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Cheap LiFePO4 BMS?


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I've been doing lots of research into LiFePO4 based power systems and although I made the case for a LA based system if you're mostly a summer boater, since having plenty of solar would keep your batteries topped up to 100%, I'm actually not at all convinced that LAs are a cheaper solution anymore. LA seems not cheaper than Li per cycle, and maybe not even per kWh if you manage to source cheap second hand LiFePOs.

 

The one comment I've heard from @peterboat @Tom and Bex @Dr Bob and @MoominPapa (our resident Lithium users) is that to make it affordable you need to DIY your BMS (which can mean "battery monitoring system" or "battery management system" depending on how advanced it is...).

 

I've had a look around and I'm really not sure how you came to that conclusion. Here is an off-the-shelf 60A 12V LiFePO BMS for 23 USD. https://www.aliexpress.com/item/32921429106.html

Could someone educate me as to why you need to DIY a BMS when dirt cheap devices like this are available?

 

Is 60A not going to be enough - if not, I gather that you can fit a relay to these units which would allow the BMS to remotely turn on and off a load which you connect directly to the battery, so that the power doesn't have to flow through the BMS.

Or is it that off the shelf BMSes are unreliable or inflexible for boater's needs?


If I was to go for an "expensive" LiFePO4 BMS off the shelf, what device would I be looking at and how much would it cost? I'm just trying to factor in these costs when I'm doing my costings trying to determine the right battery chemistry for me - so a rough idea is fine if you don't know specific models and prices.
 

I apologise for all the questions, I'm trying my best to research all this on my own as well!

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60A is certiainly too low for the boat houseboat application. No support for a bistable relay, so using a relay will cost you a continuous current draw for the coil.

 

Charge termination just disconnects the battery, which risks frying the alternator and leaves the boat loads connected to the alternator or other sources without the battery in circuit, likely frying those too.

 

The voltage limits are too high/low and not adjustable.

 

The balancing function is probably not able to shunt enough current for big cells, and the balancing algorithm is likely based simply on cell voltage at top-of-charge.

 

There's no "fuel gauge" functionality.

 

There's no temperature sensing and alarm function.

 

It's a lot of reading, but this: http://nordkyndesign.com/category/marine-engineering/electrical/lithium-battery-systems/ is the best source I've found on the requirements for this.

 

MP.

 

 

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4 minutes ago, MoominPapa said:

60A is certiainly too low for the boat houseboat application. No support for a bistable relay, so using a relay will cost you a continuous current draw for the coil.

 

Charge termination just disconnects the battery, which risks frying the alternator and leaves the boat loads connected to the alternator or other sources without the battery in circuit, likely frying those too.

 

The voltage limits are too high/low and not adjustable.

 

The balancing function is probably not able to shunt enough current for big cells, and the balancing algorithm is likely based simply on cell voltage at top-of-charge.

 

There's no "fuel gauge" functionality.

 

There's no temperature sensing and alarm function.

 

 

Not a lot going for it then !

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27 minutes ago, MoominPapa said:

60A is certiainly too low for the boat houseboat application. No support for a bistable relay, so using a relay will cost you a continuous current draw for the coil.

 

Charge termination just disconnects the battery, which risks frying the alternator and leaves the boat loads connected to the alternator or other sources without the battery in circuit, likely frying those too.

 

The voltage limits are too high/low and not adjustable.

 

There's no "fuel gauge" functionality.

 

There's no temperature sensing and alarm function.

Thanks for your reply. I envisage this device to only be for top balancing and over/under charge protection. And according to the spec it does temperature protection? Perhaps I misunderstand though - I only want it to shut off charging if temperature is below say 5 degrees.

 

Couldn't I connect relays to disconnect/connect whatever I want when the BMS turns itself off? For example killing all boat loads and dumping the alternator load into a LA sink?

I hadn't considered needing bistable relay support to save current - thanks for that I'll keep it in mind. "Fuel gauge" would have to be managed separately.

 

28 minutes ago, MoominPapa said:

The balancing function is probably not able to shunt enough current for big cells, and the balancing algorithm is likely based simply on cell voltage at top-of-charge.

 

Bottom balancing I would prefer to do manually every couple of months (or more frequently until I get to know the batteries). And I imagine that yes the balancing is slow, but so what?

