Work around for common port BMS

Rosco4271

New Member
I have been assembling parts for my off road caravan just waiting in 16 3.2v EVE cells and common port from China. I have 3 24v 350 solar panels, victron smart solar 100/50, victron smart battery protect24v/220amp, 24/12 converter, and a relay same as shown on his video. After watching the video again I just realised that the system will completely shut the solar input once the voltage drops low. Is there a work around for this or am i up for a seperate port BMS. Thanks in advance
 

HRTKD

Boondocker
LIke pierre said, it depends on the BMS. A cheap one may shut off both ports. My Overkill Solar 4s 120 amp BMS will control the charge/discharge ports independently. That's handy for me as the LiFePO4 batteries sit in my trailer all year. During the winter the warming pads continue to get power even when the temperature is below the low temperature (charging) cutoff of the BMS.
 

HaldorEE

Photon Sorcerer
I have been assembling parts for my off road caravan just waiting in 16 3.2v EVE cells and common port from China. I have 3 24v 350 solar panels, victron smart solar 100/50, victron smart battery protect24v/220amp, 24/12 converter, and a relay same as shown on his video. After watching the video again I just realised that the system will completely shut the solar input once the voltage drops low. Is there a work around for this or am i up for a seperate port BMS. Thanks in advance
Depends on the BMS. The Daly BMS are essentially a hybrid. My understanding is they are all separate port, but they can internally connect the charge and discharge port in parallel. So if the discharge port is disabled due to low battery voltage, the charge port is still conducting, and vise-versa.

I do not know if all BMS work this way. It seems like a reasonable way to do things, but I didn't design them, so I don't know.

My electrical system is designed to ensure the BMS never sees a battery voltage that is outside operating thresholds except in the event something breaks. Probably the single most import job the BMS has is preventing battery damage due to an unbalanced cell. The rest of the system should be able to handle charging and discharging in a way that the BMS never engages in normal operation.

With cheap 280 AH cells, it makes sense to seriously underuse the battery capacity, cycling between 25% and 75% SOC. I bet if you did this, those batteries would last so long that they would be finding new reasons why batteries break down. Only issue with doing this is your BMS is not going to be able to rebalance cells if you don't charge them to 100% regularly. You could manually rebalance the cells every so often or use an active balancer that automatically rebalances cells at every voltage. Cell balance becomes less important the smaller a percentage of battery capacity you use.

I just talked myself into setting my charge discharge limits to 75% and 25% respectively.
 
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Rosco4271

New Member
Hi HaldorEE,
Thanks for the information I am considering that as well, I have ordered a bottom balancing card and voltage display unit so will use it from time to time.
Will has removed the Victron Battery protect in his book on Solar RV but recent he was using a victron smart Solar regulator and a smart Battery Protect. any Ideal why? As I have set to use Victron gear in my Caravan. Thanks for your help
 

Johncfii

Solar Addict
My Overkill Solar 4s 120 amp BMS will control the charge/discharge ports independently.

HRTKD, this is interesting. Can you elaborate?

Looking carefully again, just now, at any of the Overkill Solar BMS currently offered, there is no information indicating that it is a separate port BMS? I’m wondering if I missed something important, or am I not understanding your post correctly?
 
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pierre

Somewhere down South
HRTKD, this is interesting. Can you elaborate?

Looking carefully again, just now, at any of the Overkill Solar BMS currently offered, there is no information indicating that it is a separate port BMS? I’m wondering if I missed something important, or am I not understanding your post correctly?
You are correct. This BMS is a common port. What happens is that once the Pack UV or individual cell UV is triggered the BMS will disconnect the load. If you have a charging source connected this source will supply the load and any surplus current will flow into the battery via the BMS. Once the UV recovery points are reached the BMS will turn the load current on again. Hope this helps.
 

HRTKD

Boondocker
You are correct. This BMS is a common port. What happens is that once the Pack UV or individual cell UV is triggered the BMS will disconnect the load. If you have a charging source connected this source will supply the load and any surplus current will flow into the battery via the BMS. Once the UV recovery points are reached the BMS will turn the load current on again. Hope this helps.

Correct. Similarly, if the high voltage thresholds are reached, the charge stops, but the loads can still pull from the battery.
 

Johncfii

Solar Addict
Wow, I am feeling really dense in not understanding how you are accomplishing this with a single common-port BMS. It is the above statement that "If you have a charging source connected this source will supply the load and any surplus current will flow into the battery via the BMS." that confuses me. How can the charge current flow "via the BMS" if the BMS has disconnected the load, by opening the negative lead connection through the BMS? I can see how this can work if the charger is wired around the BMS, but not via.

