Li** battery thoughts and testing.

Hi all,
As I know some here are into RC electric flight and other battery powered kit like power tools ...
A friend has asked me to have a look at an electric scooter that apparently runs on 36 volts worth of LiFe cells.
However, it's not working and we are not sure if it's a faulty cell or not but as I've not really played with these multi-cell Li** cells I thought I'd ask the panel for some general tips please?
Now, I understand some of these packs come with the main power leads and also individual monitoring and control (?) connections to each cell but I'm not sure if these would (typically, in case there are variations) be just wires connected to the inter-cell joints or going to some control / monitoring electronics in each cell?
I have also seen the little monitoring gadgets that display the total voltage and the voltage of each cell but they only seem to go up to 7 cells or so. If this battery is 36V and each fully charged LiFe cell 3.6V, we would need a 10 cell device to be able to monitor them all at once?
Or, if this battery was equipped with said monitoring leads, could you simply wire some LEDs (and current limiting resistors) between each cell and then say look for the LED that goes out when you put it on load?
Would it be possible (and advised) to remove (or short) [1] a bad cell from a pack to see if the rest of the scooter was still working ok and replacement packs are pretty expensive and this one is only a few months old?
Cheers, T i m
[1] Or ideally replace of course.
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T i m wrote:

If the battery is less than a year old is there any warranty on the product? check before opening. You might find some internal cell management but these are often omitted in commercial products as a cost saving I'd measure the voltage of each cell after a period of charging and repeat with modest discharge load say C/100 if that does not indicate any differences between cells, move on to a heavier load ideally not the scooter unless you can prop it 100% safely and concentrate on the measurements. I don't think a simple led approach will tell you much unless a cell is almost totally dead. I really don't suggest you short out any type of Li cell although LiFe are meant to be more benign than other Li based ones.
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On Fri, 25 Mar 2016 14:43:44 +0000, Bob Minchin

Ah, good point, I'll check. I can't remember if he said it was only a few months old or has only been used for a few months (but bought 2 years ago etc).

Ok.

Understood.

That was my thought re the test Bob. Sort of a go / no-go.

I was only thinking of doing that if the cell appeared complete o/c (or very high resistance) but not my preferred solution.
I'll report back once I've actually had a chance to see it.
Cheers, T i m
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On Fri, 25 Mar 2016 14:43:44 +0000, Bob Minchin
Update:

Bought directly from China so 'yes' but not cost effective to use.

I have found a controller / monitor and it seems it's that that isn't allowing the output. Short the input and output of that board and you see 36V on the output (it only has 2 main wires with the battery sense wires disconnected).

I did that when we started and all the cells looked 'ok'.

It turns out it wouldn't in this case especially.

So, I now need to split the two boards that make up this controller unit (sandwich) and check what the output device it and if it's failed.
There is a thermal fuse but that tests ok (in situ anyway).
Cheers, T i m
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T i m wrote:

Most likely to be a power FET or sometimes several in parallel. I had reason to buy some of both sexes recently and impressed that there are quite a range of meaty devices available from CPC for a quid each.
If not a fet problem, then maybe some other fault telling the fet not to turn on. Maybe it thinks the cells are flat and protecting them by not turning on.
Good luck
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On Sat, 26 Mar 2016 21:15:52 +0000, Bob Minchin

<snip> >Most likely to be a power FET or sometimes several in parallel.
Hi Bob, I have since taken some pictures of the BCM:
Main controller:
https://dl.dropboxusercontent.com/u/5772409/BCM%201.jpg
The monitoring board (front and back):
https://dl.dropboxusercontent.com/u/5772409/BCM%202.jpg
https://dl.dropboxusercontent.com/u/5772409/BCM%203.jpg

The FETs on there are: https://dl.dropboxusercontent.com/u/5772409/RU75N08R%20datasheet.pdf

That was my worry. I do have one of those little component testers and could de-solder them for testing if they can't be tested in-circuit.
FWIW the FETs are driven by opto-couplers: https://dl.dropboxusercontent.com/u/5772409/EL817%20datasheet.pdf

Yes, it might be that sort of issue but without a cct diagram (and sufficient skill and good eyes) I'm not sure I can work it out myself. ;-(
It looks like it's all referenced around the battery -ve with the board both managing the charge current via the monitoring board (I assume it works en-masse, there isn't anything that can actually affect the charge applied to each cell individually, just start ramping the charge current down if it senses one cell is at full charge?) and enabling the output (as you say), if the battery is above a preset threshold (it was around 40V on charge when we last tested it as a complete unit).