 

32 minutes ago, MoominPapa said:

It's a lot of reading, but this: http://nordkyndesign.com/category/marine-engineering/electrical/lithium-battery-systems/ is the best source I've found on the requirements for this.

For some reason that website is blocked on my current connection. But I'll check it out, thank you.

 

Do you have any idea how much an off-the-shelf BMS would cost me? Or what models I should look at? I just want to get an idea as to how much this is going to increase the cost of the system over LAs.

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5 minutes ago, ivan&alice said:

Do you have any idea how much an off-the-shelf BMS would cost me?

 

I'm not sure there is such a thing. I've been keeping a weather eye open for one and seen nothing. 

 

I suspect it is because there is no consensus on what it needs to do. Even the max current switching capacity for a boat is debatable. Boater A says 150A would be fine, boater B says 300A is not enough, for example. You say you want to bottom balance every two months, peterboat says he NEVER balances his, for another example.

 

 

 

 

 

 

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Just now, Mike the Boilerman said:

 

I'm not sure there is such a thing. I've been keeping a weather eye open for one and seen nothing. 

 

I suspect it is because there is no consensus on what it needs to do. Even the max current switching capacity for a boat is debatable. Boater A says 150A would be fine, boater B says 300A is not enough, for example. You say you want to bottom balance every two months, peterboat says he NEVER balances his, for another example.

 

Hmm. Perhaps I misunderstood that DIYing a BMS was to make it affordable - perhaps it is actually just because appropriate devices don't exist?

Regarding balancing, I want to do it frequently at first so that I can a) keep an eye on the new system and b) learn as I go. But yes hopefully they won't require balancing nearly that often, especially with an 80/20 charge range.

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1 hour ago, ivan&alice said:

I've been doing lots of research into LiFePO4 based power systems and although I made the case for a LA based system if you're mostly a summer boater, since having plenty of solar would keep your batteries topped up to 100%, I'm actually not at all convinced that LAs are a cheaper solution anymore. LA seems not cheaper than Li per cycle, and maybe not even per kWh if you manage to source cheap second hand LiFePOs.

 

The one comment I've heard from @peterboat @Tom and Bex @Dr Bob and @MoominPapa (our resident Lithium users) is that to make it affordable you need to DIY your BMS (which can mean "battery monitoring system" or "battery management system" depending on how advanced it is...).

 

I've had a look around and I'm really not sure how you came to that conclusion. Here is an off-the-shelf 60A 12V LiFePO BMS for 23 USD. https://www.aliexpress.com/item/32921429106.html

Could someone educate me as to why you need to DIY a BMS when dirt cheap devices like this are available?

 

Is 60A not going to be enough - if not, I gather that you can fit a relay to these units which would allow the BMS to remotely turn on and off a load which you connect directly to the battery, so that the power doesn't have to flow through the BMS.

Or is it that off the shelf BMSes are unreliable or inflexible for boater's needs?


If I was to go for an "expensive" LiFePO4 BMS off the shelf, what device would I be looking at and how much would it cost? I'm just trying to factor in these costs when I'm doing my costings trying to determine the right battery chemistry for me - so a rough idea is fine if you don't know specific models and prices.
 

I apologise for all the questions, I'm trying my best to research all this on my own as well!

The problem here is 'what is a bms'. We cant even decide if it is 'monitoring' or 'management'.

 I agree with MP's assessment.

The 'box' you have identified is not the 'whole' BMS. It is a box that measures voltage and then takes an action if limits are exceeded.....plus a bit of self balancing. To look after your LiFePo4s, you need a bit more than that one box .....hence the need to DIY your own BMS that could include this box.....but I wouldnt as it is only 60A (not enough).

 

MP's system is based on his DIY Arduino brain. It is a great solution but a bit complex for us muppets.

My system is based on multiple pieces viz

1) a manual battery disconnect switch  (£50)

2) a electrically operated battery disconnect switch (500A) ....but they cost (£150). Tyco ones are much cheaper but need an 'activation' circuit

3) a voltage/temp sensor that activates a relay that drives the disconnect switch on over/under conditions (based on the total bank ie 13V). This also has audible alarms. I use my Victron BMV monitor

4) a voltage sensor working at the cell level, monitoring all 4 cells which then activates the disconnect (£80). A cheaper one was identified a few weeks ago.

