Please recommend a good BMS for LifePO4 batteries

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Re: Please recommend a good BMS for LifePO4 batteries

Postby offgridQLD » Sun Apr 13, 2014 10:07 am

I think the KIS rule is needed in a BMS . It needs to be 100% relable . wasting a few watts at the end of absorb when you have several kW charge potential conected to it is trivial. Like i mentioned before once they are setup and initially balanced as a pack the small trim balance at the end of each cycle is only small.

I have taken some pics of a small 12 Channel 36v off cell (remote) BMS and will post it up in a sec and point out the basic components on the board ( I only have basic electronic skills) but you can fill in the gaps and get the idea.

Gordon you will see in the pic there are tiny little SM resistors on each Channel where that have calibrated each one . Room on the board for 6 resistors PR Chanel and you can see how they are all varied on each Chanel.

you can see the thick wire loop is the shunt for the main current limit I think this was a 30A limited BMS/balancer. The two black wires are the negative feed from load to battery that the BMS can interrupt. You can see the white pin plug for the 12 channels for each cell. along with the bleed resistors down the bottom with 470 on them for each Chanel. There is 4 fets on the aluminium block up the top I guess they are used to pull the pin of anything triggers a bms shutdown event , over voltage, under voltage and over temp. Notice how they have tacked a small board onto the side of the main board to fit the last Channel. Typical chines after thought, reminds me of when my daughter writes a birthday card and underestimates her font size having to bend her words around the edge of the card to fit the width :lol:
Image

You can see the tiny surface mount resistors used to trim each Channel they all vary in value and number of resistors so they must have fine tuned it. Space for 6 resistors pr Channel.
Image

Underside of board more bleed resistors and you can see the temperature probe. The BMS is designed to sit ontop of a small 36v battery pack so the temp probe is in direct contact with the cell (not ideal as it's only toching one cell but considering its just a rough temp safety feature its ok.
Image

Out of interest the BMS on my electric car has 88 channels and trims the cells to 5mv each cycle ( even after a 90% DOD Cycle at just 10% SOC they are within 10mv and back to 0mv discrepancy at the end of charge though the diagnostic tool only has a 5mv resolution.

All 88 cells within 5mv (resolution of gauge)
Image

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Re: Please recommend a good BMS for LifePO4 batteries

Postby Gordon-Loomberah » Sun Apr 13, 2014 11:48 am

Interesting little BMS Kurt. Temp probe isn't something I need to worry about though, as I have never noticed my pack to be anything other than at ambient. If I charged at 800A it might get slightly warm though ;)

I've just done a 1.8hr Absorb, and last I checked about 20 mins before it was finished, dV was just under 20mV. It does jump around a bit due to the resolution of the Junsi, but maybe I'll import the log into open Office and smooth it out to see how it looks. During Bulk charging is when dV can go over 100mV, Float is often around 50-60mV, but discharging overnight it is typcially under 20mV.
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Re: Please recommend a good BMS for LifePO4 batteries

Postby offgridQLD » Sun Apr 13, 2014 12:00 pm

20mv sound great and is typically what I shoot for with all my other lithium cells, actualy its about as good as I can get with them (somtimes 12- 15mv) but not often.

The Imiev must have a real nice OEM BMS that or its the Japaneses Toshiba cells or a combination of both as it always blows me away with its balance considering how the cells are treated in a EV. I am happy I have a window into what is going on with the pack on a cell level and keep tabs on it (Mitsubishi usually charge and arm and a leg for this service data) the joys of android tablets and On board diagnostic scanners :D

I guess with your cells Gordon the ultimate aim isn't 0mv balance but to keep the cells within a range and to have the ability to keep tabs on any cells that get away from what you consider normal. 100mv on the low end and 20- 30mv on the upper end . For example If you see one at 300mv on the low end or 80mv on the top end it could indicate its time for some manual intervention.
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Re: Please recommend a good BMS for LifePO4 batteries

Postby offgridQLD » Sun Apr 13, 2014 8:45 pm

I was under the impression you were only absorbing for 20mins or so. Why the long absorb. Was 20mins not giving it enough time to ballance.

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Re: Please recommend a good BMS for LifePO4 batteries

Postby Gordon-Loomberah » Sun Apr 13, 2014 10:17 pm

Drifting a bit OT here, but I was using 24min for a fair while, but at a higher voltage than I'm testing now. Cell pair #3, or the pair of cell balance modules on it, always goes to a higher voltage than the rest before dropping back, and I'm wanting to keep the peak down a bit.

dV did get down to under 10mV at the end of the 1.8hours @28.9V though!
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Re: Please recommend a good BMS for LifePO4 batteries

Postby Whitespeed » Sun Apr 20, 2014 8:45 am

Hi guys, I have been looking at these BMS units on offer to go with my batteries and I appear to be missing something.

What do they actually do except for act as an interface for the signal wire from the cell units. So cell unit detects the problem, cuts the current on the "daisy chain" wire and this drops the relay out.

If I have this right, what am I paying the extra $300 difference between the basic and the better unit for. Surely this is not just for a plastic box and ten LED display and its driver ?
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Re: Please recommend a good BMS for LifePO4 batteries

Postby Gordon-Loomberah » Sun Apr 20, 2014 9:27 am

Indeed, if I did it again, I think I'd just go with cell modules, relay driver and contactor, as suggested earlier:
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Re: Please recommend a good BMS for LifePO4 batteries

Postby Whitespeed » Sun Apr 20, 2014 10:27 am

Thank you,

I had to ask the question as I do not have the grasp on electronics that you have, and as I previously stated I thought I very well may have missed part of their function. The relay driver with relay is only around $100 and my CC will give me a reasonable idea of their SOC through resting voltage.

If I combine this with kw in from the panels and power out from the steplight meter, I should be somewhere close.

Now I just have to be patient and wait for a response from one of the suppliers before battery purchase. Unfortunately he is not so good with emails.
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Re: Please recommend a good BMS for LifePO4 batteries

Postby tom rickard » Thu May 29, 2014 7:10 pm

I went down the same path with looking for a suitable BMS for the lifepo4 batteries. I ended up using the Batrium system. I wanted a system that disconnected the batteries from load in an undervoltage situation, but only disconnected the charge components in an overvoltage situation.

If i'm away, and the charge controller has a fault causing it to overcharge, I don't really want my inverter to be disconnected and turn off the fridge etc..

The other issue I had with some units was that the contactor they used was DC directional, and the anti-arc only works in one current flow. Not ideal if you have an overcharge situation, and your contactor welds itself closed because it's not designed to work in that polarity.

I've gone the other way to a lot of people, I have a 400aH battery bank, and only a 2.5kW PV array. But I have a 100Amp charger, and a 11KV diesel generator. I run my generator on average for 6hours / week. That costs me less than $1000/year, and based on the PV recommendation when I bought the system, would take me around 6 years to break even, and I kept my capital..

There are definitely a lot of ways to skin this cat! Good luck.
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Re: Please recommend a good BMS for LifePO4 batteries

Postby offgridQLD » Thu May 29, 2014 10:20 pm

$6000 + the cost of a 11kv diesel gen $5000 minimum ? $11,000+ over 6 years that would purchase a lot of solar. Almost zero mantanance and zero noise. I'm struggling to see the value .

Perhaps I have overlooked something in your particular situation.

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