diy bms stuart pittaway

Batteries, Chargers, and Battery Management Systems.
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whatever   100 kW

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diy bms stuart pittaway

Post by whatever » May 24 2020 7:50am

just doing a bit of research on powerwall style battery pack, and came across some really nice diy bms boards, very suitable for ebike applications. Pretty much an cheap alternative to something like Batrium.
Anyone else had any experience with these?

this video shows how they are used
https://www.youtube.com/watch?v=DKz48WeSGlU

this is designers youtube chanel ( stuart pittaway)
https://www.youtube.com/user/spit2k1/videos

here is designers chat page ( for people already using the boards etc)
https://community.openenergymonitor.org ... 11292/1520

john61ct   100 GW

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Re: diy bms stuart pittaway

Post by john61ct » May 24 2020 9:03am

What makes them similar to Batrium, or particularly suited to ebikes?

whatever   100 kW

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Re: diy bms stuart pittaway

Post by whatever » May 25 2020 3:24am

lets say you have 7 lithiums in series, number of parallel in each group can be quite large example: 7s20p ( using 18560 or similar),
each group is individually monitored. Number of groups in series can also be a large number.
Each bms board can control each group ( balance, lvc etc), each board is connected to output monitoring (serial link or wifi), so you have control over each separate group of cells.
Same principle as batrium but much much cheaper solution.
As far as I'm aware theres no system can monitor between parallel cells as yet.

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Re: diy bms stuart pittaway

Post by john61ct » May 25 2020 9:49am

Every BMS does what you are talking about.

I'm not saying Batrium or this guy's BMS aren't great

nor that cheaper ones are good enough.

Just that, in order to compare BMSs, you need to be aware of real differences to distinguish between them.

_____
And so long as cells are joined in a parallel group, they will act as a single cell. They must be atomized in order to reveal any differences in health between individual cells.

That has nothing to do with BMSs, just a fundamental fact.


whatever   100 kW

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Re: diy bms stuart pittaway

Post by whatever » May 25 2020 12:17pm

every bms allows you to adjust/monitor each group of series cells individually....me thinks not.
that is the challenge ahead: how to monitor cells in parallel

john61ct   100 GW

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Re: diy bms stuart pittaway

Post by john61ct » May 25 2020 2:17pm


whatever wrote:every bms allows you to adjust/monitor each group of series cells individually....me thinks not.
Think what you like, does not make you right.

What could you possibly mean by "BMS adjusting cell/groups"?

But monitoring yes, if that is not done at the cell/group level, it's not a BMS, even the $20 ones (try to) do that, inherent in the definition.

> that is the challenge ahead: how to monitor cells in parallel

No, that is not a thing, as I said, so long as they are connected in parallel it can't be done, they are all by definition at the same voltage.

Just buy cells at the Ah capacity you need if you want that.

Or build modules from 1P strings to get the voltage you want,

then parallel those strings into the size AH pack you need.

whatever   100 kW

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Re: diy bms stuart pittaway

Post by whatever » May 26 2020 12:58pm

you could set different variables for each series group, lets say one group has a bad cell, you might want to take it easy on that group and set lvc higher, all sorts of situations could be dealt with.
You say its not possible to monitor parallel cells individually they have to be atomised, yes that is the challenge, how to monitor them without dismantling the pack.
Lets say each cell has a tiny mosfet in the top of the cell ( assume 18650 size), it would be possible to turn off all but one cell in a large parallel group of cells, and monitor that cells condition, or test that cells condition. So its is possible to monitor cells in parallel, how to build such a thing though is a different story, as each mosfet would need a separate connection to turn its mosfet on/off.
It is not that far fetched to concieve of such a method being built. It might be possible to switch the mosfets on/off via some wireless method in near future. It is one of the weak points in battery packs, how to monitor cells in parallel.
Isolating a bad cell is possible using overtemperature on/off valves in the top of the cells ( fairly common), but a bad cell that doesn't get hot is going to be a bit of a problem. It will of course lower capacity of that group, but if it shorts out ( which can happen) it can cause fuses to blow.

john61ct   100 GW

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Re: diy bms stuart pittaway

Post by john61ct » May 27 2020 9:17am

whatever wrote:you could set different variables for each series group, lets say one group has a bad cell, you might want to take it easy on that group and set lvc higher, all sorts of situations could be dealt with..
A "group" means cells in parallel, at 1S voltage.

A string is cells (or groups) in series, higher voltages.

Are you talking here about multiple higher-voltage modules that get combined into a bigger pack

each with their own separate BMS?

> You say its not possible to monitor parallel cells individually they have to be atomised, yes that is the challenge, how to monitor them without dismantling the pack.

No one is trying to do that within a paralleled group. Again, it is only possible to treat the group as if it were a bigger cell.

> Lets say each cell has a tiny mosfet in the top of the cell ( assume 18650 size), it would be possible to turn off all but one cell in a large parallel group of cells, and monitor that cells condition, or test that cells condition.

Sure, it is easy to isolate a cell from the rest of its group.

But you don't have any valid voltage information to spot a problem, trigger that happening.

> So its is possible to monitor cells in parallel

You have only spoken of the isolation mechanism, again, there is no capability of **monitoring** intra-group cell


> how to build such a thing though is a different story, as each mosfet would need a separate connection to turn its mosfet on/off.
It is not that far fetched to concieve of such a method being built. It might be possible to switch the mosfets on/off via some wireless method in near future.

Same again.

I thought we were discussing reality as it is, not dreaming of "what is possible." The ability to do the latter usefully requires a good grasp of the former.

wired vs wireless is an irrelevant separate topic.

> It is one of the weak points in battery packs, how to monitor cells in parallel.

Easily overcome, b going to higher capacity cells, you get many advantages, and not being "blind" to intra-group cell failures is a big one.

> Isolating a bad cell is possible using overtemperature on/off valves in the top of the cells ( fairly common), but a bad cell that doesn't get hot is going to be a bit of a problem. It will of course lower capacity of that group, but if it shorts out ( which can happen) it can cause fuses to blow

The fusible link per cell idea that Tesla has used for years is one approach, based on current.

But a temp sensor per cell?

Remember KISS complexity introduces new problems, reduces reliability.

john61ct   100 GW

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Re: diy bms stuart pittaway

Post by john61ct » May 27 2020 9:19am

In any case, do you know enough to be able to summarize why this fellow's implementation is good compared to other BMS?

What is similar to Bactrium's approach, that is not inherent to all BMS?

whatever   100 kW

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Re: diy bms stuart pittaway

Post by whatever » May 31 2020 3:02am

regarding how to read parallel cells that have gone bad.
Here one guy uses an infrared thermal camera to find which cell has gone bad ( no need to dismantle the packs)
see youtube vid at 7:06 min. This is probably the only way to do it at the moment, without taking pack apart.
Interestingly he cant tell which cell it is using just his hands to feel the heat.
https://www.youtube.com/watch?v=KnXqwiaAxV4

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