LiFePo4 -- Parallel or Not?

[Dockhead]

I swore the next time I had a significant battery fail on my boat, that I'm chucking it all and going lithium. Well, it's happened. My Trojans are only 3 years old, but the aft bank almost went into thermal runaway in the hot weather, with the Victron Multiplus repeatedly going back into absorption after withdrawing small amounts of power for Power Boost


Then on top of that we were rafted up just last night to a gorgeous Colin Archer pilot ketch which burned down (!) due to thermal runaway and fire in the bow thruster gel battery.



Clearly a sign from the gods. So here we go.



I think I have a pretty good handle on most of the design questions from the several excellent conversations we've had on here, but there are still a few things I don't understand.


One of them is whether it makes sense to parallel cells or not. You do everything to avoid that with lead for the obvious reasons, but does the same thing apply to LiFePo4 managed with a BMS? I would think the balance issues would be solved by the BMS, no? And isn't there an advantage to being able to pull cells offline if necessary in case one fails, and retain functionality of the bank? Or is it better to just carry a spare cell?


I'm looking at the Winston 160 amp/hour cells. I would need 16 of them to make up a 2P8S 24v bank with 320 amp/hours of capacity. Alternatively, I could do 8x of the 300 amp/hour cells, but making the battery box to fit my space would be somewhat harder.


Any tips?


Also, any tips on how to construct the battery box?

[mbartosch]

Originally Posted by Dockhead

One of them is whether it makes sense to parallel cells or not. You do everything to avoid that with lead for the obvious reasons, but does the same thing apply to LiFePo4 managed with a BMS? I would think the balance issues would be solved by the BMS, no? And isn't there an advantage to being able to pull cells offline if necessary in case one fails, and retain functionality of the bank? Or is it better to just carry a spare cell?

We have a 4P4S setup (16 * 100 Ah Winston) on Entropy, and if I'd do it again I would personally try to avoid having more than 3 parallel cells now.
I would probably choose a 2P setup or even 1P, depending available space, of course.

The reason for this is that the more parallel cells you have the more wiring and connectors are involved, all introducing wiring complexity, possible connector problems and additional resistances for each connection.

With a 4P setup, I suspect the "outer" cells will be significantly less stressed than the "inner" cells, possibly leading to internal imbalances in the parallel set. You won't be able to detect this easily, though, cell monitoring gets something a mean value.
Connecting the main cables to a 3P or higher setup will also probably cause imbalance within the parallel set "outside" of the cable connection.

Getting a spare cell for a 1PxS or 2PxS setup would probably suffice to address most problems, IMO.

Keep in mind that if you let the BMS balance the cells, it will only balance the serial cell packs with each other. Balance within the individual parallel packs are impossible to address.

I'm looking at the Winston 160 amp/hour cells. I would need 16 of them to make up a 2P8S 24v bank with 320 amp/hours of capacity. Alternatively, I could do 8x of the 300 amp/hour cells, but making the battery box to fit my space would be somewhat harder.

If I had to decide this for myself I'd go with 1P8S of the 300 Ah cells. You may have more hassle with the physical installation on the boat but electrically it is simpler.

[evm1024]

Hi Dock, Welcome the the LiFePO4 crowd...

I was shooting for 600 ah to 800 ah for my 12 volt house bank and ended up with four 700 AH cells. These cells were on "sale".

If I were to spec a bank again I think I would go for eight 400 AH cells 2P4S. The idea there would be that with any 1P design a single cell failure leaves you (at 12 volts) very under voltage. This may be much less of a problem with your 24 volt bank.

I have also considered using 200 AH cells as the basic building block. This has some interesting options. For example a set of 4 or 8 (12 or 24 volts) cells at 200 AH could be used to power a bow thruster. Then if any house bank cell started under performing (as in capacity loss) then that cell could be swapped for a better one in the thruster bank.

As for mounting I opted for a simple set of 1/4" thick aluminum plates bolted to an L bracket that was bolted down. The photo shows my 700 AH 12 volt house bank. It could be a 400 AH 24 volt bank as well.

Regards

[Texlan]

Originally Posted by evm1024

If I were to spec a bank again I think I would go for eight 400 AH cells 2P4S. The idea there would be that with any 1P design a single cell failure leaves you (at 12 volts) very under voltage. This may be much less of a problem with your 24 volt bank.

I would go 4s2p. Separate strings that were paralleled. You don't pay much more for a decent BMS like Batrium, and if one cell goes bad in one string, you won't destroy the other string with it. With 2p4s, one cell developing a short will drag the paralleled cell down with it. With separate strings in 4s2p, you're only pulling the other *whole string* down by 3.2 volts, which gives you an opportunity to isolate the string with the bad cell and operate on the good string until you can replace the bad cell.

