Lifepo4 replacement quandary

karrak · August 2018

Dave said:

undefined I am tired of servicing the LA batteries every month and was hoping that moving to a LiFeO4 system would be a bit more maintenance free, especially to free me up for traveling. I did not realize it would be this complicated. It sounded like with a good BMS and a well-designed system, you can go through the balancing process up front and then be set for a few years, Now, that doesn't sound too promising.

As you have said "all you do need is a good BMS and a well designed system". This can be simple if you buy a well integrated system like Schneider gear hooked up to a LG battery or Victron LFP batteries and other Victron gear. It becomes a little harder if you want to use equipment that has not been designed to work with the BMS.

The simplest way to do this is using a BMS like I have described in earlier posts that is fully self contained and will disconnect the battery if any cell goes out of it safe operating range and will also keep the battery balanced. With this setup if the inverter and charge controller are properly setup and no other fault occurs the BMS should never have to disconnect the battery.

I am curious about a couple things I've seen here. McGivor is planning on 4 parallel strings. Earlier I have read that a person wants to parallel at most 2 strings, that was for LA batteries, does it not apply to Li?
I was looking at costs for running 2 strings of 180 or 200Ah batteries versus one string of 400 Ah and they don't seem all that different. Is there any good reason not to run one string of the higher capacity batteries? WOuldn't doing so, greatly alleviate balancing problems because each cell would actually have it's own sensor/balancer?
I also don't understand why it's considered OK to have a single sensor for two batteries in parallel. It seems to me that both batteries could be off V in different directions and the sensor could indicate that their combined V was correct even though each cell was incorrect.

You don't wire the cells up as four parallel strings you wire them up as blocks of 4 parallel cells and then make up one string from the blocks of 4 cells. As long as you wire the blocks of 4 cells up so they share the current evenly you won't have any problems.

When 4 cells are paralleled together they become like one cell because all the individual cell have exactly the same voltage across them. There can be an advantage in having cells paralleled up any variations in the individual cells get averaged out over the paralleled cells.

Simon

westbranch · August 2018

When/where did you get yours, it seems the inventor has dropped out of sight...??

mcgivor · August 2018

@BB said
I also have a concern with hacking the temperature sensor to drop output voltage on the Charge Controller in the event of an overcharge. It sort of depends on what is happening with the Solar Charge controller... If the computer lost its mind, then adjusting the temperature sensor will probably not stop the over charging. Also, since charge controllers usually have a series pass transistor of some sort (between the solar array and the battery bank), a failed (shorted) transistor would not be affected by a temp-sensor hack shutdown circuit.

Actually this is a recommended method by Morningstar at least for their controllers, they explain this in detail here's a link https://www.morningstarcorp.com/morningstar-best-practices-battery-chemistry/
they also state it can be done with software. The shutting down of the battery is also discussed which Simon @Karrak, or anyone else for that matter, may find informitive, I'm assuming this information relates to their equipment, as they cannot speak for others.

mcgivor · August 2018

@Dave said, Speaking to Auto Parts Electric, it sounded like it was not a huge deal to make the change to Li, i.e. one did not have to be an electrical engineer to do a good job on a system. Was I being led down the primrose path or are posters here just holding their systems to a higher standard?

There are drop in replacement LiPo4 batteries with intrergated BMS, that could be what is being referred to, there are also larger battery packs available, usually sealed units made up of many small cells in many cases, choosing to go the build it yourself route dose require some work and research to ensure success, that's the stage I'm currently in, the open cell arrangement allows one to be able to at least test individual cells with a meter to confirm the BMS values, replace a bad cell, add or reduce capacity as needed, one has to weigh in all the factors before making a decision IMHO.

Dave · August 2018

> @mcgivor said:
> @Dave said, Speaking to Auto Parts Electric, it sounded like it was not a huge deal to make the change to Li, i.e. one did not have to be an electrical engineer to do a good job on a system. Was I being led down the primrose path or are posters here just holding their systems to a higher standard?
>
> There are drop in replacement LiPo4 batteries with intrergated BMS, that could be what is being referred to, there are also larger battery packs available, usually sealed units made up of many small cells in many cases, choosing to go the build it yourself route dose require some work and research to ensure success, that's the stage I'm currently in, the open cell arrangement allows one to be able to at least test individual cells with a meter to confirm the BMS values, replace a bad cell, add or reduce capacity as needed, one has to weigh in all the factors before making a decision IMHO.

thanks @mcgivor but this was individual batteries and a separate designed to work on a system of that size and characteristics. i wanted to avoid an all-in-one system for the reasons you mentioned, thanks.

I'll try and remember which bms raj recommended and post it.

i am still curios pros/cons of multiple strings versus bigger capacity batteries in single string. one negative for the latter would be greater expense if a battery failed. others?

we don't need huge reserve, 350 usable Ah would be plenty so either option should be viable i think.

