BMS disconnect question?

[tanglewood]

Originally Posted by s/v Jedi

Because it is not the task of the BMS to control the charging process, which is done by the charger.

The BMS must monitor cells and send out warnings if limits are near and protect the battery with HVC or LVC events when limits are crossed.

If a warning is issued and used to shutdown a charge source, something is seriously wrong.

The exception would be if you are using DVCC or similar charger control.

[tanglewood]

Originally Posted by fxykty

While not a core function of a BMS, having the BMS manage all the different charge sources present on a boat is certainly a nice set of functionality. Each individual charger does it’s thing and should be set up correctly, so that if the BMS goes offline each charger behaves correctly. But having the BMS control when charging happens and what voltage and current values to use is very handy. Any charger that understands CanBUS (such as Victron equipment via a Cerbo GX) can be controlled by a BMS that uses CanBUS as a communication method.

As an example, if it’s very cold or very hot, you can still charge, but at lower current values than in nominal temperatures. Do many (any?) chargers have temperature settings for charge parameters?

It's more than just Canbus. It's a specific protocol over Canbus that is supported by all the devices involved. You can't just plug it in and assume it will work. In fact I think you need to assume it's not working until tested and demonstrated that it does work.

[SaylorMade]

Not an expert on this, just been watching videos and reading this forum, but from what I have seen so far, charging of LifePo4 packs is terminated when the first cell reaches a set max. voltage. i.e. monitoring individual cell voltage, and not pack voltage.

If you for some reason have a very unbalanced pack, it would be a very bad idea to just go on pack voltage, wouldn't it?

Or maybe thats where the whole communication thing comes in, but then why not just let the BMS decide on when to stop charging in the first place?

[BAD ORCA]

[ Post deleted at author's request ]

[BAD ORCA]

Originally Posted by Nate24

This is a link to Jim Healy’s site and one of three write ups on lithium batteries he put together. Jim advises the ABYC electrical group. I would suggest reading all three of his articles. This link is to the most recent. They are well written and explain the issues and advantages of lithium very well.

https://gilwellbear.wordpress.com/ca...s-1q23-update/

Thanks for that. I'll give that a read.

[hjohnson]

Originally Posted by fxykty

While not a core function of a BMS, having the BMS manage all the different charge sources present on a boat is certainly a nice set of functionality.

Bingo. Jedi is dead set on his old school independent system. And that’s cool for him.

But it’s absolutely not the only way to skin this cat. I’ve been running a tightly integrated system for the past 18 months, and it’s been absolutely fantastic. Everything just works, my charging sources adapt on the fly to whatever the BMS wants, and I can monitor and control my system from anywhere on earth. If the BMS decided that the cells need balancing, it will back off the chargers to let that happen. If it’s cold or hot, it will back them off. If there’s an alarm state, it will shut down the charging sources, including the alternator, prior to an HVC disconnect.

Lastly, if the BMS itself takes a crap and fails, everything will revert and drop to a limp voltage of 13.2 volts.

[BAD ORCA]

Originally Posted by SaylorMade

Not an expert on this, just been watching videos and reading this forum, but from what I have seen so far, charging of LifePo4 packs is terminated when the first cell reaches a set max. voltage. i.e. monitoring individual cell voltage, and not pack voltage.

If you for some reason have a very unbalanced pack, it would be a very bad idea to just go on pack voltage, wouldn't it?

Or maybe thats where the whole communication thing comes in, but then why not just let the BMS decide on when to stop charging in the first place?

I think that it depends on the battery/BMS. From my understanding, most BMS's (probably all) do this but only as a way to protect the battery cells in an emergency. The terminating cell voltage being 3.65 volts per cell. When it reaches this voltage the BMS turns off and disconnects the battery from the electrical system.

From my understanding so far, you absolutely want to avoid doing this for the health and longevity of the battery not to mention the obvious problems associated with an abrupt electrical dissconection. You want to terminate the charge and discharge at voltages that wont allow this to happen.

I agree, i think this is where the whole communication thing comes in because you can edit and change the voltage parameters for charging and discharging to suit your particular battery pack.

