Trident off grid charging system


I have a trip coming up where I will be spending several weeks at sea and am looking to take my Trident

We do have a generator on board but rather than having to fire that up for a few hours just to charge up the Trident after use I am thinking about getting a way of using the standard 12V (Deep Cycle Gel Cell standard boat) “house” batteries (yes I know you can get inverters for 12VDC to 230VAC but then throttling this back down to 12VDC for charging seams wasteful – we don’t do this for any other electrical gear [laptops and alike which just have a DC to DC adapter eg this or underwater lighting] )

Given the Charger supplied with the Trident is a Shenzhen Modiary 12.6V 4A output I am assuming that inside the Trident is a Battery Management System (BMS) chip/card for charging the (I assume) 3S Li-ion batteries inside the unit.

I have measured the Voltage on a charged Trident’s tether connection 3 pins (looking from the rear) (and also on the charger) it looks like the 6 O’clock pin is the neutral and the 10 O’clock pin is the positive (from the charger) and I (assume) the 2 O’clock pin is the feed of voltage back up the tether (12.31VDC measured on the ROV).

Given this (I assume) the Shenzhen Modiary 12.6V 4A (approx. 50W) Charger is “semi” smart doing something like a programmed auto 4 stages (Ip-Im-Um-cutoff) charging process

Charge process:

Stage 1 Slow start, precharge function for deeply discharged packs (pre-charge small current)

Stage 2 Bulk charge stage. 3.3 amps is applied to the battery until the voltage rises to 12.6 volts (will give bulk current charging (up to 50W))

Stage 3 Constant voltage state until the charge current falls to 400mA

Stage 4 The charger turns off

So my question (most likely to @Walt_Holm or @Eric_Stackpole ) do all of these assumption sound right to those who have seen the inside of a unit

Given this with a fairly standard LiPO charger that accepts a DC input like this (or generic) set up in the Lithium Battery mode (ie no balance cable to the charger and assuming the balancing is done onboard the ROV by the assumed BMS on the unit) for 3S and 4A (not sure of batteries for 1C charging but assuming from the charger 4A) and then cobble together a way to pin this into the Trident

Am I crazy or is this in the ball park for an off grid charging system


Scott W


Thank you Scott_W,

I have been meaning to follow up on previous discussions and ask this very same question since my boat has plenty of 12vdc on board but no generator.

It would be nice to know how to charge Trident in the field using 12vdc systems including deep cycle batteries and solar panels. I agree that low cost inverters are available however in addition to the losses you have mentioned, 240vac creates additional risks that I am not yet ready to add to my certificates of survey/operation.

I am also keen to know more about the charging and operating voltages within Trident. Based on previous discussions, I have been led to believe that voltages in the tether were likely to be higher than 12vdc to help manage voltage drop. Although perhaps not related to charging, Trident may adopt intelligent switching on the 2 O’clock pin (similar to USB PD!?) so this would be something to watch out for.

Further to this, would it be possible to power the topside unit during dives by connecting its USB port to a USB charger to help conserve/extend Trident’s power supply? I would imagine the topside unit’s power consumption would be significantly less than Trident’s however every mAh makes a difference.


Following this, I would love a 12v charging option for use on overnight boat trips. A way to supplement power from a 12v connection to the topside unit would be even better.

My plan for now is the 100W inverter, while inefficient, we run the engines enough to make it a non-issue.


Hi All:

Here at OpenROV we’re looking at the issue of charging in the field and have prototyped some hardware, but so far we haven’t come up with any products that meet our quality standard while still being affordable.

The simplest method is to use an inverter to convert 12 volts DC (or whatever your field voltage is) to 110 VAC, and then plug your existing charger into that. That solution is, however, bulky and inefficient.

As @Scott_W mentions, the charger for Trident is a standard unit from Modiary for 3S lithium-ion battery packs, that charges at 4 amperes maximum. There is no cell balancing built into the charger- that function is handled on the battery protection modules inside the Trident. So the Modiary charger acts like a 12.60V power supply that is current-limited at 4A. Note that no charge cutoff happens on Trident, because the Trident vehicle is kept on during charging (which allows download of data during the charge process), so the charge current never tapers to the point where the charger would cut off.

