How do I Size a BMS for a LiFePO4 Battery Pack

[Calothrix]

Electrical rookie here! I am building a campervan and considering purchasing the 3.2V, 190Ah cells that Will and others have recommended. If I do so, I will construct a 12.8V, 190Ah battery to use as a house battery. I plan on having a 2000-2200 watt inverter but the maximum load on it at any one time would be about 1800 watts for 5 minutes, once per day. I am trying to figure out what size BMS to use with this battery. Eg., specs for the 100A Daly BMS that Will recommends indicate 100A continuous discharge and 200A peak discharge. So, my first question - would the 100A Daly BMS work with this battery. Or another way to ask the question would be, when does a peak discharge become a continuous discharge?.....after 1 sec, 10 sec, 5 min, 10 min, etc.? My second question, is a BMS also rated for the charge side of the equation? In this case, could I charge the battery at over 100A or would the BMS limit the charge to 100A or completely stop charging if the charge rate exceeded 100A? Thanks!

 

[FilterGuy]

A bunch of good questions.

Your inverter can probably peak out at a lot more than the 2200 watt (as an example, the 2000W Victron can surge 3000W). So, if the rest of the components can't supply that you are in danger of frying them. However, since you don't need the higher wattage, you may be able to get away with sizing your other components smaller and using a smaller breaker. I say breaker because with what we are talking about the likelyhood of it popping is higher due to human error (Doing more than planed). You want to be able to reset it.

> maximum load on it at any one time would be about 1800 watts for 5 minutes, once per day.
5 minutes is a long time in electron years. You should consider this steady state for your system sizing. Consequently the BMS will need to handle something greater than 150 amps. (170A would cover the load plus inverter loss). However, do you know if there is any turn-on surge associated with this load. As an example, when you first turn on a 1500W hair dryer, the heating elements are cold and actually present a much lower resistance. Therefore, it will draw a very high current for a few moments when you first turn it on. In most system, this would not even be noticed by the fuse and wiring. However, a good BMS is monitoring the current and will detect the over-current almost instantly. You say the BMS can handle a 200W peak, but the turn on surge might be much higher. The BMS might see the turn-on surge as a short and instantly shut off.

Now comes the real ugly part. @Will Prowse is recommending we don't run the BMS at it's full rating. I have been thinking running at 80% rating would work nicely but in a recent video @Will Prowse recommended a 50-60% number!

That makes if very difficult to find a good 12 volt BMS with all the features you want and can handle the current. I am struggling with this very problem right now and will probably end up putting a breaker on my inverter that will pop before the inverter reaches it's full surge potential.
Even then, I am using two banks of cells in parallel, each with its own 120A BMS.

There has been a lot of chater recently about using the BMS to control the inverter and not running the current through the BMS. (Check out Will's latest video on it.) The situation you are in with the boat probably means that is not a complete solution for you because you are going to have several loads and charge sources that you must shut off.

 

[tictag]

So, my first question - would the 100A Daly BMS work with this battery.

No. You should always spec your system at the rated value of your components - you never know, you might only use 1,000W but then a friend visits, sees a mains outlet and plugs his 3,000W electric toothbrush into it. A 2,000W rated inverter will try to pull 167A from a 12V battery, even your anticipated 1,800W will pull 150A.

Or another way to ask the question would be, when does a peak discharge become a continuous discharge?

Surge current is usually specified for the device e.g. "200A for x seconds", but if it is not, I would consider anything less than a second to be 'surge', anything longer, 'continuous'. For example, a microwave will surge at 3X it's operating power ... but only for about 2-500ms.

My second question, is a BMS also rated for the charge side of the equation?

Yes, but not in the way you're thinking. The BMS will have both a maximum charge rate and a maximum discharge rate, the latter more to protect the BMS, the former more to protect the battery. For example, if your battery specs state a max charge rate of 0.5C then the BMS for a 100AH battery should be set at 50A.

In this case, could I charge the battery at over 100A or would the BMS limit the charge to 100A or completely stop charging if the charge rate exceeded 100A?

