Lithium-ion 12v Battery Replacement

[redpoint5]

The silly thing with motorcycles is they use generators and shunt excess power generation to a heat sink. I'd like to shut my generator off too. I wonder how much power it robs from the engine? Probably should have done that during track days.

[serialk11r]

Originally Posted by redpoint5

The silly thing with motorcycles is they use generators and shunt excess power generation to a heat sink. I'd like to shut my generator off too. I wonder how much power it robs from the engine? Probably should have done that during track days.

No, it uses PWM to short current through the stator to cancel out the rotor's magnetic field. That gives increasing copper losses as the rpms go up, but removes excess iron losses. The regulator itself is pretty efficient, and they could make it more efficient with bigger MOSFETs. If it used the regulator to burn off power that thing would go up in flames in seconds even if it had the biggest heatsink ever.

This control strategy is actually more efficient than an alternator up to a certain point, but the stator losses go up quadratically with speed due to the permanent magnet and start wrecking efficiency. I would guess that at the rev limit, a motorcycle generator is probably under 30% efficient vs. maybe 40% on a car alternator.

Since the stator is so undersized on a motorcycle, I think the highest efficiency option is to wire in a synchronous rectifier and DC-DC converter. I'm kinda curious how many motorcycles already have synchronous rectifiers, since you can use the same MOSFETs to do the PWM, so it would reduce the number of parts.

The actual best thing to do is to put in a bigger stator with lower winding resistance, but I'm guessing you'd need to do a lot of cutting to make that happen

[Ecky]

Originally Posted by teoman

Were you able to test the voltages?


I am talking to the chinese seller to work on my new order. And he sent me a picture saing new caps 650 usd, refurbished ones 135 usd. (Asking are these the ones)

So they are not new (despite the guy claimed they were in the beginning).

So, measured the caps and one of the end caps is a dead short. That's definitely not a great way to fail, a shorted cap will cause the other caps to go up in voltage and have a cascading failure. Eventually the whole thing becomes a short across the positive and negative terminals on the battery. I was lucky enough to actually be looking at the pack when it failed, and pulled it off when I saw the LEDs on the other balancing boards start to get brighter.

I wonder what caused the failure?

[teoman]

Are the others at the same voltage?

[Ecky]

Originally Posted by teoman

Are the others at the same voltage?

They're all still very close, yes.

[freddyzdead]

Originally Posted by Ecky

I was under that impression as well. Hard to know what to make of all of the Amazon customer images of LiFePO4 batteries which caught on fire when they're not supposed to be able to.

https://www.amazon.com/Battery-Tende...70_&dpSrc=srch

Edit: my electrical theory is a bit weak but isn't it the case that since capacitors self-discharge, leaving one connected to the battery all the time would eventually drain it?

The self-discharge of capacitor bank is very small. I have charged an 83F one to 15 volts and left it standing. A week later it was at 13.8. The car itself would drain it a lot more than that. I have had such a capacitor bank and a LiFePo4 battery in my little Daihatsu Copen for nearly 2 years now, and it hasn't missed a beat. I happen to be blessed to live somewhere that doesn't freeze, so I don't have any concerns about charging. I did accidently flatten it once, though, and that is a problem. Some people gave me a push, and it wouldn't clutch start. I wasn't expecting that. I now have a jumpstart pack in the glove box, in case that ever happens again. No, I don't know either, where those pictures of flaming LiFePo4 batteries come from.

[redpoint5]

Yes, just going to reiterate that caps don't self-discharge much. When they are near their rated voltage maximum, they will discharge at a higher rate, but I wouldn't call it fast. As they drop just a couple tenths of a volt, the self-discharge slows to a crawl. I've had caps with most of the voltage remaining 6 months after charging them, and that's with LED balancers on them. The LEDs themselves glowing for months.

[RedDevil]

Originally Posted by freddyzdead

... I did accidently flatten it once, though, and that is a problem. Some people gave me a push, and it wouldn't clutch start. I wasn't expecting that. I now have a jumpstart pack in the glove box, in case that ever happens again.

That's because the caps, once discharged, are at 0 Volt and will pull any power the alternator provides during the jump start down.
The LiFePO4 battery should have its own BMS to protect it.

Hooking up a separate jumpstart pack will bring it back up, but as the caps can draw near infinite amperage it needs to have some kind of current limiter.

[Tahoe_Hybrid]

Originally Posted by redpoint5

I have the supercap built, 1 battery (for proof of concept), 10 gauge copper wire, and a 300A breaker. The breaker might be rated a little too high, but I wanted to give myself the option of starting the vehicle using the battery if I wanted to, while still giving some protection against a short. I figure shorting a lithium-ion battery has fire potential, and with the battery living in the cabin, some precaution should be made. I might throw in a power resistor to limit current and then put a lower rated breaker or fuse for added safety. I'm not even sure if the 5Ah battery can supply 300A and trip the breaker. That's somewhere around 4,500 watts before the thing trips, assuming no voltage sag.

The test mule will be my Acura TSX.

I'll probably start a new thread if I get serious about this project, include some photos, a build description, and some data about various electrical draws that impact how long I can operate the car in "virtual alternator delete" mode.

Don't hold your breath; I have to find out where to live in a couple months, which might include buying a house. Lots of projects on my table, and I'm among the worst with time management (which is why I'm on here and not doing something useful).

you so crazy to use #10 with a 300amp breaker
or you just wanted a incandescent bulb under the hood?

you need at lest 4/0 wire for 300amp breaker (short run) longer runs will need 350 gauge wire(350 is a industrial sized wire)

trying to start a fire?

[hayden55]

Originally Posted by Tahoe_Hybrid

you so crazy to use #10 with a 300amp breaker
or you just wanted a incandescent bulb under the hood?

you need at lest 4/0 wire for 300amp breaker (short run) longer runs will need 350 gauge wire(350 is a industrial sized wire)

trying to start a fire?

Probably just too big. It will never ever even get close. The Prius hardly uses the 12v system.