Thanks, I'm going to need some of that to get any further. ;-)
Cheers, T i m
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T i m wrote:

If you can find the data sheet of IC1 to 10 and it is some special purpose IC you might find they have used a standard application circuit from the data sheet which might give you an idea of what is going on.
Also on the Fet board you should be able to drive the output side of he isolators (carefully) to see if the fets switch ok. Gates can be driven via 1k resistor.
After a while effort required these jobs become a balance of how good a friend this chap is and how determined you are not to be beaten by the thing.
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On Sun, 27 Mar 2016 19:30:38 +0100, Bob Minchin

I've had a quick Google from what I can read off the tops and nothing comes up.

I must admit I've been Googling about re testing FETs and they seem to be a bit of a mystic art (for someone who grew up with germanium transistors).

Quite ... and if there weren't so many unknowns I'd be more keen to delve but as you say, it can soon become very time consuming.
FWIW the owner of the scooter would just like it working and I don't suppose would mind spending a few quid to get there. So, I've since seen 'generic' theoretically compatible BMS boards on eBay for less than 20 quid ... assuming they are actually compatible etc.
I'd also say that my patience for such things is less than it used to be, partly because of my eyes, the size of the components (and them being SM) and not having a nice clear workshop setup to do such things in.
Anyway, I'll see if I can summon up some more effort tomorrow but thanks again for your thoughts Bob. ;-)
Cheers, T i m
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T i m wrote:

Tim, I don't know if you are still playing with this but I had an idea to evaluate the monitor cct and by seeing what it does under various conditions, you might be able to check out the fet switch board.
Assemble a string of 9 equal resistors say 330 ohm. somewhere in the middle of that string put a 470 ohm pot, initially set to 330 ohms so that each resistor represents a cell. 10mA through the chain should give 3.3v across each "cell". enough to make the monitor work and not likely to be enough to let the smoke out. Connect the monitor board nodes to each resistor junction and connect to a variable voltage supply ( up to 40v or so) You then can vary the applied voltage and see what the monitor does and if each stage is consistent in operation. Then with the voltage set to "i am happy level ~36v" you can adjust the pot to simulate an over or under voltage cell and again see what the output of the monitor board does. Although you can't drawn any current out of this set up, you should see if the FETS will switch and if anything looks dud.
hth
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On Wed, 30 Mar 2016 12:11:56 +0100, Bob Minchin
Hi Bob,

Sort of. We have a replacement board on order but I would like (OOI) to see if I can fix the old board if possible.

Ok ....

Hey, I wonder if we had the same moment at the same time! I thought of exactly the same thing because I didn't like the idea of playing with live cells.

Understood.

He he.

They are typically on a std header plug so I could probably make up a reasonable test jig quite easily (and with an intermediate plug to allow for different combinations of cells.

My main bench PSU only goes up to 30V but I'm sure I could sort something.

Yup. ;-)

I hadn't thought that far ahead but 'brilliant'. ;-)

Quite.

It may well do Bob (so thanks again). ;-)
The 'good thing' about most of this setup is the symmetry. If you can monitor the goings-on on one channel you can easily also see / check that the next does the same thing. I believe the charging side is working (well, I saw the battery charging and the current ramping down towards the end that I believe is how it works), it's just that the output wasn't ever enabled. So (as you are suggesting and assuming a linear system here) I should be able to at least simulate the load monitoring side and may find either a single channel dead or the actual monitoring side ok and the actual output / switching side dead (excluding the power FETs if the results of my tests / tester are valid).
Cheers, T i m
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Only a few months old? return it, and no do not short out cells even if you are sure its dead. Not sure if the resistor led thing would be of great use, but only on load I'd assume as the duff one if shorted would go out. I'd most certainly talk to the suppliers before you do anything and if possible try to find some source of at least 24 volts to see if the device itself actually works first. Brian
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On Fri, 25 Mar 2016 17:50:47 -0000, "Brian Gaff"
Hi Brian. As mentioned elsewhere we have now actually done some tests and it seems like the battery itself is ok, or at least not dead, accepts charge and can support a two amp load.
We think the problem now lies with the battery controller module that sits inside the battery pack heat shrink and specifically the bit that enables the output, enabled presumably when certain conditions are met, like the battery voltage above a certain threshold.
Unfortunately without a circuit diagram (or even with) I'm not very familiar with the use of fets nor this sort of dual function circuitry (I get the concepts but may not be able to figure out the detail) so I'm not sure where to go next.
I think the position of this board in the negative battery feed makes sense, especially with reference to the cell monitoring role, enabling the output and monitoring and controlling the charge, but it's the bit between that and how it affects the actual output that is my hundred dollar question.
Thinking about it and as the ten way monitoring board connects to the main board by just four connections, the output from that is probably just the summation of the condition of each of the cells individually but I wouldn't know what to expect to see. eg, Is it a go / no-go type output or an analogue one based on the overall voltage (if the latter you wouldn't need the ten way monitoring board I guess)?
I could rig it back up and monitor the four way connection and see if I can see any correlation between charge and the signals on the pins?
Cheers, T i m
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