5) a cell voltage meter showing all 4 cell voltages (with its own audible alarm) as (4) does not have a voltage display. (£25)

6) I also have a separate voltage monitoring board (13V) from amazon of £12 to do additional audible alarms but never fitted it.

7) Some form of termination for charge sources. The MPPT shuts down at what ever voltage I set it at. The Alternator shuts down at whatever setting I choose on my AtoB.

 

The box you identified does a mix of 2 and 3 above but is not high enough power and needs a bigger number 2, and it looks like the over and under configuration is not good enough.

 

My system does not have inbuilt balancing....so I think of my system as a monitoring system. MP's does balancing so his is a real management system.

You build your BMS from a number of different boxes. All mine are available on t'internet.

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6 minutes ago, Mike the Boilerman said:

 

I'm not sure there is such a thing. I've been keeping a weather eye open for one and seen nothing.

.....and you will not find one unless you go to the commercial systems which have to be muppet proof under ANY condition, ie what you get with an electric car. Once you understand what has to be done, you can design your own system (multiple 'boxes') to monitor and react.

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5 minutes ago, Dr Bob said:

1) a manual battery disconnect switch  (£50)

2) a electrically operated battery disconnect switch (500A) ....but they cost (£150). Tyco ones are much cheaper but need an 'activation' circuit

3) a voltage/temp sensor that activates a relay that drives the disconnect switch on over/under conditions (based on the total bank ie 13V). This also has audible alarms. I use my Victron BMV monitor

4) a voltage sensor working at the cell level, monitoring all 4 cells which then activates the disconnect (£80). A cheaper one was identified a few weeks ago.

5) a cell voltage meter showing all 4 cell voltages (with its own audible alarm) as (4) does not have a voltage display. (£25)

6) I also have a separate voltage monitoring board (13V) from amazon of £12 to do additional audible alarms but never fitted it.

7) Some form of termination for charge sources. The MPPT shuts down at what ever voltage I set it at. The Alternator shuts down at whatever setting I choose on my AtoB.

 

The box you identified does a mix of 2 and 3 above but is not high enough power and needs a bigger number 2, and it looks like the over and under configuration is not good enough.

For (2) I'm still not entirely sure why you can't use that little BMS board to drive a relay (or a few relays) for over/under protection, turning off and on whatever you want?

 

Also I think the main thing that this little BMS board is for is to do (4), which is to activate the disconnect if any individual cell drops down too low in voltage. Without 4 you might end up overdischarging an individual cell, which is the primary thing you are trying to avoid with LiFePOs, right?

I'd consider the temperature sensor an optional but recommended. And if you have victron kit I understand it's just as simple as sticking it on a battery and connecting it to the MPPT WiFi.

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Balancing the cells is an issue.

I ran mine for 3 months never going above 80% SoC. No problem with balance. I then took them up to 100% using a battery charger (to reset amp hour counter) and found 2 cells well out of balance. Spent a few days top rebalancing. Back out for a couple of months and I now see I am a bit out of balance on one cell at the bottom end....but no issues between 30-90% SoC. Likely I will hook up to a 240V line for a few days to look at top rebalancing again just to see how  bad the balance is. If you stay between 30 and 80% ALWAYS then you will never need to rebalance as the inbalance is only important as the voltage gets into the knees at the top and bottome of the voltage curve. That's why Peterboat never has a balance issue.

MP solves it by a clever rebalance circuit.

I reckon I will spend maybe four hours twice a year just tinkering with the balancing this year and then probably not bother after that.

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Is it fair to say then (adding up those numbers) that a DIY BMS will cost me around 320 GBP? That's a fair amount but not enough to make Lis completely uncompetitive...

1 minute ago, Dr Bob said:

If you stay between 30 and 80% ALWAYS then you will never need to rebalance as the inbalance is only important as the voltage gets into the knees at the top and bottome of the voltage curve. That's why Peterboat never has a balance issue.

MP solves it by a clever rebalance circuit.

I reckon I will spend maybe four hours twice a year just tinkering with the balancing this year and then probably not bother after that.

Yeah, so I plan to design the system to stay between 20% and 80%, but of course that leaves you with only 60% usable charge which starts making Li's look a lot more like LAs.

