Series / Parallel Battery Banks

Mountain Don · November 2014

When connecting FLA batteries in a series / parallel system the usual thing is to connect batteries in series first. Then connect those series strings in parallel using a variety of schemes to try to mitigate resistance differences. Often this is done with 6 volt batteries but can also be done with 2 volt cells.

Reading about LifePO4 cells being connected in series / parallel to achieve the needed voltage and amp-hours capacity I have noticed it is done differently. The main difference is that these LFP's are being assembled from cells, not from prepackaged multi-cell batteries. They connect the cells in parallel packs first. Then connect those parallel packs in series. As illustrated by this drawing...

(12 cells or batteries, four sets of 3 in parallel and then connected in series...)

Would this be beneficial to a bank of FLA's? Beneficial in evening out the varying resistances between cells when configured in the 'normal' FLA off grid battery bak configuration? ???

BB. · November 2014

Re: Series / Parallel Battery Banks

It is actually a very good question...

What is a large 200 AH cell vs 2x 100 AH cells in parallel? In theory, not much.

From my point of view, I have always preferred series connected cells. It allows us with a meter to measure individual cell voltage in parallel strings.

Also we can measure string current with a DC Current Clamp DMM you can also measure per battery string current. This allows you to pretty easily and quickly identify poor performing cells (open cells that don't pass current, shorted cells that drag down overall string voltages).

When you have paralleled connections and then series for cell group to cell group--It makes it almost impossible to find a bad cell until other cells are damaged or the overall bank performance starts to fade.

I have always suspected (not that I have researched the reasons) that the LiFePO4 batteries are connected in this method is to allow the use of fewer BMS (battery monitor sensors/system) connections. You have once cell monitor for 1/2/4/etc. cells (however many are in parallel).

Whereas most lead acid systems are either not going to have per cell BMS setups, or that large Lead Acid Cells can be in the 1,000+ AH range per cell--Vs the 10's of AH ranges for the typical LiFePO4 cells we have seen so far.

Having more BMS "taps" on a battery bank would make an already "expensive" LiFePO4 installation even more costly (and with more components that could fail over time).

However, if the folks here who have already experimented/used LiFePO4 batteries in their (relatively) low power solar off grid power systems today don't find a need for a BMS system--Then perhaps it would be worth revisiting the series/parallel per cell connection question on our off grid power systems (assuming the BMS system are more needed on high discharge rate battery banks--such as for electric vehicles).

-Bill "my thoughts" B.

karrak · November 2014

Re: Series / Parallel Battery Banks

Mountain Don wrote: »

(12 cells or batteries, four sets of 3 in parallel and then connected in series...)

Would this be beneficial to a bank of FLA's? Beneficial in evening out the varying resistances between cells when configured in the 'normal' FLA off grid battery bak configuration? ???

This configuration is not a good one for sharing current evenly. Here is a link to a very good article on this subject http://www.smartgauge.co.uk/batt_con.html I am using 'method 2' for my 4p8s battery

BB. wrote: »

From my point of view, I have always preferred series connected cells. It allows us with a meter to measure individual cell voltage in parallel strings.

Also we can measure string current with a DC Current Clamp DMM you can also measure per battery string current. This allows you to pretty easily and quickly identify poor performing cells (open cells that don't pass current, shorted cells that drag down overall string voltages).

When you have paralleled connections and then series for cell group to cell group--It makes it almost impossible to find a bad cell until other cells are damaged or the overall bank performance starts to fade.

I agree with this if you don't have an BMS measuring single cells or groups of cells in parallel. If you do have a BMS you would pick up the weak group of cells and could then check the individual cells in the parallel block.

I have always suspected (not that I have researched the reasons) that the LiFePO4 batteries are connected in this method is to allow the use of fewer BMS (battery monitor sensors/system) connections. You have once cell monitor for 1/2/4/etc. cells (however many are in parallel).

Having more BMS "taps" on a battery bank would make an already "expensive" LiFePO4 installation even more costly (and with more components that could fail over time).

I think you are right about this

However, if the folks here who have already experimented/used LiFePO4 batteries in their (relatively) low power solar off grid power systems today don't find a need for a BMS system--Then perhaps it would be worth revisiting the series/parallel per cell connection question on our off grid power systems (assuming the BMS system are more needed on high discharge rate battery banks--such as for electric vehicles).

I am not convinced about not having an individual cell Battery Monitoring System for off-grid LiFePO4 (LFP) systems especially for systems over twelve volts. I think a BMS could also benefit Lead Acid battery banks as well. If they cost a fortune or would cause more problems than they might solve I would agree not to have them.

Simon

BB. · November 2014

Re: Series / Parallel Battery Banks

Yea--That has always been a concern of mine... 24, 48 volt, and higher voltage battery banks and the possibility of individual cell failure.

A 12 volt bank and you use a 2 volt cell (lead acid example), the difference between a 10 volt vs 12 volt battery bank voltage is obvious.

A 48 volt bank with a cell failure, you have 48 vs 46 volts--Not near as obvious cell failure.

