Calculating run time on an 18ah battery

[FryDog62]

What’s your best guess on how many hours of run time I might get on an 18ah lithium battery drawing 2.5 amps.  My guess is approx 7 hours.  Anyone come up with something different than that? 

[MN Fisher]

Sounds right - if you draw it down to near zero charge, just a bit over 7 hours for that battery/draw.

[padlin]

a little digging shows a 12v lithium goes below 12v starting at the 10% level, so depending on the devices low voltage cutoff, you should get about 6.5 hrs

[Tennessee Boy]

If it’s truly a 18 ah battery then 7.2 hours.  Most manufacturer underrate their batteries some and every cell will vary in capacity some.  It will probably hold more than 18 ah initially.   The capacity will drop with each cycle.   The numbers I’ve seen say that a LifePo4 battery will loose half it’s capacity after 5000 cycles.  When a manufacturer says a battery will last 2000 cycles that usually means 2000 cycles before it looses 20% of it’s capacity.  There are no standards that I’m aware of about what these specifications mean.

[FryDog62]

29 minutes ago, Tennessee Boy said:

If it’s truly a 18 ah battery then 7.2 hours.  Most manufacturer underrate their batteries some and every cell will vary in capacity some.  It will probably hold more than 18 ah initially.   The capacity will drop with each cycle.   The numbers I’ve seen say that a LifePo4 battery will loose half it’s capacity after 5000 cycles.  When a manufacturer says a battery will last 2000 cycles that usually means 2000 cycles before it looses 20% of it’s capacity.  There are no standards that I’m aware of about what these specifications mean.

Agreed on the no standards thing... this entire technology is still new enough that it's hard to know capacities, or exactly how long some of these charges, and even batteries will last. No one I know has had a Dakota Lithium for 11 years anyway ~ 

 

I was on the water 7 hours with the battery last week and it lasted the entire time. But I know I was right near the end, and just curious how much longer it may have lasted.  I may have been on borrowed time, am thinking of getting a larger battery so I don't have to find out the hard way...

[TOXIC]

Isn’t it also true that a lithium battery will not allow you to fully discharge it? Don’t they shut down at a certain point? That’s one of the negatives I’ve read about them.  Also everybody including Walmart is now in the lithium game.  Opens the door to some better deals.  It was only a matter of time before the discounters got into the pockets of the first wave retailers. 

[MN Fisher]

36 minutes ago, TOXIC said:

Isn’t it also true that a lithium battery will not allow you to fully discharge it? Don’t they shut down at a certain point? That’s one of the negatives I’ve read about them.

I actually consider that a positive since draining one to zero charge might cause it to not want to charge up again

[TOXIC]

@MN Fisher agreed but my point is that when trying to calculate run time you have to allow for the amount of charge the battery holds in reserve and shuts down at, it’s not 100%.  The complaint is that they don’t get weak, they just switch off and if you are running just 1, you are SOL😉

[FryDog62]

1 hour ago, TOXIC said:

Isn’t it also true that a lithium battery will not allow you to fully discharge it? Don’t they shut down at a certain point? That’s one of the negatives I’ve read about them.  Also everybody including Walmart is now in the lithium game.  Opens the door to some better deals.  It was only a matter of time before the discounters got into the pockets of the first wave retailers. 

There's some good deals out there as prices have come down since Covid.  If I could find a 25ah or 30ah lithium as small as my other Amped Outdoors 32ah (5 7/8 x 3 29/32 x 3 5/8) I'd buy it.  Any bigger dimensions than that - and it won't fit in the front hatch of my kayak.    ... Looking.. 👀

[casts_by_fly]

2 hours ago, TOXIC said:

Isn’t it also true that a lithium battery will not allow you to fully discharge it? Don’t they shut down at a certain point? That’s one of the negatives I’ve read about them.  Also everybody including Walmart is now in the lithium game.  Opens the door to some better deals.  It was only a matter of time before the discounters got into the pockets of the first wave retailers. 

