Lipo vs "deep cycle" marine battery capacity question

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LanM

10 mW
Joined
Oct 25, 2019
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29
Hello guys,
I'd like to ask a question (or several) about the capacity difference consideration when trying to calculate how large of a battery you would want to replace a deep cycle marine battery. I understand that a deep cycle battery is designed for more discharge than a regular lead acid battery. Is there any difference in standard usable Ah between a deep cycle and the total calculated Ah of a pack i would build?

For example, if I would like to replace a group 31 100AH 12v deep cycle with lipo 3.7v so a 4S configuration will be fine for the trolling motor I will be using, do I calculate out to match the 100ah for my P, or do I go bigger since lipo isn't a deep cycle cell? If I'm only charging to 4.15 and discharging to 3.6v as read in the sticky, how do you calculate the actual usable Ah of your battery to make sure the range is there?

Thanks.
 
Well first off, that label is used fraudulently in mass market retail in the US, 99.99% of the time just slightly more robust Starter batteries. Especially in 12V automotive sizes.

Next, even with more expensive true deep cycling lead batteries, actual Ah capacity is often well below that claimed.

OK, now, say you have 225Ah rated of decent true deep cycling lead in golf car size, so say 200Ah in reality.

You won't get good longevity if you frequently go below 50% of capacity, IOW pulling 100Ah out and putting say 110Ah back each cycle.

Capiche?

Being a quality deep cycle bank as opposed to big boc crap, means might get 800 or 1000 cycles, as opposed to 2-400. EoL being capacity has dropped to say 75%.

Now LiPo is subject to the same Avg DoD vs cycle life curve, and everything else being equal will likely die a fair bit earlier.

The main advantage is less weight and bulk. Also will discharge at a higher C-rate, but that will also reduce longevity.

Lead will cost around $1/Ah @12V nominal, LiPo lots more.

LanM said:
how do you calculate the actual usable Ah of your battery to make sure the range is there?
Click to expand...
Usually, at lower C-rates, will be close to rated going all the way down to 3V.

For longevity leave 20% SoC there.

Precision requires an Ah counter, or even better a CC load and precisely timed discharge tests.

 
john61ct said:
.....
Now LiPo is subject to the same Avg DoD vs cycle life curve, and everything else being equal will likely die a fair bit earlier.
........
Click to expand...
Are you suggesting that the Lipo will not last as long as the deep cycle lead ? :shock:
That is definitely incorrect !
Lipo will outlast DS lead every time if used correctly.
 
You can get by with less ah on the lipo pack. Lead acid at high c rate loses a lot of usable capacity to the Peukert effect.

Re deep cycle lead... It depends on whether you're talking about level of discharge, or rate of discharge. Deep cycle lead is built with thicker plates, which inhibits it's ability to put out amps, but doesn't flake off during long slow discharge like you get with lights etc on a boat. For cranking amps, you want thinner plates, but they're less tolerant of going really really low on voltage.
But lipo is a whole different animal, so the deep cycle comparison isn't relevant to the ah thing like between different kinds of lead batteries.
 
Good quality lead banks regularly go well over a dozen years before hitting 75% SoH, and that's deep cycled very frequently even daily in some cases.

Not talking big box special of course, more like Rolls / Surrette, big 2-4V cells

Yes if it's a high discharge use case, say over 0.5C is regularly required, then LiPo may be worth it.

Neglected to mention the fire hazard - is this for use inside a boat or van?
 
Thanks for the replies guys.

Use will be the front deck of a kayak. Likely enclosed in an aluminum or kevlar housing.

The thing I wanted to wrap my head around the most is the rated Ah of a pack vs its actual usage Ah available if you're trying to charge to 4.15 V and discharge to 3.6 since that's what I've read are the best range for long pack life.

So, If a pack of 3.7v/ 3600mah cells in a 4S then say 10P giving me a 40 cell pack:
3600MahX40=144,000 Mah, or 144ah (532Wh)

A) is that math correct? Do I now divide the by 4, meaning this is a 36Ah pack?

