LiFePO4 Batteries - Okay Tear Me Apart ;-)

[electric1]

Okay, then... so the only real annoyance with the MiniBMS at this point is that it doesn't distinguish between over-charge and over-discharge... so, at LVC, it'd open the relay and you wouldn't be able to charge your batteries until it has been reset. Kind of a pain in the ass if you manage to get yourself into this situation, but more than doable. Better would be to differentiate the two states and provide a relay/contactor control for each like the Genasun.

I would argue this point. There is no real need to distinguish LVC from HVC, both of those events are harmful for the battery and require immediate disconnect from the circuit. Imagine BMS as simple device that has single binary output "All is Good" / "Something is Wrong". That is all you need to know to take action. On a boat with 14.5V charging sources HVC is virtually not possible, so in reality BMS only trips on LVC anyway. Since you have to take action to start the generator, its not difficult to press the BMS Reset button while you at it. This button along with the buzzer can be wired into the cabin or the deck or both in parallel if you want.

I also maintain my position at this point that RC filter across the contactor will save diodes on the alternator in the highly unlikely event of BMS tripping while alternator is running at high load.

[s/v Jedi]

Hi Electric,

Thanks for that excellent explanation.

About sizing: so it is actually normal to cycle between 10% and 90% SOC? Is the life span of the bank significantly improved when cycling between 20% and 90%? I never found any diagrams showing number of cycles against cycle depth so I'm thinking there's little difference.

What is the max. current without throwing cells out of balance or other bad things happening? 2C? If so... wow!

What is the Ah rating of the cells based on (like 20h or 10h Ah ratings for LA)? Is that also 2C???

About current battery monitors: they are being fooled when that shunting activates of course, plus it only works when you can change the Peukert settings (all good ones can do that I think). But I think it'll work just fine in combination with your mini-BMS.

cheers,
Nick.

[s/v Jedi]

Originally Posted by electric1

I also maintain my position at this point that RC filter across the contactor will save diodes on the alternator in the highly unlikely event of BMS tripping while alternator is running at high load.

You mean to put the RC network in parallel with the output winding or the field winding? On the output winding I would like to see test results ;-)

On the field winding, if you have access to the point between regulator and winding (which many alternators will not allow as the regulator is part of the physical construction) you can also just interrupt that circuit instead of adding the RC network.

All external regulators will have an on/off input which is the way to go imo.

ciao!
Nick.

[electric1]

Originally Posted by s/v Jedi

About sizing: so it is actually normal to cycle between 10% and 90% SOC? Is the life span of the bank significantly improved when cycling between 20% and 90%? I never found any diagrams showing number of cycles against cycle depth so I'm thinking there's little difference.

Lifespan of LFP cells varies in different reports from different manufacturers and its very difficult to confirm in real life since no one had these cells long enough to find out. Like my favorite saying goes ( I should make it my signature line ) "There are 3 kind of liars out there: liars, damn liars and battery manufacturers."

There is also hidden logic in lifecycle numbers, you get more shallow cycles than deep cycles, but since deep cycles last longer, over the same battery life you will end up with less deep cycles, so it all evens out. For example, one battery is cycled 365 times a year and each cycle provides 24 hours of use (deeper cycle), while the other battery is cycled 730 times a year and each cycle provides 12 hours of use ( shallower cycle ). Both batteries are used the same way and provide the same useful work over the same time period, so their number of cycles is not very relevant from this point of view, as long as you stay within normal operating conditions.

What is the max. current without throwing cells out of balance or other bad things happening? 2C? If so... wow!

My personal experience with Thundersky cells ( which are the cheapest LFP cells on the market ) is that continuous rate under 1C and bursts under 2C have no impact on their health. Further you stay below those rates the better. So again, this makes them a perfect choice for house banks.

What is the Ah rating of the cells based on (like 20h or 10h Ah ratings for LA)? Is that also 2C???

LFP cells are always rated at 1C, but have virtually no derating up to 2C. So, when calculating bank size, you can bet that 200AH cell will deliver 200 Amps for an hour, or 100 Amps for 2 hours, or 20 Amps for 10 hours, etc etc.

About current battery monitors: they are being fooled when that shunting activates of course, plus it only works when you can change the Peukert settings (all good ones can do that I think). But I think it'll work just fine in combination with your mini-BMS.