 

 

1 hour ago, TOXIC said:

@MN Fisher agreed but my point is that when trying to calculate run time you have to allow for the amount of charge the battery holds in reserve and shuts down at, it’s not 100%.  The complaint is that they don’t get weak, they just switch off and if you are running just 1, you are SOL😉

 

 

That is all true, however a couple points.  1- you can put a meter on one if its a critical application and you need to know.  And a lot of batteries are now coming with a built in meter.  2- yes you have to plan for the usable capacity.  90% of the stated capacity is usually safe.  If the battery is understated for capacity (like some manufacturers do) then you get the full stated capacity as usable capacity.  Either way, you're getting a LOT more than lead acid of the same capacity.

 

 

42 minutes ago, FryDog62 said:

There's some good deals out there as prices have come down since Covid.  If I could find a 25ah or 30ah lithium as small as my other Amped Outdoors 32ah (5 7/8 x 3 29/32 x 3 5/8) I'd buy it.  Any bigger dimensions than that - and it won't fit in the front hatch of my kayak.    ... Looking.. 👀

 

 

Have you tried the trick of removing the hatch and also the hatch rim?  Unscrew the rim and you have an extra inch of hole to work with.  Battery in, rim back on, close the hatch.  I think I've mentioned that before but just checking.

 

[FryDog62]

36 minutes ago, casts_by_fly said:

Have you tried the trick of removing the hatch and also the hatch rim?  Unscrew the rim and you have an extra inch of hole to work with.  Battery in, rim back on, close the hatch.  I think I've mentioned that before but just checking.

 

You and I think alike… I did remove the rim to get my 32ah in there… I could fit the battery itself in except for the battery connect tabs and had to remove that rim.  
 

Maybe I should buy a bigger hatch lid, I’ve seen them somewhere. It would open up a lot more battery options. But I’d rather find a 25 or 30 ah with small

enough dimensions first…

[casts_by_fly]

46 minutes ago, FryDog62 said:

You and I think alike… I did remove the rim to get my 32ah in there… I could fit the battery itself in except for the battery connect tabs and had to remove that rim.  
 

Maybe I should buy a bigger hatch lid, I’ve seen them somewhere. It would open up a lot more battery options. But I’d rather find a 25 or 30 ah with small

enough dimensions first…

a hatch lid might work but why not just get another 32 from amped?  Or even the 25/30?

[FryDog62]

34 minutes ago, casts_by_fly said:

a hatch lid might work but why not just get another 32 from amped?  Or even the 25/30?

I could but I really just don’t want to spend another $270 on the 32ah. Oddly, their 30ah is less powerful and slightly larger dimensions and won’t fit in the front hatch. Their next size battery down from a 30ah drops to 20ah (and is the same dimensions as the 32ah). But I think a 20ah will only give me about another hour of run time over my current 18ah. 
 

You’d think with all the new lithium batteries to the market in the last couple years, along with dropping prices, I could find something… so far, coming up dry - 

[Bankc]

Is it just the hatch lid that's the issue?  Do you have more space down blew that? 

 

Like on my kayak, I can't fit a 100Ah battery in my front hatch.  But I can fit a 50Ah.  And I've got room for 2x 50Ah.  So I can get my 100Ah battery through the hatch if I run two 50Ah batteries in parallel.  Though, I haven't needed to do that, as the one 50Ah works just fine for my needs.

 

One more thing, are you sure you're actually drawing a continuous 2.5 amps?  It's not uncommon for them to list the maximum current it might draw in a quick burst, like maybe just during startup.  Or even overestimate that figure to make the lawyers happy, so no one can sue them in the even that one rogue unit just squeezed over the line. 

[FryDog62]

18 minutes ago, Bankc said:

Is it just the hatch lid that's the issue?  Do you have more space down below that? 

 

Like on my kayak, I can't fit a 100Ah battery in my front hatch.  But I can fit a 50Ah.  And I've got room for 2x 50Ah.  So I can get my 100Ah battery through the hatch if I run two 50Ah batteries in parallel.  Though, I haven't needed to do that, as the one 50Ah works just fine for my needs.