You'll need to check the specs but if there is a general "100A" rating, I doubt very much it would be able to charge or discharge beyond that value.


If you are building this battery yourself, there's nothing stopping you building a 24V (nominal) variant. A 100A BMS would happily deliver 2,000W at 24V all day long, and maybe even squeeze a bit more out on the weekends. You'll need a DC-DC converter for 'normal' 12V loads but these are not expensive.

 

[Calothrix]

Thanks guys. Kind of interesting FilterGuy that you used a 1500W hair dryer in your example because that is essentially what my (wife's) largest draw would be. Sounds like I have a few more Will videos to watch!

tictag - Initially, I will be charging solely from my Ford Transit's 230 amp alternator (yeah, I know, that's another topic!) and perhaps later add solar. Can you somehow charge a 24 volt battery with the vehicles 12 volt system? Even if I could, I'm not sure my lights, water pump, etc. will work with 24 volts.

Bottom line - sounds like the 100A BMS ain't gonna cut it! Thanks again guys for your replies.

 

[tictag]

It is very ill-advised to connect even a drop-in replacement Lithium-ion battery directly to a car/van's alternator so you're going to need a DC-DC converter regardless and, yes, 12V to 24V or 48V DC-DC converters are quite common. Victron Orion products jump to mind but there will be other brands.

In order to maintain a 12V system, you might want to consider a higher spec BMS. According to Daly's website, they offer 100A, 200A and 300A product variants. I have no direct experience with such products.

 

[Calothrix]

On my Transit, the alternator charges 2, 70Ah, deep-cycle AGM starting batteries. These batteries are connected to 3, 60A CCP's (customer connection points) to be used for coach power; thus, offering a total (theoretically) of 180 amps for charging. Research on the Ford Transit forum indicates users measuring about 14.6V, and I have seen anywhere from 90A to 150A total coming from the 3 CCP's. Wouldn't these be very good bulk charging parameters for the 190Ah LiFePO4 battery I am considering? Some on the Transit forum have connected a 120A Sterling B to B charger to the CCPs and use this to charge their LiFePO4 coach batteries but I was wondering if the B to B is even needed, since the charging parameters are already what the 190 Ah LiFePO4 battery wants? I'm just a beginner here, but why couldn't a person just connect the LiFePO4 battery to the CCPs (with appropriate fuse in between) and then also include a manual on/off switch? I could start the van, turn the charge switch on, start driving and monitor the battery and before it is overcharged, turn the switch off. This is all probably just wishful thinking on my part but why would this plan not work? Thanks tictag!

 

[FilterGuy]

Kind of interesting FilterGuy that you used a 1500W hair dryer in your example because that is essentially what my (wife's) largest draw would be.

It's not as big of a coincidence as it seems..... 1800W for 5 min once a day sounded a lot like a hair dryer and some lights..... I had to do that math for my wife!! That is why I picked it for the example.

 

[Danny]

It's not as big of a coincidence as it seems..... 1800W for 5 min once a day sounded a lot like a hair dryer and some lights..... I had to do that math for my wife!! That is why I picked it for the example.

My wife's hair dryer is why I'm working on my inverter project too! I got her a "special" boat hair dryer with reduced wattage. I"m working on my 2.0 design now with the BMS bypass. Good post!

 

[tictag]

@Calothrix, there are a number of challenges here.

Firstly 14.6V (or 3.65V per cell) is the theoretical maximum charge voltage for LiFePO4 chemistry before damage occurs but you'll need to check your battery specs to see whether they are rated for such a high voltage. Secondly, an alternator is not a constant current source and LiFePO4 batteries have a very low internal resistance, which means the alternator will just dump as much current as it can into them. Most LiFePO4 batteries can be technically charged at C Amps (i.e. 190A for a 190AH battery) but check your battery specs and your BMS would need to support this (most tend to rated for 100A max). Additionally you would need a minimum of 0AWG cable from the alternator to the battery terminals if you are to avoid a potential electrical fire, preferably bigger to avoid losses. Furthermore, alternators tend to be directly cooled by the engine meaning that they receive minimum cooling at idle, drawing such a current in addition to charging the starter battery(ies) may damage the alternator unless you can somehow directly cool it. And further still, if your coach battery is directly connected to the alternator, it will also be directly connected to the starting battery, meaning that when you crank the engine, many hundreds of amps will be drawn from your coach battery, through potentially under-specified cable and BMS.

