Lifepo4 replacement quandary

mcgivor · August 2018

@Dave Angelini said, sounds like you (mcgivor) are still thinking this out? Maybe just wait another year and see? I would not jump into this if you question all the work. As I have said, this (homebrew) is not for my clients and I just do not have the time for it.

Yes Dave there are many thoughts of how I'll achieve my goal, there are various ways each with its pros and cons, eventually I'll come up with a plan, then turn theroy into practice so no rush on my behalf, understandably when dealing with clients home brew is an invitation to legal issues should something go wrong, especially in the US.

@Karrak said, The one problem with using mechanical relays is that they use quite allot of power. For your system if you are using the Omron relays I would think you would have to use a G9EC-1 as the main inverter relay and G9EA-1 for the charge controller. Combined these draw ~16W which equates to energy use of ~0.4kWh/day. Maybe a better arrangement would be to use MOSFET switches or can you shut down your inverter and charge controller via external inputs or switches?

Simon, understandably power consumption of the relays is a consideration, I've thought about using SSR which is essentially a mosfet or similar switch but when higher currents are involved they too consume energy. Finding a means to comand the charge controller or inverter to shut down is something I'll look into. The BMS you linked looks promising but limited to 30A, what I need is something in the 100 A range discharge, 60A charge, perhaps they can customize according to needs but as of yet I've not perused that route.

Ivor

mcgivor · August 2018

After searching I came across this https://www.aliexpress.com/store/product/8S-24V-Lifepo4-Battery-PCB-and-BMS-for-electric-motorcycle-and-29-2V-scooter-Lifepo4-back/1379203_32727619771.html?spm=2114.12010615.8148356.13.755c63a7Op8rOQ
Same company as Simon @Karrak linked previously just a larger capacity version at 100A, any thoughts?

Raj174 · August 2018

So you will need 4 of these since this is an 8S BMS? Also, I would guess standard charge/discharge current is about .33C for each 100 amp hour string.

mcgivor · August 2018

Raj174 said:

So you will need 4 of these since this is an 8S BMS?

There will be 32 cells 4P 8S, so 4in parallel becomes one cell, that's my understanding, perhaps I'm missing something, wouldn't be the first time.
Ivor

Raj174 · August 2018

I'm pretty sure that this BMS is designed to manage individual 3.2 volt cells.

Raj174 · August 2018

A BMS that is designed to handle 8S4P is a bit more complicated and a bit more expensive. I was hoping the place where you bought the cells had a solution.

mcgivor · August 2018

The information from the attached pdf posted earlier said this,

NOTE: When referring to a “cell” in this paper, we refer to either single cell or a group of cells connected in parallel to gain higher AH pack capacity. For example, if your pack is configured as

50S2P, it means you have 100 cells, all cells are grouped in pairs, each pair is connected in

parallel, and then all 50 groups connected in series. In this case we refer to a paired group as a

single “cell”, and each BMS module is managing a “cell”, so there would be 50 BMS cell

modules in this pack.

I'll attach pdf again

Raj174 · August 2018

I agree that cells in parallel can be looked at as single cells, but I don't think they can be protected that way. IMO a BMS would need to measure and manage individual cells to be effective.

mcgivor · August 2018

Raj174 said:

A BMS that is designed to handle 8S4P is a bit more complicated and a bit more expensive. I was hoping the place where you bought the cells had a solution.

I did ask, they responded they do not have a 24V nominal BMS, suggesting to look elsewhere, which is not a big deal, their services were excellent I might add, I'm not intimidated, as you said it seems like rocket science at first, but not really once the understanding kicks in, so to speak.

mcgivor · August 2018

So for arguments sake, not that I'm arguing, if 4 cells were in parallel the voltage of the parallel cells would be equal, assuming they are ballanced, they would be a single cell in the eyes of the BMS. I'm having difficulty trying to understand how each cell could be monitored in this configuration. But willing to learn.

karrak · August 2018

mcgivor said:

After searching I came across this https://www.aliexpress.com/store/product/8S-24V-Lifepo4-Battery-PCB-and-BMS-for-electric-motorcycle-and-29-2V-scooter-Lifepo4-back/1379203_32727619771.html?spm=2114.12010615.8148356.13.755c63a7Op8rOQ
Same company as Simon @Karrak linked previously just a larger capacity version at 100A, any thoughts?