 

One more thing, are you sure you're actually drawing a continuous 2.5 amps?  It's not uncommon for them to list the maximum current it might draw in a quick burst, like maybe just during startup.  Or even overestimate that figure to make the lawyers happy, so no one can sue them in the even that one rogue unit just squeezed over the line. 

There is room beneath the front hatch lid for sure - I could get another 32ah in there.  But not enough room to run two smaller, i.e. 15ah or similar, in parallel. 

[casts_by_fly]

13 minutes ago, FryDog62 said:

I could but I really just don’t want to spend another $270 on the 32ah. Oddly, their 30ah is less powerful and slightly larger dimensions and won’t fit in the front hatch. Their next size battery down from a 30ah drops to 20ah (and is the same dimensions as the 32ah). But I think a 20ah will only give me about another hour of run time over my current 18ah. 
 

You’d think with all the new lithium batteries to the market in the last couple years, along with dropping prices, I could find something… so far, coming up dry - 

 

The 32 AH from amped is a NMC chemistry at 14+V.  It's more energy dense so that's why its smaller.  The technology development is in LiFePo batteries since they are going into electric cars.  They are the ones getting cheaper and why the 30 is slightly larger (though only 2 dimensions really matter.

 

Any reason you can't put it in the under seat hatch and run a cable?  That's where my 30 wide is.  It fits between the ribs exactly.  The 30 wide fits exactly through the hatch hole, but you could put a couple of them down there in parallel.

[FryDog62]

9 minutes ago, casts_by_fly said:

 

The 32 AH from amped is a NMC chemistry at 14+V.  It's more energy dense so that's why its smaller.  The technology development is in LiFePo batteries since they are going into electric cars.  They are the ones getting cheaper and why the 30 is slightly larger (though only 2 dimensions really matter.

 

Any reason you can't put it in the under seat hatch and run a cable?  That's where my 30 wide is.  It fits between the ribs exactly.  The 30 wide fits exactly through the hatch hole, but you could put a couple of them down there in parallel.

I could but I just wired everything to the front hatch next to the other battery for the Garmin/Livescope. That's where I was running the 18ah battery for my other Helix before I upgraded the screen and added 360 Imaging.  A second battery with more ah that fits would be easier, or I'm just lazy and tired of already re-wiring and want to get back to fishing lol... 

[gim]

6 hours ago, FryDog62 said:

No one I know has had a Dakota Lithium for 11 years anyway ~ 

 

Exactly.  I doubt anyone has had a lithium battery long enough to say whether its lasted through the warranty provided.  Don't you just love it when someone says they've had a lithium battery for 2 years without any issues.  That provides absolutely no help on longevity of product in reference.

[Tennessee Boy]

4 hours ago, TOXIC said:

@MN Fisher agreed but my point is that when trying to calculate run time you have to allow for the amount of charge the battery holds in reserve and shuts down at, it’s not 100%

There are countless reviews on YouTube where they load test batteries.  They test them against the listed capacity.  A new battery should deliver it’s rated capacity or more before shutting off.   Here’s an example of a review where a battery delivers 11% more than it’s listed capacity.

 

 

[FryDog62]

28 minutes ago, Tennessee Boy said:

There are countless reviews on YouTube where they load test batteries.  They test them against the listed capacity.  A new battery should deliver it’s rated capacity or more before shutting off.   Here’s an example of a review where a battery delivers 11% more than it’s listed capacity.

 

 

Makes me think I just turn the brightness of my Helix down 20% and I get the 10 hour run time I want…

[Tennessee Boy]

33 minutes ago, FryDog62 said:

Makes me think I just turn the brightness of my Helix down 20% and I get the 10 hour run time I want…

If it was me I would run it until the BMS shuts off to see what you get in a real world test.  My guess is in reality the Helix probably uses less than it’s maximum current rating most of the time.