1 Large battery better than 2 smaller ones?

[JeremyFowler]

I'm sure this question has been asked often before but I can't find the answer in the 10 mins that I've been searching. I probably don't know how to search the forums properly...
Assuming both 'banks' have the same Amp hours, are 2 smaller batteries the exact equivalent of 1 larger one?
Pros that occur to me are: if there is a cell failure it is cheaper to replace only the 1 battery. Easier to lift.
Cons: extra connections, extra complexity. But maybe there is some other factor that I don't know, something like the batteries interacting with each other or something.
Cheers!

[Ballsnall]

Originally Posted by JeremyFowler

are 2 smaller batteries the exact equivalent of 1 larger one?

Yes they are equivalent but a single battery is always preferable to multiple smaller batteries when creating a house bank.

[JBP]

Originally Posted by Ballsnall

Yes they are equivalent but a single battery is always preferable to multiple smaller batteries when creating a house bank.

Why would a single battery be always preferable? Lots of people use multiple batteries even though huge single batteries are available. I'm going to be facing this choice soon too.

[ItDepends]

All other things being equal, which they never are…

The primary cause of early death of a battery bank is a shorted cell in one of the individual batteries. Assuming that the failure rate is random, and not related to cell capacity, a battery bank made of 6 cells would have half the failure rate of one with 12 cells. The key is the number of cells, not the number of batteries.

Of course practicality rules. You couldn’t lift a 500 amp-hr 12 V battery with 6 cells, so we have to split them up.

This is why large battery banks are frequently put together with 6 volt batteries (3 cell) or even 2 volt batteries (single cell).

[s/v Jedi]

It is not just a question of 1 big or 2 smaller batteries: actual capacity is a main factor.

Step 1: you must have a battery that has enough capacity to make it through the night comfortably.

Step 2: add another identical capacity battery. Give each battery it’s own monitor, BMS, main switch etc. (so the negative of each battery goes through a shunt for that battery’s monitor before anything else)

Step 3: add one extra battery monitor (I recommend a Victron SmartShunt) that, on the battery side, ties all batteries together (interconnecting theirs hunts) and on the load side connects to the main negative bus bar. This is the monitor that you configure to show SOC for the combined batteries.

Now the advantages:

- You have ample battery power available. Even when it’s a cloudy day (solar) or little to no wind (wind generator), you can probably go through another night without starting an ICE to recharge the batteries.

- You have redundancy. Even when a battery fails, the one you have left still allows you to function normally.

- Easy maintenance. Install the 2nd (new) battery when the 1st is at 50% of it’s expected lifespan. You can replace a battery while staying fully operational.

- In case of LiFePO4: you can create a battery out of cells that only connect in series. This allows monitoring of each individual cell, reduces complexity and wiring, optimizes keeping the cells in balance and doesn’t require enormous investment in large cells double the capacity.

[JeremyFowler]

Thanks for the input! As this is a new design, and I've not got much experience with this as yet, I think I'll go for simple Flooded Lead Acid batteries now. I've enough space, and I'm not too concerned about the weight. I'll take sv jedi's design advice into account. Cheers!

[Tetepare]

Originally Posted by s/v Jedi

Give each battery it’s own monitor, BMS, main switch etc. (so the negative of each battery goes through a shunt for that battery’s monitor before anything else)

Suggestions for 12v lead acid? I find monitors from $18 to $250 from name brands to no names. For three batteries at $200 for each monitor plus the shunt it would be closing in on $1000, not including batteries. It’s what I’d like to do, but is it cost prohibitive, and which devices? Thanks.

[s/v Jedi]

Originally Posted by Tetepare

Suggestions for 12v lead acid? I find monitors from $18 to $250 from name brands to no names. For three batteries at $200 for each monitor plus the shunt it would be closing in on $1000, not including batteries. It’s what I’d like to do, but is it cost prohibitive, and which devices? Thanks.

Unfortunately, my recommendations are based on technical and practical factors, not considering cost. Two out of three ain’t bad, right?

Seriously, the systems I propose only make sense for larger systems, aiming at 10kWh batteries, large solar arrays, all electric galleys etc. If you have a smaller system, where LiFePO4 doesn’t make sense, then my recommendations may not fit.

That said, how will you know SOC of each battery and of them switched together? I guess simple voltmeters plus a table showing SOC for a voltage range will be the best fitting method. You switch a battery off with it’s main switch, wait a couple minutes to let it settle, then measure voltage and using the table you have a good indication of it’s state of charge. This method works well for lead acid and voltmeters are easy on the budget.

[PaulCrawhorn]

Only one BANK for House is better than multiple.

But one series string of cells as opposed to two? I like the redundancy aspect, including two BMS but yes more points of failure, so maybe only if planning to cruise in primitive places.

At the cell/group level, ideal is cells at the Ah capacity you want for the whole string, so 1P8S better than 14P8S

but not a huge risk factor, assuming your cells are brand new, top notch quality tested as well matched before commissioning.