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LiFePo4 battery with BMS and vehicle control.

757 Views 0 Replies 1 Participant Last post by  MuttMutt
So I am done with lead acid batteries. I can build a Lithium Iron Phosphate battery for less than the high end spiral wound lead batteries and include a ton of features as well. So I have the cells and basically built the battery but there are a couple things I want to do.

  1. The battery will disconnect main power automatically which will also disconnect the power draw from the relay and a fan.
  2. The battery will have a manual kill switch which will allow the interior functions, i.e. lighting, power locks, alarm, etc to function but as soon as the starter hits a fuse will blow.
  3. The battery will have a voltage monitor that reports to my HomeAssistant install when at home so I can keep an eye on things.
The main reason why I am posting this is because I figure others will like the idea and want to duplicate it but I also need to find a switched power source to trigger a relay. I figure someone here may already know the exact line I need to tap into to make this happen. Anyway here is a basic diagram of things:

Rectangle Product Slope Font Line

Everything here for the most part will be included in the single case as the relays, buck converter and ESP8266 are not huge. The relay with the 40A fuse crossing the main legs is rated at 500 amps with a 400 amp fuse protecting the circuit, the 40 amp fuse is to maintain power output but heavily limit it so should the relay fail that fuse will also fail, if the main fuse fails it will also kill the circuit. The BMS shunts to ground for the relay leg so I am also going to install a 120MM fan to move air across the cells and keep them happy. There is a switch in the leg from power to the first relay to act as a kill switch. When it's off you can unlock the car but once you try to start the 40Amp fuse will fail since that relay is not powered there is no driving the car. What I am searching for is a good place for switched power to trigger the relay between the switch and the main power relay, I can easily build a latching relay that I could trigger with something and keep on until a door pin shorts to ground or something similar but ideally I would like it to be something simple for instance something that will trigger when you step on the brakes to start the car which is also an option. I have a manual for all the circuits and such but was hoping I would not need to dive into that stuff and see if someone else knows of a good idea. The BMS I am using can act as a main relay but mine is rated to 30Amps so I am using the BMS to trigger a heavy duty contactor/relay for main power.

If I can't find a good power source to power the relay and keep it active like I need I will tap into a brake light power and use a latching relay and use a door pin to break the contact similar to something here: Latched Output - Momentary to Constant Output - Positive Input/Positive Output Relay Wiring Diagram

The reason why I plan to disable the contactor is because when it is active it will draw power. This one draws .13 amps while active but I would rather have that reduced to 0 outside of when the car is running for those who wonder. I did a test over about 10 hours and it dropped the total voltage about .12 volts over night but still that adds up over a week if you don't drive often, easier to just have it not be there at all since there will also be other little parasitic draws from the remote start and security system anyway.

Here are the obligatory words of warning for anyone interested in doing something similar:
  • LiFePo4 batteries have HUGE amounts of power available and can dump that power nearly as fast as a capacitor be careful if you play with them. 1200 amps can kill you and you really should have a fuse in the circuit to protect your vehicle.​
  • LiFePo4 does not play well with high heat or super cold. I am lucky to be in an area which gets warm but not so cold. However I have to mount the battery inside the cabin as high heat kills them quick.​
  • You must have a BMS to keep the cells safe, it does a ton of things to prevent issues and at 200 bucks for the build it's not a ton of cash but I expect this battery to last for 15 to 20 years so the investment related to replacing a lead acid battery every 5 years or so is a ton better but replacing a 200 dollar battery for derp mistakes will suck worse.​
  • I am sure other issues may exist as well but this is a blanket warning that I am not going to save you from yourself, if you build one you are on your own. Kinda self explanatory when things are DIY but if you kill your car, yourself, your kids, your dog, or destroy the world it's on you not me, LOL.​
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