Battery runtime calculation

[helios]

Does anyone have a good way of calculating the runtime of an external battery?

I was looking at either a 14Ah or 24Ah battery, to run a 50W light and 36W gloves.

Thanks

 

[grantctobin]

Bottoms of feet cold (drysuit, 39F / 3.9C water)

I'm trying to dial in my exposure protection for winter Lake Tahoe diving. It gets down to (and is currently) 39F, pretty much from the surface on down. I've taken a number of steps to boost my thermal protection and have successfully done 90 minute dives at this temp without getting too chilly...

scubaboard.com

See PDF in this thread

 

[grantctobin]

24Ah*11.1V = 266.4 Wh
50+36=86*1.1= ~94.6
266.4/94.6=2hr 49min
Call it 2.5hr or less if everything on full and new battery etc etc etc

 

[tursiops]

W = VxA = V x Ah/h; rearranging, h = VxAh/W
Assume V=12V and Ah=14
h = 12x14/86 = 1.95 h.
This is an absolute maximum; I'd plan on no more than 50% of that.
So, an hour tops with the 14Ah battery; maybe 1.5h with the 24Ah battery.
All in theory....

 

[helios]

@tursiops @grantctobin Thanks very much, and especially for the calculations.

 

[tbone1004]

@tursiops @grantctobin Thanks very much, and especially for the calculations.

We need to know which battery pack you are looking at specifically to get real numbers. Also this is why quoting battery packs in Ah instead of Wh is annoying.
@grantctobin quoted pack voltage at nominal 11.1v which is a 3s lithium pack and is highly likely to be the voltage that the pack is run on. There are a couple of exceptions that run higher pack voltages, but 11.1v is most common with modern battery packs.
@tursiops quoted older pack voltages from the NiMH and Lead days, but it makes a big difference when trying to figure out what the Wh of the pack really are.

I'll amend Grants math a bit to say that the BMS system in the battery is likely only going to allow you about 80% of the capacity as usable, especially with that high of a watt draw.

24ah capacity *11.1v nominal voltage=266.4Wh total but only about 213wh available for use.
86w total draw +10% for efficiency losses in connectors and what not=94.6w total draw
213/94.6~2.25hrs ish and about 1.25hrs for the 14ah pack.

 

[helios]

We need to know which battery pack you are looking at specifically to get real numbers. Also this is why quoting battery packs in Ah instead of Wh is annoying.

I was looking at this one:

Accu Thermo Type 24

Accu Thermo Type 24 - durrable and reliable power source to power your heating gear and torch for long hours of underwater exploration.

ammonitesystem.com

 

[tbone1004]

I was looking at this one:

Accu Thermo Type 24

Accu Thermo Type 24 - durrable and reliable power source to power your heating gear and torch for long hours of underwater exploration.

ammonitesystem.com

They call out at the bottom of that page that the pack is 242wh so the math that Grant and I did is pretty close to reality. Their graphs for the gloves+vest call out 2h50m but that is 100% depth of discharge and the BMS shouldn't allow quite that much. I would be comfortable planning that for 2h15m

 

[rjack321]

What heated gloves are 36w? That seems very generous - at least compared to my 45w vest that is usually running at about 30w