Series/parallel vs parallel/series?

[Harold2940]

Greetings.

This is just a general battery question. A friend of mine purchased four 6-volt golf cart batteries and wired them in series to run a 24V inverter. This is just for emergency backup power, and he will be charging them via a charger plugged into the grid.

Here's what he has now:

He is planning to purchase four more and connect them in parallel for double the capacity. He asked me how he should wire them, so I told him, like this:

Create another 24V serial string, and connect the two strings together in parallel. This is what I would call Series/parallel.

I worked for "Ma Bell" for over forty years, and was very familiar with central office power. We used twenty-four 2V lead-acid cells wired together to produce 48V battery strings. We had several (sometimes 5 or 6) 48V series strings connected in parallel to create a massive 48V power plant that could run the entire central office for a minimum of eight hours. In the case of a commercial power loss, the backup generator would usually auto start within seconds. The generator was always engineered to carry the full load of the office and keep the batteries charged. The eight hour window was for a real emergency situation where commercial power was lost, AND the generator failed to start. We had eight hours of battery time to either get the generator running, or bring in a portable generator.

All of our individual 48v strings of batteries were not connected together with wire. We used solid copper bars to interconnect the batteries and chargers.

Getting back to my friend's question, there is another way to connect them together, which is what I would call Parallel/series:

In this case, each 6V battery is connected in parallel with another, and the four 6V paralleled batteries are then wired in series. I have never seen this done, and in all the years of my time with Ma Bell, I've never seen her do it either. Other than considerable more interconnecting wire being more expensive, what are the ramifications of series/parallel vs parallel/series?


Thanks!

 

[DIYrich]

In option 2, all the wires have to handle max amps.

In option 1, each bank can have wire sized for amps of that bank.

Option 1 let's you take a battery out of service and keep power.

Option 2 can be done with one bms.

 

[Hedges]

A third option, another form of 2p4s, is to connect both strings in series but also connect in parallel. (only one of two dotted lines in drawing below.)
With two series jumpers in each location, each jumper carries half the current rather than all. Unless it's twin becomes disconnected.
Each parallel connection carries nominally zero amps (I think). But with the diagonal corner connection rather than busbars or matched cable lengths off the ends, the ones on the ends do carry current, and I haven't figured out about the ones in the middle. I might be tempted to skip on parallel wire gauge, but in case of a fault it would get too much current.

I'm not sure the pros and cons. So long as batteries remain identical SoC and voltage, nothing flows through the cross connects. Does this help them remain balanced? Averages out slight variations in capacity? (People DIY LiFePO4 and bin cells according to capacity, trying to make parallel groups which add up to the same.)

I have 16x 6V batteries I'll be using at 48V. Presently they are wired 8s2p and will be floated for storage.
Then I plan to connect one parallel connection for each intermediate point (I've seen drawings with two parallel connections, i.e. each two batteries are paralleled with two wires, then those pairs are connected in series with two wires.) I plan to buy a crimper and make 4/0 jumpers. Cables to multiple inverters likely 2/0 or smaller.

From FullRiver manual:

 

[Harold2940]

Thanks for your answers. You guys did bring out a point that I did not consider, that in the parallel/series configuration, all jumpers must carry full load current. That configuration also requires more jumpers, which would also be more expensive. I know we always used several serial strings of batteries in parallel at the phone company. That makes the most sense to me for home backup power as well.

Thanks again for your comments.