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Old 08-04-2022, 04:52 PM   #1
Not Yet Wild
Join Date: Apr 2020
Location: Texas
Posts: 7
Default DIY Lithium LiFePO4 w/ Sergio method HV/LV Relay – lessons learned

Thank you Sergio for the HV/LV relay design and molfpedal for providing schematics and RXV specific information.

I recently build a LiFePO4 battery pack for a RXV with a Navitas controller using Sergio’s HV/LV method. It was learning experience and I wanted to share some of the lessons learned in case you are considering a lithium build.

I used EVE LF105 LiFePO4 batteries because used EV / leaf batteries were very hard to find. I found a Chinese company that had a supply of LF105’s in their warehouse in Houston. This was ideal because I didn’t have to wait 30+ days for shipping, I saved $10 a cell by picking them up, and I could test the batteries before taking them home. Here are my lessons learned:

The batteries come with busbars that are very undersized depending on your power needs. Undersized buss bars will generate a lot of heat in your battery box. The busbar that came with the batteries were 2mm thick and 12.5mm wide rated for 140 – 155 amps. Per Navitas recommendation I am running their RowPow S51105P battery profile with a max discharge limit of 315 which is 3C the battery max pulse discharge. Even if you double up your busbars to 4mm x 12.5mm is it only rate for 210 – 240 amps.

I ended up making my own busbars 1/4” thick by 3/4 “wide out of copper bar stock which were rated for 320 – 425 amps. Just make sure you nickel plate them for longer durability and corrosion resistance. I have not had any excessive heat or overvoltage issues since the new busbars.

Here is a link to copper ampacity charts:

BMS location:
Consider the location of your BMS and potential air flow. I mounted my BMS in a sealed aluminum box 2” above my batteries. Because my undersized busbars were generating so much heat my BMS would show false high battery voltage when it got too hot. Even when charging the BMS would shut down the charger due to ‘single cell overvoltage.’

I installed a 48v 120mm x120mm computer fan with vent holes which helped keep the temperatures around 105 -110 F. After installing the new oversized busbars I do think the fan is needed anymore even in the Central Texas heat.

DC Amp meter shunt:
I would highly recommend using a DC Amp meter shunt and I have found it very accurate at keeping track of your amp hours and the battery SOC. It is only $35 on amazon and its well worth it. You can also monitor your voltage, amps your are currently pulling, and amp hours left in the battery pack.
I will also adjust / add amp hours with regen power going back into the batteries on the RXV.

RXV Specific lessons learned:

Stock solenoid:
Your stock solenoid will not last long with the Navitas set at 315 amps max discharge. My solenoid lasted a couple weeks then I replaced it with the gigavac MZJ-400A. Remember to change the voltage from 36v to 48v in the Navitas app if you have an older RXV.

RXV resister heat:
The RXV resister will get VERY hot especially with the regen of going down hills and heavy breaking. If the resistor gets too hot it will shut down the cart, without brakes BTW, until the temperature gets back to an acceptable operating range.

I was able to resolve this issue by adding an additional resistor wired in series and haven’t had an issue since. I did find out the hard way to wire it in series not in parallel.

I be curious to know what the regen cut off is for the RowPowS51105P battery profile and would guess its either 0.5c or 1c. It doesn’t take much to regen over 100amps going down a hill with 4 people in the cart.

Overall, it was a fun experience and wasn’t that hard if you take your time, do you research on this site, and follow the wiring diagram. A lithium pack is well worth it being 270lbs lighter, no voltage sag, only charge 1x a week, and easily going 35-40 mph with a Navitas controller. This lithium build might have saved me a few hundred dollars compared to buying a build battery pack like RowPow or Eco. The main benefit of DIY is I knowing its build right, with quality components, and specifically set up for the RXV with the LV accelerator cut off.

I hope this helps if you are considering building a DIY lithium battery pack
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