Solar, 18650 Powerwall, RVs, etc, 14 packs store 560 cells, 560 x 2000mAh 18650 cells is about 4 kWh, max current is 40A which can supply 2 kW @ 48V.

Video 1: https://www.youtube.com/watch?v=XNzXbSjXd8k
Video 2: https://www.youtube.com/watch?v=JJnd1URNQVw
Video 3: https://www.youtube.com/watch?v=78bsf8_4UzA

Price is £18($25) per pack + £15 for the bits & cables.

BMS (£25) - You can buy this cheaper separately, currently supplying Daly 40A BMS.

Breaker (£5/10) - This also doubles as an isolation switch - so you don't get a spark when you connect to your inverter.

No Spare Pack! - Sorry, the box can only hold 14 of these.