New Recommended Batteries


Hello OpenROVers:

The OpenROV team and community have been experimenting continuously with different battery configurations and chemistry. We are ready to publish our conclusions about what we think will be a better source of power for OpenROVs.

What you need to know in brief:

  • We're changing our recommendation to safer, more reliable batteries
  • The new batteries will work with all 2.5, and 2.6 ROVs but require an adapter (see below), new ROVs acquired through our webstore will include adapters
  • The new batteries use the same charger as the old batteries but with caution (see below)
  • The new batteries, chargers, and adapters can now be found on the webstore

We're changing our recommended batteries!

For the last few months we've been recommending the Trustfire 26650 Li-Ion batteries to our users. These batteries are commonly available, offer good charging and discharging profile and contain a protection circuit. While these batteries perform well under laboratory conditions there were two main problems with them in practice. First, the protection circuitry was too sensitive for us and this would frequently trip and require a reset. Many ROV users have experienced this symptom without realizing that this was the cause. Second, the physical package of this battery is very easily compromised by water, sand, sunscreen, margaritas, and falls from moderate heights. Based on feedback from the community and tireless in-house testing we're now recommending a different battery chemistry.

Enter the Batteryspace Li-FePO4. Switching from a Li-Ion to a Li-FePO4 chemistry is good for a number of reasons. They are more reliable, safer, and have a better charging cycle lifespan.

These batteries will work inside existing 2.4, 2.5, and 2.6 OpenROVs.

The Li-PO4 batteries have two main drawbacks. One, they are slightly shorter than the Li-Ions. Two, they don't have a built in protection circuit. In order to solve both these problems, we've developed a battery adapter with a polyfuse built in. The polyfuse will protect against over-current situations and the adapter's arrangement will mean that users wanting to use the new battery configuration can do so without rebuilding their battery tubes. The battery adapter build instructions can be found on our dozuki's upgrades page, here.

The new batteries use the same charger as the old batteries, but LOOK OUT FOR THIS:

Charging the Li-FePO4 batteries is the same as the Li-Ion batteries from Trustfire. BUT: the battery chargers must be set to the lower of the two voltages (3.0V instead of 4.2V) on the toggle switch. CHARGING THE Li-FePO4 BATTERIES ON 4.2V MAY DAMAGE THE BATTERIES! Please refer to the picture below for clarification.

Safety regarding Lithium batteries should be considered.

Even though we've moved to a safer chemistry of lithium battery, these aren't things to mess around with. Regardless of whichever power source you choose to power your ROV, take time and precaution to consider the safety. Batteryspace has a great section about these batteries here.

These things are available on the webstore.

6x Li-FePO4 Batteries

3x Chargers

Battery Tube Adapters -- You only need to buy this IF you are planning to use the Li-FePO4 batteries AND you have a version 2.5 or 2.6 with a serial number below 1200. ROVs purchased through the webstore as of the date of this post (these will have serial numbers 1200 and higher) will have these adapters included.

OpenRov 2.7+ PolyFuse
On board Battery - Lithium-ion a no go?

Do the new Li-FePO4 batteries last as long as the Li-ion batteries?


Thanks for info's on the new batteries!


How many amp is the fuse?


I'm not too sure about the type of Chargers you use but from what i get they might not charge the proposed batteries completely, yet not even remotely complete.

Let me quote Wikipedia for a second:

Batteries with a lithium iron phosphate positive and graphite negative electrodes have a nominal open-circuit voltage of 3.2 V and a typical charging voltage of 3.6 V. Lithium nickel manganese cobalt (NMC) oxide positives with graphite negatives have a 3.7 V nominal voltage with a 4.2 V maximum while charging.

The latter is equivalent to the old used cells, the first is what the new batteries are alike. We need to be charging these best at 3.65V (max is 3.9V (look here from this site).

I know that charging batteries with lower voltages won't harm them but from what i understand this won't charge the new batteries very muchas cut-off voltage is 2.5V anyways.

If i miss something please correct me!

@Sam no they wont last the same (comparison:

old: 3.7V * 5Ah = 18.5Wh

new: 3.2V * 3.3Ah = 10.56Wh

that's about 57% of the old batteries Energy.

Greetings, Nils


Hi Nils, Kjetil, Sam:


The FastTech chargers that we have been using put out about 3.6V when the switch is in the 3.0V setting. Why they say 3.0V is unknown to us, but they work just fine for charging LiFePO4 cells.


The polyfuses we use in the adapters are rated for 16V, with a 10A trip current and a 6A hold current.


As Nils calculated, a 26650 LiFePO4 cell stores noticeably less energy than a LiCo 26650 cell. The problem with the Trustfire batteries is that we were rarely ever able to use the full charge of the battery- at some point or another we would get a nuisance trip of the protection circuit, and you would have to pop open the battery tube to clear it. With the new LiFePO4 cells, we still get plenty of battery life (~2 hours), so we really haven't missed the added capacity of the trustfire cells.

Hope this helps.



When comparing batteries an independent review of most 26650 batteries can be found on the flashlight forums and well worth a read

Consider the current Li-ion (Lithium-Cobalt) UltraFire UF26650 5000mAh (Flame) battery which trips on a current at around 5.9 Amps and has a measured stored energy of 13.395 Whr @5amp

Whist Walt has indicated that the new 3300mAh Li-FePO4 will trip at a 10A current and is OK on 6A hold current and given this is a safer battery chemistry with an albeit lower energy density with a somewhat lower stored energy than the UltraFire

However, you can also consider at a protected Keeppower 26650 4500mAh (or others) which has a max constant discharge current: 13.0A (hence greater draw then Li-FePO4 fuse) and a larger measured stored energy 16.019 Whr @5amp then both the UltraFire (about 20% greater but also more accessible) and the Li-FePO4 (around 50% greater theoretically but in practice most likely more)


@Walt thank you so much for clearing this up. I already thought that this can't be true but was wondering after all. With 3.6V output this should be rather exactly where we need it to be. I guess battery life of 2hrs is okay for most of us, but still can be easily extended by adding either more batteries or changing the type of battery. Has anyone ever thought about using unprotected cells? I am coming from a standpoint of Multirotors where we just keep an eye on the voltage and amperage by ourselves (or rather a beeper tells us when cell voltage comes down to a specific point).

Also how much Amps does the ROV draw at peaks?




For your problems, we will reply as below,

  1. For the sensitivity of the battery protection board, we could use the more functional battery PCM, and that would be a good solution for you;
  2. And the watertight or sandtight for the battery, maybe you could consider that using the battery case for your battery pack, the PVC is not enough if for the extreme situation.

Hope above would be some kinds of help for your problem.