Yes that should do the job. If you wire the cells into 4P blocks and then wire the 4P block in series you will only need one BMS. The MOSFETs on these boards must allow for bidirectional current flow so are configured like the bottom diagram. Each of the MOSFETs is separately controlled so that the switch can shutoff the inverter but allow the charge controller to charge the battery and visa versa. For the systems I am installing I have modified the BMS PCB to split the two sets of FETs and put high voltage FETs in the "charge side switch" so that I can disconnect the solar panels on one side and the inverter on the other. The modifications are described in this thread

If you do use this BMS you can hook it up to any computer that will run the Python programming language and run my BMS software.

There are a number of functionally similar BMS boards, some of which have displays and can accommodate higher balance currents. There is a good discussion although rather lengthy which looks at the different Bluetooth BMSs here

Simon

karrak · August 2018

mcgivor said:

So for arguments sake, not that I'm arguing, if 4 cells were in parallel the voltage of the parallel cells would be equal, assuming they are ballanced, they would be a single cell in the eyes of the BMS. I'm having difficulty trying to understand how each cell could be monitored in this configuration. But willing to learn.

You are correct, to the outside world the four individual cells become one. All four cells will have the same voltage across them so by protecting all four as one is the same as protecting each individual cell.

Simon

Raj174 · August 2018

karrak said:

mcgivor said:

So for arguments sake, not that I'm arguing, if 4 cells were in parallel the voltage of the parallel cells would be equal, assuming they are ballanced, they would be a single cell in the eyes of the BMS. I'm having difficulty trying to understand how each cell could be monitored in this configuration. But willing to learn.

You are correct, to the outside world the four individual cells become one. All four cells will have the same voltage across them so by protecting all four as one is the same as protecting each individual cell.

Simon

Thanks for clearing that up Simon, looking at expensive BMS's for multiple parallel strings had me thinking each cell need protection. Sorry for the confusion @mc@mcgivor.

Rick

mcgivor · August 2018

Thank you Simon, I was beginning to question myself into the theroy of running 4 strings of 8 cells series, each with its own BMS paralleled at each end + to+ & - to- to achieve independent monitoring of each cell. But the problem I see with this is the total current would be divided by the number of strings, so each would monitor 25% of the current, so assuming 100A each would handle 25A discharge, should one fault there would be a cascade effect, resulting in the same result with added complexity. Haven't even considered the charging aspect, I like to walk around pondering these questions, one of which is the use of the Chargery BMS, I've come up with a solution without the use of relays, which I will experiment with in the near future. I'm thinking along the lines of double layer protection.

mcgivor · August 2018

Raj174 said:

karrak said:

mcgivor said:

So for arguments sake, not that I'm arguing, if 4 cells were in parallel the voltage of the parallel cells would be equal, assuming they are ballanced, they would be a single cell in the eyes of the BMS. I'm having difficulty trying to understand how each cell could be monitored in this configuration. But willing to learn.

You are correct, to the outside world the four individual cells become one. All four cells will have the same voltage across them so by protecting all four as one is the same as protecting each individual cell.

Simon

Thanks for clearing that up Simon, looking at expensive BMS's for multiple parallel strings had me thinking each cell need protection. Sorry for the confusion @mc@mcgivor.

Rick

Not at all Rick, we're all here to learn from one another, otherwise what would be the point.

mcgivor · August 2018

Been researching all avenues with regards to how to disconnect charge/discharge without causing damage to equipment, one option for the charge side is to use the remote temperature sensor input, which will now be redundant and replace it with a fixed resistance to simulate 25°C then use a resistor in parallel controlled by a relay to fool the controller into thinking the temperature has increased to a value greater than its control parameters thereby reducing voltage below the battery voltage, essentially shutting down the charging. The load side would require the solar to be disconnected before the battery, which powers the controller, to prevent spikes which could potentially damage the controller or loads. A work around would be to disconnect both simultaneously using a linear actuator or solenoid to trip DC circuit breakers using a timer like an Omron H3CR- A which can provide a one shot signal of 0.5 seconds. These are non software solutions, will look into software as well, just not my cup of tea. Curious to hear how others overcome these hurdles, albeit not a very popular subject, for now at least.

just starting · August 2018

Mcgivor, have you looked at a electrodacus sbms it's all integrated so it can shut down at user set points and it can have a external temp sensor at the battery

mcgivor · August 2018

just starting said:

Mcgivor, have you looked at a electrodacus sbms it's all integrated so it can shut down at user set points and it can have a external temp sensor at the battery

No I've not, but will look into it, i leave no stones unturned, thanks for the input

just starting · August 2018

Here is a photo of a sbms40 witch is a 40 amp

mcgivor · August 2018

Yes I've seen these but the last update was about 2 years ago, they were working on a system where the BMS was actually the charge controller, are they still in business ? Sorry I missed the Electrodacus probably because it was in lower case.

