Adding smaller amp lithium to existing setup?

[zac]

Can you add a smaller amp lithium battery to a larger one ie I currently have a 460amp battery but not enough space for an additional 460amp so can you add say a 230amp or 100amp battery as long as from same manufacture?
Just want to maximise the space I have while I can afford it.
All the searches on this site seem to just reference 2 of the same size in parallel.

Hope this is not a stupid question

 

[Gromett]

Hope this is not a stupid question

There is no such thing as stupid question. Only stupid people think there are

On your question. I don't know categorically, But from memory I recall someone did an experiment with this and found that both batteries charged/discharge at a commensurate rate.

Theoretically the voltage falling as they discharged would keep them roughly in sync and they wouldn't discharge into each other. You would need to start with both independently fully charged.
For the best chance of success both batteries should be identical chemistry, preferably same manufacturers cells inside the battery. All EVE cells for example and you would need to ask.

That said. Should you do it? Would I do it? Should you depends on how desperate you are for extra power. Would I do it? No.

If I wanted a second but different sized battery. I would be tempted to have it totally separate running separate circuits to the bigger one.
For instance I would have the smaller one running my heater and lights and the bigger one running everything else such as inverters. Or visa versa if that was more suitable.

The above is just my view/opinion. I can't remember the source and there will no doubt be some on here who disagree.

 

[zac]

I don't need to do it but then a lot of what I do I don't need too just interested to see if it's possible without causing any issues

 

[Gromett]

PS: Ensure you fuse the link between the batteries if you do this just to be on the safe(r) side.

 

[LithiumConvert]

Only stupid people think there are

I don't think that came out the way you intended.

Hope this is not a stupid question

No, it certainly is not.

I have a bank of Sterling lithium batteries at home. Some twice as big as the others. They have been working well together for over five years. But check what your manufacturer says. Some limit how many.

Only need fuses between them if they are not physically next to each other and then you need a fuse at each end of the cable close to each battery. (to protect the cable)

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[MisterB]

this might not make financial sense, but could you sell your 460 and buy two 300Ah? of course its dependant on size available and getting a good price for your 460?

another alternative might be to build your own battery, so it fits in a specific space - Roger Ivy built one on his youtube channel (he is a member on here)

 

[RogerIvy]

For the past year we’ve had four different batteries connected in parallel without any issue. Smallest is 240ah on fairly cheap cells and largest is 300ah.
No problems.

edit: we’ve done this deliberately to see how it all goes

 

[Roger S]

I have a 230a Fogstar a nd was thinking of adding another but smaller battery and on the Fogstar website when someone asked if you can add a different size battery their site reads "No, all batteries connected in series and/or parallel must be the same capacity rating, same make and same model."

 

[LithiumConvert]

Lithium, in parallel, is far more tolerant. But, I do not think I will mix different manufactures.

 

[zac]

I have a 230a Fogstar a nd was thinking of adding another but smaller battery and on the Fogstar website when someone asked if you can add a different size battery their site reads "No, all batteries connected in series and/or parallel must be the same capacity rating, same make and same model."

Thanks, oh well that's out as don't have capacity for another massive 460ah battery.

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[bigtwin]

For the past year we’ve had four different batteries connected in parallel without any issue. Smallest is 240ah on fairly cheap cells and largest is 300ah.
No problems.

edit: we’ve done this deliberately to see how it all goes

That doesn’t surprise me; have you data logged the information to demonstrate to the naysayers?

Others have undertaken the same experiment with, IIRC, a 5Ah battery and a 100Ah battery and demonstrated shared charge/discharge in direct proportion to their capacities.

Lithium, in parallel, is far more tolerant. But, I do not think I will mix different manufactures.

Whyever not? They will charge/discharge in proportion to each of their internal resistances. Physics doesn’t recognise who manufactured them.

Ian

 

[RogerIvy]

That doesn’t surprise me; have you data logged the information to demonstrate to the naysayers?

