Questions on BMS use with alternator

[baltik]

My charging setup is going to be: Alternator -> Renogy 40a Dc to DC converter -> BMS -> 4x 150ah aluminum cells.

I like the idea of using one of the 120A alibaba BMS that others have had success with: https://www.aliexpress.com/item/4000425316662.html?spm=a2g0s.8937460.0.0.4ac82e0es4u6A2 Seems robust for my use case and I love the ability to monitor remotely via bluetooth

My question is how to best handle the situation when batteries are fully charged. My understanding is that continuing to charge when batteries are full creates heat and will degrade battery life. Is it safe to have the bms shut off the charging load when batteries are full? Can it damage the DC-DC charger? if charging is shut off, is discharing also disabled?

thanks!

 

[baltik]

anyone?

 

[grizzzman]

WIll there be a lead-acid starting battery involved? If not then it will be very likely that the alternator will go up in smoke.

 

[baltik]

That’s what the DC to DC converter does, caps the alternator output

 

[grizzzman]

That’s what the DC to DC converter does, caps the alternator output

If the BMS disconnects it will fry the alt without a battery period. With that said a marine external alt. voltage controller will work by turning the "control circuit" off instead of a sudden current off event.

 

[baltik]

the DC to DC is wired via a starting lead acid battery

 

[grizzzman]

Most dc to dc chargers will have no issues with a BMS disconnect.

 

[Dzl]

In my opinion its the responsibility of the DCC50S charger to handle the details of charging via the alternator, including determining when to cutoff charging (and restart charging). The BMS can do so as well but its not really its role in my opinion, its a protection device, not a day to day charge manager. I think of it as the last line of defense, that steps in only when necessary. Just my 2c

 

[Airtime]

In my opinion its the responsibility of the DCC50S to handle the details of charging via the alternator, including determining when to cutoff charging (and restart charging). The BMS can do so as well but its not really its role in my opinion, its a protection device, not a day to day charge manager. I think of it as the last line of defense, that steps in only when necessary. Just my 2c

I think it is the reverse. The BMS is the only component that knows the actual state of the battery, so it is best equipped to control charging. You can stop charging when highest cell reaches your desired target. Having the DC-DC design when to cut off is more of a blunt instrument.

 

[John Frum]

I think it is the reverse. The BMS is the only component that knows the actual state of the battery, so it is best equipped to control charging. You can stop charging when highest cell reaches your desired target. Having the DC-DC design when to cut off is more of a blunt instrument.

The BMS is the blunt instrument.
Its either on or off.
The charger can control voltage directly and current via voltage.
UPDATED to correct thinko.

 

[Airtime]

The BMS is the blunt instrument.
Its either on or off.
The charger can control current voltage directly and current via voltage.

Curious why that is needed for LFP? No fancy lead-acid based charge algorithms needed, just set the voltage and charge with all the current that the DC-DC can deliver and stop charging when you reach target cell voltage. Just bulk and then stop.

All I want out of a DC-DC for alternator charging is for it to deliver its rated current with a fixed settable voltage limit and a remote on/off control.

 

[John Frum]

Curious why that is needed for LFP? No fancy lead-acid based charge algorithms needed, just set the voltage and charge with all the current that the DC-DC can deliver and stop charging when you reach target cell voltage. Just bulk and then stop.

All I want out of a DC-DC for alternator charging is for it to deliver its rated current with a fixed settable voltage limit and a remote on/off control.

LFP requires CC=constant current/CV=constant voltage.
The way CC is achieved is by adjusting voltage to regulate current.
LFP requires this because their internal resistance is so low that they can gobble up enough charge to hurt themselves.
Luckily they usually kill the alternator along the way.

 

[John Frum]

All I want out of a DC-DC for alternator charging is for it to deliver its rated current with a fixed settable voltage limit and a remote on/off control.

The DCC50S should be able to do this provided the alternator can keep up.

 

[Airtime]

LFP requires CC=constant current/CV=constant voltage.
The way CC is achieved is by adjusting voltage to regulate current.
LFP requires this because their internal resistance is so low that they can gobble up enough charge to hurt themselves.
Luckily they usually kill the alternator along the way.

40A from the OP’s Renogy DC-DC is not going to stress LFP at all, and will be fine with the alternator. Or at least for Sprinter vans, Mercedes specifies up to 40A is ok and use a DC-DC.

CC/CV is achieved by just charging at the DC-DC 40A rate, until it reaches the voltage set for the DC-DC and then it is CV until you terminate charging.

it will work fine either way, I just prefer BMS control because it knows cell voltages, also knows total current coming into the battery from all charging sources and loads. The DC-DC only knows total battery voltage and what current it is delivering. A much more coarse picture of the battery state.

 

[John Frum]

40A from the OP’s Renogy DC-DC is not going to stress LFP at all, and will be fine with the alternator. Or at least for Sprinter vans, Mercedes specifies up to 40A is ok and use a DC-DC.

