BMS Decision Variables (+ 1 Quick Q re Battery Spacing and Safety)

[dmcdetroit]

We inherited a DIY'd solar system when we bought a Sprinter 2500, and while documenting and moving the system to the rear of the vehicle, we discovered that the system is missing a battery management system. There is a lot of BMS advice here, but everyone seems to put their systems together differently, with different goals and requirements, and we find ourselves a bit stymied about how to make this decision in the context of the system we are working with.

Our cells are LiFePO4 3.2V, 135Ah, configured in a paired 8-battery array yielding 270Ah. You can see from the red-dot labels on the photo that the battery pairs at full charge are slightly over 3.2V each (3.37, 3.41, 3.33, 3.43).



Our questions:

1. What size BMS do we need for this configuration (and what variables are important in this decision)?
2. Does our configuration indicate that we need more than one BMS, as some posts suggest? On what would we base this decision?
3. What are the other important variables that we should consider?
4. Do we have to equalize the battery loads before we hook up the BMS? and/or should we first discharge the batteries until they are below 3.2V, then turn on the BMS?

One final safety question: Where can we find information on spacing requirements for LiFePO4 batteries? We inherited the existing battery spacing, which is governed by DIY copper strapping, and found a Home Depot crate that worked nicely as a battery box for that configuration. (See above photo.)

Then we were advised that strapping the cells together would produce overheating and we needed cooling space between each individual cell to avoid a fire. We had not heard this before, and could not find any photos of a battery array that showed spacing between the individual cells. But if our configuration is unsafe, we want to fix it now, before we finalize the new installation. How have you handled this in your systems?

Thanks, all. We really want to get off-grid safely — and soon!

 

[Craig]

The size of the BMS would depend as much as your inverters potential to pull from the batteries as it does the batteries capacity. What are your inverters specs?

Since you are in a parallel-series configuration 1 BMS is all you need.
I think the BMS would balance the cells in time without prior balancing.

these cells can be contained without a gap between them as long as you use them within their intended discharge rate. Again this would depend on your inverter.

 

[dmcdetroit]

Thank you very much. The inverter is a pure sine wave with 1500w continuous and 3000 watt surge.

 

[Dzl]

To the best of my understanding:

Ah, excellent! This is a great example of information that is non-obvious to someone who does not already know what to ask. If I may test your patience a bit further ...

1. When you say "in one direction," do you mean the maximum current available for an aggregate of all loads (AC and DC, including the DC used by the inverter)? Does this mean I would need to know -- before choosing a BMS -- the current required by every load I intend to connect to the system (water pump, refrigerator, coffee grinder, microwave, fan, etc.)?

As I understand it, the BMS has to be rated for the maximum current that will pass through it at a given time (in most cases this will be discharge current: AC + DC loads + inverter conversion losses). So yes, you need to account for every load that might be running at one time.

The easiest way to calculate total AC current is to take the continuous power rating of your inverter / battery voltage / 0.85 to approximate inverter inefficiency, then add approximate total DC loads to this to get total current.

It is generally believed around here that the FET based commodity BMS' should not be used at 100% of their rated output very often or at all as a lot of these sellers/manufacturers tend to inflate the numbers, sometimes quite substantially.

2. Assuming the answer is yes, is it possible to take that spec -- maximum total current -- along with my other system specs (12V, 270Ah, etc.), and match them up to a commercial BMS offering? In other words, do BMS vendors generally use these specifications as a framework for buyers, so they can decide what meets their system's requirements?

More or less. At least with respect to inline FET based BMS (current limits don't apply in the same way to non-inline BMS' since current doesn't flow through them).

Most FET based BMS', will give a current rating. That can be matched to the max current you calculated.
And they will also state something along the lines of 4S, 8S, 7S-16S. This is shorthand for 4 cells in series, 8 cells in series, and so on. 4S=12v, 8S=24V, 16S=48V.

Its also important to make sure a BMS is compatible with Lifepo4. Many BMS are made for other lithium chemistries, make sure it mentions lifepo4 or 3.2V cells.

To be clear, I am looking for the basic BMS functionality that a couple of members have listed here; i.e., cell-level monitoring capabilities, cell-level low-voltage and high-voltage discharging, and cell balancing.

Thanks in advance. I am frustrated and just want to get my system reconnected and on the road.

A basic Daly BMS will be the simplest, most basic set and forget type BMS that accomplishes all of the above. There are zero user configurable parameters and no way for users to monitor the system or cell level voltages, but it has the basic feature set you list above.

If you have a 12V system, want a somewhat more full featured BMS and can live with a maximum of 120A rating you could also consider the JBD BMS sold by Overkill Solar on amazon and DYKB on Aliexpress. Its more full featured than the Daly, has a bluetooth option, and more user congifurable parameters. Its cheaper on Aliexpress, but I've heard Overkill solar offers good customer support, has a support forum on reddit, and is easier to deal with as a newbie.

Personally I really like the SBMS0, but the learning curve is steeper and the design and setup is much more involved. It has lots of strengths, its very full featured, and its a unique design model. But I don't recommend it to those wanting simplicity.

 

[Dzl]

If any of the above isn't clear or is incomplete, I'm happy to elaborate or clarify.

 

[dmcdetroit]

If any of the above isn't clear or is incomplete, I'm happy to elaborate or clarify.

It seems both clear and parse-able, thanks very much. It’s late here, so fresh eyes tomorrow and then a decision!

 

[Dzl]

One final thing, the BMS' I mentioned above are not necessarily reccomendations, just a couple examples of BMS' that might be suitable for your situation.