Grounding two isolated battery banks

[heritage]

Someone asked "how could I protect the alternator from being fried if the bms shut down your lfp house bank".

Firstly; my house bank has three lfp banks in parallel. Each has its own bms. If one shuts down the other two pick up the load, and if two are shut down the third is still there.

Secondly; the reason a bms shuts down a battery is typically because the battery is full or empty. While the alternator is charging, the bms will never see an empty state, and if the battery is approaching full, the charge acceptance of the battery is very low. So if the bms disconnects the battery, the low current produced by the alternator will unlikely be strong enough to damage the alternator diodes.

Thirdly; Balmar now sells a diode protection gizmo that I have installed as a last line of defense. Balmar APM-12 Alternator Protection Module for 12 volt systems

 

[cagreen75]

[*]Alternator will be getting sense voltage from AGM so that if LFP cuts out the alter will still be under control instead of going to max charge.

First, let me say that my intent here is not to criticize the system you have designed it is to try and explain how you might consider changing it to make the system work more efficiently and improve your battery's life.
Let me try to clarify what I was trying to explain. As you say the sense voltage is coming from the AGM and in that case when the engine is started the alternator will rapidly bring the AGM up to near a full charge rather quickly at which point the regulator will adjust the field current in order to not overcharge the AGM battery. Even though both banks are getting charged through the Argo FET, the regulator is monitoring the voltage of the AGM battery and the LFP batteries will then be charging at a reduced rate and it will take a very long time to get the LFP bank back to a full charge. The problem with that is extended engine run time and the potential to damage your LFP battery bank. I apologize for any confusion regarding my post.

 

[Stu Jackson]

and if the battery is approaching full, the charge acceptance of the battery is very low.

Except for LiFePo batteries that is not true. You are mixing FLA characteristics with LiFePo operations.
Have you read Maine Sail's excellent article? Drop-In LiFePo4- Be an Educated Consumer

 

[DArcy]

First, let me say that my intent here is not to criticize the system you have designed it is to try and explain how you might consider changing it to make the system work more efficiently and improve your battery's life.
Let me try to clarify what I was trying to explain. As you say the sense voltage is coming from the AGM and in that case when the engine is started the alternator will rapidly bring the AGM up to near a full charge rather quickly at which point the regulator will adjust the field current in order to not overcharge the AGM battery. Even though both banks are getting charged through the Argo FET, the regulator is monitoring the voltage of the AGM battery and the LFP batteries will then be charging at a reduced rate and it will take a very long time to get the LFP bank back to a full charge. The problem with that is extended engine run time and the potential to damage your LFP battery bank. I apologize for any confusion regarding my post.

This is an important point.

I'm not sure your setup will directly damage the LFP but they may not fully charge. The AGM internal resistance is much lower than LFP so they will charge faster and the regulator will adjust accordingly so the LFP may never get a full charge. They just don't have the same charge profile so charging will be very inefficient. Drop in LFP batteries are touted to be replacements for LA but that doesn't mean you can simply mix the two together in parallel.

On the negative terminal side, I have a common negative bus bar with one cable to the engine. I like the idea stated earlier of keeping the engine block out of the current path.

 

[Hayden Watson]

First, let me say that my intent here is not to criticize the system you have designed it is to try and explain how you might consider changing it to make the system work more efficiently and improve your battery's life.
Let me try to clarify what I was trying to explain. As you say the sense voltage is coming from the AGM and in that case when the engine is started the alternator will rapidly bring the AGM up to near a full charge rather quickly at which point the regulator will adjust the field current in order to not overcharge the AGM battery. Even though both banks are getting charged through the Argo FET, the regulator is monitoring the voltage of the AGM battery and the LFP batteries will then be charging at a reduced rate and it will take a very long time to get the LFP bank back to a full charge. The problem with that is extended engine run time and the potential to damage your LFP battery bank. I apologize for any confusion regarding my post.

