LiFePO4 - The future for off-grid battery banks?

[Sunking]

David the off the shel BMS out there now are aimed at the DIY EV market, Currently there is no demand for the RE market. So you are correct the AH capacity is smaller because th eEV market is a much higher voltage application.

From the engineering POV the RE technology or I should say the Off-Grid Battery, is really dysfunctional and make no sense operating at such low voltages. The only justification is the DIY nature of the off-grid nature and the safety factor associated with the lower voltages.

[david3]

david3 you certainly are going to need a BMS or the cells are going to have an unhappy life. Easy to make for cells below 20ah but harder for ones you need for over 700ah. You should consult the manufacturer of the cells see what they recommend. I have noticed they seem a bit overpriced all of them but in the total scheme mabe not that much extra expense. But you are going to have to use one.

Yeah, there's no question about that. A BMS is required. Do you have any suggestions as to which one would be best?

David the off the shel BMS out there now are aimed at the DIY EV market, Currently there is no demand for the RE market. So you are correct the AH capacity is smaller because th eEV market is a much higher voltage application.

From the engineering POV the RE technology or I should say the Off-Grid Battery, is really dysfunctional and make no sense operating at such low voltages. The only justification is the DIY nature of the off-grid nature and the safety factor associated with the lower voltages.

The plan would be to use a 16 cell 48V battery system. It would be great to operate at an even higher voltage, but most of the inverters available operate at 48V max.

To be safe, the BMS should be able to handle 250 amps across the whole 48V battery system, to deal with surges.

I imagine there are at least a few out there that would work, but I'm not sure which ones. It would be nice if there was a nice, neat, shrink wrapped BMS for this sort of application. And if I could dream, it could even come in a case.

[Sunking]

This one might suit your needs

[john p]

here is some information also on ones available here in Australia. I havent used one of theirs as have made my own as mine were only need for small system,,,

http://lithbattoz.com.au/index.php?page=battery-diagnostics

Battery Management and Diagnostics
What is the difference between a Battery Management System (BMS) and the BMI Voltage Management System (VMS)?

A BMS is an electronic circuit board used in conjunction with a lithium ion battery which serves to prevent over charge, over discharge and it provides a balancing function for the cells in a battery pack. By doing this it ensures the longest life and best performance from the battery. For certain chemistries of lithium ion batteries the BMS provides a vital safety function since with over charge, over discharge and cell imbalance are conditions where certain more hazardous chemistries of lithium ion batteries could possibly catch on fire (or even explode under certain circumstances). This discussion is primarily aimed at the only safe chemistry of lithium ion i.e., Lithium Iron Phosphate (LiFePO4). BMI only manufacture safe LiFePO4 batteries. A BMS is especially important where multiple cells are used to construct a much larger battery pack. The BMS manages cell balance during charging and/or discharging as well as having the ability to disconnect the battery from the load when the batteries charge has been fully exhausted. This is very important since LiFePO4 cells can be severely (or even permanently) damaged if excessively over-discharged. Likewise a BMS should disconnect the battery from a charging source when the battery has received a complete charge so as to not become excessively overcharged since this can also lead to a reduction in the life of the battery.



A VMS and BMS both manage and provide the cell balancing function in a battery pack. This is where the similarity ends.


A VMS differs fundamentally from a BMS in that it will not disconnect a battery automatically from external devices connected to it during over-charge or over discharge conditions. It will only provide a signal output or some other visual or audible warning when approaching these conditions. This signal output is used by electric vehicle (EV) manufacturers to interface with their own external control systems in different ways. For example the signal generated by the VMS when the battery becomes fully discharged might be used to drive an external relay or contactor via suitable switching transistors. The relay contacts then disconnect the battery from its load thereby preventing further discharge which could lead to battery damage and thus maintaining long battery life. The signal outputs from the VMS can be used in many different ways and are only limited by the imagination of the EV designer.





The BMI VMS is built into the HPS series of battery packs. We do however supply the VMS in kit form for those experienced battery constructors who would like to assemble their own custom battery. The complete VMS kit includes two circuit boards, interconnecting ribbon cables, temperature sensing thermistor and wiring loom to connect the VMS board to the individual cell terminals. There is no soldering required since all wiring and sensors are terminated with connectors and simply plug into the main VMS circuit board. The second circuit board is optional since it is required only for advanced applications but is useful to have due to the warnings it provides by indicator LED's and warning buzzer. This board is a display board and has two DB9 data connectors fitted so that batteries can be monitored on a computer via an RS-232 data link. A battery data converter and software is available which interfaces the VMS with a PC. Each VMS board connects to and monitors four cells and several VMS boards can be linked together "daisy chain" fashion to accommodate any number of cells. So for example if a 36V battery was to be assembled this would require twelve cells and three VMS boards.



Please enquire if you require additional information about the BMI VMS system.

LiFePO4 Battery Balancers


Lithium Batteries Australia supplies the world's highest quality battery balancer for those who do not want to use a BMS and would prefer to balance their cells via an external balancer rather than a BMS. This balancer is the highest quality unit on the market and is very fast and accurate in its balancing ability due to its high 1.5 amps shunt current while charging. These are a very high quality unit made in an extruded aluminium case. They are made in Europe exclusively for LBA by our european partners.

