Dual 4S in parallel or 2P4S Configuration

[Nigel Snook]

I have 8 prismatic 280Ah cells on order for a system to be installed on a sailing yacht. I keep changing my mind between 2 separate batteries in parallel or a single 2P4S arrangement. I realise there are pros and cons between each configuration but the redundancy I get with 2 separate batteries is a feature that I perhaps value above all others, however, it crosses my mind that there could be unanticipated consequences of having different states of charge between both batteries when re-establishing the 2 batteries in parallel e.g. after a BMS trip with one. Presumably there will be a great deal of current flowing between batteries until a balance has been achieved – do I care??

I’d appreciate the thoughts of someone who has gone through the same thought process and the conclusions reached.

 

[JRG]

I'm going through this EXACT same thought process, so I'm curious to the feedback you get.

 

[markver]

Hi Nigel, in my 5th wheel RV I have two 4s 12v LiFePO4 batteries in parallel. It has only been 2 months and works quite well. Pros include: 1) redundancy in case of a single battery failure. 2) with two 120a BMS(Overkill Solar) units you can monitor and balance each of the 8 cells via the Bluetooth App. 3) They stay very equal in SOC(state of charge), both when charging and discharging. And the big one is... 4) With two BMS units the load is split equally so each one does half the work and extends the life of my batteries & BMS. The only con would be if one of your batteries is lower in max SOC then it will cut-off a bit sooner than the other. However, as you stated they do balance between each other after the charger turns off. My two batteries are only off by +-.010v and pass power between each other for about 10 minutes to a final balanced SOC.

Note: I set up my system for long cell life by top balancing each battery before hooking them in parallel, then configuring the BMS to operate from 90% SOC down to 20% SOC. This conservative setup is giving me +-30-40% more run time than my previous lead-acid batteries depending on loads...

 

[Nigel Snook]

Hi Nigel, in my 5th wheel RV I have two 4s 12v LiFePO4 batteries in parallel. It has only been 2 months and works quite well. Pros include: 1) redundancy in case of a single battery failure. 2) with two 120a BMS(Overkill Solar) units you can monitor and balance each of the 8 cells via the Bluetooth App. 3) They stay very equal in SOC(state of charge), both when charging and discharging. And the big one is... 4) With two BMS units the load is split equally so each one does half the work and extends the life of my batteries & BMS. The only con would be if one of your batteries is lower in max SOC then it will cut-off a bit sooner than the other. However, as you stated they do balance between each other after the charger turns off. My two batteries are only off by +-.010v and pass power between each other for about 10 minutes to a final balanced SOC.

Note: I set up my system for long cell life by top balancing each battery before hooking them in parallel, then configuring the BMS to operate from 90% SOC down to 20% SOC. This conservative setup is giving me +-30-40% more run time than my previous lead-acid batteries depending on loads...

Thanks Mark. My concern is when there is a significant difference in SOC of each battery, so for example if one BMS cuts out and the other takes all the load for some time - what happens when you reinstate the tripped battery. I am presuming there will be a thumping great current flowing from the higher SOC battery to the other. This could potentially trip the BMS on over charging parameters??

 

[jonstret]

Nigel I am also planning exactly this configuration and my research / opinion is having the redundancy of parallel operations is a big plus. I was looking to spec my BMS to allow all "normal" loads and normal "charge" to run from one battery pack. In my case I have 3000/24/80 victron multiplus and 600watts of solar. Load and charge is going to be less than 150 amps.

This is a long way to answer your specific question. When a pair of batteries reconnect and equalise there may be a momentary high current but would expect the bms to handle that any then the charging and balancing there after. Which BMS are you planning to use ?

 

[MisterSandals]

then configuring the BMS to operate from 90% SOC down to 20% SOC

How are you configuring your BMS to do this?
Are you cutting of charging with your BMS at the top of every charge cycle? Presumably at some voltage?