B) is that pack really that capacity if I'm only going to discharge to 3.6V? How do we calculate a % of actual usable power based on that conservative ideal? As in; does the 4.15 to 3.6V use only 80% of the packs capacity so I should look at a 144ah pack as 115Ah?

c) If my motor will draw approximately 170-180 watts and 15 amps at the level I'll be using it, how would I accurately plan for a 5 hour usable trip length.

Thanks.
 
LanM said:
Thanks for the replies guys.

Use will be the front deck of a kayak. Likely enclosed in an aluminum or kevlar housing.

The thing I wanted to wrap my head around the most is the rated Ah of a pack vs its actual usage Ah available if you're trying to charge to 4.15 V and discharge to 3.6 since that's what I've read are the best range for long pack life.

So, If a pack of 3.7v/ 3600mah cells in a 4S then say 10P giving me a 40 cell pack:
3600MahX40=144,000 Mah, or 144ah (532Wh)

A) is that math correct? Do I now divide the by 4, meaning this is a 36Ah pack?
This would be a nominal (3.6 x 10) = 36Ah pack at that 4s voltage. It would have a nominal Wh capacity of (4 x 3.7 x 36) = 532.8 Wh. (note that if replacing a 12V lead acid pack, a 3s lithium config might do the job).

B) is that pack really that capacity if I'm only going to discharge to 3.6V? How do we calculate a % of actual usable power based on that conservative ideal? As in; does the 4.15 to 3.6V use only 80% of the packs capacity so I should look at a 144ah pack as 115Ah?
A table I have seen for 48V (13s)packs suggest that discharging cells to 3.6V would provide about 50% of the nominal capacity. Here's a link to the discharge curve for a 3500mAh cell. It shows more or less the same.
https://www.orbtronic.com/content/Datasheet-specs-Sanyo-Panasonic-NCR18650GA-3500mah.pdf

c) If my motor will draw approximately 170-180 watts and 15 amps at the level I'll be using it, how would I accurately plan for a 5 hour usable trip length.
If you are drawing a 180 watts for 5 hours continuously, you need 900Wh of usable capacity.

Thanks.
Click to expand...
 
dilkes,

Would the data in the link apply to the lipo cells I intend to use? I'd go with 18650's if I didn't recently purchase a manufacturer liquidation stock of over 300 lipo packs originally intended for tablet use.

"If you are drawing a 180 watts for 5 hours continuously, you need 900Wh of usable capacity"

when referring to the link, and your suggestion that discharging to 3.6V provides about 50% of nominal capacity mean that for that 900Wh I'd need 1350Wh of electricity?

Thanks.
 
You have not told us what lipo cells you plan to use,..there are many in that 3500 mAh range.
As dilkes said , for 5 hrs, you are looking to need 900 Wh of energy ..so that is the size of pack you will have to assemble. At 4s ( nominal 3.8v = 15.2v) that means you will want 900/15.2 = 60 Ah of useable capacity.
And at 180 W draw @15.2v ,..it will be 12 amps ( note ..for 60Ah you will need at least a 20p pack , so that will be less than 1 amp per cell !)
Now, you need to look at what capacity each cell can deliver..
Go here, https://lygte-info.dk/review/batteries2012/Common18650comparator.php
From the drop down list at the top left choose a possible cell .. (i loaded Sanyou GA 3500 as a typical example. But you can change that .
Then tick the 1.0 amp box to see the discharge curve.
You can see that if only discharging to 3.6v the cell will give less than 2 Ah of capacity..so you would need 30+p to make that 60Ah.
BUT most of these cells can happily be cycled down to 3.2v or less, with little affect on life.
SO if you chose to use a range of 4.1 to 3.3v say, you can get 2.75 Ah per cell and only need a 22p pack for the same 60 Ah capacity. !
Thats a 4s, 22p, 88 cell pack with 4.4kg of cells.
 
18650 cells have their place, but this is a boat that has been running on lead bricks. Big pouch or prismatic cells make a lot more sense. I'd look for something from the automotive world.
 
Use will be the front deck of a kayak.
Click to expand...
The OP is comparing a single 12v, 100Ah lead , to an equivalent lipo...for a trolling motor on a kayak !
That is 25+kg of lead , compared to 4.5 kg of lipo .
Which would you rather have on the front deck of a kayak ?
But there certainly are other options.
 
