Which is "better", using single 100Ah LiFePO4 cells or multiple 5Ah cells?

[crossy]

Our 6.5kWP grid-tie solar is working fine, it just sits on the car-port and offsets our bill nicely.

Note that we are located in sunny Thailand so regulations are somewhat more relaxed than many are used to.
Our grid tie is currently net-metering (spinning the conventional disc meter backwards when exporting) which is technically illegal but there are many thousands of small systems doing just that.
In the words of our local PEA (the electricty authority) supervisor "if the meter reader doesn't see it going backwards ..." (eye, heart, grief)

Of course the engineer in my head is bored, I'm looking at "hybridising" the system whilst at the same time adding more solar capacity thus alleviating the "risk" of net metering here. This would also add a degree of "UPS" functionality and reduce the necessity for our (noisy) genset.

A hybrid system of course needs batteries which are the subject of this thread (discussion applies equally for those going totally off-grid too).

I can assemble 100Ah packs of 32650 cells for about the same cost as single 100Ah (golf cart) cells.

So, which is better, using single 100Ah LiFePO4 cells or the "Tesla" method of using multiple 5Ah cells?

A few thoughts:-

• Smaller cells cost less each, so gradual expansion of the pack can be done on an "available funds" basis.
• Smaller cells should allow for better cooling.
• The big cells need less effort to assemble packs.
• Failure of a single small cell just removes a small chunk of capacity, loss of a big cell removes a big chunk (or even the whole pack).

 

[snoobler]

Welcome to the forum.

I can assemble 100Ah packs of 32650 cells for about the same cost as single 100Ah (golf cart) cells.

So, which is better, using single 100Ah LiFePO4 cells or the "Tesla" method of using multiple 5Ah cells?

A few thoughts:-

• Smaller cells cost less each, so gradual expansion of the pack can be done on an "available funds" basis.

Pro

• Smaller cells should allow for better cooling.

False Pro. If cooling is an issue, you're over-using your big cells.

• The big cells need less effort to assemble packs.

HUGE Pro

• Failure of a single small cell just removes a small chunk of capacity, loss of a big cell removes a big chunk (or even the whole pack).

Only true if the cell is fused, and the fuse opens to take the cell out of the group. They can fail in a way that allows them to just bleed down their cellmates. Even when it fuses, you now have a disparity in capacities in your series string. One out of 20 is no big deal, but lose a few, and you'll be dealing with imbalance.

 

[crossy]

Thanks for the welcome

Yeah, the more I think about it the more attractive the larger cells become. I like the granularity of the smaller cells but multiple connections and the potential need for cell fuses makes it less attractive.

I suspect I will end up with multiple packs of 16S X 100Ah each with it's own BMS and fuse/breaker then the packs in parallel at the inverter connection.

You are of course correct about cooling although the environment here is around 30C (86F) all year, the packs would be shaded but no additional cooling.

We're probably looking at about 20kWh of storage at 48V, around 400Ah of available capacity although the maximum power draw would be about 3kW.

 

[snoobler]

The ~280Ah cells seem to be the sweet spot in terms of $/kWh. 20kWh would only need 24 cells, but for 48V, you'd need 32 - 29kWh. About $4000 for 2X 16S packs..

 

[time2roll]

What chemistry are the 5A cells? I like the safety of the LiFeSO4 for a home brew.

 

[crossy]

What chemistry are the 5A cells? I like the safety of the LiFeSO4 for a home brew.

LiFePO4 same as the bigger cells.
They are pretty low cost here from the Chinese manufacturers.

 

[crossy]

I was thinking of something like this, workable?