Dave · August 2018

i meant "separate bms" but left out bms.

mcgivor · August 2018

just starting said:

When you are looking for all in one solution I believe this fits.

The Electrodacus looks to be a solution, only problem is they seem unavailable, last website information is dated 2016, last Facebook post Jan 2017.

Edit. Took the time today to email Dacian, the developer of the ElectroDacus to ask the question if he was still in business this was his reply

I currently have no stock of SBMS but I will do another Kickstarter around the end of this month or early next month and if that is successful I will build another batch around the end of the year.

For details you can check the user manual here http://electrodacus.com/SBMS120/manual/SBMSmanual.pdf

And latest news on my google+ page here https://plus.google.com/+electrodacus

mcgivor · August 2018

@Dave The choice of smaller cells was mainly due to price per Ah, as Simon points out post #62 it really doesn't matter as the parrallel cells are in blocks so essentially single cells.

just starting · August 2018

I could possibly sell you one I have 3 of each, one of each right now, I'm waiting on the dmppt450

karrak · August 2018

BB. said:

Solar Charge Controllers do not really regulate voltage (on short time scales)--The battery bank does the major voltage regulation (buffering current).

When you disconnect the battery bank, but (for example) leave the DC loads still connected and running, it is certainly possible for the DC bus (without battery) to go out of regulation (high or low, or, hopefully directly to OFF).

For standard operational conditions I agree that the battery plays a large part in the voltage regulation and that the standard charge controller voltage regulation mechanism would probably be too slow to react to the battery being suddenly disconnected. If the output voltage rises to a point where it would damage circuitry within the charge controller there should be an internal emergency shutdown mechanism which would immediately (in a matter of microseconds) shutdown any power coming from the solar panels.

I had a look a quick look at the operation manuals for the Tristar MPPT controllers which warns that damage can be done to the controller if the battery is disconnected while charging, the Outback Flexmax manual doesn't say anything and gives no warnings and the Schneider Conext manual give this rather encouraging message.

"If the solar charge controller is charging when the batteries are disconnected

with a DC disconnect switch, an over-voltage output fault occurs due to the load

on the solar charge controller output being removed. After approximately 10

seconds the solar charge controller LCD goes blank and the solar charge

controller loses power.

Normal operation resumes when the battery is reconnected."

Simon

karrak · August 2018

Dave said:

i am still curios pros/cons of multiple strings versus bigger capacity batteries in single string. one negative for the latter would be greater expense if a battery failed. others?

I think 2PxS is a good configuration as it means you still have a working battery if one cell goes open circuit or looses capacity. It also means that you can easily expand the battery size by 50% rather than doubling it if you wanted to.

Simon

just starting · August 2018

Karrak Wow a 720 ah Winston bank I'm so jealous.Or is it 360?

karrak · August 2018

It is a 360Ah@24V, soon to be 180Ah@48V or around 9.3kWh battery. Enough to run the household including cooking.

Big improvement on the old NiCd battery I used to own!

Simon

Dave Angelini · August 2018

That should get you around 10KWH Simon. Very nice! The Resu I have is about 9 useable and is perfect for us. I would not mind more but for 2 years it has been the cats meow. Still 99% Sol and we have been seriously cycling in this wild fire for nearly a month.

mcgivor · August 2018

The search for a BMS has been tedious, primarily due to the specifications needed, 60A charge 110A discharge, most seem poorly made, based on images, with no heat sink or actual terminals, the one settled on is in attached pdf, any thoughts?

karrak · August 2018

Were you planning on separate charge and discharge disconnects?

Lack of heat sinks can actually be a sign of using MOSFTES with lower resistance and thus not needing the large heat sinks.

From my post in the Endless Spheres forum

"From the voltage drop I measured I calculated the drain source resistance of the original HY3008 FETs to be ~7mOhms, so total resistance of the ten FETs in parallel is 0.7mOhms. This gives a power loss of ~7W with a 100A load."

The board you are looking at says the MOSFET resistance is <20mOhms.

Simon

mcgivor · August 2018

Wouldn't the number of mosfets have an influence over the overall power loss, it appears to have 56 in total, how they are configured is an unknown, to me at least, this is a new avenue of learning on my behalf.

mcgivor · August 2018

With regards to separate disconnects, the manufacturer will send all wiring information, so I'll take it from there when it arrives as part of a samples package, not sure what's included but the price is right.
Ivor

karrak · August 2018

To stop current in both directions you need two FETs back to back in series as per my earlier post

From the pictures in the pdf you linked to I would say that there is a total of 48 FETS in two blocks of 24.

Each block of 24 FETs are wired in parallel so the resistance of each block would be ~0.8mOhms. The two blocks are wired is series so the total resistance is ~1.6mOhms.

If you split the FET switch into two halves, one blocking the charge current and one blocking the discharge current the current only has to go through one set of FETs so it halves the power dissipation.

Other issues I see with this board are that the connect, disconnect and balance voltages are not programmable and that there is no display or output to a mobile phone or computer so there is no way for you to easily monitor the cell voltages and battery current.

