Firefly Electric Conversion

[Stubby79]

Weather has finally warmed up...and I've finally stopped working on other projects to see if the battery(and therefore the motor) perform better above freezing.

Yup.

She was managing to maintain 400a while accelerating in 2nd at 5 centigrade, instead of barely above 200a at -2c to -5c.

Take the effect of temperature...


And then add it (or more like multiply it) to the effect of heavy discharge...


...and you end up with your motor controller trying to protect your batteries from over-discharge.

Wouldn't be an issue with a higher voltage battery pack (72v at the moment)

Wouldn't be an issue if it was room temperature outside.

Wouldn't have been so off-putting if I had been sure that was all it was!

So, since she was a good girl, and it was warm enough out, I opted to rig together my little charger and give her some juice as a treat:


Good to see the Link10 can read finer amperages through it's "prescaler". Would suck if it couldn't read the charging properly...

Side note, the potential heart of a bigger charger that I won on ebay for cheap has arrived. That will be a project for down the road, though.

[thingstodo]

Originally Posted by Stubby79

So, since she was a good girl, and it was warm enough out, I opted to rig together my little charger and give her some juice as a treat:

Is there a problem charging below 0C with your battery chemistry? If so, disabling the charger if the batteries are too cold may be a 'feature' to add to your new battery charger

[Stubby79]

Originally Posted by thingstodo

Is there a problem charging below 0C with your battery chemistry? If so, disabling the charger if the batteries are too cold may be a 'feature' to add to your new battery charger

Apparently. I thought what I read was a load of crock until I looked at the data sheet for my batteries, which show charging temp range of 0 to whatever.

LiFePO4 will, from what others have said, "plate" the cathode if charged below freezing, resulting in a 20% loss of capacity. The only good thing is that it can only occur once...so if you mess up, well, it won't get any worse if you do it again.

Don't quote me on it, but obviously there's a good reason not to do it.

So, yes, I will take steps to avoid charging below freezing. I'm eventually going to see how warm the batteries get on my daily commute, and if they don't warm up much, I'll insulate them. And have some form of simple, safe heating, so that in the winter, I can still charge them if it gets below freezing. And the car will be more drivable if I keep the batteries above a certain minimum temp.

[Daox]

Shouldn't be too massively hard to create a circuit to check the temperature of the pack, and if so activate a relay to power the charger and enable it to charge. If it doesn't pass the temperature check, it activates another relay to a heating pad of some sort to heat the pack up above freezing.

[RedDevil]

Originally Posted by Daox

Shouldn't be too massively hard to create a circuit to check the temperature of the pack, and if so activate a relay to power the charger and enable it to charge. If it doesn't pass the temperature check, it activates another relay to a heating pad of some sort to heat the pack up above freezing.

I was thinking along those lines for a LiFePO4 12V battery replacement.
Have diodes block the charging.
Bridge the diodes by a relay to power the pad when it is engaged and short the diodes when it is not.

Engage the relay when the temperature is below the threshold.
There are thermostat switches that can do that; these come in any temperature range.
One from an old fridge would be in range and adjustable too. Problem is they engage when it gets warmer, we want it the other way round.

My 12V battery is deteriorating. When I key on (and not start the engine yet) it drops below 11 Volt quite fast. Time is running out

[Stubby79]

Originally Posted by Daox

Shouldn't be too massively hard to create a circuit to check the temperature of the pack, and if so activate a relay to power the charger and enable it to charge. If it doesn't pass the temperature check, it activates another relay to a heating pad of some sort to heat the pack up above freezing.

It won't be difficult, IMO. I'll set it to 5 degrees, just to be sure. And, if I've gone that far, I'll probably have it set on a timer to pre-warm the battery before I go to work, so I don't experience the voltage/capacity drop in the winter.

[Daox]

That sounds like a great idea.

[Stubby79]

Not much to look at (cheap action cam doesn't do low light worth poop), but I went for a "proper" test drive this morning:

https://www.youtube.com/watch?v=cpBNK-ppKBc

2nd gear the whole time. Peaked a bit over 50 km/hr near the start, then slowed down closer to the 40k speed limit. Climbing the hill 2/3rds through, it didn't slow down below 40, even though the controller was kicking in the low-voltage protection.

Route, according to google maps, was 6.0kms. AH meter said I used 11.7ah. (though I forgot to check that it was zero'd out before starting...charging will confirm it one way or the other). at ~72v, that's about 140wh/km from the batteries. That's pretty decent, if it's true.

Fun little drive. Would have been better with brakes. Yes, I can stop the car, but I need plenty of space. Time to add vacuum pump to the list of to-do's. Anyway, motor was luke warm after the drive. As in room temp kind of warm. Nothing appreciable on the temp of the controller or batteries. I'll have to take some actual temp readings next time.

But ahead of that is to start in on the second half of the battery pack...I want to see how it does with less voltage drop at 72v from having them in parallel before confirming the switch over to 144v. Oh and field voltage control...she was pretty gutless on takeoff in 2nd, in spite of the motor ammeter being pegged. (I've got that one half done, just need to put it all together.)

And, if I'm still not satisfied with the performance, well, I have that FB1 motor and the 1000a controller sitting around...that should get it moving in a hurry!

[Stubby79]

Charge proved the numbers above accurate.

[Stubby79]

So...I discovered this while checking that my battery was still in balance:



Wasn't on very tight. At least not tight enough to compress the bus bar down flat against the cell's terminal. Looks like most of the current was flowing through the bolt, and some passing through the aluminum screw-on collar (giant nut) for the terminal.

How much current can one little bolt handle for how long?

Maybe that's why I was getting more of a voltage drop than I had originally expected. Resistance is not futile, apparently.

I've changed out the bolt, leveled out the bus bar & hand checked/re-tightened every terminal's bolt.

I'm at the beginning stages of the second half of the battery pack. Currently have the cells charging again in parallel. Had to borrow a couple of bus bars (though I still have more than I paid for!) from them, so 2 or 3 cells weren't in perfect balance with the rest...though it was only barely measurable, in the thousandths of a volt. Maybe I'm being a bit too paranoid?