A lead acid battery with a cell getting (some) over voltage/over charging is not fatal to the cell (as long as the water is still covering the plates).

For both lead acid and LiFePO4 batteries a cell going "dead" is fatal to either cell's life. Difference is that on a group of series cells, it obvious when you hit the "dead/over discharged cell stage". With an LiFeP04 battery, it only a few millivolts (or 10's of millivolts) between "good" and "bad" level of discharge in a cell.

Neat thing with the "large" LiFePO4 cells is that they can be taken apart and the bad cell(s) can be replaced. Most multi cell lead acid batteries--You are forced to replace the "battery" or entire 4/6/8/12 volt group of cells.

And that is always the scary part of LiFePO4 ownership (for me)--Knowing the state of charge of individual cells by measuring the bank voltage--The values between good and bad are so small. And the cost of failure is so close.

With lead acid, taking a cell to zero volts does pretty much kill the cell--But you can usually "limp" through for a few days/weeks until the cell is replaced (or, if you have two or three parallel battery strings, you can disconnect the bad string).

With paralleled cells in LiFePO4--You are taking out multiple cells because they are paralleled. Same reason I do not like to parallel lead acid cells.

A question about LiFePO4 cells... Do they fail "shorted"? Lead acid cells can, and that is another reason I like series strings of cells--One fuse per series string to limit excessive current flow. When you have cells bolted in parallel, no chance for fusing (at least, I have not read about "unprotected Li cells" having fusing).

Because LiFePO4 batteries are "more perfect" (less losses (no gassing losses) and cell leakage of energy (low self discharge)) than Lead Acid--Monitoring a pilot cell and using a Battery Monitor might give you "good enough" daily status of the battery bank. And all one would have to do is a weekly (or monthly) check of individual cells with a DMM for any cell equalization issues.

-Bill

Cariboocoot · November 2014

Re: Series / Parallel Battery Banks

Mountain Don wrote: »

Would this be beneficial to a bank of FLA's? Beneficial in evening out the varying resistances between cells when configured in the 'normal' FLA off grid battery bank configuration? ???

Answering this question: no.
For FLA or AGM do not parallel first and then connect in series.
Especially do not do both; the last person who tried that got magic smoke coming out.

karrak · November 2014

Re: Series / Parallel Battery Banks

Here is a simple thought experiment.

If we have a 48 volt battery made up in this configuration, 2p16s for LFP and 2p20s for Lead, i.e. cells in parallel and then in series. Say one cell dies completely when the battery is at 100%SOC. If we now discharge the battery down to 40%SOC we get the situation where the partner cell of the dead cell will be at -10%SOC, i.e. reverse biased, death for cells of any technology. Now if we have individual cell monitoring the alarm bells would go off as soon as the partner cell got to a voltage that would damage it.

Simon

Blackcherry04 · November 2014

Re: Series / Parallel Battery Banks

karrak wrote: »

Here is a simple thought experiment.

If we have a 48 volt battery made up in this configuration, 2p16s for LFP and 2p20s for Lead, i.e. cells in parallel and then in series. Say one cell dies completely when the battery is at 100%SOC. If we now discharge the battery down to 40%SOC we get the situation where the partner cell of the dead cell will be at -10%SOC, i.e. reverse biased, death for cells of any technology. Now if we have individual cell monitoring the alarm bells would go off as soon as the partner cell got to a voltage that would damage it.

Simon

Great explanation karrak of the " Cascading Effect ". It shows why a a proper BMS is required on a LiFeP04 bank, but could be used on any technology.

Did someone say a lot of wires ?? No, I don't have a link, search Google images.

Attachment not found.

BB. · November 2014

Re: Series / Parallel Battery Banks

And each of those wires should have a fuse/resistor/fusible link of some sort to protect against short circuits themselves.

It gets to be a mess to install BMS per cell systems.

And the problem with parallel bused/connected cells, is you cannot measure and determine a good cell from a bad cell. And a bad cell could take its paralleled brother down too.

-Bill

Blackcherry04 · November 2014

Re: Series / Parallel Battery Banks

BB. wrote: »

And each of those wires should have a fuse/resistor/fusible link of some sort to protect against short circuits themselves.

It gets to be a mess to install BMS per cell systems.

And the problem with parallel bused/connected cells, is you cannot measure and determine a good cell from a bad cell. And a bad cell could take its paralleled brother down too.

-Bill

As the bank voltage goes up the problems magnify. A 48v bank has 16 Cell Lines ( paralleled cells, expressed as xxP16S), depending on the Amp Hour capacity and the size of the cells, the number of Parallel connections become a real problem to keep it all balanced. How many cells do you want ? 16, 32, 48, 64, 80, 96 etc. and on and on.

There are plenty of large cells available in the 300 to 1200 amp hr capacity, one should build a bank with the largest cells to get the capacity you want and cut the connections down. The other side of that is no one wants to replace a $1,000 cell.

Everything comes at a Price, then comes the tradeoffs.

Mountain Don · November 2014

Re: Series / Parallel Battery Banks

So once again it comes down to the simplest solution is likely the best; nothing parallel, only serial connections.

Series / Parallel Battery Banks

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