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Parallel/ series Vs Series/parallel

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IslandStu

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Sep 24, 2020
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Hi there, I'm trying to find out which is the best way to wire up my battery bank of 8 12v 225ah deep cycle gel batteries for a 24 v output which will achieve the most balanced load for charging and discharging without the need to swap around batteries so that any particular cell isn't overly stressed. I look around the internet but keep finding conflicting information about whether to use Parallel/Series or Series/Parallel. Could someone please enlighten me and put forward a diagram of most efficient method.

Many thanks in advance

Stu
 
2 batteries in series, then 4 sets in parallel
connect in/out cables from alternate ends
 
Yes I read this but it didn't explain how it works for eight batteries in a 24v load. I emailed the author of this but it was posted over ten years ago and the email might not be valid anymore, I'm quite new to all of this and would just like a straightforward diagram for my 8 12v batteries.
Thanks for your reply

Stu
 
If you want 24V this is the absolute minimum number of cables or busbars you can have, there IS no better way.
 
IslandStu said:
I got told this is the best way (see diagram)View attachment 23657
Click to expand...
I think this is the best way as well. this way you only have 2 cells to worry about being balanced. If you do 2s4p you have 8 cells to worry about keeping in balance. I had a 3p2s setup and I did see them get a little out of balance now and then so I just independently charged the low set.
 
IslandStu said:
I got told this is the best way (see diagram)
Click to expand...

I may be seeing this wrong.

When I look at the two banks in parallel with 4 batteries, and then put in series, what I see is two banks that are set up either as method one or method 2 in the smartgauge link and are pretty imbalanced. Seems to me the method 3 or 4 would still be needed to balance whatever batteries are in parallel to minimize the imbalance. Whether 4P2S or 2S4P, there's still imbalance, its just with the 4P2S, there's 2 parallel strings to deal with.

Here's excerpts from the smartgauge link:

Method 1.JPG​
Method 1
Battery internal resistance = 0.02 Ohms
Interconnecting lead resistance = 0.0015 Ohms per link
Total load on batteries = 100 amps
The bottom battery provides 35.9 amps of this.
The next battery up provides 26.2 amps.
The next battery up provides 20.4 amps.
The top battery provides 17.8 amps.
Method 2.JPG

Method 2
with the same 100 amp load....
The bottom battery provides 26.7 amps of this.
The next battery up provides 23.2 amps.
The next battery up provides 23.2 amps.
The top battery provides 26.7 amps.
 
mopat said:
12 cables
Click to expand...

Way I'm seeing this is still wired by method 2 and the middle two cells are drawing 10% less amps than the cells connected to wither the positive or negative cable going to the bussbar.

For me it's not about the bare minimum amount of cables, but getting the batteries receiving and sending a balanced amperage.

This is what I'm thinking, 4S2P.
4S2P.jpg
I certainly don't have the least amount of cables, but as much as possible, the middle batteries are balanced with sending and receiving the same amps, and not the difference listed in past 9.

I am still on my first install, but a this is my upgrade plan from 12 VDC to 24 VDC and how I plan on wring the batteries.
 
Just passing on an "expert's" opinion quoting from Cruisers Forum:

"I would not parallel then series connect lead acid batteries.

I would series each pair to make 24 volts, then parallel each pair using #3 on the smart gauge site.

This comes from 2 recent experiences, the last one was 10 L-16 Trojan 6 volts, that were 5 parallel each and series connected at what would be your #4 & #8 batteries. (See Post #5) Battery #8 was the first to die from warped plates, then battery #4. This bank was less then 8 months old, and deep cycled(50% SOC) less then 20 times.

The other bank was 20 X L-16 Trojans in a 5 each parallel, then series to make 24 volt, then each bank of 10 bats parallel connected. They died at 9 months, with the exact same symptoms 4 batteries cooked and the rest under charged.

Both banks could not keep water in the 2 bats that made the series connection. Each of these suffered from warped plates, the rest were hard sulphated. Trojan did not warrant any of the bats, because of the misconfiguration.

Not one singe Lead Acid Battery Manufacture I know, recommends first parallel then series. As a matter of fact they each show series pairs then parallel, each pair.

Lloyd"
 
Classic example of what happens when you have imbalance with any series connected battery. Under charge one battery (which may be a single battery or two + in a parallel configuration) will rise to a higher voltage than the others in the string, in fact there will be a range of voltages across the batteries depending on their exact state of charge and present amp hour rating. If the voltages across the batteries isn't kept in check the voltage across a battery could easily go over 15V* for the entire charge cycle. Not good for a sealed battery of any type, worse for an AGM and a disaster for a gel. Even an open cell battery will be degraded by frequent over-voltage but it will be a different mode of death because unlike a seal battery the lost water can be replaced.

Failure to recognise and control this voltage issue is what killed the batteries.

*of course scale battery voltages to suit the specifc case, obviously referring to 12V batteries in my post.
 
Last edited:
Craig said:
I think this is the best way as well. this way you only have 2 cells to worry about being balanced. If you do 2s4p you have 8 cells to worry about keeping in balance. I had a 3p2s setup and I did see them get a little out of balance now and then so I just independently charged the low set.
Click to expand...
And would it be a good idea every six months to swap the 'purple' lead with the two leads that go to the installation and vice versa?
 
mopat said:
I would series each pair to make 24 volts, then parallel each pair using #3 on the smart gauge site.
Click to expand...

Seems like this recommendation is the same as post #12, picture 1.

I am a few months away from putting together this battery bank, but I want to be sure this is the way to do this without getting the charging all out of balance.

The pics from post #5 and Post #10 look like the batteries when charging and discharging would be out of wack.

This will be six or eight batteries in this bank I’m putting together, so probably $1500 or $2000 for lead acid batteries, so I want to get this correct.
 
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