 

[jonstret]

All in planning but I will configure my victron charger to cut the charge at just below the max LIFEPO4 top voltage and have the BMS to protect and balance. Same with victron MTTP Solar. I am going to add a new second alternator to my yanmar engine. I am leaning towards a balmar 24v 70amp alternator and their Max Charge MC-624 Voltage Regulator which can be programmed for LifePO4 and can deal with the BMS shutting down the battery and not damaging the alternator or regulator. The regulator also deals with managing heat in the alternator and reducing the charge rate as it get too hot.

 

[MisterSandals]

I will configure my victron charger to cut the charge at just below the max LIFEPO4 top voltage

But you said you were doing this with the BMS which did not make any sense.

 

[jonstret]

sorry didnt mean to confuse. Are you planning a different solution? What BMS are you using as I have not purchase that

 

[MisterSandals]

No worries, i was trying to get you to think about your system and to see if you understood what it REALLY takes to keep your SoC between 20-90%.

 

[jonstret]

Did I get it right ?

 

[MisterSandals]

Did I get it right ?

You did not say how you would do it so i cannot say. I think it is more complicated than you think.

 

[jonstret]

OK Won't hijack this thread and will read more but need to get it right.

 

[Rocketman]

I choose to go with 2p4s - all one battery on my MotorHome. We will see how it goes this spring.

My reasoning is I figured the batteries themselves are fine with either approach. But because I want to retain the emergency boost ability (being able to use the house batteries to help start the diesel motor), I needed a bms that can handle whatever the batteries can deliver. I also wanted to bms to interface with my Victron equipment. I choose Batrium for my bms.

When in boost mode my 270ah *2 = 540ah battery can deliver power at 2C house that will give me the 1100CCA the diesel will need to start if it is cold. (Although I doubt if it would use that much - paralleled with the starting battery +solar+generator charging).

The choice of your bms determines the battery.

 

[Nigel Snook]

Nigel I am also planning exactly this configuration and my research / opinion is having the redundancy of parallel operations is a big plus. I was looking to spec my BMS to allow all "normal" loads and normal "charge" to run from one battery pack. In my case I have 3000/24/80 victron multiplus and 600watts of solar. Load and charge is going to be less than 150 amps.

This is a long way to answer your specific question. When a pair of batteries reconnect and equalise there may be a momentary high current but would expect the bms to handle that any then the charging and balancing there after. Which BMS are you planning to use ?

Thanks Jon - I am planning on an "external control" type BMS, probably the REC. I am still concerned that reinstating a parallel connection with a battery that has a higher state of charge will simply over current trip the battery that is on line. I don't believe a BMS has the capability to limit current - just switch it off.....

 

[Rocketman]

Nigel- look at the Batrium bms - it is a lot like Rec-bms - but it seemed at lot easier to make changes.

 

[markver]

How are you configuring your BMS to do this?
Are you cutting of charging with your BMS at the top of every charge cycle? Presumably at some voltage?

How are you configuring your BMS to do this?
Are you cutting of charging with your BMS at the top of every charge cycle? Presumably at some voltage?

Yes, I set the BMS charge only to 3.5v/cell, and discharge to 2.8v/cell.

 

[MisterSandals]

BMS charge only to 3.5v/cell, and discharge to 2.8v/cell.

The BMS will not charge. But you can set it to disconnect your battery at 3.5v as well as 2.8v.
But a BMS is designed and intended to be a safety disconnect and not a charge manager.

Why would you not set your charge controller to stop charging at 3.5v per cell? That is what a configurable SCC is designed and intended to do.

 

[markver]

The BMS will not charge. But you can set it to disconnect your battery at 3.5v as well as 2.8v.
But a BMS is designed and intended to be a safety disconnect and not a charge manager.

Why would you not set your charge controller to stop charging at 3.5v per cell? That is what a configurable SCC is designed and intended to do.

I have a basic pwm solar charger and an RV lithium 14.4v shore power charger that cuts off at full battery charge. Both are non-configurable, it works great for my application.

 

[markver]

a BMS is designed and intended to be a safety disconnect and not a charge manager.

Why not? Now it does both jobs and all the cells stay well balanced thru charge and discharge cycles.