Unfortunately, it seems to me that, almost all of the cheaper LFP batteries dont have communication capability where you can see the individual cell voltages so using overall pack voltage with a good battery monitor becomes the only viable option. I think Battleborn recommends charging to 14.2 volts and holding the voltage at 14.2 for 15 minutes per battery pack. I think this is one area where spending the extra money for a high quality, marine grade LFP will pay dividends in the long term. Not only should the cell voltages be very close and stable over the long term, but the ability to monitor and keep them there is possible

I bought an E-Poch battery that shows individual cell voltages on the bluetooth app , but im not sure if the Victron equiptment can read and use this info. Still waiting for the battery to ship. This will be my learning battery so i can figure all this stuff out before upgrading the entire house bank.

[Statistical]

Originally Posted by BAD ORCA

I bought an E-Poch battery that shows individual cell voltages on the bluetooth app , but im not sure if the Victron equiptment can read and use this info. Still waiting for the battery to ship. This will be my learning battery so i can figure all this stuff out before upgrading the entire house bank.

It can't. Even if you could get that info to the charger the charger can't charge individual cells.

So even in DVCC systems (where the BMS coordinates charging between multiple charge sources) based on the cell voltage but without the chargers acting on the cell voltage.

One aspect where this can be useful is as one cell approaches peak voltage programmed into the BMS if the other cells are behind it will call for a low but not zero charging current. That helps in balancing the cells. The charger don't need to know "why" this is being asked just that it is.

Note while 3.6V is considered peak voltage for maximum longevity many custom systems don't charge beyond 3.4V or 3.45V. Resting voltage of LFP is ~3.4V per cell anything beyond that just accelerates charging but that higher cell voltage is a surface charge only and quickly dissipates.

As someone else said there are more than one way of doing things. Do you need DVCC with the BMS controlling everything? No. However the BMS has access to more information and more accurate information than anything else so there is value in letting the BMS run the show.

[s/v Jedi]

Originally Posted by hjohnson

Bingo. Jedi is dead set on his old school independent system. And that’s cool for him.

But it’s absolutely not the only way to skin this cat. I’ve been running a tightly integrated system for the past 18 months, and it’s been absolutely fantastic. Everything just works, my charging sources adapt on the fly to whatever the BMS wants, and I can monitor and control my system from anywhere on earth. If the BMS decided that the cells need balancing, it will back off the chargers to let that happen. If it’s cold or hot, it will back them off. If there’s an alarm state, it will shut down the charging sources, including the alternator, prior to an HVC disconnect.

Lastly, if the BMS itself takes a crap and fails, everything will revert and drop to a limp voltage of 13.2 volts.

The additional functionality is bloatware, designed to feed the buyers need for more gadgets that add absolutely nothing to the battery or how well it gets charged. Complete failure is one small bug away (let’s update the firmware) and you get nothing back. I can also see my system from all over the world, but why is that needed? And that’s done by my Cerbo GX, without batteries that control the chargers with CANbus protocols.

What if you have several brands of batteries and they want different charge parameters? Or are you gonna change all your batteries when one fails so that it all keeps working?!

[GreenWave]

Originally Posted by hjohnson

Bingo. Jedi is dead set on his old school independent system. And that’s cool for him.

But it’s absolutely not the only way to skin this cat. I’ve been running a tightly integrated system for the past 18 months, and it’s been absolutely fantastic. Everything just works, my charging sources adapt on the fly to whatever the BMS wants, and I can monitor and control my system from anywhere on earth. If the BMS decided that the cells need balancing, it will back off the chargers to let that happen. If it’s cold or hot, it will back them off. If there’s an alarm state, it will shut down the charging sources, including the alternator, prior to an HVC disconnect.

Lastly, if the BMS itself takes a crap and fails, everything will revert and drop to a limp voltage of 13.2 volts.

Can you summarize your setup to achieve this? And what battery are you using?

[SaylorMade]

Originally Posted by BAD ORCA

I think that it depends on the battery/BMS. From my understanding, most BMS's (probably all) do this but only as a way to protect the battery cells in an emergency. The terminating cell voltage being 3.65 volts per cell. When it reaches this voltage the BMS turns off and disconnects the battery from the electrical system.

From my understanding so far, you absolutely want to avoid doing this for the health and longevity of the battery not to mention the obvious problems associated with an abrupt electrical dissconection. You want to terminate the charge and discharge at voltages that wont allow this to happen.

I agree, i think this is where the whole communication thing comes in because you can edit and change the voltage parameters for charging and discharging to suit your particular battery pack.