A challenge, but not impossible, I guess.

Or you could buy a DC-DC converter.

 

[Calothrix]

Batteries are rated for 3.65V per cell. The spec sheet is conflicting on charge current: one location it shows 1C(190A) and another location .5C(95A). I have a message into the seller for clarification on this......just received reply from seller saying .5C is recommended and 1C is maximum. The below photo can help you better envision the setup I described in my previous post. Basically, from what I have read, the alternator charges the starting batteries and these power the CCPs.

Photos courtesy of “Morey’s In Transit” website. Thanks Morey!

Below are the 3, 60A CCPs on the rear left side of the driver’s seat. These are connected to the 2, 70Ah, deep cycle AGM vehicle starting batteries located under the seat.



Below is the back of the driver’s seat, where the 3 CCPs are combined into one 180A power source and it is protected via a 150A fuse.



I would have a manual on/off switch between the fuse and the house battery which would be about 4' away. So, not sure if I would need a cable as large as 0AWG but I would verify this. This switch would primarily be turned on when the vehicle is moving. I would charge at idle if that was my only option and for only a short period. I have plans to add some cooling fins to the alternator to help prevent overheating. As long as I have some way of monitoring current, volts and LiFePO4 battery temperature, I am not even sure I would need a BMS or B to B charger? The charge current/voltage (~90-150A/~14.6V) that the vehicle would supply is within specs for the battery. I would not let the LiFePO4 battery discharge below 10 % SOC or get above 90% SOC (will have to check the corresponding battery voltages for these specs) and I would only charge/discharge the battery when it is within temperature specs. The main unknown for me is how the vehicle's dual AGM starting batteries are interacting with the house battery when I am charging it.....that is a big "black box" to me! I don't know, maybe I am way out in left field here but does what I am presenting seem reasonable, or am I way off base? Thanks!

 

[tictag]

Copper cable specifications are well established - check here.

The idea of split-charging is not new (check here, for example), but it is not something I would ever recommend (due to the challenges highlighted above).

Good luck with you project!

 

[Calothrix]

tictag, I have to admit, the brain-swell I received when reading the split-charging article you referenced was over-the-top! Of all the research I have done for this campervan, this was the best bang-for-the-buck (or shall I say quid!) read. Thank you my UK friend.....if I ever get over there again, I owe you a pint.....or two!

 

[tictag]

You're welcome. Just note that the article was written for lead-acid batteries, you will need to apply your Lithium-ion knowledge to it.

"Bang-for-your-quid" doesn't quite have the same ring to it!

 

[GmanFree]

Hi all

Great info here. Could I please add, with a upcoming 8x280 eve 3.2v cell build for 4S2P Setup with a total of 560ah at 12v.

What would be the recommended BMS size to suit. I have a 3000w renogy inverter and will be using a induction cooker or air fryer.

This is to power a camper trailer.

Would a 330amp Heltec bms or Daligreen 350amp bms be the correct size to use?

Cheers

 

[FilterGuy]

Hi all

Great info here. Could I please add, with a upcoming 8x280 eve 3.2v cell build for 4S2P Setup with a total of 560ah at 12v.

What would be the recommended BMS size to suit. I have a 3000w renogy inverter and will be using a induction cooker or air fryer.

This is to power a camper trailer.

Would a 330amp Heltec bms or Daligreen 350amp bms be the correct size to use?

Cheers

The amp rating for the BMS is driven by the possible loads that the battery will see..... not the battery size.

The Renogy 3K is probably going to be 85%-90% efficient. I'll use 90%. That means at full load it will draw 3000W/.9= 3333.33W.
If the batteries are low the voltage will be low and will require more current for the same wattage so this will be the worst case. If we use 12V for the battery voltage this means the current will be 3333.33/12 = 278A.