CC/CV is achieved by just charging at the DC-DC 40A rate, until it reaches the voltage set for the DC-DC and then it is CV until you terminate charging.

it will work fine either way, I just prefer BMS control because it knows cell voltages, also knows total current coming into the battery from all charging sources and loads. The DC-DC only knows total battery voltage and what current it is delivering. A much more coarse picture of the battery state.

I think you are confused.
If you are not I am.

 

[Dzl]

I think it is the reverse. The BMS is the only component that knows the actual state of the battery, so it is best equipped to control charging. You can stop charging when highest cell reaches your desired target. Having the DC-DC design when to cut off is more of a blunt instrument.

While I see where you are coming from, I think you have this backwards, unless you are thinking purely theoretically. I know that you prefer/advocate a design where the BMS controls all charging and the chargers are the blunt on/off devices, and I think there is a lot to be said for that model, but that's not the norm, and maybe isn't well suited for Op's already purchased components.

Practically speaking OP's BMS can switch charging (all charging) on or off, that's it. It isn't its role to control charging. It can step in if an individual cell voltage becomes a problem, but why not let the charger do its thing so long as the pack is well balanced and there are no problems. I agree with Joey the BMS is definitely the blunt instrument between the two.

 

[grizzzman]

Also if you want you can control 40amp dc to dc charger using the BMS(assuming the BMS can control an on)(power) or (off) no power command.

 

[Airtime]

To answer the OP’s questions:

My question is how to best handle the situation when batteries are fully charged. My understanding is that continuing to charge when batteries are full creates heat and will degrade battery life. Is it safe to have the bms shut off the charging load when batteries are full? Can it damage the DC-DC charger? if charging is shut off, is discharing also disabled?

Yes at 40A it is most likely safe to have BMS cut off DC-DC, unless you have a small alternator. At least for Sprinter vans it is safe according to Mercedes Builder Equipment Guide recommendation.

No it should not damage your DC-DC, but to be sure read the manual and ask Renogy.

As for shutting off charging separate from discharge, that depends on what BMS you pick and also on how you design your disconnects. But in general, yes you can disable charging without disabling discharge, as long as you design for that.

 

[Airtime]

While I see where you are coming from, I think you have this backwards, unless you are thinking purely theoretically. I know that you prefer/advocate a design where the BMS controls all charging and the chargers are the blunt on/off devices, and I think there is a lot to be said for that model, but that's not the norm, and maybe isn't well suited for Op's already purchased components.

Practically speaking OP's BMS can switch charging (all charging) on or off, that's it. It isn't its role to control charging. It can step in if an individual cell voltage becomes a problem, but why not let the charger do its thing so long as the pack is well balanced and there are no problems. I agree with Joey the BMS is definitely the blunt instrument between the two.

We’ll agree to disagree on that. Most chargers use halfway modified lead acid algorithms that are not great for LFP. And they have no idea what other chargers are doing. But yes they will work fine and are necessary if doing drop-in with internal BMS.

OP is doing DIY LFP and has not yet bought his BMS so he does still have the opportunity to make these choices himself.

OP—if you are doing a DIY LFP bank in a mobile application, a good reference source of knowledge is Nordkyn Design.
http://nordkyndesign.com/category/marine-engineering/electrical/lithium-battery-systems/

 

[Dzl]

OP is doing DIY LFP and has not yet bought his BMS so he does still have the opportunity to make these choices himself.

Oops you are correct, thought the linked BMS was already purchased.

We’ll agree to disagree on that.

Agreed

Most chargers use halfway modified lead acid algorithms that are not great for LFP. And they have no idea what other chargers are doing. But yes they will work fine and are necessary if doing drop-in with internal BMS.

Based on my limited understanding, this class of DC-DC charger exists largely to address lithium batteries. I could well be misunderstanding the situation though.

What BMS would have the necessary awareness of the vehicle side of the system to intelligently control charging? Or are you only talking about using the BMS to control the high voltage cutoff?

 

[Dzl]

@Airtime
To clarify, this excerpt taken from the Nordkyn series is basically the perspective I've tried to articulate here and elsewhere:

A protection system with automated disconnection is just that: a last line of defence that should never be activated. Using the disconnection device(s) to terminate charging (a suggestion often formulated by DIY implementers) is out of the question. It breaches the system design boundaries, where the battery protection layer’s role is to mitigate any failure in the charge control system. More specifically, one role of the BMS is defending against a failure in maximum charging voltage regulation.

 

[Airtime]

To clarify, this excerpt taken from the Nordkyn series is basically the perspective I've tried to articulate here and elsewhere:
"A protection system with automated disconnection is just that: a last line of defence that should never be activated. Using the disconnection device(s) to terminate charging (a suggestion often formulated by DIY implementers) is out of the question. It breaches the system design boundaries, where the battery protection layer’s role is to mitigate any failure in the charge control system. More specifically, one role of the BMS is defending against a failure in maximum charging voltage regulation. "

Yes, I've had a dialog with Nordkyn Design specifically on that topic. It is relevant to the topic of this thread based on the OP questions on BMS control, so I'd like to clarify that since you quoted it. I initially misunderstood his meaning as well. What he is talking about in that statement is battery disconnects for safety as a separate and additional protection layer from normal charge control, in the system architecture. Not that the BMS should do only protection and not charge control. On the contrary, he is very firm in his view that the BMS should control charging, not chargers.

