Building a 7.2 KW LiFePo4 Battery

After installing a dual-input pow­er trans­fer pan­el for pow­er out­ages at the house, I was in need of a large bat­tery for pow­er­ing the sec­ond UPS input on the pan­el. The first input on the pan­el is for my back­up gen­er­a­tor, but I want­ed some pow­er avail­able until the gen­er­a­tor was able to be start­ed and at night when pow­er usage would be low and the gen­er­a­tor could be shut off.

My ini­tial cal­cu­la­tions for pow­er­ing the house for the night was around 5 kWh and I want­ed a buffer so that I was­n’t com­plete­ly dis­charg­ing the bat­tery bank. So I decid­ed a 6.5 kWh bat­tery would suf­fice.
I was able to acquire some new sur­plus Navitas / Topband LiFePo4 bat­ter­ies at a real­ly good price.

The bat­ter­ies came in box­es of 30 each with sil­i­cone ter­mi­nal cov­ers. There were a few cells that had their alu­minum vent cap pro­tec­tors slight­ly bent, but over­all were in excel­lent condition.

I used my DIY Battery Capacity Tester to test sev­er­al of the bat­ter­ies, and all of them test­ed well above their rat­ed capac­i­ty. A lit­tle bit of a caveat to those results was that the test­ing was at ⅒ C or 2.5 Amps which is the lim­it of the bat­tery tester. In my typ­i­cal use case I would be dis­charg­ing the bat­ter­ies at that ⅒ C rate anyway.

After a bit of cal­cu­la­tions and phys­i­cal design I end­ed up going with a 24 volt nom­i­nal bat­tery with 8 banks in series and 11 cells in par­al­lel
( 8s11p ).
The next step for me was build­ing a stur­dy case to house the bat­tery pack and it’s asso­ci­at­ed electronics.

The bat­ter­ies them­selves would weigh around 128 pounds (58 Kg) along with cop­per bus bars, elec­tron­ics, cables, and case I was expect­ing a total weight of around 150 pounds (68 Kg).
The case was made from 80/20 Inc. 10 series alu­minum extru­sions and 3/4″ marine grade ply­wood. the ply­wood was bolt­ed to the case at each of the six ver­ti­cal extru­sions using coun­ter­sunk low-pro­file sock­et head screws and wash­ers. The side and top pan­els are Aluminum-Polyethylene com­pos­ite sheets.

The bus bars were made from type 101 Oxygen Free Copper (OFC) and were 1/8″ X 1″ for the end bus bars and 1/16″ X 2″ for the 7 con­nect­ing bus bars. I laser cut an acrylic tem­plate for cen­ter punch­ing the drill holes for con­sis­ten­cy. There were 176 holes drilled and then beveled on each side for the bus bars,

The sides, top, bot­tom, and space between the 11 cell banks were insu­lat­ed with 1/32″ Arc-Resistant GPO3 fiber­glass sheets and the side pan­els were also padded with poly­car­bon­ate sheets for cell com­pres­sion. The Battery Manage­ment System (BMS) is a mod­i­fied xiaox­i­ang 100 Amp unit with a wire­less Bluetooth mod­ule. The mod­i­fi­ca­tion I made was the addi­tion of four bolt on M5 ter­mi­nals vs the orig­i­nal sol­der pads. There is also a reset­table 100 Amp ‘Blue Sea’ cir­cuit break­er rat­ed for DC volt­ages on the pos­i­tive line.

As of writ­ing this post the bat­tery has been installed and oper­a­tional for just over a month. The charg­er for the sys­tem is a Victron Blue Smart IP22 unit capa­ble of 16 Amps at 24 Volts DC and set to a LiFePO₄ charge pro­file. The invert­er is a 1200 VA 24 VDC VE.Direct Phoenix unit which plugs direct­ly into my AC trans­fer switch panel.

So far oth­er than some short term tests, the sys­tem has not been called upon to pro­vide for any out­ages yet. Hopefully with this bat­tery addi­tion I will be ready for the next extend­ed outage.