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Old 05-05-2019, 11:04   #1
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Lithium battery C-rate question

OK, so if I wanted to add a lithium battery bank, at 48-volts. And I'm using twelve 12-volt lithium batteries, put into groups of 4 in series, and those three groups are in parallel to increase the power stored to 480 ah. I want to confirm how the C-Rate works.

If each of these individual batteries is rated for 160 ah at 12-volts (so four in series gives me 160 ah at 48-volts) and these batteries are rated at a continuous discharge rate of 3C, how many amps can these four in series discharge, at maximum continuous?

I'm guessing 480 amps (for 20 minutes).

Is that right?

And, if I put three of these 4 battery series groups together in parallel, for a total of 480 ah at 48-volts, these 3C-Rate batteries should discharge at their maximum continuous rate for one hour, right?

Finally, if the batteries' charge rate is 1C, and have a large generator, how many amps can these batteries take back in, at a maximum charge? I'm guessing 160 amps, but I'm confused by the batteries in series now. If each one can take in 160 amps per hour, can I push 160 amps per battery?

For example, looking at just one set of four of these batteries in series, can I push 480 amps from the generator (160 amps per battery), therefore recharging the four battery bank in one hour? Or, can I only push 160 amps requiring four hours to recharge this bank of four 160 ah batteries at 48-volts?

Help is appreciated. Feel free to unconfuse me.
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Old 05-05-2019, 11:12   #2
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Re: Lithium battery C-rate question

First off, by LI do you mean LFP? Link to cell specs?

Is this for propulsion, or House bank usage?

If 480Ah is the total capacity, 240A is .5C, 960A is 2C.

But series does not increase Ah only V.

Parallel increases Ah only not V.

3C will lose a **lot** of potential longevity, and possibly lead to overheating. Same with charging, even more so.

Most vendor C-ratings are pure fiction, needs to be well tested.
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Old 05-05-2019, 11:34   #3
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Re: Lithium battery C-rate question

Quote:
Originally Posted by john61ct View Post
First off, by LI do you mean LFP? Link to cell specs?

https://www.super-b.com/en/marine/sb12v160e-zc

Is this for propulsion, or House bank usage?

Propulsion, 15 kw electric motors (Oceanvolt SD 15, servoprops)

If 480Ah is the total capacity, 240A is .5C, 960A is 2C.
So the C-Rate applies to the total ah in the bank. I have to assume E-Rate does as well? So, if I want, I can put 480 ah back in within one hour if the E-rate is 1E? (Some folk still call this C-rate). I understand this may not be desirable, I just want to know what can be done.

But series does not increase Ah only V.
Yes, as I stated above, I'm putting four 12-volt batteries in series to get to 48 volts.

Parallel increases Ah only not V.
Yes, as I stated above, I'm putting three groups (of four in series) into parallel to get to 480 ah at 48 volts

3C will lose a **lot** of potential longevity, and possibly lead to overheating. Same with charging, even more so.
This is for emergencies, not continuous operation. Still, in an emergency, I want to know I can go full out and for how long, and damn the longevity (after all, it's an emergency). Normally, I won't push them anywhere near the max.

Most vendor C-ratings are pure fiction, needs to be well tested.
Understood. And thanks for the reply.
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Old 05-05-2019, 11:37   #4
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Re: Lithium battery C-rate question

Quote:
Originally Posted by john61ct View Post
First off, by LI do you mean LFP? Link to cell specs?

Is this for propulsion, or House bank usage?

If 480Ah is the total capacity, 240A is .5C, 960A is 2C.

But series does not increase Ah only V.

Parallel increases Ah only not V.

3C will lose a **lot** of potential longevity, and possibly lead to overheating. Same with charging, even more so.

Most vendor C-ratings are pure fiction, needs to be well tested.
Just why would you say "pure fiction"?
These are maximum capacities, necessary for design, with consideration for factors of safety. Running electrical components at maximum ratings will degrade them quickly, but some specialized users might have good reasons for this type of use.
I doubt any cruising sailor would want to be anywhere near maximums, as recreational users we typically want maximum life/least cost.
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Old 05-05-2019, 11:48   #5
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Re: Lithium battery C-rate question

Especially with Li, regular balancing of all the cells / batteries in the bank is very very important. With just putting 12 V units in parallel and series, you may develop unbalancies on charge state over time and Li does not like that. So - you need to be sure the whole setup is kept well balanced.
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Old 05-05-2019, 12:03   #6
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Re: Lithium battery C-rate question

You should determine the internal resistance of batteries and this will give you an ideea about C-factor. If you have relatively cheap Chinese LFPs, stay on very low C's, nevermind 0.5C, 1C or such, even 0.3C might be too much. So you need to measure.
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Old 05-05-2019, 12:18   #7
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Re: Lithium battery C-rate question

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You should determine the internal resistance of batteries and this will give you an ideea about C-factor. If you have relatively cheap Chinese LFPs, stay on very low C's, nevermind 0.5C, 1C or such, even 0.3C might be too much. So you need to measure.
I won't be using cheap Chinese batteries. I'm currently interested in quality batteries made in the Netherlands (SuperB) or batteries made in Norway(Corvus). Both are expensive but are highly rated batteries.

