Choosing the correct battery bank size

[Lindsay]

Hi,

Hoping someone can help me size my battery bank as I move from deep-cycle lead-acid to Lithiums.

On my 24V setup on average I use 40Ah in a 24hr cycle, so about 1kWh. This is all DC and powers my security cameras, internet, laptop, DC water pump, 12V lighting.

I am wanting to add a fridge that uses 1.8kWh as well as my TV, which I estimate 1kWh (TV is set to a power saving mode, but is on most of the time when I get home at night). So 2.8kWh total on AC (I got to these values using a power meter on the mains).

I have a 24V Mecer/Axpert 3kVA (SOL-I-AX-3MPLUS24) and I already know the inverter uses a bit over 20W with no load attached, but what % will I need to add to the 2.8kWh to allow for inefficiencies in the DC to AC conversion? 

I was looking at the lithiumbatterysa diy kit: https://lithiumbatteriessa.co.za/collections/diy-kit/products/24v-200ah-8s-daly-smart-100a-bms-lifepo4-lithium-iron-phosphate-diy-kit-new-a-grade-cells

This works out to 4.8kWh (with 100% DoD), but I'm not sure it would it be enough with allowing for the occasional rainy/cloudy days and not draining it fully? My load over 24hr would be at least 3.8kWh total for DC and AC loads, excluding inverter inefficiencies (I have just over 1500W solar array).

This kit seems to be connected 2p8s, however I asked if I bought 8 more batteries for additional capacity, could I arrange it in 3p8s. They said it was not recommended on the included Daly BMS. Assuming that's correct, then is there another BMS I could get that would support this arrangement?

Thanks in advance!

Edited May 24, 2021 by Lindsay

[Kalahari Meerkat]

1 hour ago, Lindsay said:

arrange it in 3p8s.

I see no reason why not, the BMS should limit the current draw/charge current to 100A and adding 50% or even doubling up on capacity should only lengthen your run and charge times! note also the charging time would be impacted by the larger capacity, but, on the other hand each cell will only be subjected to 2/3 or the max current draw in your scenario, compared to the 2P8S setup.

As long as your current draw requirements don't exceed 2.4kW aka 100A at the 24V end, then you should be ok with this.

[Lindsay]

19 hours ago, Kalahari Meerkat said:

As long as your current draw requirements don't exceed 2.4kW aka 100A at the 24V end, then you should be ok with this.

Yeah, I'm not going to be drawing more than about 1500W from my appliances and only then if I plug in the induction cooker or mini-oven.

Thanks for the response, I was hoping some others might let me know what they think as well. I just don't want to spend R 20k and then find I can't up-size it if I need to

I already felt like I wasted money last time on deep-cycle lead-acid, but a few years ago options were more limited.

[Lindsay]

There seem to be only a couple of concerns I could find online for adding more cells in parallel. That a faulty cell (shorted out) would lead to the multiple cells in parallel with it discharging into this cell causing a potential fire (although I'm not sure it would actually cause a fire with only 2 other cells in parallel and LifePo4 chemistry) and that you might need different charging settings for the BMS/charger if you have more cells in parallel (not sure what these would be in my case going from 2 to 3 in parallel):

https://electronics.stackexchange.com/questions/431036/can-i-make-endless-parallel-li-ion-connections

https://electronics.stackexchange.com/questions/161437/how-many-lifepo4-cells-can-be-in-parallel

Ran into this while googling, which may be useful to people wanting to understand series/parallel battery packs:

http://liionbms.com/php/wp_series_parallel.php