Shunting has so little impact on capacity that it can be ignored. For example, in my experience MiniBMS shunts for 20 minutes at 0.75 Amp, which corresponds to only 0.25 AH

While SOC meters designed for Lead Acid do not provide very accurate reading on LFP since LFP has much flatter curve, they are good enough to provide meaningful reading and when combined with simple BMS it makes LFP bank very easy to use.

When using LA SOC meter on LFP you will notice that SOC reading stays in the middle longer and then drops faster at the end, so as long as you know this and expect it, you will be fine. It only takes a few cycles to get use to it and get comfortable with it. Its somewhat similar to typical car's fuel gauges, they spend more time in the middle and then drop fast.

[s/v Thea]

Nick,

Regarding cycling, the Thunder Sky brochure shows 3000 cycles at 80% DOD, 5000 @ 70% DOD, all at 0.5C discharge rate.

Does anybody have a chargiung curve (charge rate vs % charged) for charging these batteries at 14.5/4=3.625 V/Cell?

Doug

[jallum]

I think it's also important to point out that our usage pattern on boats is fundamentally different than in an electric car, where the bank is being used for propulsion and being drained hard -- in contrast, our batteries on sailboats have a pretty cushy life. We have pretty modest loads most of the time and we'll tend to "sneak up" on the LVC and HVC points. I think that something simple (like the MiniBMS) will probably prove to be a great solution for the vast majority of people contemplating LiFePO4 batteries. If I didn't like big dumps of nerdy data, I probably would have gone the simple route myself.

[jallum]

Originally Posted by s/v Thea

Does anybody have a chargiung curve (charge rate vs % charged) for charging these batteries at 14.5/4=3.625 V/Cell?

The big limitation here is the capacity of your charging system... unless you have a truly massive charger (an amperage rating of 2-3x the Ah-capacity of the batteries), these batteries are going to take current pretty much as fast as you can throw it at them until full. For most of us, the "curve" will be more of a straight line.

[jallum]

Originally Posted by electric1

While SOC meters designed for Lead Acid do not provide very accurate reading on LFP since LFP has much flatter curve, they are good enough to provide meaningful reading and when combined with simple BMS it makes LFP bank very easy to use.

When using LA SOC meter on LFP you will notice that SOC reading stays in the middle longer and then drops faster at the end, so as long as you know this and expect it, you will be fine. It only takes a few cycles to get use to it and get comfortable with it. Its somewhat similar to typical car's fuel gauges, they spend more time in the middle and then drop fast.

The meters from Victron (600 & 602) can be programmed with the appropriate peukert (and other) numbers and will then read accurately.

Victron's Multiplus and Quattro inverter/chargers can also be loaded with a battery profile appropriate for LFP, according to their tech people.

[s/v Thea]

Originally Posted by jallum

The big limitation here is the capacity of your charging system... unless you have a truly massive charger (an amperage rating of 2-3x the Ah-capacity of the batteries), these batteries are going to take current pretty much as fast as you can throw it at them until full. For most of us, the "curve" will be more of a straight line.

I'm interested in the charge rate @ 3.6V when batteries are 90% or more charged, it must drop off at some point, be nice to see a curve.

[electric1]

You mean to put the RC network in parallel with the output winding or the field winding? On the output winding I would like to see test results ;-)

This is what I have in mind.... This RC time circuit will give 10ms for alternator field to gradually collapse while charging the cap. These values were picked for the sake of argument and might need adjustment based on actual energy stored in the alternator field when circuit is broken, which I don't know how to calculate precicely. I will ask my buddy who has more experience with this type of thing.

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[electric1]

Originally Posted by s/v Thea

I'm interested in the charge rate @ 3.6V when batteries are 90% or more charged, it must drop off at some point, be nice to see a curve.

Actually 3.6V is the "knee" in CC/CV curve for Thundersky cells. So, when using chargers designed for LA it will be basically a straight line, just like Jallum said. Once they reach 3.6V they shoot up in voltage super fast, that is why most chargers and BMS's don't go to the max voltage, its too much risk.