In our email dialog, I asked this question:
"One question I have for you. You advocate that BMS disconnects be used not for charge control, but as a separate additional protection layer. Charge devices should be set up with lithium-appropriate charge profiles and settings for normal operation. BMS disconnects only used if needed for protection of the battery bank. If the charging system is well designed, the BMS should never disconnect unless there is some system fault.

"I have seen other arguments to reverse this approach. I.e., use the BMS as primary for charge control, and use it to control the power switch of charging sources rather than directly disconnecting high current paths. Any charging source built in protections used with wider limits, as a second second layer. ... I wonder if you could comment on this approach?"

His response:
"I don’t advocate using “appropriate charge profiles” anywhere. It doesn’t work properly because the only component that has a correct view of the state of the battery is the BMS. The BMS should control charging, but the protection layer, i.e. battery disconnect, should never be used for charge control. In this topology, the BMS enables chargers and the chargers simply limit the voltage they deliver to the battery, so it can charge safely. You still need to be able to break the connection to the battery if something goes wrong for safety reasons."

So what he advocates is two things:
1) Normal operation: The BMS should control charging (meaning enabling and disabling charging sources) as it it the only thing that knows the battery state.
2) Fault conditions: Have an additional safety layer that disconnects the battery from the system when needed for safety. Should never occur in normal operation.

I agree with point 1). Only the BMS knows the battery state, including cell voltages and what total current the battery is actually seeing. As for 2), I think in his marine area it makes sense due to the life/death nature of an electrical failure at sea. I think it is more optional on land, and specifically for LiFePO4 chemistry vs. other lithium chemistries. But it depends on the user and their risk tolerance.

Back to the DC-DC topic--yes it is safe to have the BMS shut off DC-DC converter charging when the battery is full. In fact I think that is preferred.

 

[Dzl]

Thank you for sharing this e-mail exchange, I'm going to take a bit of time to consider it rather than responding here/now.

To make sure I understand what you/he are advocating, will you give an example of a BMS + SCC + Alternator charging solution that fits your model?

 

[Airtime]

Thank you for sharing this e-mail exchange, I'm going to take a bit of time to consider it rather than responding here/now.

To make sure I understand what you/he are advocating, will you give an example of a BMS + SCC + Alternator charging solution that fits your model?

I'll say that while I'm an EE with long R&D experience, I'm learning in this area which is why I approached Nordkyn on this topic. Glad I did because I had misunderstood his points. But I've come to agree, the BMS in control just makes more sense to me.

My choice (voting with my wallet) is Electrodacus SBMS0 for BMS, Electrodacus DSSR20 for SCC (whole different topic from BMS--will test vs. Victron MPPT side by side just out of curiosity) and Victron Orion 12/24-20 DC-DC Converter (simple--set voltage, remote control). Also Victron Multiplus inverter/charger.

The Electrodacus SBMS0 BMS is most aligned with this approach. But it can be implemented with any BMS that has separate control lines for charge enable and discharge enable. I've also used the Chargery BMS as well but I like the Electrodacus better. Various reasons, but one is that it has settings and control lines both for normal charge/discharge enable (default settings 3.55 OV and 2.8V UV, settable) as well as wider limits for fault protection (OVLock 3.7 and UV Lock 2.5, settable) that can trigger separate protection devices like a battery disconnect.

Any charger can be used, but much simpler and more elegant if it has a control line that can turn it on or off, without requiring high current relays or SSRs. Victron products in general have this, it is a mixed bag with other products. Without that, you can use SSRs or relays but less ideal.

On SCCs, you can use an MPPT controller which will work to optimize received solar power, but let the BMS decide when the SCC is allowed to charge the battery. Do that by setting a nice safe cell over voltage limit in the BMS like 3.55V. And then set the SCC Absorption setting higher, say 14.3 or 14.4 as an example. The SCC can then become a 'second layer' of protection but the BMS controls charging in normal operation based on actual battery state. (Electrodacus SCC is a whole different topic and different approach, but it is optional in this BMS charging control discussion.) You can use normal charge controllers as well--just need that control line.

Same with DC-DC converters for alternator charging. Just need a control line, and set Absorption higher than BMS settings so BMS can control.

 

[John Frum]

Having re-read the thread the OP seems to be talking about this unit https://www.renogy.com/content/RNG-DCC1212-20-BC/DCC1212-204060-Manual.pdf not the DCC50S after all.

On page 22 of the manual it says that absorption is limited to 3 hours.
So the product does do charge termination albeit crudely.

I agree that the BMS is in the best position to decide on battery SOC and health.
If the BMS can control the charger, that is optimal.