel-friendly sizes, due to the potentially hazardous nature, large Lithium-ion batteries are not welcome on airplanes.
While the FAA allows up to two 300 Wh batteries per person in carry-on luggage, there is only one single scooter manufacturer that offers such a travel battery option. International air travel restrictions set up by the IATA are even more restrictive and do not allow Lithium-ion battery packs beyond 160 Wh on passenger aircraft. Unfortunately it does not stop there. Shipping large Lithium-ion battery packs by air freight and even by ship can a cumbersome experience too. Assuming your potential plans for cave diving trips involve any form of air travel, a lightweight NiMH scooter would be a much wiser choice.
When doing your research, you may also want to look into DPV models that offer battery upgrade options. Some manufacturers provide a small/medium-sized scooter with a basic entry NiMH battery, plus an upgrade option at a later point in time to Lithium-ion. There are also models that come with two different density Lithium-ion batteries, a reasonably priced entry model and higher range upgrade option. Further reading and research will likely lead you to the Tahoe Benchmark (http://www.tahoebenchmark.com) which is an in-depth project covering virtually all DPV’s currently on the market. It is highly recommended literature.
Whatever technology you end up choosing, may your DPV always bring you back to the open water and provide you with many enjoyable and safe dives!