LFP (LiFePo4) Battery for Parking, 192Wh (DIY)
- Thread starter GPak
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Hack_man
Well-Known Member
Your design is very interesting, nice job!
I'm a little confused why you are saying a Schottky diode has a higher voltage drop than "ideal"? Any real diode has a higher voltage drop than "ideal" so what are you comparing it to? Are you referring to the increased voltage drop at higher forward currents due to the series resistance with respect to silicon? Schottky tends to have a lower voltage drop than silicon or germanium.
I think you are correct that Schottky might not be the right solution at higher currents but I'm just trying to understand the logic.
From Wikipedia
A silicon p–n diode has a typical forward voltage of 600–700 mV, while the Schottky's forward voltage is 150–450 mV. This lower forward voltage requirement allows higher switching speeds and better system efficiency.
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GPak
Well-Known Member
Thanks,
My post #26 was a response to 98nchen's previous post #25, where he mentioned that he had a schottky diode, and if that was okay? - I suggested an "ideal diode" as a most efficient option instead of a schottky.
I actually tested a schottky diode, and it got so hot I couldn't touch it.
In terms of lower voltage drop and efficiency, a schottky diode is better than a silicon diode, and an "ideal diode" is better than a schottky diode.
My post #26 was a response to 98nchen's previous post #25, where he mentioned that he had a schottky diode, and if that was okay? - I suggested an "ideal diode" as a most efficient option instead of a schottky.
I actually tested a schottky diode, and it got so hot I couldn't touch it.
In terms of lower voltage drop and efficiency, a schottky diode is better than a silicon diode, and an "ideal diode" is better than a schottky diode.
98nchen
New Member
From my experience, the schottky diode got obscenely hot in a few seconds and the current dropped quite a bit after going through it.
im currently looking at relays to stop power draw from the car battery when car is off. wonder if theres a power efficient one. saw this one, it seems to use 5mA? Im capping my current at 12a for charging so I dont overload anything. I also saw a design for reducing current draw by 1/3. might attempt to replicate it if theres nothing good on the market currently
im currently looking at relays to stop power draw from the car battery when car is off. wonder if theres a power efficient one. saw this one, it seems to use 5mA? Im capping my current at 12a for charging so I dont overload anything. I also saw a design for reducing current draw by 1/3. might attempt to replicate it if theres nothing good on the market currently
EricSan
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Yep, Schottky diodes will get enormously hot at these current levels. Definitely not what you want. I used this purple anti-backflow diode with parallel ICs inside my LTO battery pack. It prevents my parking battery from dumping significant current back into the car's electrical system before the alternator starts to charge the battery.
Ideal Diode - 80V 50A black board, based on an NCEP023N10LL mosfet. This one requires having a ground wire attached (no big deal) and stays nice and cool. With an 8A charge rate, the temp rise is only 4-5c over ambient with a voltage drop of about 32mV with an 8A load.
https://www.aliexpress.us/item/3256803401816483.html
I also used this Time Delay Relay to control the charging voltage for my parking battery:
https://www.aliexpress.us/item/3256802724327712.html
It seems to provide 6-8 seconds of delay before turning on, which lets the car's charging system stabilize before drawing significant charging current. I don't really care what its current draw is (likely around 100-150mA I haven't measured it), it is powered from the fuse box with an add-a-fuse. Might want to add a diode to the coil leads to suppress the voltage spike when the coil is de-energized. They are cheap, order an extra one or two as spares.
Be sure the fuse harness that you plug the relay into has 12g wiring for the power leads. Something like this:
https://www.amazon.com/gp/B074FSZWVT
If you are targeting 12A charging current for your parking battery, you need to wire as shown below. This is too much current to pull from a fuse box or from a utility outlet. Use an super heavy-duty (8g) inline fuse holder like this to tap directly from the car's battery positive terminal:
https://www.amazon.com/gp/B0B6R8J932
I've found that similar 12g fuse holder housings get warm while passing only 8-10A of current. Warm wires are too thin. Connect the parking battery ground directly your chassis ground (not the battery negative terminal) so the BMS in your car knows you are pulling battery current and keeps it properly charged. I've been using the following wiring configuration in two different cars for about 6-7 months without any issue.
Ideal Diode - 80V 50A black board, based on an NCEP023N10LL mosfet. This one requires having a ground wire attached (no big deal) and stays nice and cool. With an 8A charge rate, the temp rise is only 4-5c over ambient with a voltage drop of about 32mV with an 8A load.
https://www.aliexpress.us/item/3256803401816483.html
I also used this Time Delay Relay to control the charging voltage for my parking battery:
https://www.aliexpress.us/item/3256802724327712.html
It seems to provide 6-8 seconds of delay before turning on, which lets the car's charging system stabilize before drawing significant charging current. I don't really care what its current draw is (likely around 100-150mA I haven't measured it), it is powered from the fuse box with an add-a-fuse. Might want to add a diode to the coil leads to suppress the voltage spike when the coil is de-energized. They are cheap, order an extra one or two as spares.
Be sure the fuse harness that you plug the relay into has 12g wiring for the power leads. Something like this:
https://www.amazon.com/gp/B074FSZWVT
If you are targeting 12A charging current for your parking battery, you need to wire as shown below. This is too much current to pull from a fuse box or from a utility outlet. Use an super heavy-duty (8g) inline fuse holder like this to tap directly from the car's battery positive terminal:
https://www.amazon.com/gp/B0B6R8J932
I've found that similar 12g fuse holder housings get warm while passing only 8-10A of current. Warm wires are too thin. Connect the parking battery ground directly your chassis ground (not the battery negative terminal) so the BMS in your car knows you are pulling battery current and keeps it properly charged. I've been using the following wiring configuration in two different cars for about 6-7 months without any issue.
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GPak
Well-Known Member
Upgrade - Added Voltage Sensitive Relay (VSR), see attached video for explanation.
So now we have 2 redundant Relays in series, the VSR and the original one in the battery, which is not optimal, but a simple upgrade.
A better solution could be to use this Voltage Sensing Relay instead of the regular Relay inside the Battery without having to use the ACC input to control charging, if there was enough space.
So now we have 2 redundant Relays in series, the VSR and the original one in the battery, which is not optimal, but a simple upgrade.
A better solution could be to use this Voltage Sensing Relay instead of the regular Relay inside the Battery without having to use the ACC input to control charging, if there was enough space.
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