Battery balancing question

[aragonx]

Hi,

I put together my 2nd battery from 4 new Echo-Worthy 100Ah 12v batteries (to make 48v). I have 4 existing 100Ah 12v (in series for 48v) batteries that are currently running my system.

Here is my thinking. I think I need a balancer for the 4 existing batteries to keep them in sync with each other. Then I need another balancer for the 4 new batteries to keep them in sync with each other. Then I need a 3rd balancer to keep the two sets in sync with each other. I was thinking of buying 3 of these to do the job:

https://www.amazon.com/dp/B0CKTQF6J9/?coliid=IHBD3A2N9JBOA&colid=23DPE0349MDID&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

I was hoping I could get a BMS/balancer for all 8 batteries. My goal is to add 8 more at some point so they will all be slightly different. But it seems most of the BMS/balancers only work on cells that are 3.5v ish. Not on 12v "cells" like what I have.

So my question is: Is that they best way to do this or is there a better way?

 

[sunshine_eggo]

You need to individually charge each 12V to full (14.4V for 2 hours). Note if any trigger charge protection before you get to 14.4V. Then place in parallel @ 14.4V for 2 hours.

Not familiar with that balancer, but 12V balancers are commonly used to help maintain balance. I used a bunch with my 12V/48V Lead acid bank.

Contrary to their diagram, I would get a separate balancer for each 48V string.

 

[aragonx]

You need to individually charge each 12V to full (14.4V for 2 hours). Note if any trigger charge protection before you get to 14.4V. Then place in parallel @ 14.4V for 2 hours.

Not familiar with that balancer, but 12V balancers are commonly used to help maintain balance. I used a bunch with my 12V/48V Lead acid bank.

Contrary to their diagram, I would get a separate balancer for each 48V string.

I think I understand you are saying I should get at least a balancer for each string? Adding the 3rd balancer to keep each string in sync seems like the logical next step to that. So I need 3 balancers today and one for each string if I keep using the same type of batteries?

 

[sunshine_eggo]

Only one balancer per 4S string. The fact that each string is connected in parallel at their main terminals means each 4S string will be in balance with the other 4S strings by default.

4S2P bank needs 2 balancers, 1 per string.
4S3P bank needs 3 balancers, 1 per string.

I don't understand why you think a 3rd balancer is needed. I hope I've addressed that above.

 

[aragonx]

Hi,

I just assumed each string would get out of sync with each other string. But what you are saying makes perfect sense. Sorry, I'm a little slow at times but I'll get there. lol

 

[sunshine_eggo]

Hi,

I just assumed each string would get out of sync with each other string. But what you are saying makes perfect sense. Sorry, I'm a little slow at times but I'll get there. lol

I can definitely take awhile to jam all this stuff into what looks like intuition. I still have several headdesk moments where I forget something very simple or basic...

 

[aragonx]

Oh, I just realized that I never answered your question.

The reason I think I need the 3rd balancer is because the 2 parallel strings will drain and charge at different rates. So, for instance, string 1 could fill up before string 2. The voltage would kick the charge controller into float and never fully charge string 2.

Is that not correct?

 

[MisterSandals]

The voltage would kick the charge controller into float and never fully charge string 2.

With the 2 batteries connected in parallel, their voltages will be the same. Does your SCC charge determination operate on voltage or SoC?

 

[aragonx]

With the 2 batteries connected in parallel, their voltages will be the same. Does your SCC charge determination operate on voltage or SoC?

I think it's voltage. I think the smart shunt is just sharing voltage and current to the SCC. And the batteries are dumb, without any communication interface at all.

 

[MisterSandals]

I think it's voltage. I think the smart shunt is just sharing voltage and current to the SCC. And the batteries are dumb, without any communication interface at all.

So the batteries will always be the same voltage. What would a third balancer do then?

 

[aragonx]

So the batteries will always be the same voltage. What would a third balancer do then?