Hillhater said:
Use will be the front deck of a kayak.
Click to expand...
The OP is comparing a single 12v, 100Ah lead , to an equivalent lipo...for a trolling motor on a kayak !
That is 25+kg of lead , compared to 4.5 kg of lipo .
Which would you rather have on the front deck of a kayak ?
But there certainly are other options.
Click to expand...

Yes, deck of a kayak, which previously ran a torqeedo 403 so I've never used a heavy marine or auto battery. The rig is a Jackson Big Tuna, and with my heavy self in the solo seat positiin I still prefer to put the battery up front.

Hillhater, the cells will be TCL Hyperpower cells from a manufacturer liquidation auction, similar to my other thread. I figured with over 300 of the things I'm going to use them wherever I can.

No idea on the specs other than what's printed. I'm still researching what equipment is the best blend of affordability and utility to begin testing.
 
Yes that use case, lead would be silly.

Even LFP would be half the density of LiPo.
 
LanM said:
dilkes,

Would the data in the link apply to the lipo cells I intend to use? I'd go with 18650's if I didn't recently purchase a manufacturer liquidation stock of over 300 lipo packs originally intended for tablet use.
Depends what cells you are using. But the discharge curve will likely be similar.

"If you are drawing a 180 watts for 5 hours continuously, you need 900Wh of usable capacity"

when referring to the link, and your suggestion that discharging to 3.6V provides about 50% of nominal capacity mean that for that 900Wh I'd need 1350Wh of electricity?
Not my suggestion. It's what the discharge curves say.
50% means you would need an 1800Wh rated pack
I wouldn't restrict myself to discharging down to only 3.6V.

Thanks.
Click to expand...
 
At high C-rates discharge, you could allow as low as 3.2Vpc sag point.

The goal should be **resting** recovery, back to 3.4Vpc

Of course, using 3.6V instead does give much longer life-cycle longevity than going to lower average DoD%.

But you do need to double the pack size and weight to get the same range.
 
. the cells will be TCL Hyperpower cells from a manufacturer liquidation auction,...
Click to expand...
Other than these are 1s, 2p 10.4Ah lipo pouch cells, designed for Tablets, we know little about them, and certainly dont have any detail discharge performance curves.
Its risky to speculate but,...
Its unlikely they will have the discharge profile or voltage range of 18650 can type cells. (IE< . 4.2 - 2.5v for full capacity.)
But we should not automatically assume they they behave like most other pouch cells (4.2 - 3.6v for 95% capacity)
But this does sound like a viable application for those cells ,
...but we need test data to estimate a suitable pack .
HOWEVER..shooring from the hip,.. i would speculate that a 4s, 10p, arrangement of 40 of those tablet 10.4 Ah packs, (8.0 kg approx), would comfortably do the job with a 0.15 C discharge rate. !
With favourable test data that could well become a 4s, 8p, 32 unit, 6.5kg, pack :mrgreen:

Ahh ! ...I Just realised you are thinking of using those single 3.6Ah TCL pouches !..
So that would mean about a 4s, 30p 120 cell, pack ..for similar capacity and low 0.15C load.
A much more complex assembly job than using the higher capacity cells. :wink:
 
I'd happily use either of the 2 cell types. If it'd be much easier to use the 1S2P 10,400 Mah, I would.

I just trying to drum up uses for the ones I have (and recently sold the torqeedo so the timing was right for a new battery setup for the minn kota thats replacing it.

Question- I see the discharge profile of the 18650''s and from what's posted here discharged to 3.6v leaves 50% on the table.

But reading Neptronix's rc lipo sticky-he shows and states that very little capacity is left below 3.6. A review of his most recent post on the thread shows him reiterating that sentiment, and that it leaves 5-10% in reserve as a tradeoff for much longer life.

Should the takeaway from this be that the data in that thread isn't chiseled in stone as true, and that pack capacity is 50% at that point instead of 80% as stated in that thread?
 
