Do I keep my BMV? When is the BMS good enough?

Interesting … my BMS and BMV are pretty much on par.

The only difference comes in when the BMS sets its own SOC, on volts. Normally when charging, BMS SOC is higher than BMV. But in the end, both end at 100% SOC time and time again.

But AH in and out, should be the same. BMV needs some tuning.

My system is controlled by the BMV, not the BMS SOC.

Just trying to keep it as simple as possible.

I’m hoping/trusting that I will be ok with just the BMS taking care of things because I get low voltage alarms now at 60% SOC and I certainly don’t want to go syncing the 2 every once in a while.

From what I’ve read and understood from you, is that the BMV is the more accurate of the 2, but it begs the question as to why it must then sync to what the BMS is saying (being the less accurate of the 2 and only working on volts for its findings)?

Maybe my thinking is flawed, but it just seems like a redundant piece of kit currently.

That’s always going to depend on what you are comparing it to, and how it is being used.

There are two things you use to determine SOC. The one is purely amp-hour counting, which is subject to some drift (due to accuracy of reading, how often you get a reading, accumulating floating point errors, etc). The second is “waypoints”, voltages that are known to correspond to specific SOCs.

The BMV has exceptionally good Amp-hour counting, but it only has one waypoint: The “charged voltage”. This is when it resets to 100%.

Now a good BMS should be at least as accurate as the BMV in terms of Ah counting, and it should have more than one waypoint for correcting the SOC. To use an example of what I mean by waypoint: If your lowest cell measures below 3.2V (assuming LiFePO4), you are definitely below 50% SoC… not above it. So a good BMS can use such voltage readings – and it has readings of each cell, which the BMV doesn’t have – to correct the SOC estimate.

On top of that, the usage pattern makes a difference. If the built-in current sensor in your battery cannot accurately measure below 4A, for example, loads below 200W for extended periods of time go unnoticed. Your SOC estimate will tend to drift upward. There are actual well-known commercially-available batteries that have trouble measuring below 4A. I will not mention them, to protect the guilty.

With all that said: The bulk of the SOC-estimating work is done using Ah counting. The bit in the middle, between 30% SOC and 80% SOC, that sits in that flat part of the voltage curve, and that is done almost entirely with Ah counting because there isn’t much in the way of useful waypoints. If you recharge the battery to 100% daily, your BMV should be 100% accurate, as there is little time for drift, and it hits that 100% waypoint every day.

But if the battery lives in that space in the middle for days on end, you will always see a difference developing, and in theory the BMS can make adjustments with the additional waypoints that the BMV cannot.

So… what it comes down to is this: If the BMV appears more accurate than your BMS… you need a better BMS :slight_smile:

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For sharing what I thnk I may know …

Jip. like right now. BMS SOC at 77%, BMV at 47% … the volts as you say.
I’m charging hard now.

Now if I assumed the BMS SOC is the one, I would be rather disappointed due to the lack of Ah’s that did not follow when I use it again and the volts fall off the cliff.

As mentioned, my BMS does not read lower than ±30w going into the batt. The BMV on the other hand sees very VERY low wattages going into the batt.
The BMS Ah counting is pretty accurate compared to the BMV overall though.

SOC for BMS, comes from here:

Based on this:

Thank you for an excellent explanation Plonkster.

I am currently in that scenario as a result of winter. Battery does not get recharged to 100% everyday and this has been the case for a month or 2.

What BMS does the Freedom Won Etowers have? Presumably a good one if the price tag is anything to go by?

And yes, my base load during the entire night is less than 200W

So my scenario will change because I am adding more PV very soon to get the battery full during the day, and also adding more battery capacity so my base load will be higher during the night with more things being able to run.

Just wondering if it would really make sense to keep the BMV (when the BMS will do) from a financial perspective as it will be a nice cash boost for my intended expansion.

Personally, from my own experiences in the last few years, having a BMV as a “check” for the SOC is a good tool to have with the big bucks one has spent on them batts.

If you have it already, even better.

My 2 cents.

I believe it has the same BMS as the BSL batteries. It doesn’t have the nice Orion BMS you get in the Lite models. It is a good BMS, but not great. Personal opinion though… I don’t mind if someone wants to tell me different. I have very little experience with those batteries. Pretty much limited to what I learned in testing, and that is usually limited to hard charging- and discharging over a few days.