Dave · August 2018

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.

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.

Will be out of touch most of the day but wanted to get some thoughts.

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?

just starting · August 2018

LiFePO4 is not fla, in a 100 amp 3.2 v cell there are 5 20ah cells in Parallel in the battery like a Winston or calb cell if you can get a good bms it can put say 200mv per cell for balancing, if you look at my photo that's cells that were not balanced before, I wanted to see how long it took to balance with the 200mv per cell it's looking like less than a year The Electrodacus will balance all cells, with 200 mv if there less than 150mv apart it stops charging when the first cell hits 3.55 and won't turn on until it drops to 3.399 I have my lvdc set for first cell to hit 2.8, also has temp settings to stop charging and discharge if to high or low temp

BB. · August 2018

JS,

Is "... stops charging when the first cell hits 3.55 and won't turn on until it drops to 3.99" supposed to be 2.99 volts?

-Bill

karrak · August 2018

mcgivor said:

Been researching all avenues with regards to how to disconnect charge/discharge without causing damage to equipment, one option for the charge side is to use the remote temperature sensor input, which will now be redundant and replace it with a fixed resistance to simulate 25°C then use a resistor in parallel controlled by a relay to fool the controller into thinking the temperature has increased to a value greater than its control parameters thereby reducing voltage below the battery voltage, essentially shutting down the charging. The load side would require the solar to be disconnected before the battery, which powers the controller, to prevent spikes which could potentially damage the controller or loads. A work around would be to disconnect both simultaneously using a linear actuator or solenoid to trip DC circuit breakers using a timer like an Omron H3CR- A which can provide a one shot signal of 0.5 seconds. These are non software solutions, will look into software as well, just not my cup of tea. Curious to hear how others overcome these hurdles, albeit not a very popular subject, for now at least.

Using the temperature input to the solar controller as you have indicated is a good way to shut it down.

If the charge controller is well designed it should not cause voltage spikes and should not be damaged if the battery is disconnected. I would think that most of the modern MPPT charge controllers work at frequencies of 100kHz or more, working at these frequencies reduces the size of the inductors and capacitors, it also reduces the amount of energy stored in the inductors that will cause any voltage spikes. When designing a charge controller it would be easy to calculate the maximum amount of energy stored in the inductors and place a transient voltage suppression diode in the circuit to dissipate this energy if need be.

I cannot believe that well respected solar controllers on the market would not be able to cope with a simple failure like the battery being disconnected from them because of someone accidentally turning off a circuit breaker, a fuse blowing or some other wiring fault.

The other thing to bear in mind that the BMS should only disconnect the battery as a last resort when all else has failed and will probably never happen.

As far as I can see if you have separate charge and load disconnects the load disconnect will only stop current going to the loads and still allow charge coming from the solar controller and visa versa so this becomes a non issue.

Simon

just starting · August 2018

My bad bb here are some photos was supposed to be 3.399

just starting · August 2018

And there are no caps or inductors just mosfet no pwm or mppt just cc

Dave Angelini · August 2018

The Schneider mppts can be shut down with xanbus or modbus. It really is the best way as it soft starts or soft stops the solar.
I believe the new Outback CC's do this. Not sure of the FM series.

just starting · August 2018

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

BB. · August 2018

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).

There are so many devices out there (charging, loads, etc.), that it is impossible to give an accurate general answer... For example, older cars with alternators could go >>12 VDC if the battery was disconnected while the car was running and pop lights and damage other electronics.

With a BMS disconnecting the battery bank, I am not sure I would lump it into the worst thing in the world--More like a pull the alarm as something is already going badly wrong (one or more cells is out of the "safe/long life" voltage range). Would it 100% cause other failures in the DC Bus devices. On average probably not. Would I do this type of function multiple times per day as an "on/off" switch--No...

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.

This is one of the killers with adding complexity to a system to increase reliability/fault tolerance... Very frequently the addition of the system(s) to make the system "more reliable" make the system less reliable/more fault prone.

Using a BMS with integral disconnect switch/MOSFETs/etc... Probably one of those necessary evils. If it trips, hopefully it was for a good reason, and on average, nothing else is damaged.

-Bill

just starting · August 2018

I like just having a sbms40 and a inverter and batteries w solar panels not a whole lot of different items just 4 pieces for the whole system keeps it simple. Less to go wrong.

Lifepo4 replacement quandary

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