I'm partial to the Orca ESS batteries from Covus, but I'm not sure they will sell to me (they tend to outfit very large ships, ferries, tug boats, etc.) Though they do list 'yachts' in their applications and do say they offer down to 50v and 5.7 kw batteries. We'll see.

I want to buy a 30kw, 48-volt bank. I've sent them an email to see if they are amendable.
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Old 05-05-2019, 12:46   #8
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Re: Lithium battery C-rate question

Yes quality vendors's specs are closer to reality, ideally even publish C-rate vs thermal-gain charts.

In the hobbyist world, very knowledgable forum members with good testing gear post the detailed charts and videos that indicate actual IRL performance.

Just because batteries are sourced from China does not automatically mean poor quality, but word of mouth verification of a trusted supply chain is critical.

BTW the questions I asked above are very relevant to the discussion, and it seemed a recalculation of Ah capacity was needed?

For House bank usage, if you care about longevity, energy density and C-rates are pretty irrelevant, and should really stick to LFP for safety rather than the other LI chemistries.
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Old 05-05-2019, 13:04   #9
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Re: Lithium battery C-rate question

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If you have relatively cheap Chinese LFPs, stay on very low C's, nevermind 0.5C, 1C or such, even 0.3C might be too much.
Yes for sub-par cheap stuff, and doubly so for longevity.

But there are many cells that can routinely go to 10-20C, even higher for short bursts **in a propulsion context**. A123 is top-notch for example, K2 more of an unknown, but then they are apparently OEM for Trojan Trillium, so are presumably at least capable of making quality cells.

Not that such high rates are what I'd advise for a boat with humans aboard, just sayin', propulsion is a very different use case compared to House banks.
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Old 05-05-2019, 13:09   #10
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Re: Lithium battery C-rate question

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Originally Posted by john61ct View Post

BTW the questions I asked above are very relevant to the discussion, and it seemed a recalculation of Ah capacity was needed?

For House bank usage, if you care about longevity, energy density and C-rates are pretty irrelevant, and should really stick to LFP for safety rather than the other LI chemistries.

No, the examples I gave were sufficient to have my question answered. No Ah recalculation was needed.

and... I'm not talking about a House bank... nor about Chinese cells.

I'm assuming quality batteries for propulsion systems requiring large amperages (at rare times) to move the boat at full speed.
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Old 05-05-2019, 13:25   #11
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Re: Lithium battery C-rate question

Even in this context, good quality batts from trustworthy vendors, the answer to original questions lies with their internal resistance. Ask the vendor for their data sheets and check their IR specs. The higher IR, the lower the actual C in real world usage. However their control systems should take care of this parameters and cut power usage above a certain limit.
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Old 05-05-2019, 13:31   #12
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Re: Lithium battery C-rate question

OK, 480Ah @48V seemed huge for a House bank, but for propulsion will be a very limited range, especially in chop upwind or against the tide.

Likely you will want a diesel genset that keeps up with a majority of the max power required, to have a margin for safety when SHTF.

Professional advice is called for to get a practical system, not a use case for a DIY approach based on Internet advice.
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Old 05-05-2019, 13:49   #13
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Re: Lithium battery C-rate question

Quote:
Originally Posted by john61ct View Post
OK, 480Ah @48V seemed huge for a House bank, but for propulsion will be a very limited range, especially in chop upwind or against the tide.

Likely you will want a diesel genset that keeps up with a majority of the max power required, to have a margin for safety when SHTF.

Professional advice is called for to get a practical system, not a use case for a DIY approach based on Internet advice.
As much as land EV use is increasing and can make sense, using batteries for propulsion on a larger boat intended to go to sea seems the height of irresponsibility. As John pointed out, you still would need an ICE for emergency situations - and unless that generator is approximately of the same power as an all ICE propulsion system, it isn't going to provide enough power to keep you moving against adverse currents or seas.

A land EV runs out of juice - you coast to the shoulder and wait for a tow.
A boat runs out of juice - maybe you end up wrecked on the coast.

Keep the ICE engine for propulsion, mount enough solar panels that you don't need to burn dino juice for your regular electrical loads.
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Old 05-05-2019, 13:57   #14
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Re: Lithium battery C-rate question

Well there are those with zero ICE on their RTW boats.

Some only want power to help the last few hundred feet getting back to marina.

But trying to get the same functionality as a big diesel motor and a full tank of fuel, yes is not cost-effective, and only "green" beyond a PR sense, with a boat specifically designed for that purpose.
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Old 05-05-2019, 14:06   #15
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Re: Lithium battery C-rate question

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OK, 480Ah @48V seemed huge for a House bank, but for propulsion will be a very limited range, especially in chop upwind or against the tide.

Likely you will want a diesel genset that keeps up with a majority of the max power required, to have a margin for safety when SHTF.

Professional advice is called for to get a practical system, not a use case for a DIY approach based on Internet advice.
As I keep saying, 480 ah was only an example to get a simple answer to a simple question about C-rates. This is NOT my planned bank size, nor does what size I intend to use matter to the question I asked about C-Rates. Don't read anything into that size. Thanks!
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