Thanks for the comments.
What you describe will only happen if the AGM is at the bulk voltage threshold. The resting voltage of a 100% SOC AGM is somewhere between 12.7v & 13.0v. The LFP at 13.0v is about 30% SOC. When the regulator is calling for CC bulk charge because the voltage is less than the CV threshold of 14.0v, almost all of the current is going to the LFP because its internal resistance in much lower than the AGM. The AGM (assuming adequately sized wires) cannot get to a high voltage than that of the LFP because the LFP will take all it can get. If anything, this system will undercharge the AGM but the shore power charger will correct that eventually.
The biggest worry would be from cooking the alternator because it is charging too hard. For that, I have PWM control on the field wire (externally regulated alternator) which will limit the output to a level that will keep the alt case temp under 90ºC. If it gets to 95ºC, I have thermostatic control that will shut down the alternator until it cools to 80ºC.
The thing to remember is that a voltage regulator does not determine the voltage that is being charge with. It only tells the alternator how hard to charge and in the bulk CC phase, that is "Give me everything you got". It is the lowest level of internal resistance of the any of the batteries that determines the voltage. Until the LFP gets to somewhere north of 95% I cannot generate enough current to move the LFP much above 13.6v so it will produce as much power as I will allow with my PWM control. In my case, that will be about 60A until I have a chance to do more testing.

 

[Hayden Watson]

Except for LiFePo batteries that is not true. You are mixing FLA characteristics with LiFePo operations.
Have you read Maine Sail's excellent article? Drop-In LiFePo4- Be an Educated Consumer

Correct. That is why I will be limiting my alternator to about 60% of its rated output.

 

[Hayden Watson]

This is an important point.

I'm not sure your setup will directly damage the LFP but they may not fully charge. The AGM internal resistance is much lower than LFP so they will charge faster and the regulator will adjust accordingly so the LFP may never get a full charge. They just don't have the same charge profile so charging will be very inefficient. Drop in LFP batteries are touted to be replacements for LA but that doesn't mean you can simply mix the two together in parallel.

On the negative terminal side, I have a common negative bus bar with one cable to the engine. I like the idea stated earlier of keeping the engine block out of the current path.

This is not correct. the LFP has much lower IR than the AGM. Because of that, almost all of the current will go to it until it approaches full charge when the LFP IR rapidly increases and more of the current will go to the AGM. Towards the end of charging the AGM will be getting most of the current and then it will drop to a short absorption and then to float.

 

[cagreen75]

Thanks for the comments.
What you describe will only happen if the AGM is at the bulk voltage threshold. The resting voltage of a 100% SOC AGM is somewhere between 12.7v & 13.0v. The LFP at 13.0v is about 30% SOC. When the regulator is calling for CC bulk charge because the voltage is less than the CV threshold of 14.0v, almost all of the current is going to the LFP because its internal resistance in much lower than the AGM. The AGM (assuming adequately sized wires) cannot get to a high voltage than that of the LFP because the LFP will take all it can get. If anything, this system will undercharge the AGM but the shore power charger will correct that eventually.
The biggest worry would be from cooking the alternator because it is charging too hard. For that, I have PWM control on the field wire (externally regulated alternator) which will limit the output to a level that will keep the alt case temp under 90ºC. If it gets to 95ºC, I have thermostatic control that will shut down the alternator until it cools to 80ºC.
The thing to remember is that a voltage regulator does not determine the voltage that is being charge with. It only tells the alternator how hard to charge and in the bulk CC phase, that is "Give me everything you got". It is the lowest level of internal resistance of the any of the batteries that determines the voltage. Until the LFP gets to somewhere north of 95% I cannot generate enough current to move the LFP much above 13.6v so it will produce as much power as I will allow with my PWM control. In my case, that will be about 60A until I have a chance to do more testing.

As boat owners and tinkerers, we all make mistakes and later learn about better ways to do things. The system I described earlier will charge both banks with the proper charge current and voltages based on the battery types and DOD. The important thing here is to charge the batteries in a way that will extend their life. It does not matter if those batteries are AGM or LFP or a combination of both. The bottom line is the need to be charged properly. The system you describe in my opinion will charge the batteries but will not charge them properly. As you say you have read tons of articles and you seem to know what you are doing, but In my opinion, there are better methods to get the job done. That is all I was attempting to explain. By the way, I have also spent lots of time reading articles and spent my entire life working in the field and I still make mistakes sometimes that later I find myself correcting usually at great cost. Again my intent was to help you build a better system, however, it seems you have made up your mind that your method is best for you and I respect that. That said I will be silent going forward.

 

[Maine Sail]

This is an important point.

The AGM internal resistance is much lower than LFP so they will charge faster and the regulator will adjust accordingly so the LFP may never get a full charge.

.

No true at all and LFP can hit 100% SoC at just 3.42V per cell or 13.7V. If anything the AGM's will be slightly under charged..