I charge my 160AH pack every night with 20 Amp Zivan charger. The pack stays between 3.3V-3.6V per cell while charging at 20 Amps for several hours ( 4-7 hours, depending on how much it was depleted during the day ). Then once it reaches 3.6V per cell it would shoot up to 3.8V HVC level within 2 minutes if current is not reduced. Zivan is programmed to switch to CV phase at 3.6V , so current is dropping super fast after 3.6V and BMS is there as insurance policy in case charger doesn't slow down fast enough. In my usual routine I don't have HVC tripping at all, but in your case of using LA chargers HVC is virtually impossible and your charging curve will be essentially a straight line.

[jallum]

Originally Posted by s/v Thea

I'm interested in the charge rate @ 3.6V when batteries are 90% or more charged, it must drop off at some point, be nice to see a curve.

You can grab the info for any cells you may be considering from the thundersky site:

http://www.thunder-sky.com

...and by all means ask them questions. I've found their support people to be pretty responsive, allowing for timezone differences.

[hellosailor]

Is everyone sure that "LFP" and "LiFePO4" are the same thing?

And last I'd heard, GM had chosen conventional LiOn batteries made by LGChem for the volt. Not LFP, not LiFePO4, but conventional LiOn. Same as Tesla. ( Well, not quite the same, since Tesla uses, what 3200 "AA" cells? )

[imcbride]

Originally Posted by hellosailor

Is everyone sure that "LFP" and "LiFePO4" are the same thing?

Again with a Wikipedia reference, but consensus is yes. Lithium iron phosphate battery - Wikipedia, the free encyclopedia

And last I'd heard, GM had chosen conventional LiOn batteries made by LGChem for the volt.

You're right, announced Jan/09. Only Lishen (Chinese) is making EVs with LFP. Here's a short article with a table of the production EV cars and their chosen battery chemistry. Even Chevy Volt Battery Suppliers Surprised at GM’s Volt Aggressiveness

Not LFP, not LiFePO4, but conventional LiOn. Same as Tesla. (Well, not quite the same, since Tesla uses, what 3200 "AA" cells? )

While in general here we've been discussing large capacity prismatic cells, there are also cylindrical LFP cells that are designed with significantly higher current ratings. They're used by EV folks for propulsion, and also for their small size in R/C planes, etc.

Iain

[OceanPlanet]

Hi folks,

Wow, what a great discussion on BMS issues! I haven't been able to read all the excellent posts, though I hope to soon (especially those by electric1). However, I wanted to point out that the Genasun BMS was designed specifically for marine use by an experienced sailor (Alex MeVey) who also happens to be an MIT guy.

He has been at this for a while, and originally designed BMS systems for lithium-cobalt cells which proved to be a daunting task and risky chemistry for marine use. Several of the issues that have come up here have occurred in real-world instances, so he has learned from experience.

Regarding the issue of preventing damage to the alternator when a BMS cuts off a charge buss contactor...(note that the Genasun BMS controlss both charging side and and discharging busses, so there are two contactors)...the Genasun BMS has an on-board switch for the alternator field current. In an over-voltage situation (regulator malfunction, or whatever) this allows the BMS to open the field current circuit BEFORE cutting the charge-side contactor.

Also, something to consider is that most BMS's out there only balance at or near the top of the charging cycle. The Genasun BMS (and also the Mastervolt I believe) can activate the shunting for balancing at any time...discharging, charging, sitting still, whenever. This is a good thing for marine use where we might rarely do manually-controlled balancing charging procedures. Since the shunting ability of the cell-mounted diodes is limited, one shouldn't pump in too much current at the "balancing voltage" at the very top of the charge cycle, or you might overwhelm what the diodes can shunt off and thus overcharge a cell or cells.

Now, using typical "top-off" balancing BMS boards, the balancing voltage of the TS cells is rather high (approx 16.8V or 4.2V per cell on a 12V battery), so the only way you would get there using a typical marine charger is to use the "equalization" mode. So, if you know how to do that and monitor the current and cell voltages this can be done just fine. However, if you don't want to mess with it, the full-time balancing of the Genasun/Mastervolt/etc type BMS is probably worth the extra $.

Attached is a suggested typical Genasun system wiring diagram. Note that on the diagram there are "circuit breaker-battery disconnects" between the batteries and charge/discharge busses. This is going to be corrected, to be OCP fuses, NOT switches that someone could accidentally open while the alternator is charging (which could result in the aforementioned alternator output-to-nowhere problem).