 

I'm OK to manually balance often while I'm learning and then leave it up to my BMS once I'm satisfied that it's stable under my normal operating conditions.

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1 minute ago, ivan&alice said:

For (2) I'm still not entirely sure why you can't use that little BMS board to drive a relay (or a few relays) for over/under protection, turning off and on whatever you want?

 

 

You can. But apart from my £150 switch, all the high powered switches use latching relays with MP refers to - and they cant be driven from a normal relay.

 

 

 

1 minute ago, ivan&alice said:

Also I think the main thing that this little BMS board is for is to do (4), which is to activate the disconnect if any individual cell drops down too low in voltage. Without 4 you might end up overdischarging an individual cell, which is the primary thing you are trying to avoid with LiFePOs, right?

I'd consider the temperature sensor an optional but recommended. And if you have victron kit I understand it's just as simple as sticking it on a battery and connecting it to the MPPT WiFi.

So yes, the board can be used but it is not much use if you cant set the over or undervoltage to what you want. It looks like the values quoted are too high. There was a box identified a couple of weeks ago that was more configurable. My box 4 has a cell overvoltage cut off of 3.8V which is good.

 

No, I would have a temperature sensor just in case. Mine is done via the BMV.

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16 minutes ago, Mike the Boilerman said:


Boater A says 150A would be fine, boater B says 300A is not enough, for example.

If you're paralleling with a the start battery, it's whatever the starter-motor takes on a cold morning when the oil is thick, which is a lot (and possibly a lot more then with a LA battery alone as the battery impedance is lower.)

 

This is a problem on my system because the bistable contactor is marginal in rating for a 300A starter load, and I wanted to fuse the Lithium bank at around 200A anyway. I solved the problem by adding  a starter interlock to the computer. When the start button is pressed, the computer opens the contactor and then enables the starter motor, so it runs on the starter battery only. 10 seconds after the start, the contactor closes again. There are various solutions to this problem, none are perfect, including mine.

 

MP.

 

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2 minutes ago, MoominPapa said:

If you're paralleling with a the start battery, it's whatever the starter-motor takes on a cold morning when the oil is thick, which is a lot (and possibly a lot more then with a LA battery alone as the battery impedance is lower.)

 

This is a problem on my system because the bistable contactor is marginal in rating for a 300A starter load, and I wanted to fuse the Lithium bank at around 200A anyway. I solved the problem by adding  a starter interlock to the computer. When the start button is pressed, the computer opens the contactor and then enables the starter motor, so it runs on the starter battery only. 10 seconds after the start, the contactor closes again. There are various solutions to this problem, none are perfect, including mine.

 

MP.

 

My solution was to use a 500A switch:)

 

8 minutes ago, ivan&alice said:


Is it fair to say then (adding up those numbers) that a DIY BMS will cost me around 320 GBP? That's a fair amount but not enough to make Lis completely uncompetitive...

Yeah, so I plan to design the system to stay between 20% and 80%, but of course that leaves you with only 60% usable charge which starts making Li's look a lot more like LAs.

 

 

Yes you should be able to do it easily for that.

I wouldnt get too hooked up on usable charge. We use 150Ahrs per day and that has to be put back in running the engine for 1, 2 or 3 hours at 50A that the solar doesnt put in. You dont have to do it everyday but we need to run the engine at least 1 hr per day to heat the water.

The cost of Li vs LA must take into account the likely 10 hrs more engine running per week in Nov-Feb on OUR systmem/habits. That is a lot of engine time/diesel over a 5 year period.

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2 hours ago, Mike the Boilerman said:

 

I'm not sure there is such a thing. I've been keeping a weather eye open for one and seen nothing. 

 

I suspect it is because there is no consensus on what it needs to do. Even the max current switching capacity for a boat is debatable. Boater A says 150A would be fine, boater B says 300A is not enough, for example. You say you want to bottom balance every two months, peterboat says he NEVER balances his, for another example.