So when lifepo4 batteries get full, don't they up the voltage that they are putting out? I'm guessing the bus will have the average of whatever strings are connected to it. So if one string is full and putting out 58.4 and one string is putting out 54.4, then the bus would read 56.4?

 

[MisterSandals]

So if one string is full and putting out 58.4 and one string is putting out 54.4, then the bus would read 56.4?

If they are connected in parallel they will have the same voltage.

 

[robbob2112]

So when lifepo4 batteries get full, don't they up the voltage that they are putting out? I'm guessing the bus will have the average of whatever strings are connected to it. So if one string is full and putting out 58.4 and one string is putting out 54.4, then the bus would read 56.4?

This is why you connect the batteries in parallel at the battery level verse just at the ends. So you have 4 packs of 2 batteries strung in series verse 4 batteries in series twice then parallel. i.e. they are or should be 2p4s verse 4s2p.

IMO
It is also why you need to charge all the batteries fully before connecting, then just connect up 4 sets of 2p and let them equalize a few hours. Then string the parallel sets into a series string.

There are diagrams of how to do it so you pull an equal amount of current from each battery verse the batteries at one end being at a different state of charge than the other end. Some simple as the negative on one end and the positive on the other end pulling from the opposite side. Some take more wire and connect in a Z fashion to keep things the most equal. Do yourself a favor and research some on how to do it for the most efficient way.

I assume you started with 12v series for some reason and are sticking with it because you have it. Would have been simpler with just 2 x 48v batteries in parallel.

 

[sunshine_eggo]

Provided you connect the batteries in parallel via best practices, there shouldn't be a detrimental imbalance in operation.

See link #6 in my sig

 

[robbob2112]

Provided you connect the batteries in parallel via best practices, there shouldn't be a detrimental imbalance in operation.

See link #6 in my sig

So, I read the section on battery bank wiring

3. Battery bank wiring

www.victronenergy.com

Section 3.3 is what I was talking about as far as the different ways to connect things. How does that work with 8 batteries? just simple 4 sets of 12v then each set lines link the diagrams? or is there some slicker way to do it?

 

[sunshine_eggo]

So, I read the section on battery bank wiring

3. Battery bank wiring

www.victronenergy.com

Section 3.3 is what I was talking about as far as the different ways to connect things. How does that work with 8 batteries? just simple 4 sets of 12v then each set lines link the diagrams?

Yep. Those battery voltages aren't labeled.

or is there some slicker way to do it?

Technically, there are only two batteries - two 48V batteries. It doesn't matter that they're in 12V packages. All 12V in each string MUST flow exactly the same amount of current, or you violate physics, so the only consideration is that you get equal current between the two strings using best practices to make that happen. It can be more challenging because you have more physical connections to deal with, crimp and torque properly.

The balancers are then used to ensure the individual 12V maintain a top balance.

 

[aragonx]

The only reason I stuck with these types of batteries is because they were cheap and easy to get on-line.

Here is a diagram of what I have and my proposed wiring.

 

[robbob2112]

So, based on the victron docs...

Connect every post on the bottom row to the post on the top row. You end up with a box at each junction and either 1 larger wire from oposite corners or still the two wires.

 

[sunshine_eggo]

So, based on the victron docs...

Connect every post on the bottom row to the post on the top row. You end up with a box at each junction and either 1 larger wire from oposite corners or still the two wires.

No.

This:


Is consistent with the Victron best practice. Victron does not label those batteries as 12V, and if you want to split hairs, they are using 24V inverters, so each of those parallel batteries must be 24V.

The two strings of 4S 12V are 48V batteries like this:

 

[robbob2112]

Not arguing at all, but wouldn't cross-connecting all of the batteries allow the parallel batteries to stay equilized with each other then you only need 1 of the equalizer units? If you series connect without the cross--connects won't you need 2?

When I do my system it will just be 48v batteries, but want to understand everything.