Please recommend a good BMS for LifePO4 batteries

New to renewable energy? Have questions? Here's a great place to ask them and view information about wind and solar power basics.

Re: Please recommend a good BMS for LifePO4 batteries

Postby offgridQLD » Fri Apr 11, 2014 8:48 pm

Well my view is watts, whrs and kwh are much more accurate way to measure output and consumption . Particularly considering battery's have a voltage spread across there SOC range. Battery at 50 volts X 10amps for one hr = 10ah. battery at 56 volts x 10 amps for one hr = 10Ah. In reality one is 0.5kwh and the other is 0.56kwh. more than 10% difference.

I don't think anyone will flame you for using KWH on EM.

I use AH when referring to a particular battery spec as people know what battery you are talking about . I guess when referring to loads through cables and so on to put some kw numbers in perspective. All consumption in and out I use kWh.Though that's just me. I'm not bothered what people use its up to them.

Regarding the battery size for your system in the end I guess you just have to work out how much you want to consume each night and on average how low DOD you want to cycle them to. Find some middle ground to be on the safe side Until the chemistry proves its self to you. What I was getting at with you having the grid is DOD might have some flexibility in real bad PV conditions. If it was a off grid system it might result in Now water pump, hot beer and candles or diesel at $1.65L. You might have a bit more flexability given you have the grid and its a secondary system.

Kurt
offgridQLD
Solar Crusader
Solar Crusader
 
Posts: 2117
Joined: Thu Jan 26, 2012 11:50 am
Location: South Australia, Fleurieu Peninsula

Re: Please recommend a good BMS for LifePO4 batteries

Postby Whitespeed » Sat Apr 12, 2014 9:04 am

This is all true,

Our house uses on average, 12kwh per day. When we go off grid I have no problem buying a set of 1000ah cells. These would be perfect for us, allowing two full days of normal consumption without sun.

With both off and on grid panels together we have 6.5kw, this should come very close to covering us (maybe a couple more kw just cause I can)

I have to be honest the temptation to buy at least 400ah now is strong. If I was confident they behaved in a proven way like Pb and the less I discharged them the longer they last I would have no problem laying out the extra.

Given that the general consensus appeares to be that they might have a "Best before" date of around 8-10 years regardless of DOD (within reason) I think that best value for me will be the 200ah cells, this way I can just take my 5kwh per day with the occasional dip to 7 if needed. With the expectation of replacement when we disconnect.

As you have stated, my situation is different to the average off grid. I have grid as backup and can take whatever I choose from the batteries. This is an educational tool for us that happens to pay for itself (thanks to generous feed in tariff) To teach us the fundamentals of off grid living and how the technology operates in detail.

Thank you all for the good advice on the subject. Though I still need some info on best BMS, with EV works offering at least two that sound like they might do the job, and EV power offering others.

As a Start, what is the difference between latching and non latching BMS and what does that mean to my system.

What features should I be looking for when comparing, that will benifit me off grid (not in an EV)
Whitespeed
Solar Crusader
Solar Crusader
 
Posts: 172
Joined: Sat Apr 05, 2014 9:16 pm
Location: Central Vic

Re: Please recommend a good BMS for LifePO4 batteries

Postby Gordon-Loomberah » Sat Apr 12, 2014 9:16 am

Latching in a contactor just means no power is required in the on position, ie if they are on and you disconnect the power to the relay electronics (BMS/BCU), they stay on.

I got through winter with an average daily use of about 12kWh (DC, so ~85% of that in AC kWh) with 3.5kW of PV (tracked), with no generator use. If you have 6.5kW of PV, that would be easy, assuming your clouds aren't much different to mine ;)
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5763
Joined: Thu Sep 24, 2009 9:41 pm
Location: Loomberah NSW Australia

Re: Please recommend a good BMS for LifePO4 batteries

Postby Whitespeed » Sat Apr 12, 2014 12:12 pm

So given that my power should never be disconnected, this is not a feature I should need to worry about ?Anything else I should be looking for in the BMS's that the locals supply.
Whitespeed
Solar Crusader
Solar Crusader
 
Posts: 172
Joined: Sat Apr 05, 2014 9:16 pm
Location: Central Vic

Re: Please recommend a good BMS for LifePO4 batteries

Postby Gordon-Loomberah » Sat Apr 12, 2014 12:35 pm

I think the main feature is that it isn't draining your batteries when it is on. I take my BMS energising power from the battery side of the contactor to ensure control under all situations- that allows a re-connect when any fault condition resolves- HVD/LVD etc.

The main actions the BMS needs to do are HVD and LVD, but only as backups to your primary means of HVD and LVD, and disconnection of the battery when an individual cell goes high or low voltage.

The "fuel gauge" display is quite handy for a quick check of your SOC. As I've written previously, I had to do a fair bit of experimentation to get mine to function correctly with an 800AH battery, as it was only desigend for up to 511AH (which seems a rather odd number!).

IMO the various alarm features are of marginal value, as you need to be near the BMS to notice them- unlikely when it is in a power shed some distance from the house.

For me, I think a multi-channel relay driver could be configured to perform the useful (for me) actions of a BMS.
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5763
Joined: Thu Sep 24, 2009 9:41 pm
Location: Loomberah NSW Australia

Re: Please recommend a good BMS for LifePO4 batteries

Postby Helipos » Sat Apr 12, 2014 11:26 pm

as it was only desigend for up to 511AH (which seems a rather odd number!).