 

 

 

 

 

 

I don't Mike I have 4 in series on the electric van I checked them the other day which is the first time since I fitted them, all showed 13.8 volts.  The Bms on each battery is connected together so they must communicate? The cells seem to be in balance as well so even with a cheap mppt controller 13.8 volts doesn't seem to stress the batteries and keeps them balanced. I will check them in a year or so to make sure they are okay 

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58 minutes ago, peterboat said:

I don't Mike I have 4 in series on the electric van I checked them the other day which is the first time since I fitted them, all showed 13.8 volts.  The Bms on each battery is connected together so they must communicate? The cells seem to be in balance as well so even with a cheap mppt controller 13.8 volts doesn't seem to stress the batteries and keeps them balanced. I will check them in a year or so to make sure they are okay 

So your batteries have built in BMS's. A bit like the ones that Sterling offer, (at £1200 for 100Ah!!!)?

 

https://sterling-power.com/products/lithium-batteries?_pos=1&_sid=8012b349e&_ss=r&variant=50393785109

 

Are they just charged directly from the charger/alternator, or is there some other kind of gizmo in between?

 

Or is this a bespoke system that came with the van..... a bit like you might get with a Nissan Leaf or a Tesla?  

 

 

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4 hours ago, ivan&alice said:


Is it fair to say then (adding up those numbers) that a DIY BMS will cost me around 320 GBP? That's a fair amount but not enough to make Lis completely uncompetitive...

Yeah, so I plan to design the system to stay between 20% and 80%, but of course that leaves you with only 60% usable charge which starts making Li's look a lot more like LAs.

 

I'm OK to manually balance often while I'm learning and then leave it up to my BMS once I'm satisfied that it's stable under my normal operating conditions.

In amongst all your number crunching just remember that life changes all the time. If you invest heavily in a Li battery set up and decide to sell that particular boat you will not get the money back, ninety percent of boaters will just disregard the extra cost of your choice of batteries, I know I would. There is more to it than just banging a lot of technical kit in the boat. This is one of many reasons I am continuing with my tried and tested much lower outlay battery set up until such time as Li or some other set up is truly viable, at present it aint.

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2 hours ago, Richard10002 said:

So your batteries have built in BMS's. A bit like the ones that Sterling offer, (at £1200 for 100Ah!!!)?

 

https://sterling-power.com/products/lithium-batteries?_pos=1&_sid=8012b349e&_ss=r&variant=50393785109

 

Are they just charged directly from the charger/alternator, or is there some other kind of gizmo in between?

 

Or is this a bespoke system that came with the van..... a bit like you might get with a Nissan Leaf or a Tesla?  

 

 

I have an onboard charger which is set to Gel which the truck was designed for, however it has 900 watts of solar on the roof so it's never plugged in it just charges from the sun! Cheap and clean to run 

Richard this is a full electric truck no engine in it

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3 hours ago, peterboat said:

I have an onboard charger which is set to Gel which the truck was designed for, however it has 900 watts of solar on the roof so it's never plugged in it just charges from the sun! Cheap and clean to run 

Richard this is a full electric truck no engine in it

Yes, I think we knew it is all electric. From what you say, if they need no attention, the batteries must each have a built in BMS which seems to control things fine, and the Gel setting is ideal for them.

4 hours ago, mrsmelly said:

In amongst all your number crunching just remember that life changes all the time. If you invest heavily in a Li battery set up and decide to sell that particular boat you will not get the money back, ninety percent of boaters will just disregard the extra cost of your choice of batteries, I know I would. There is more to it than just banging a lot of technical kit in the boat. This is one of many reasons I am continuing with my tried and tested much lower outlay battery set up until such time as Li or some other set up is truly viable, at present it aint.

I've tried Trojan T105s, and Rolls batteries, and have destroyed both, (sulphated), in about 2 years each :(. This despite being incredibly anal about battery charging - getting them to 100% at least every 2 days, and usually every day :( 

 

So I'm going to revert to using the "Smelly battery buying and managing system" when I buy my next set, probably some time next Spring :) 

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8 minutes ago, Richard10002 said:

Yes, I think we knew it is all electric. From what you say, if they need no attention, the batteries must each have a built in BMS which seems to control things fine, and the Gel setting is ideal for them.

They do have an inbuilt BMS the problem is no one knows whether its live or not it was designed to work with a master controlling BMS which you cannot buy/ I can read the battery with the puter and all I ever see is batteries balanced, but I never take my batteries above 80% because that gives them 5000 cycles plus of life, also the new price of these batteries is 2.5K and I have 40 of them so I do like to take care of them

I only mentioned the electric bit because you said alternator

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