If one pokes around the internet, they can find 3S car chargers (with a 12V cigarette-lighter plug) available. We’ve looked at many of them and it turns out it’s surprisingly hard to find one of decent quality. Most of the cheap ones will only drop voltage from 12V (in electrical circuit terms, they are buck converters, as opposed to buck-boost converters). The significance of this is that you will never get a full charge on your Trident if you are operating from a bare 12V lead-acid battery (which will be at something like 12.6-12.8 volts), but instead must be running an alternator feeding in the battery ( a 12V lead-acid battery while being charged is generally at 13.8 volts or so.). Short answer- the cheap E-bay 3S car chargers do not work well.

There is a solution out there for folks who are comfortable enough with electronics to understand how to tinker with stuff, and don’t mind voiding the warranty of their Trident. I’ll say this again: OpenROV can’t warrant what I’m about to write below, since we have no way of knowing how its been done. Use at your own risk.

The DJI Phantom 2 used a 3S lithium battery pack that charged at 4 amperes. They made a car charger for the Phantom 2, and you can still find these surplus on Amazon and E-bay, or buy them new from DJI. From DJI they cost about $100, but on E-bay they can be found as cheap as $20 or so. If you lop off the DJI battery connector, you’re left with a high-quality 3S 4 ampere battery charger that runs fine off of a 12V battery.

The Phantom 2 car charger is a pretty-well made device, and I have used it a number of times to charge a Trident. Your mileage may vary, or course, and you do this absolutely at your own risk. You’ll have to jimmy up some way of attaching the charger leads to the connector pads of the Trident- at this point we’re not planning on selling bare connectors for the vehicle. I suppose if you were always going to charge from 12V you could just lop the connector off of your existing Trident AC charger.

Note that later Phantom car chargers (the 3 and the 4) are for 4S lithium systems and will absolutely kill your Trident!

As I mentioned above, this technique is most definitely not approved by OpenROV. But if you’re willing to tinker with things a bit you might find that it works well for your application. Use at your own risk!

Getting back to the issue of selling 12v chargers (car chargers) for Trident in the store, we have had a person poking around in China trying to find an OEM version of the Phantom 2 charger, but so far have had no luck. We found one company that advertised that they still made such a unit, but when pressed they admitted that they no longer produced it. Sigh…

Walt Holm


Thanks @Walt_Holm for the great background

Timing might be a bit tight to get something ready before we head off

Yes I get the ability to use 12VDC to 230VAC inverters but I have had really poor results from these at least 4 of them have failed whist on boats (1 failed giving a live AC feed to the housing)

Now all of the gear side is running on DC to DC systems (3 laptops 12V to 19V, Side Scan 12V to 24V & Multibeam 12V native)

I’ve had a quick look around and the LTC3780 DC-DC Buck-Boost or the Drok Auto Boost Buck Converter, 35W Power Supply look like contenders

I fully understand that it may be voiding the OpenROV warranty



Hi Scott:

So if you’re on a boat with a 12VDC bus running around, there’s no reason why you need to use a car charger per se- but the car charger format is handy for those who have a cigarette lighter on their car or boat and don’t want to do any extra wiring.

If you’re handy with electronics stuff, pretty much any of the Drok adjustable-current-limit buck-boost converters should work. Set it for an output voltage of 12.60 volts, and then set the current limit for a maximum of 4 amperes or less. This is pretty easy to do with any DMM that’s got a 10 ampere current range.

The Drok unit you linked to seems to only be rated to 3A, which, given my past experience with their stuff, means that it will work just fine up to about 2A or so- their hardware is notorious for having over-generous specifications. But a Trident will charge just fine at 2A, it will just take a little longer to get to full charge.


Solar/Power Bank Charging
Solar panel charging

Thank you Walt_Holm,

This is gold…pure gold. A quality 12.60vdc power supply is now on my shopping list.

I am now also VERY interested in your comment re: the possibility of downloading data during charging. My post dive workflow is a bit time consuming at the moment since I need to wait for the 1080p data download process to finish before connecting the charger. This typically requires Trident to be charged after a longer dive to give it enough power to complete the data transfer followed by a second shorter charge after data download to top it up again before the next dive.

When might this additional functionality be made available (or documented)?


Hi @pforperry:

See my response in the other charging thread on the forum.

In order to download while charging, you’ll first need to have a charger that keeps the Trident turned on during the charge process. If you’ve got a very early Trident, that won’t be the case. Contact customer support to help you out on this one.