Todays useless fact :)

Its a function of counting from 0 with a 9 bit binary counter.
User avatar
Helipos
Solar Crusader
Solar Crusader
 
Posts: 361
Joined: Tue Oct 06, 2009 9:18 pm

Re: Please recommend a good BMS for LifePO4 batteries

Postby Warpspeed » Sun Apr 13, 2014 7:50 am

One aspect of Lithium cell management nobody has mentioned, is the equalisation of individual cell voltages.

I have no personal experience with Lithium, but I have read that over time, and a large number of charge/discharge cycles, the individual cell voltages can begin to diverge by gradually increasing amounts.

The solution is to fit some kind of active voltage equalisation circuitry across every cell.
Pretty much a giant octupus that constantly shuffles a very small amount of charge between cells to keep all the cell voltages identical.

I am told this is another aspect of Lithium that makes battery management a bit different to lead acid cells, as you cannot just crank up the charging voltage of Lithiums to give them an "equalising" charge as wirth lead acids.

As the terminals to each cell are readily accessible, I can see several technical ways to accomplish this, but have no idea how the commecial units might work, or even if it is necessary at all, which I suspect it might be.
Warpspeed
Solar Crusader
Solar Crusader
 
Posts: 435
Joined: Fri Jan 03, 2014 3:18 pm

Re: Please recommend a good BMS for LifePO4 batteries

Postby offgridQLD » Sun Apr 13, 2014 8:53 am

Yes you more or less hit the nail on the head. The. Common cell top balancers one for each cell just shunt excess power/charge once cells reach a set voltage and give the runt cells a chance to catch up to the high cells without over charging them.

One you have bank that's ballanced it doesn't take much to keep it trimmed and in. Check.

On smaller batterys like the lithium cells I use for electric bikes 10-20ah the cell balancing shunts are not attached to the cells As there just isn't room instead there is a wire going to each cell and the little BMS unit has the blead resisters on its PCB. It,s not as robust as your relying on a spegety of wires.

Kurt
offgridQLD
Solar Crusader
Solar Crusader
 
Posts: 2117
Joined: Thu Jan 26, 2012 11:50 am
Location: South Australia, Fleurieu Peninsula

Re: Please recommend a good BMS for LifePO4 batteries

Postby Gordon-Loomberah » Sun Apr 13, 2014 8:56 am

The balancing is done with individual cell modules, which I'm sure I have pics of in the Lithium thread. They can bypass some of the current charging from each cell, up to 4A in the case of the cell modules I use. Starting at 3.6V, a red LED comes on at 3.65V indicating a significant current, ie not just a few mA, is being shunted by the module. These same modules also indicate to the BCU that high or low voltage conditions exist, and if too low or high, break the daisy chain connection which triggers the BCU to make the contactor disconnect.
The charge current isn't shuffled around as such, just reduced on those cells that are highest, when above 3.6V.
ATM I'm doing some testing with various Absorb voltages and times, and currently have it set it to just under 3.62V/cell, which is working well.

I don't think all the cell modules are precisely identical, so may have to buy a few more to match them a bit more closely, or perhaps just swap them around for some more testing. I'm a bit suspicious of the way the current varies on a few of the cells when in Absorb.
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5763
Joined: Thu Sep 24, 2009 9:41 pm
Location: Loomberah NSW Australia

Re: Please recommend a good BMS for LifePO4 batteries

Postby Warpspeed » Sun Apr 13, 2014 9:21 am

That is most interesting, but I think it could be done much more effectively.

Two methods come to mind, the simplest being to connect a series connected pair of ultra capacitors sequentially across each cell in a random pattern. Significant power would flow into these capacitors from higher voltage cells, and flow out, charging up any lower voltage cells.

The whole thing hops around from cell to cell randomly, eventually bringing every cell to the same exact voltage under all conditions of charge or discharge. The actual switching could be done by relays (horror) or much better electronically with MOSFETs.
There are simple methods to generate a random digital hopping sequence.

Another nicer method would be to use four bridge connected MOSFETs at each cell, to generate a peak to peak square wave voltage at twice the exact cell voltage, and transformer couple every cell onto a common ac bus interconnection with readily available 1:1 pulse transformers.
The higher voltage cells would feed square wave ac power into the common bus.
The switching MOSFETs working across lower voltage cells would act as synchronous rectifiers, directly feeding dc current into the low cells.

All these MOSFETS would be driven synchronously together through opto isolators from a master oscillator.
Power flow through all these bridge connected MOSFETs would be bidirectional and completely automatic with no other control system required.
Power would shuffle continuously around the system as needed, to keep every cell at the exact same voltage.

As each of our precious cells could be worth up to several hundred dollars, I dont see the cost of adding something like this being a significant barrier, especially if it prolongs battery life and lowers maintenance requirements.

The other advantage is, its essentially a zero power loss system, nothing is given up or dissipated to do the balancing, so it can be run continuously requiring very little actual power transfer to be very effective.

Its all just an idea at this stage...........
Warpspeed
Solar Crusader
Solar Crusader
 
Posts: 435
Joined: Fri Jan 03, 2014 3:18 pm

PreviousNext

Return to Q&A - wind and solar power basics

Who is online

Users browsing this forum: Majestic-12 [Bot] and 5 guests

new solar power specials