We're trying to decide what batteries and supporting circuitry to use as an optimal on-board power source for OpenROV. Here's what you need to know to get up to speed:
OpenROV runs on battery power. The idea behind doing it this way is that 1) the tether for the ROV can be much thinner because it doesn't have to source all the power for the system, and 2) the ROV can easily be made to work autonomously with no tether at all.
Originally, the design of the ROV called for 8 alkaline C cells to be used (four in each of the two battery packs), and the dimensions and placement of the main electronics tube, motors, and battery packs were made such that the ROV would be neutrally buoyant and balanced this way.
Later on, it was realized that there is a problem with using alkaline C batteries- although they have great capacity and are readily available even in very remote stores (where one might be deploying an ROV), they have a high internal resistance which causes their voltage to drop dramatically if they are made to source more then about 2 amps of current. The main problem with this behavior is that since the on-board computer runs off the same battery power as the thrusters (there are various reasons why it would be impractical to do otherwise), maneuvering can cause the voltage of the system to drop so low that the computer shuts down and control of the ROV is lost.
There are many other battery chemistries out there which have lower internal resistance and can therefore source more current without loosing voltage. Happily, many of these alternate chemistries are also rechargeable which is good for several reasons:
1) are less wasteful
2) are lower cost over time
3) can be left inside their tubes while being charged
4) can be charged through the tether
The two main chemistries that seem most appealing are NiMH (Nickle-Metal Hydride) and LiMnNi (Lithium Manganese Nickle).
The main reason we like these battery chemistries is that they are both fairly easy to come by, and they both are available as round 26.2mm diameter cells (same diameter as a C battery) so they could essentially be a drop-in replacement for the alkalines with out any heavy redesign. In the case of NiMH batteries, the length of a typical cell is 50mm (which meets the standard "C" size specification), and in the case of the LiMnNi batteries we've looked at, the length is typically 65.8mm (about 1.3 times the length of a standard "C" battery, meaning that three LiMnNi cells could fit in the space four regular sized Cs would fit). It should be noted that other lithium-based chemistries are also available in the "long C" form factor, but LiMnNi offers the best compromise of energy density and safety.
Since LiMnNi batteries have a voltage of about 3.7v per cell and NiMH have a cell voltage of about 1.2v per cell, LiMnNi battery packs would probably be configured with the two tubes in parralell and NiMH packs would be configured with the tubes in series. For LiMnNi, both tubes with 3 batteries per tube would have 3.7v*3=11.1v in parallel so capacity/current sourcing ability is doubled, and for NiMH, both tubes with four batteries each would be in series so 1.2v*(4+4) = 9.8v with the capacity/ current sourcing ability of one cell.
Here's where some of the challenges lie:
LiMnNi batteries (and in fact all lithium based rechargeable batteries for that matter) require special charging circuitry for each battery, which means that if they are placed in series, separate wires going to each battery are needed, and a fairly complex circuit must be used to manage the current going in and out of each cell. Lithium batteries also usually require over charge/ over discharge protection circuitry but there are several batteries available that have this built in. Here's a document we've created that lists the performance and sources of several LiMnNi "long C" batteries with built in voltage protection electronics:
NiMH batteries can be charged in series directly and are relatively low cost, but are much more dense then alkaline batteries, so using them with the current OpenROV design would require the addition of about 150g of flotation for fresh water (not an easy thing to add). NiMH batteries also don't have quite as good capacity or current sourcing ability as lithium batteries.
Weight (density) of the on-board batteries is a fairly important factor because ultimately we'd like the battery tubes to be liquid compensated with an inert, non-conductive fluid such as mineral oil or silicone fluid which will add weight but make it so the tubes can hold out water without requiring complex endcaps. If the weight saved by using ligher batteries can make up for the weight added from the liquid compensation, no additional flotation will need to be added elsewhere on the ROV.
Here are the weights we've measured for various battery types:
8x Alkaline = 522g
8x NiMH = 654g
6x LiMnNi = 542g
While wanting to save weight makes LiMnNi appealing, having the battery packs filled with fluid makes frequent removal of batteries from their tubes impractical, so in-tube charging (without needing to remove the batteries) is desired. Doing this with LiMnNi requires an on-board balanced charger, where as NiMH batteries can be charged directly with little more then a current-limiting resister.
Both batteries can be charged from any state (i.e. neither have to be discharged completely in order to be recharged). This is important for a longer-term goal -item 4 on the list of advantages of rechargeable batteries- which is to send nominal power to the ROV through the tether, but let the batteries handle power surges. If a charging circuit (and likely a DC-DC converter) could be placed on-board the ROV, it may be possible to send enough power down the tether so that the ROV could be run indefinitely. Although we're still collecting data about typical use, it seems that on average, an actively driven OpenROV draws on the order of 40-50w.
So... based on all these factors and information, we're trying to decide what the architecture for OpenROV's power system should look like.