I don't like to drive BMS at their limit so I would look for a minimum of a 300A BMS but ideally, I would use something that can take 278A x 125% = 347A. (This would also help cover for the extra current if the efficentcy is closer to 85%)

 

[GmanFree]

Thanks so much for your comments.

Many have been telling me to get a 250a Daly, it's been so confusing as everyone has such a differing opinion. I thought the 250a would be too small.

Heltec have a 330a common port BMS available locally, would that be an idea choice?

Only think that concerns me is that the heltec have small heat sinks, can a extra heat sink be fitted over or to help with heat?

Many are also telling me it's better to run two separate batteries with dual bms units. But I'm trying to keep things simple.

 

[740GLE]

Keep things simple and upgrade to a 24v system ?

 

[v_green57]

My wife's hair dryer is why I'm working on my inverter project too! I got her a "special" boat hair dryer with reduced wattage. I"m working on my 2.0 design now with the BMS bypass. Good post!

Had to deal with the "significant other blow dryer" issue in 2005 when we set up our off-grid cabin on a Trace DR1524 1500W MSW inverter. Still in use.

Solution: find a "travel" blow dryer that has a 110/230V selector switch. Set it to 230V. Power consumption is halved.

The "blow drying" process now takes a little longer, but the inverter is not stressed. We are on "cabin time" there anyway and don't care if things take a little longer.

Significant other still marvels that she can run a blow dryer at all out in the middle of nowhere.

 

[Gette]

I came across this researching the hell out of designing my system.

I too am trying to figure out best size BMS.

I'm planning 12v (existing RV keep it simple as opposed to 24v?) Lithium upgrade. DIY building qty 2x 4s 280ah 3.2v EVE's (560ah 12v total - 8 cells, 4 per battery)

From what little I know, it seems we are pretty much stuck with qty 2x 4s BMS's for 12v so the BMS can be connected to individual cells.
So if I understand it correctly that will "double" the BMS's rated charge/discharge, correct?

So that being said qty 2x 200amp 4s BMS's (1 on each 4s 12v battery of course) should do it, right with some surge headroom if needed?

I am trying to run my AC (with soft start) off these batteries occasionally (or hair dryer/microwave lol).
Planning on Victron MP II 12/3000 and 400-600 watt solar. But we mainly camp with hookups. But need occasional AC use in paring lots and travel to and from camp grounds (we have pets). I also have Diesel Gen as last resort (scares pets lol)

Also want to control/monitor everything via Victron CerboGX or more then likely Rpi4 with VenusOS and @Louisvdw Victron VenusOS Serial Driver to expand on the BMS's that communicate with Victron (see below).

My current head scratcher is WHICH 2x 4s BMS's (thinking 200amp +). lol JK, JDM, Daley, ANT, etc

I believe REC and Batrium interface directly with Victron (without Louis's awesome driver) but too much $$$$ esp buying 2x.

Am I on the right track? Thoughts/comments?

https://diysolarforum.com/threads/v...kbms-heltec-renogy-tian-ecs.17847/post-205916

 

[Gette]

The amp rating for the BMS is driven by the possible loads that the battery will see..... not the battery size.

The Renogy 3K is probably going to be 85%-90% efficient. I'll use 90%. That means at full load it will draw 3000W/.9= 3333.33W.
If the batteries are low the voltage will be low and will require more current for the same wattage so this will be the worst case. If we use 12V for the battery voltage this means the current will be 3333.33/12 = 278A.

I don't like to drive BMS at their limit so I would look for a minimum of a 300A BMS but ideally, I would use something that can take 278A x 125% = 347A. (This would also help cover for the extra current if the efficentcy is closer to 85%)

Noob here learning so please excuse my confusion...

But I thought 3000w inverters tend to put out less (not more). So would it then be "multiplied" not divided?
3000W x .9 = 2700w. (seems to be more inline with Victron MP (II) 12/3000 anyway which they say is 93%).
"Constant power "2400w, though "peak power" says 5500W (I assume that is split second surge?).

BTW @FilterGuy learning a lot from you posts and docs! Can I buy you a drink?