That said, it's pretty unlikely you'll develop a bad/shorted cell, it's a fairly mature tech at this point. I've read recommendations that you keep it at or under 200AH/cell in a high vibration environment, as the lower mass of the internal components mean less likely to damage due to mechanical stresses.

Sean

[Viking Sailor]

DH


You might want to consider these 24 volt LiFePo4 batteries with a built-in BMS. They offer ease of installation and fail-safe operation.


https://battlebornbatteries.com/shop...cycle-battery/


The only additional item you would need is a 24 volt Alternator Protection Device (APD24).


https://sterling-power.com/products/...tection-device


I have no relationship with either of these companies.

[alctel]

I have a 400 AH bank of 8 cells, 2P4S. (12V)

Much like mbart said, I'd go for the 1P over the 2P. Less wiring and simpler, although I haven't seen any balance issues with my cells (I don't balance them with the BMS either)

If you are worried about redundancy, I'd just carry a spare cell.

As for the battery box, most people seem to use two blocks of wood with threaded rod in a kind of press, with the batteries in between them

[CapnBazza]

Originally Posted by Dockhead

I swore the next time I had a significant battery fail on my boat, that I'm chucking it all and going lithium. Well, it's happened. My Trojans are only 3 years old, but the aft bank almost went into thermal runaway in the hot weather, with the Victron Multiplus repeatedly going back into absorption after withdrawing small amounts of power for Power Boost


Then on top of that we were rafted up just last night to a gorgeous Colin Archer pilot ketch which burned down (!) due to thermal runaway and fire in the bow thruster gel battery.



Clearly a sign from the gods. So here we go.



I think I have a pretty good handle on most of the design questions from the several excellent conversations we've had on here, but there are still a few things I don't understand.


One of them is whether it makes sense to parallel cells or not. You do everything to avoid that with lead for the obvious reasons, but does the same thing apply to LiFePo4 managed with a BMS? I would think the balance issues would be solved by the BMS, no? And isn't there an advantage to being able to pull cells offline if necessary in case one fails, and retain functionality of the bank? Or is it better to just carry a spare cell?


I'm looking at the Winston 160 amp/hour cells. I would need 16 of them to make up a 2P8S 24v bank with 320 amp/hours of capacity. Alternatively, I could do 8x of the 300 amp/hour cells, but making the battery box to fit my space would be somewhat harder.


Any tips?


Also, any tips on how to construct the battery box?

Save yourself a lot of headaches and expense. The hard work has already been done for you. There a number of “drop in” choices and getting cheaper all the time. Make sure you get ones capable of starting your engine if that is what you need. Some have quite low charging and output amperage.

[AndyEss]

Well, parallel, serial, whatever, your back, knuckles, arms will thank you a million times over for chucking all that lead weight.
My battery boxes were in my engine compartment - Torquemada's dream self-inflicted contortion torture chamber.
The LFP house bank I cobbled together makes it almost a pleasure to duck down and see what new improvement project I can dream up.
If it ain't broke, don't fix it, is not my way of swinging. I always look for ways to improve (even if it's only in my mind) my ride.
Of course, only a sailor would thrill at a ride of 5-8 mph as being thrilling.

[Delfin]

Originally Posted by Dockhead

I swore the next time I had a significant battery fail on my boat, that I'm chucking it all and going lithium. Well, it's happened. My Trojans are only 3 years old, but the aft bank almost went into thermal runaway in the hot weather, with the Victron Multiplus repeatedly going back into absorption after withdrawing small amounts of power for Power Boost


Then on top of that we were rafted up just last night to a gorgeous Colin Archer pilot ketch which burned down (!) due to thermal runaway and fire in the bow thruster gel battery.



Clearly a sign from the gods. So here we go.



I think I have a pretty good handle on most of the design questions from the several excellent conversations we've had on here, but there are still a few things I don't understand.


One of them is whether it makes sense to parallel cells or not. You do everything to avoid that with lead for the obvious reasons, but does the same thing apply to LiFePo4 managed with a BMS? I would think the balance issues would be solved by the BMS, no? And isn't there an advantage to being able to pull cells offline if necessary in case one fails, and retain functionality of the bank? Or is it better to just carry a spare cell?


I'm looking at the Winston 160 amp/hour cells. I would need 16 of them to make up a 2P8S 24v bank with 320 amp/hours of capacity. Alternatively, I could do 8x of the 300 amp/hour cells, but making the battery box to fit my space would be somewhat harder.


Any tips?


Also, any tips on how to construct the battery box?

If you are not entirely budget limited, may I suggest going with a configured set of cells from Lithionics, with their BMS? They balance the cells for resistance, install them in a container and you can pretty call your tune with respect to capacity. I have two 300 Ah 24 vdc paralleled, and with their BMS managing balancing, safety shut-offs etc. You can save a bunch with a DYI project, which may be worth it, depending on the user. It wasn't for me and I couldn't be happier.