Can you post a link to the referenced video? Hopefully, I will understand after seeing the complete circuit wiring of all the components.
 

pierre

Somewhere down South
Wow, I am feeling really dense in not understanding how you are accomplishing this with a single common-port BMS. It is the above statement that "If you have a charging source connected this source will supply the load and any surplus current will flow into the battery via the BMS." that confuses me. How can the charge current flow "via the BMS" if the BMS has disconnected the load, by opening the negative lead connection through the BMS? I can see how this can work if the charger is wired around the BMS, but not via.

Can you post a link to the referenced video? Hopefully, I will understand after seeing the complete circuit wiring of all the components.
Each BMS has charge and discharge FET's which are controlled by the onboard electronics. The negative lead does not get disconnected at all. Simply put , look at it as two one directional switches in parallel - the one will only allow discharge current to flow and the other charge current.
 

Horsefly

Photon Sorcerer
This article explains using MOSFETs for bidirectional current flow using a single channel: https://www.homemade-circuits.com/bidirectional-switch/

I'll admit I was confused about this as well, as I thought that was the difference between "separate port" and "common port" BMSs. Some BMSs use a relay / contactor to disconnect the charger / loads, and that obviously won't allow separate control of charging vs discharging.
 

Johncfii

Solar Addict
Thanks for the explanations. I understand. I did not realize that my BMS incorporated a bi-polar switch. There is nothing in the Overkill Solar manual about this that I can find. Amazing how I missed this detail all these months, and it opens up some interesting new possibilities.
 
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HaldorEE

Photon Sorcerer
Hi HaldorEE,
Thanks for the information I am considering that as well, I have ordered a bottom balancing card and voltage display unit so will use it from time to time.
Will has removed the Victron Battery protect in his book on Solar RV but recent he was using a victron smart Solar regulator and a smart Battery Protect. any Ideal why? As I have set to use Victron gear in my Caravan. Thanks for your help
The Victron battery protect is not designed to work with an inverter (no Victron inverter would need it).

One common question here about a low voltage battery protect is using them with a cheap inverter that lacks programmability. The Victron is not the option you want to use for that.
 

HaldorEE

Photon Sorcerer
Thanks for the explanations. I understand. I did not realize that my BMS incorporated a bi-polar switch. There is nothing in the Overkill Solar manual about this that I can find. Amazing how I missed this detail all these months, and it opens up some interesting new possibilities.
Verify before you count on this.
 

Horsefly

Photon Sorcerer
Thanks for the explanations. I understand. I did not realize that my BMS incorporated a bi-polar switch. There is nothing in the Overkill Solar manual about this that I can find. Amazing how I missed this detail all these months, and it opens up some interesting new possibilities.
Like I mentioned earlier, I also was under the impression that the whole reason for a separate charge and discharge port on a BMS was so they could be controlled separately. It was reading that the Overkill manual that convinced me they were separate on this common port BMS.

The manual certainly does imply that the charging and discharging are controlled separately. For example, in section 2.4.5 it talks about disabling charging below a certain temperature, disabling discharging at a different low temperature, disabling charging above a high temperature, and disabling discharging at a different high temperature. All of section 3.1 talks about the parameters controlling charging or discharging separately.

I verified it does work separately. I stuck the temperature probe in an ice brine while charging. I watched as the charging was stopped. I then immediately turned on a load and verified discharging still was working. I turned off the load and verified charging was still cut off.
 

Johncfii

Solar Addict
The manual certainly does imply that the charging and discharging are controlled separately. For example, in section 2.4.5 it talks about disabling charging below a certain temperature, disabling discharging at a different low temperature, disabling charging above a high temperature, and disabling discharging at a different high temperature. All of section 3.1 talks about the parameters controlling charging or discharging separately.

Thank you. You're certainly right about the manual. Thanks for the pointers. I didn't read it carefully enough. A good example of pre-conceived notions getting in the way of "seeing". And I’ve now verified that my BMS performs this way.
 
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Rosco4271

New Member
The Victron battery protect is not designed to work with an inverter (no Victron inverter would need it).

One common question here about a low voltage battery protect is using them with a cheap inverter that lacks programmability. The Victron is not the option you want to use for that.
Hi again i have wired the battery protect in to the system but am waiting patiently on EVE batteries, so whats the worst can happen, i am trying to protect the battery. Going to cycle the battery 30 to 85%. So will the battery protect shutdown on inrush current? Thanks again for your advice
 
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