Batteries venting & Waterproof Hull

[Jem]

Hello everybody,

in my project I would need to have the batteries inside the hull, but I am worried about batteries venting: having my hull ( an old pressure-cooker ) exploding in my face sounds like a dangerous thing.

Battery venting most likely happens when charging the batteries, or when the batteries are discharging a lot. The first is not a problem, as I will charge them outside the ROV, but the second can become a problem: event if I carefully monitor the level of the batteries, as some of them will be put serially, I can't prevent a particular one to accidentally discharge, causing the others to try to recharge it.

So what is your experience with batteries venting?

Do you think it's safer to use a particular type of batteries? ( Pb, Li-Po, NiMh ? )

Thank you !

[derelicte]

lipos won't vent unless you do something really bad to them (i.e. you don't lipos to vent, _ever_). I would think gel cells would be fine too as long as the discharge current isn't super high. charging is the main concern.

[Jem]

Thank you derelicte,

so you think there is no risk in using LiPos as long as they are charged outside the hull?

I've heard people in AUV students team saying that NiMh is safer to use, any idea why?

Using a pressure-cooker is a good way to have a waterproof hull, but I'm a bit affraid to have it exploding, or having a lot of hydrogen going out and explode when opening it after the mission.

[derelicte]

I think nimh, nicad, gel cel batteries are inherently safer batteries, but lipos provide a higher power density. you must also use a lipo battery charger that is correctly configured for your battery.

I use lipos for our rov, but they are in the topside control box, not the rov. I charge them outside of the box and usually keep them in lipo bags whn not in use. lipos are fine as long as you follow the rules.

google lipo fire to see what happens when you don't...

[bigbadbob]

On the AUV's i work on we have a blow off valve to release pressure. the batteries are silver zinc and prone to venting and fire if misstreated. there is also a pressure switch to shut the system down in the event of overpressurisation of the battery section.

[derelicte]

the pressure switch cut off is pretty clever. good tip!

[derelicte]

was the pressure switch an off the shelf thing? I've been thinking about this and want to try to implement it, but I haven't yet found something suitable without designing it myself.

[thegadgetguy]

Is this the type of thing you're looking for?

http://www.allelectronics.com/make-a-st ... TCH/1.html

I think it's just a diaphram that pushes against a switch at 1 psi. Not sure if this is helpful or not, sorry in advance if it's not

[derelicte]

no, that is very helpful! the only thing I found was water pump cut off switches.

the 5a limit is a bummer, though.

[thegadgetguy]

Yea, you'd probably have to use it in conjunction with a relay or microcontroller, depending on how you would set it up.