[tanglewood]

But I thought LA batteries are safer than LFP? :-)


Anyway, as I see it the only advantage to parallel strings (say 8S2P) is redundancy, quicker diagnostics, and quicker repair/time to get back in operation. But these are quite important on a distance/remote cruising boat. In all other ways, paralleled cell "blocks" wired in series (2P8S) is easier to build and doesn't require as much balancing.


So it seems you are trading off easier diagnostics with parallel strings, against less chance of problems in the first place with parallel blocks. It's a tough call.


I built our home system as 2P16S (48V), and am still undecided whether to build the boat system as 4P8S (24V), or 2P8S2P.

[CapnBazza]

Way back in 1985 when I became involved in computers career wise there were many people busy programming their own applications. I bought Multiplan and WordStar to run on MS DOS. It seems that today there are many people building LiFePO4 batteries starting from scratch with a concoction of single cells, homegrown BMSs, plastic boxes to house the lot and gauges and meters all over the place. These people seem to spend more time monitoring their batteries than they do sailing. My Chinese, 2 for AUD$900 drop ins are now in their fourth year and, touch wood, doing well. As for those unfortunate folk who still believe that LA is safer than LiFePO4, read the OPs post again. Even Boeing’s lithium battery fires appear to have been the result of faulty wiring, not the batteries.

[Dockhead]

Originally Posted by Delfin

If you are not entirely budget limited, may I suggest going with a configured set of cells from Lithionics, with their BMS? They balance the cells for resistance, install them in a container and you can pretty call your tune with respect to capacity. I have two 300 Ah 24 vdc paralleled, and with their BMS managing balancing, safety shut-offs etc. You can save a bunch with a DYI project, which may be worth it, depending on the user. It wasn't for me and I couldn't be happier.

Thanks -- I like the idea of course of a system which is professionally engineered as opposed to home brewed, although I think after a good bit of study and research that the home brewing is quite straightforward.


The 300a/h 24v module looks good to me. I'll investigate whether it will fit and whether I can actually buy it over here. It costs $8,800 in the U.S., which with VAT will be over $10,000 in Europe if not more. I'll have to think whether that's worthwhile compared to $3,000 worth of cells plus maybe $1,000 more for BMS and battery box.


What is definitely not interesting to me are lithium batteries made in lead form factors. Cobble a bunch of those together to get a bank? What quality are the electronics in those things?



Not interesting -- I don't care about the lead battery form factor and am not willing to compromise other qualities to get it. Lithium is just different and I think we need to embrace that, and forget trying to make it look like or behave like lead.

[Delfin]

Originally Posted by Dockhead

Thanks -- I like the idea of course of a system which is professionally engineered as opposed to home brewed, although I think after a good bit of study and research that the home brewing is quite straightforward.


The 300a/h 24v module looks good to me. I'll investigate whether it will fit and whether I can actually buy it over here. It costs $8,800 in the U.S., which with VAT will be over $10,000 in Europe if not more. I'll have to think whether that's worthwhile compared to $3,000 worth of cells plus maybe $1,000 more for BMS and battery box.


What is definitely not interesting to me are lithium batteries made in lead form factors. Cobble a bunch of those together to get a bank? What quality are the electronics in those things?



Not interesting -- I don't care about the lead battery form factor and am not willing to compromise other qualities to get it. Lithium is just different and I think we need to embrace that, and forget trying to make it look like or behave like lead.

It's pretty clear that you can save a lot of money with a DYI approach, and it would probably be a fun project to execute. At less than half the cost, I guess you could pooch a DYI effort once and still come out ahead. I just didn't have the time or inclination when I made the switch. In researching Lithionics, the value add seemed to be knowing I got fresh cells; they were all matched for impedance, then balanced; a BMS matched to the bank; from a company that builds these for military applications.

The 2 x 300 Ah 24 v LFP bank that replaced my 1250 Ah 24v NorthStar AGM bank is set up as 2P8S. If there was some advantage to another configuration I assume Lithionics would offer it, but they don't, I guess because of the energy density you can get in a lithium battery that avoids having to string a bunch of smaller cells together to get the total capacity you need.

[nebster]

Originally Posted by tanglewood

Anyway, as I see it the only advantage to parallel strings (say 8S2P) is redundancy, quicker diagnostics, and quicker repair/time to get back in operation.

Well, there is at least one other consideration: geometry. It can be easier or harder to wire up a particular topology, with a selected cell of a certain size, in a given space, properly.

[mbartosch]

Originally Posted by nebster

Well, there is at least one other consideration: geometry. It can be easier or harder to wire up a particular topology, with a selected cell of a certain size, in a given space, properly.

And this is a very important one. One of the things I have learned over time is that it may look very easy on paper, but it may be difficult to execute in limited and inaccessible space.
A simple connection line or a connector dot in the schematics may cost hours of work in real life.