No !,
Generally speaking 18650 and similar “Can”. Type Lithium cells, have a very different voltage /SOC profile compared to the “RC LiPo” ( Lithium Polymer) pouch cells that the Neptronix thread describes.
They are different chemistry and behave very differently.
The pouch cells tend to have most (95%) of their capacity between 4.2 and 3.6v.
The “Can” cells have only 50% of their capacity in that range, and need to be discharged down below 3.0v to realise 95% of capacity.
This is why i keep saying you need to test those pouch cells to see exactly what their capacity/voltage range is.
 
Plus the LVC under load, SoC represented by a certain voltage varies **tremendously** with the C-rate, even within a specific chemistry.

Thus the LVC voltage setpoint needs to be adjustable to accommodate changing circumstances.

You can only guesstimate the actual SoC from voltage **at rest**, at least a couple hours isolated.

And the resting-voltage-vs-SoC% table is different from one cell to the next, changes as the cell ages, etc etc

Really, you can't look for "rules" to just slavishly follow, things just aren't that simple. . .

Learn how to calibrate the general guidelines to **those** cells after testing for their unique characteristics, adapted to **that** use case, and most of all, **your** priorities and preferences.
 
John, if he builds a pack with 900Wh usable ( well over 1kWh total ). capacity, and the max draw is 180 W, then his max “C” rate is only going to be less than 0.2C,. voltage sag will be minimal, probably undetectable even!
 
You can ignore most of what I posted as at that earlier point in this thread, I assumed the OP was talking about 18650 lithium cells as he had not stated what he was using.

As mentioned later in the thread, the discharge curves for the LiPo pouch cells may be completely different.
 
Hillhater said:
John, if he builds a pack with 900Wh usable ( well over 1kWh total ). capacity, and the max draw is 180 W, then his max “C” rate is only going to be less than 0.2C,. voltage sag will be minimal, probably undetectable even!
Click to expand...
Sure, if all those things are true.

In any case, just making the point about that 3.6Vpc "rule" getting modified depending on circumstances.

I reckon for a kayak, he may not need 5 hours WOT range, figure that's too heavy a pack, and go for a higher C-rate.
 
Thanks very much for the replies guys.

John, the provided wattage/amperage is only at roughly 3mph. WOT is 5mph, drawing upwards of 30 amps from what I've seen in testing posted by others of the same motor. I wanted a 15 mile range for the "drop me off at X, pick me up at Y" trips I like to do. I'm going to be adding a PWM. 25lb battery is fine and still less than half the weight of lead acid if it gives me the range. This kayak is a tank at 35 inches wide with a 500lb capacity. I had a my 25lb blue heeler (runt of the litter) in the same spot this battery will occupy with no noticeable effect on performance until she passed in Feb.

Test data it is then, looks like I'm dancing around that need before any of these projects will get off the ground. I wish that other thread John started would populate with info. It looks like I'll need equipment that can handle 4s and 7s setups for both projects. I'd like to do it affordably.

dilkes, apologies for any confusion, I tried to clarify Lipo in the thread title and my first post however failed to specify which lipo cell. Thank you for the input regardless. It is appreciated.
 
4S lipo will cause your motor to run faster and get hotter than its design intended. Maybe that's acceptable and maybe not. It might even be a benefit if it provides improved performance without added risk of failure.

4S lithium iron phosphate is a more or less exact match for the voltage range of 6S lead-acid. You can even use lead acid dumb chargers.
 
LanM said:
Test data it is then, looks like I'm dancing around that need before any of these projects will get off the ground. I wish that other thread John started would populate with info. It looks like I'll need equipment that can handle 4s and 7s setups for both projects. I'd like to do it affordably. .
Click to expand...
Unless you want to start a career testing and analysing cells, you dont need to invest in the full lab quality/pro kit.
You can check, sort, calibrate, and record individual cells for,.. capacity, voltage profiles, DCIR, even thermal stability, ...fairly accurately with simple , cheap kit such as,....a Mmeter, RC lipo logging charger (Imax, Turnigy, etc), maybe a wattmeter. A laptop/tablet/PC with some free data analysis software is also useful, especially if you are doing many cells. Important that the charger has data logging capability.
Assuming you have a laptop, the rest could cost under $100. !
If you chose correctly, the logging charger can serve duty as your 4s pack charger.
 
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