Nevertheless: A man with a watch always knows the time. A man with two is never sure. Usually attributed to Confucius, though I’m pretty sure that is posthumously.

Similarly, unless the SOC from the battery differs significantly from the BMV, to the extent that it is a problem, I wouldn’t bother with keeping the BMV.

As an aside: In Venus 3.00 there is a new improvement, where in a system that has a BMV or SmartShunt in addition to a CAN-bus BMS, we broadcast the SOC estimate of the BMV/SmartShunt back to the BMS. Some BMSes has the ability to use that to correct their own estimate. Also, if you run a Batrium BMS without its own SmartMon shunt, this allows the Batrium to know the SOC too.

If it’s the same as the BSL, it’s the PACE BMS, which is the same as the Greenrich and Hubble. The firmware does differ slightly between manufacturers.

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Yep its the Pace. As generic as one can get. Crappy balancer.

I would not call it crappy, it’s absolutely perfect if you start out with balanced or nearly balanced cells. If it’s inadequate the problem is not with the balancer…

Sure, but its a slow passive balancer. There are much better/faster ones out there. And the FreedomWon eTower is pretty much the most expensive option of the stackable batteries.

So here are my questions:
Who knows what BMS is in their banks?
And what quality of results?
And after 2/3/5 - 10 years?

Firstly, you don’t need any faster balancing unless your battery is already beyond saving.
Secondly, if you are going to make a claim like that, you are going to have to qualify it. What do you consider a better battery and why? I agree that the etower is overpriced, you can get better value for money, but the BMS is not the reason to dismiss this battery.

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I mean, FreedomWon dont use this BMS in any one of their other batteries. On mine, I generally get a runway cell when getting to 100% charge. Sometimes it goes over 3.6v sometimes it doesn’t, but it takes forever to come down again. On the 2 batteries the BMS’s are not even in the same place, one is moved about 2cm over to the side, which is not a failing of the BMS but a tolerance that should have been catered for when buying the most expensive stackable battery option.

When I bought my second battery, it was not drama free. It was RMA’ed (BMS issue), second one, Freedom struggled for some time to get them to balance. Even when both on 100% it would cause one cell to spike to the point of shutting the battery down. All sorted in the end, but it took many of call, emails and tweaks from their end.

While I am happy with my eTowers, they just work, if I had to do it over, I would have probably gone for Pylon or BYD.

Heads-up guys: Thread split.

Once the battery settles down, the exact tech used in the BMS or how fast it balances becomes of secondary importance.

I have a brand new FreedomWon at the moment sitting with a lowest cell at 3.3V and the highest cell at 3.75V. Now since this has an OrionBMS, the BMS adapts and brings down the Charge Voltage to accommodate that high cell, but on the cheaper units you have to do that yourself. Once it has balanced out… everyone forgets about it.

It does have an impact on the initial “quality” feel and what people tell their friends about their experience with the brand though. So cannot be completely dismissed.

I hear that one of the large Lithium makers (in my home town) are moving towards paying more for cells that are already balanced, because having to balance them yourself takes time and hampers the throughput of the factory.

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I am adding a 3rd Etower shortly that has already been in use for about a month. My current two has been running for about 3 months now.

How would I best accomplish this? Get the “theoretical” SOC according to the BMS’s the same before I couple all three together and leave it on keep batteries charged for a few days?

In that case, it is already balanced.

The issue isn’t adding another module in parallel. Even if the SOC is a little different. The module with the lower SOC will naturally absorb more of the charge current and the SOC will balance out fairly quickly.

The issue is a brand new module where the cells in the series string is not perfectly balanced. Then one cell tends to jump out, and if it jumps out too much, typically above 3.6V, then the entire module disconnects, raises alarms, warnings, scares the customer, etc etc.

Since the module has been in use for some time, it should be internally balanced already.

Marvelous, thank you!

yeah, so with min, we could only stop the battery from tripping out on a cell overvoltage was by dropping the 2 packs to 10%. I do have a spikey cell from time to time but not enough to set off an alarm.

A friend of mine just installed 2 new eTowers and looks like he will need to do the same.

Edit: Updated the picture. The balancer is not coping, and funny enough the high/low is on the same battery.