question

jens-due avatar image

Parallel 65AH and 100AH battery in 48v ESS system

I have 4 strings of 4x65AH battery's can i ad a string of 4x100AH battery's to increase the total capacity for my ESS system MultiPlus II 48v 3000VA

The battery's a SUN Battery SB12-65 (12V65AH) and SB12-100 (12V100AH)

https://www.battery-kutter.de/main/additional_files/Daten%20Webshop/SUN%20Datenbl%C3%A4tter%20Nov%202019/12V/SB12-65.pdf

https://www.battery-kutter.de/main/additional_files/Daten%20Webshop/SUN%20Datenbl%C3%A4tter%20Nov%202019/12V/SB12-100.pdf


ESSbattery capacity48v battery
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If my maths is correct, you have 16 x 65Ah batteries? Stringed up as 4p x 4s ??
2 Answers
marekp avatar image
marekp answered ·

I do not see why not.

You have to make sure that they are at the same SOC when connecting them in parallel.

Edit:

The discharge current of such parallel system should be adjusted to the smaller battery in this parallel setup.

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I would agree with @MarekP, this would work and Pylontech actually allows this config but I would be hesitant to do it without a BMS. I would do it if you maybe install a Victron Battery Balancer to make sure the 2 strings stay balanced but that means you would need 6 balancers.

Hi @shaneyake

Author has a AGM batteries and they do not need BMS.

They would benefit from battery balancers.

He would need 15 battery balancers for his 12V AGM batteries. (12 for 65Ah sets and 3 for new 100Ah set)

michelg avatar image
michelg answered ·

Hi Jens-Due, in my opinion, this is a very bad idea. The batteries not being of the same capacity, it will create an unbalance between the batteries, and creating also issues with the cabling and charging. You will toast the smaller batteries as the mppt will continue to charge (unable to get the real picture of the SOC).

Michel

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Can you explain how MPPT would toast the smaller battery?

SOC of those batteries is judged by its voltage.

Two batteries, regardless of their size, connected in parallel have the same voltage.



Hello Marekp, circulating current will result from the slightest difference of cell voltages. Cell resistance is the only restraint against a dead short circuit for the difference voltage. Batteries go to hell without even being connected to a load per se.

For short-term tests , yes you can, but you you will be disappointed with this arrangement if you install it permanently - there will be a quite high self-discharge going on unless you separate the batteries (e.g. with a relay) when they are not in use or being charged.

Both batteries will lend in the extension of usable power time with the lower ampere battery dieing out first which thereafter will speed the end of usable power remaining battery.

So, definetely a no way to go ! Unless you want to change your smaller capacity battery bank soon.

Have fun. Michel

PS, forgot to mention : However a parallel circuit is also a loop, so what's actually going to happen is that the battery with the higher voltage is going to push current backwards through the weaker one, charging it in an uncontrolled and probably destructive way.

Why the lower capacity would die out first?

Battery when discharging is lowering its voltage, but when connected in parallel to the other battery, it will not lower its voltage until the other battery does it.

The other battery will provide the missing power until it is empty.

As a result both batteries will be empty at the same time.


@MarekP I did such kind of experience, and the SmartProtect did trigger, so a no go.

Two batteries of different types, if connected in parallel is not recommended. Because of these reasons:

They may have different terminal resistances and will deliver different currents to the load.

They will take different charging currents and both will not be charged to the same level. Also they will not be discharged to same level while on load.

Their terminal voltages may not be same and one having the higher voltage will deliver charging current to the other and a circulating current will arise and heat up both.

Two batteries of different types, if connected in parallel is not recommended.

I agree, but in this case batteries are of the same type (chemistry) and even the same manufacturer.

They may have different terminal resistances and will deliver different currents to the load.

I am sure that they have different internal resistance (IR). The one with lower capacity has higher IR and the one with higher capacity lower IR.

This difference assures that the smaller battery gives/takes lower current than the bigger battery when connected with it in parallel.

They will take different charging currents and both will not be charged to the same level. Also they will not be discharged to same level while on load.

That is not true.

At the end of charging/discharging smaller and bigger battery will be fully charged/discharged.

Charger finishes charging when battery reaches proper voltage, and both batteries reach this voltage at the same time. Same with discharging.

Their terminal voltages may not be same and one having the higher voltage will deliver charging current to the other and a circulating current will arise and heat up both.

This is also not true.

Batteries connected in parallel have the same terminal voltage.

Battery cannot be discharged and charged at the same time and this is what you are suggesting.

P.S.

Where did you have this SmartProtect connected?

I will stop here the discussion, as finally, Marek, you're not the one concerned by the problem. Batteries in parallel may not have the same voltage for several reasons, furthermore if a group of them is older than the other. Worse, the more powerful and charged battery (battery bank) will discharge into the others, and yes they can be charged and discharged at the same time : mppt charging the batteries and one of them discharging into the others.

It is reality, and that's one of the reasons why all the batteries must be changed together to avoid such kind of ssues.

Have a nice day.

Michel,

Your statement that batteries connected in parallel have different voltages defies logic.

The other statement that same battery can be charged and discharged at the same time is also illogical.

Current can only flow in one direction.

Current will flow from one battery to the other only when they have different voltages at the moment of joining.

It is reality, and that's one of the reasons why all the batteries must be changed together to avoid such kind of issues.

I do not disagree with this statement.

Have a great day.