Balancers are available in all the common battery voltages of 24V, 36V, and 48V. Custom units can be supplied for any quantity of cells upon special order. Please enquire if you have a special request.

mabe this be of help

I have bought cells from this company and have found them to be very good quality and their BMS appear to me to be good quality.
Their products are aimed at all markets not just the EV market.
They also make up batteries from 12 to 144 v with the BMS built in.
Their web site is not the best design and lacks detail

but if you contact them they always helpful


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[david3]

This one might suit your needs

Thanks. It looks like their 48V model only handles 15 cells (vs. 16). And there's not much information about the individual units that would connect to the cells.

It lists a "Mosfet Switch" that can handle 270A, but "Cell Balance Current" is 0 - 1.5A.

here is some information also on ones available here in Australia. I havent used one of theirs as have made my own as mine were only need for small system,,,

http://lithbattoz.com.au/index.php?p...ry-diagnostics

Thanks. Not much in the way of product information there.

[john p]

yes their web site could be better but if you email them they give usually good information

[Naptown]

Josko there is no way on earth you have 6 hours solar isolation in summer .. You are a long long long way north of the equator.. I be supprised even if you have 3hours..

The low voltage sensor is VERY important to protect the batteries.. Even one time you go below 2.2v per cell the battery can go DEAD

What are you thinking at about 70 degrees north the sun is up for almost 6 months
Ok you need a tracker that tracks at 360 degrees and there is no sun in winter but very lond summer days

[Sunking]

What are you thinking at about 70 degrees north the sun is up for almost 6 months
Ok you need a tracker that tracks at 360 degrees and there is no sun in winter but very lond summer days

Well it would require a tracker to capture, but I can only guess what the very low angles will attenuate or filter out the sun.

[Naptown]

didn't say it would produce like a gangbuster just that the days are veeeeery lond at high latitudes during the summer. And I agree atmospheric losses would be substantial

[Sunking]

didn't say it would produce like a gangbuster just that the days are veeeeery lond at high latitudes during the summer. And I agree atmospheric losses would be substantial

Well if I was not so lazy I would look at solar radiance maps up there for the month of July. I know I would not see 24 Kwh/M2 I would suspect maybe 6 Kwh/M2.

I know for the USA Hawaii and Tuscon are the champs regardless of the time of year. Never really took interest to look at Alaska.

Edit note: OK curiosity killed the cat. I looked at the tables for Alaska. Using 2-axis tracking Bettles AK peaks out to 9.5 Kwh/M2 in July. The you have the rest of the year with almost squat December and January = 0 Burrrrrr

[john p]

Obviously im wrong , but then I never go anywhere near that far noth and never likely to.
But it was always my belief that while it was "light" up there it is not sunlight as we normally think of sunlight. I thought it was like endless twilight,with just a very short time in middle of day with useful sunlight.??Does that make any sense?? but no sun "power"" .most of the day nothing to put much energy into a solar panel because the angle of the sun is very low provides light but no power/energy..

[SunElectronics]

We use to have a few of these and it worked great, just as you said the prices are unbelievable compare to the most expensive ones.

[steveg]

steveg,

Any particular reason you went with Seiden over others like Winston or CALB?

Not a lot of time has passed since you posted the information about your setup, but any new updates would be appreciated.

Hi,

I just found them helpful - and were able to ship CIF to me here in NZ.

My updates are on my blog : http://offgridnz.livejournal.com

System working great still - just measured the cells under full charge (sunny today) at 60A charge there is 70mV of voltage between all 16 cells (3.360->3.367).

Steve.

[T1 Terry]

there is 70mV of voltage between all 16 cells (3.360->3.367).

Steve.

Sorry Steve, you can't read accurate cell balance with the cells under 3.45v and the charge rate less than 10 amps, anything below full voltage except for fully discharged voltage is relatively meaningless accept to show a failing cell because the difference will be big. Fully discharged (2.8v or under) is used by a section of the EV people because it gives a more accurate out of capacity reading, saves a cell being driven into reverse charge and killing it.
As far as needing a full time active BMS on an off grid system, why? Monitor the cell voltages with a Jusi Cell Logger 8 does 8 cells and cost about $28, it can record data files you later view on your computer so you can see just how the batteries going and make minor balance adjustments if needed. I have set mine to 30 sec readings and I can record a weeks battery activity at a time. It has an alarm port that can be configured for dump loads to bring high cells down or turn off the charge till the high cell drops back. The dump load system seems to work better because it self balances the cells very quickly and they don't seem to wander much after that.
Here is a sample graph showing 4 cells of a 12v nom. 720Ah battery bank being recharged daily by a 2 kw solar bank.90hr self balance.jpg

T1 Terry

[Mike90250]

It has an alarm port that can be configured for dump loads to bring high cells down or turn off the charge till the high cell drops back. The dump load system seems to work better because it self balances the cells very quickly and they don't seem to wander much after that.

How does a dump load take excess amps out of only a few cells ??