LifePO4 charging with unfiltered DC

Squilt

100 mW
Joined
Sep 16, 2022
Messages
36
Can you charge a battery with a BLDC motor being spun by a gasoline engine without any filtering after rectification? If you rectify the 3 phase output, and select the correct motor kv, could you just hook that up to a battery? As long as the peak voltage in the ripple is lower than the maximum charge voltage, and the engine is weak enough that it can't supply more than the maximum charge current of the battery, I don't see why not. But I'm finding very little on the topic. I saw a review study on lithium batteries being charged by unfiltered sources and the differences in cell degredation were statistically insignificant. And they tested at 10-100hz. The ~150khz rectified output coming from the bldc motor at ~4000 rpm should be even less distinguishable from DC from the battery's perspective.

I'm mentioning LiFePO4 specifically because they can charge at a significantly higher rate than LiCoO2, so if this setup charged at lower c rates, there's even less risk of battery damage.
 
U need to use 3motor phases rectification to get a DC voltage/current from a spinning BLDC motor (regenerative braking or spinning torque force).
The output DC line will be a current source, outputing current & voltage). U need to connect it into a battery pack middle backup to absorb energy, than a step-up dc dc converter to match voltage/current of main battery, in order to charge. (example chargin a 42V battery from ebike motor while u're pedalling at rollers at house. regenerative energy)

Check rectification pic with diodes
3phase rectification.jpg

maybe also this rectification works with 2 rect bridges ic's (less components)

rectification2.jpg

U need to test how much current/voltage can BLDC motor supply spinning at 4000rpm. there is curve rpm vs reactive torque. the max point will produce the max current out. maybe at 1k rpm, maybe at 2k rpm or maybe at max 4k rpm.
U need to use a variable resistive load at DC out to monitor how many amps/ voltage is sourcing at 4k rpm
get the max power transfer curve
 
Last edited:
batteryGOLD,

I like your 2 schematics for the conversion of 3 phase DC to 2 line DC. You suggest “less components” for the bridge method [8 diodes] but the sorting method employed on each phase wire would take only 6 diodes. Therefore the bridge method takes more components than the sorting method.
 
His second diagram had only two IC components, not individual diodes. E.g.:
41C1fbxDeFL.jpg
 
Single components exist that rectify 3 phase AC. What I'm asking is if it would be theoretically possible to do this with no fltering, no voltage regulation, and no step-up or step-down converters. Just rectified ac fed straight to the battery.
 
I don't think it will be a problem as long as the peak current doesn't exceed the maximum charging current. With 3 phase, the average is closer to the peak.
 
Single components exist that rectify 3 phase AC. What I'm asking is if it would be theoretically possible to do this with no fltering, no voltage regulation, and no step-up or step-down converters. Just rectified ac fed straight to the battery.

Rectification is allways necessary (and very low cost). U need DC out to charge a battery. and to prevent current sent from battery to motor motor coils..

I've done a experiment with a citycoco scooter for a "regenerative braking system"
Ive used 6diodes rectification connected to a on/off mosfets module connected to brake switch signal.
at break push, switch disables motor controller, and turns on a module to pass current to a battery bank.
I used 5 parallel 1S 18650 cells to absorb regenerative current ( theoretically
a 2000W 30A motor could output 2000W 30A regenerative). soo I went into a high slope downhill road, when circuit enabled, it sends up to 30A into 1S5P cells (current mode) and the citycoco does an huge electromagnetic brake force sending 30A out into 1S5P battery

remember output from rectification is current source supply.
soo if I connect it to a 2S5P battery pack, the current will go to half 15A at the same speed and less electromagnetic force.
but if speed doubles, theorical the 30A will be sent into 2S5P battery bank. soo playin with scooter speed and battery S elements, will do a kind of match rpm to current out.

its a game to match max power transfer to a specific battery. depends on motor kv's. rpm's and more factors
but it possible to charge any battery from a bldc motor rectification, as long the max volts produced by motor at 4k rpm match max battery voltage,
or U coud also trust bms to do top cut. imagine bldc motor is producing DC 60V at 4k rmps, and U connect it into a 42V battery.
battery will charge, if bms working, the top cut will happen at 42V .

first thing to do is to see how much volts motor produces at 4k rpm


if U want, U could post some photos of your project. seems interesting
 
Rectification is allways necessary (and very low cost). U need DC out to charge a battery. and to prevent current sent from battery to motor motor coils..

I've done a experiment with a citycoco scooter for a "regenerative braking system"
Ive used 6diodes rectification connected to a on/off mosfets module connected to brake switch signal.
at break push, switch disables motor controller, and turns on a module to pass current to a battery bank.
I used 5 parallel 1S 18650 cells to absorb regenerative current ( theoretically
a 2000W 30A motor could output 2000W 30A regenerative). soo I went into a high slope downhill road, when circuit enabled, it sends up to 30A into 1S5P cells (current mode) and the citycoco does an huge electromagnetic brake force sending 30A out into 1S5P battery

remember output from rectification is current source supply.
soo if I connect it to a 2S5P battery pack, the current will go to half 15A at the same speed and less electromagnetic force.
but if speed doubles, theorical the 30A will be sent into 2S5P battery bank. soo playin with scooter speed and battery S elements, will do a kind of match rpm to current out.

its a game to match max power transfer to a specific battery. depends on motor kv's. rpm's and more factors
but it possible to charge any battery from a bldc motor rectification, as long the max volts produced by motor at 4k rpm match max battery voltage,
or U coud also trust bms to do top cut. imagine bldc motor is producing DC 60V at 4k rmps, and U connect it into a 42V battery.
battery will charge, if bms working, the top cut will happen at 42V .

first thing to do is to see how much volts motor produces at 4k rpm


if U want, U could post some photos of your project. seems interesting
Sounds good. I don't have anything in the works right now. Just brainstorming. I'll be sure to post if I ever start on this though.
 
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