Others have undertaken the same experiment with, IIRC, a 5Ah battery and a 100Ah battery and demonstrated shared charge/discharge in direct proportion to their capacities.

Whyever not? They will charge/discharge in proportion to each of their internal resistances. Physics doesn’t recognise who manufactured them.

Ian

We’ve not collected any data, just observed using the app.
In our case they’re all on the same BMS (JK) but different cells in terms of capacity, manufacturer and quality.

Because they’re different cells their voltages and internal resistance are somewhat different. They discharge and charge at different rates but that doesn’t seem to affect anything. Batteries are always at different SOC and the 240ah turns discharge off much earlier than others (to be expected).
From what we can see it’s only when they go below the lower knee that the batteries with higher SOC (voltage) discharge into the lower. But at the end of the day they all eventually discharge until the BMS cuts off to protect the cells. We’ve run the whole lot down to zero SOC a few times just to see what happens. Perhaps there is a matter of lower efficiency when the higher SOC (voltage actually) discharge into the lower … it would be interesting to do some real tests.
Above the upper knee all four batteries receive equal charge (same amps) even though different capacities and different internal resistance (and TBH different quality). We’ve pumped in 200A and each battery runs at around 50A.

I must add that the four batteries are connected to a Multiplus and used in a real world situation (our workshop). As such they are not on a test bench. It would be interesting to do new individual capacity tests and then to test the whole pack on the bench. Maybe for a future YouTube nigelivy

On the subject of mixing manufacturers, I don’t have a problem with this.
It‘s the mixing of BMS types that I would not do. Previously we tested two batteries one with a MOSFET-based BMS and the other with relay-based BMS and that particular setup I’d not recommend (seems a basic no-brainer but we tried it anyway). In my opinion you can only really mix different capacity batteries as long as they’re all MOSFET-based (Charge and Discharge must be controlled separately).

 

[bigtwin]

Because they’re different cells their voltages and internal resistance are somewhat different. They discharge and charge at different rates but that doesn’t seem to affect anything.

The above makes sense.

Above the upper knee all four batteries receive equal charge (same amps) even though different capacities and different internal resistance (and TBH different quality). We’ve pumped in 200A and each battery runs at around 50A.

The above not so; the current flow must be proportional to the resistance I = V/R.
However, the battery capacities are all similar (it’s not clear if the capacities you quoted were claimed, or measured) so currents ‘at around 50A’ may in fact be within the error bounds of the individual resistances.

Ian

 

[RogerIvy]

The above makes sense.


The above not so; the current flow must be proportional to the resistance I = V/R.
However, the battery capacities are all similar (it’s not clear if the capacities you quoted were claimed, or measured) so currents ‘at around 50A’ may in fact be within the error bounds of the individual resistances.

Ian

The capacities were measured.
When I say each battery was around 50A, that would be within about 10% variation. So some room for different internal resistance.

 

[Tombola]

Both batteries should have the same nominal voltage (usually 12.8V) to balance charging and discharging

Ideally, batteries should have similar internal resistance to ensure even cell balancing and prevent overcharging of one battery by another.

And that your bms will cover both batteries if required

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[bigtwin]

The capacities were measured.
When I say each battery was around 50A, that would be within about 10% variation. So some room for different internal resistance.

The difference between the largest battery (300Ah) and the smallest battery (240Ah) of your bank is 20% (of the largest battery), less, presumably, for the other two batteries. I would still like to believe that the current being drawn by each battery was proportional to its capacity and internal resistance.

Ian

 

[RogerIvy]

The difference between the largest battery (300Ah) and the smallest battery (240Ah) of your bank is 20% (of the largest battery), less, presumably, for the other two batteries. I would still like to believe that the current being drawn by each battery was proportional to its capacity and internal resistance.

Ian

Yes I agree.
I was just pointing out that the charge rate was surprisingly similar.
Discharge rates had bigger differences as I recall, can’t remember what they were.
As I said, it would be good to bench test individual batteries again and then as a fully charged pack.