Lithium Polymer Battery Packs?


Taken from the specifications listed with the linked battery (LiFe)

Operation Temperature

  • Charging: 0 - 45 oC (32 - 113 o F)
  • Discharging: - 10 - 60 oC (14 - 140 o F)

They also have a discharge-at-different-temperature graph, and the optimal temperature is around room temp +/- 10C, so I think they should be fine.



I was looking concerning the voltagedrop due to temperatur. My diving light (NiMh) lasts in 6 degree waters 1,5 hoiurs, but at 20 degrees it will do over 2 hours. So a big difference.

Maybe one of the choices here has a better tolerance to temperatur.



did some reading on various battery chemistry a while back and it appears to me - and i could be wrong - that the various flavors of lithium batts are not the best choice for this application for a few reasons...

* when you compare the lifespan and the time vs capacity loss curves for lithium vs nicad vs nimh, lithium starts looking a LOT less attractive

* cost

* very sensitive to over and under charging

* expensive and special charging equipment required, including balancers for multi-cell packs

* the high energy density/low weight doesn't do much for a submersible

of course using disposable batts is an extremely poor option form both environmental and wallet perspectives

in the mean time, NiMh tech keeps getting better, they have a decent track record, aren't fussy about charging and are less costly

also, changing to rechargeable batts would help solve the battery tube end cap problems and complexity - no springs or contacts needed, and the tubes could actually be permanently sealed with standard glue-on end caps. replacing packs on a job also becomes less of a hassle - un-do a connector, slide the tubes out, slide new ones in


Might I suggest looking at LiFePO4 battery packs? Though the NiMh batteries you linked look rather nice, they only have a capacity of 5Ah, which is significantly lower than the 8Ah supplied by Alkaline cells, and half the capacity of the LiFe batteries I noted a few posts up.


i must be missing something - the highest capacity cylindrical cell i see in LiFe on the page you linked to is 5000 mAh - same as NiMh

could you link directly to the cells you have in mind?



yeah, but that's HUGE battery - that's where the capacity comes from - it's over twice the length and fatter than a C :)

38mm(1.5") x 120mm(4.7")

C is 26.2 x 50 mm - that's what i was using for comparison since that's what the ROV currently uses - doesn't matter though, as long as we compare the same size batts or, better yet, energy density

good read here ... doesn't cover LiFe, but as far as i know LiFe is still sub-par compared to ion/polymer (as far as density) and apparently about the same as NiMh...

also see this and take a look at energy density - the best NiMh cells aren't that far behind the best LiOn cells...

and any other Li cell, besides LiFi, is a potential disaster. i've witnessed a LiPo's burning and lithium ion is worse...

one of the problems with lith is these chineese manufactures try to squeeze the most capacity out of the things by using thinner and thinner barriers (or whatever it's called - the stuff between the other stuff in the battery - hehe) - if the barrier has a flaw or gets damaged, the battery shorts and pOOf!


From what is noted here, LiFe batteries have ~120wh/kg compared to 80wh/kg for NiMh. As for the danger of Lithium batteries, it's largely fixed by adding iron into the mix, and as such, LiFe batteries are hardy and strong.

I'll also note price/storage capacity for the LiFes. 4 batteries at $22.50 is about the same price as the NiMhs you linked, at double the capacity. Weight and size are an issue, but with the design I'm currently working on, they could be accommodated for. Do you know the specific energy densities of those batteries you linked? They look pretty good in terms of power per kilo, but the only downside I see is their expense.


wonder how many quotes i can nest before a sentence is 1 letter in width :)

NiMh density is 60-120, LiOn is 110-160. LiFe is gonna be less than ion, but it's not listed here...

i do not know the density of the NiMh cells i linked to earlier, but i believe they were the highest capacity cells i could find at that time (couple years ago - maybe 5 - i haven't looked since)

other than sony and panasonic, i don't know anymore who makes the best cells


you know, i think i'm misleading both of us here - i don't think you can compare capacity between cells of way different voltages - at least not without some additional math :)

so maybe LiFe is a better solution that i'm giving credit for?

another resource...

and lookie here...


here's an interesting combo for alternative power...

you take some of these 25 year batteries...

and, if necessary, mix it with some of these Ultra Capacitors...

got that info from


The big thing to also consider is current supplying capability: one of the issues we've been having with alkalines is that they can't supply the amount of current that the set of motors on the ROV requires when they are all running at full speed. When the batteries can't supply enough current, their voltage drops, and in our experience, that drop can be enough to brown out (and restart) the BeagleBone.

A secondary benefit of many lithium chemistries is that they can deliver more instantaneous current so even when the nominal voltage is lower, it doesn't drop as much when you pull a lot of power.

I highly recommend these:

Three of them fit in the same tube that would hold 4 alkaline Cs, and if you reconfigure the battery tubes to be in parallel instead of in series, you get 11.something volts which runs the system just fine. Although the capacity really isn't any better, they're rechargeable so you don't have to keep getting new batteries. They have a maximum discharge current of 4 amps, which is more then twice the nominal discharge current of an alkaline C.



the TLM-1550HP handles pulses of up to 15A, 5A max continuous @ > 3.0V

here's the link to the datasheet

i have no personal experience with them however


Just a question on the side - what amp numbers have you seen on the motors and with which props?


mmmm... hard to say exactly just yet. Each motor seems to draw around an 1.25 amps "under load' but so far that's just been me gripping the shaft until the motor slows down a bunch. The kits all have Delta-V 15 69mm ducted fan props. If someone could rig up an experiment to measure current draw more appreciably- perhaps even at different RPMs- that would be really good data to have!



Ok, i've ordered the motor delivered in the kits and the DT700, so i thought to make a jig to check the diffrence between them and with diffrent props to aim for the most efficient setup between amps and thrust.

i can order some V-15's aswell for comparison. i've ordered some boat props from cornwall boats and some edf props from hobbyking to try out.


Recharging with power from the tether - another area to consider:

When I looked at various approaches to provide power over the tether and charge the onboard batteries, there were differences in sophistication levels for charging the different types of batteries. Some of the Li type had to be more carefully monitored. I thought NiMh was a reasonable compromise. If you trickle charge it, a simple charging approach/circuit is fine.

I envision the batteries being the primary source of energy, with charging power coming from the tether to extend the mission. PoE would work for this approach (12-30w can be supplied). Other approaches can be explored. Shorter missions would not need any power from the tether and you would use the ROV as is.

We will need to understand our power usage in a representative mission. Use lots of power getting there, then much less observing and maintaining position (would be charging in this phase).

Something to consider in the battery investigation.



Is there any particular reason to choose the ones you linked over the ones that seemed most popular in the RC forum?

They have significantly higher capacity with similarly compatible form factor (two will fit in a battery tube).

Does swapping in rechargeable batteries require any extra circuitry? I've no experience with Li batteries, and the last thing I want to do is fry my ROV.


I've thought of this too. I thought, as the current two-wire tether may not be able to handle PoE, that a second tether might be required. It would be ideal if this second tether was detachable. What about swappable battery tube end caps, two of which have a potted tether, and two of which simply complete a circuit for battery power?


The main differences are width and weight. The LiFe batteries I'm planning on using are a great deal larger than the ones he suggested (38mm(1.5") x 120mm(4.7")) compared to a C battery (26.2 (~1 1/8") x 50 mm) so the battery tubes must be replaced with something bigger (don't have a mastercarr link at the moment) and additional ballast (I calculated a 12" long 1.5" diameter tube) must be added to keep the ROV neutrally buoyant. The batteries he linked are essentially drop in replacements, although protection circuitry will have to be implemented for that modification.