Unfortunately, it seems to me that, almost all of the cheaper LFP batteries dont have communication capability where you can see the individual cell voltages so using overall pack voltage with a good battery monitor becomes the only viable option. I think Battleborn recommends charging to 14.2 volts and holding the voltage at 14.2 for 15 minutes per battery pack. I think this is one area where spending the extra money for a high quality, marine grade LFP will pay dividends in the long term. Not only should the cell voltages be very close and stable over the long term, but the ability to monitor and keep them there is possible

I bought an E-Poch battery that shows individual cell voltages on the bluetooth app , but im not sure if the Victron equiptment can read and use this info. Still waiting for the battery to ship. This will be my learning battery so i can figure all this stuff out before upgrading the entire house bank.

I just bought a 370 USD, 12V 100 AH pack as house battery, with BT and it let's me set all the different parameters, like cell voltage cut-off etc.
So I can just adjust the max to 3.4 if I so desire. I can see the voltage of each cell at any time as well as temperature etc.

I charge it with a cheap 12V charger, and I can see that it accepts the 10A (charger spec) until almost full, then slows down, and the BMS disconnects the charge-input when the first cell reaches the set max.

Balancing happens during AND after charge, so is typically within 1 mv an hour after full charge.

So I am reading this thread with interest, because I get the impression that I am doing something wrong now

[BAD ORCA]

Originally Posted by SaylorMade

I just bought a 370 USD, 12V 100 AH pack as house battery, with BT and it let's me set all the different parameters, like cell voltage cut-off etc.
So I can just adjust the max to 3.4 if I so desire. I can see the voltage of each cell at any time as well as temperature etc.

I charge it with a cheap 12V charger, and I can see that it accepts the 10A (charger spec) until almost full, then slows down, and the BMS disconnects the charge-input when the first cell reaches the set max.

Balancing happens during AND after charge, so is typically within 1 mv an hour after full charge.

So I am reading this thread with interest, because I get the impression that I am doing something wrong now

I will let someone with more experience than me chime in, but i would say that if your BMS allows you to do this, then i cant imagine you would be doing anything wrong as its just a feature. I dont think all BMS's allow the user to change these settings though. Im pretty sure the BMS on the one i bought does not. Probably why many people choose to build their own.

If you can change and monitor the charge settings in the BMS, and monitor it accurately. Seems a good way to save $ on extra monitoring equipment if your on a budget or just dont need all the extra devices.

[Jammer]

Originally Posted by Nicholson58

Balmar offers an alternator protection device. I believe it is a capacitor to smooth the spikes. We installed one per alternator with our LiFePo system.

I believe they are designed around a metal oxide varistor (MOV). As you may be aware, MOVs are devices that are high-impedance at low voltages but which begin to conduct when a certain threshold voltage is exceeded. There may be a capacitor in parallel with it. Typically they are used for shorter-duration surges than would occur from a BMS disconnect while the alternator is operating with full field, but if large enough the MOVs will clamp the surge and may even survive repeated hits.


https://www.mouser.com/pdfdocs/bourn...hite-paper.pdf

[hjohnson]

Originally Posted by GreenWave

Can you summarize your setup to achieve this? And what battery are you using?

My LFP battery DIY bank made up of a 2p4s configuration of EVE 230Ah cells managed by a REC ABMS. This connects to my Victron Cerbo GX via CANbus, and fully manages my electrical system. The Cerbo GX in turn relays the BMS limits to my MPPTs and MultiPlus inverter/charger. (It’s also doing a lot of boat automation stuff, and other system monitoring, including tanks, bilge, environment, N2K, etc…)

My alternator is controlled by a Wakespeed WS500, which also gets its direction from the REC, and is monitored by the Cerbo.

Overall, the system works beautifully, and I haven’t had to mess around with it or tweak anything (other than putting it into “storage mode” when I’m away for a long time) since I installed it 18 months ago. It lets us do all sorts of stupid stuff like run our hot water tank on the hook, or what we’re doing tomorrow which is starting to cook 3lbs of pulled pork using an electric sous vide cooker, while we transit to the next location.

[hjohnson]

Originally Posted by Jammer

I believe they are designed around a metal oxide varistor (MOV). As you may be aware, MOVs are devices that are high-impedance at low voltages but which begin to conduct when a certain threshold voltage is exceeded. There may be a capacitor in parallel with it. Typically they are used for shorter-duration surges than would occur from a BMS disconnect while the alternator is operating with full field, but if large enough the MOVs will clamp the surge and may even survive repeated hits.


https://www.mouser.com/pdfdocs/bourn...hite-paper.pdf

I thought they were just a bunch of zener diodes that clamped the voltage to something like 36v.