Simon

mcgivor · August 2018

The lack of display and programming ability were questions asked, that's when the question of testing samples was brought up by the seller, couldn't look a gift horse in the mouth, it would be better than nothing, would you not agree.

Ivor

westbranch · August 2018

Ivor, where did you find the extra detail, Model posted is not even listed on their site??

Dave · August 2018

mcgivor said:

The lack of display and programming ability were questions asked, that's when the question of testing samples was brought up by the seller, couldn't look a gift horse in the mouth, it would be better than nothing, would you not agree.

Ivor

, it sounds like you already have a path but I am curious what you and Simon think of this option for 24V:

https://www.electriccarpartscompany.com/24V-256V-288V-Custom-Built-Lithium-LiFePO4-LFP-PCB-PCM

bill von novak · August 2018

karrak said:

mcgivor said:
The use of mechanical relays to disconnect charge and discharge, as apposed to internal mosfets, or other means, would make it reletivaly easy to include a second BMS simply as a monitor of the primary system, interlocked via a second contact in the relay coil circuit. Perhaps I'm over thinking this a little, but redundancy is not nesesarally a bad thing.

The one problem with using mechanical relays is that they use quite allot of power. F

Not if you use a latching relay.

mcgivor · August 2018

westbranch said:

Ivor, where did you find the extra detail, Model posted is not even listed on their site??

What I've done in my search process is ask questions, providing all relevant information pertaining to my basic requirements, voltage, battery capacity and charge/discharge current. Based on this they provide suggestions, then I ask for more details, monitoring ability and so forth, very tedious, some sales staff are quick to answer, other say they need to consult thier engineering department before answering, those are the ones I tend to be more interested in, after all engineers most likely don't answer sales questions directly, unless a one person operation. This particular company didn't have a product filling all my required needs but explained they were confident in their product and offered samples to test and evaluate. They do appear to be a larger OEM supplier which may have products other than those listed on their website, over 200 pages worth, for what that's worth, so in the end I'm only out of pocket for shipment and will continue to research as I progress.

Dave · August 2018

nevermind my last post, i was reading up and this PMC really doesn't do the same job as a BMS.

karrak · August 2018

Dave said:

nevermind my last post, i was reading up and this PMC really doesn't do the same job as a BMS.

I am puzzled, it looks like a BMS to me,does all the functions of a BMS

Monitors individual cell voltages and charging current
Will disconnect battery if cell voltages or charge/discharge current goes out of operational limits
Will keep the battery balanced.

I think it will also output this information to a computer but am not sure about this. Am I missing something?

Simon

Dave · August 2018

> @karrak said:
> Dave said:
>
>
> nevermind my last post, i was reading up and this PMC really doesn't do the same job as a BMS.
>
>
>
>
>
>
> I am puzzled, it looks like a BMS to me,does all the functions of a BMS
>
>
> * Monitors individual cell voltages and charging current
> * Will disconnect battery if cell voltages or charge/discharge current goes out of operational limits
> * Will keep the battery balanced.
>
> I think it will also output this information to a computer but am not sure about this. Am I missing something?
>
>
>
>
> Simon

it makes me feel better that you're confused! they don't call it a bms and i don't know why. it's a less expensive option to their bms
https://www.electriccarpartscompany.com/3V-1S-Lithium-Lighted-Battery-Balancers

but i am struggling to work out what it lacks in comparison because, as you say, it seems pretty comprehensive.

i need to call them again.

karrak · August 2018

The part you linked to is not a BMS, it is the part of the BMS that will balance the battery but will not provide any protection if any cell goes out of its safe operating zone. Using just these balancing circuits alone can give a false sense of security and I am sure have lead to a few if not many damaged batteries.

If you do call them can you ask them

If you as a user can connect the BMS up a computer and what is needed to do so.
Is there an App for the computer that will allow you to monitor the cell voltages, battery current and temperature in real time
Is it possible for the user to reprogram the configuration on the BMS

SImon

karrak · August 2018

bill von novak said:

karrak said:

mcgivor said:
The use of mechanical relays to disconnect charge and discharge, as apposed to internal mosfets, or other means, would make it reletivaly easy to include a second BMS simply as a monitor of the primary system, interlocked via a second contact in the relay coil circuit. Perhaps I'm over thinking this a little, but redundancy is not nesesarally a bad thing.

The one problem with using mechanical relays is that they use quite allot of power. F

Not if you use a latching relay.

To my knowledge the problem is that latching relays need some sort of special interface to operate and make sure they have worked correctly. I have used a latching relay to disconnect the PV panels on one of the systems I have installed. It is being controlled via software I wrote on a Beaglebone Black microcontroller board.

Simon

Dave Angelini · August 2018

Latching relays would have to be robust to support over 100A for a large system. I have never seen one that large. One would still need a way to know it worked unattended. I suppose if you monitor power to the house you would know

Lifepo4 replacement quandary

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