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Old 01-20-2021, 07:12 PM   #11 (permalink)
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Did further testing today, and I'm fairly pleased.

First tried cranking the car through jumper cables with a full battery. No good, contact resistance too high.

30 minutes with blower, high beams, and ignition on: 54A@13V, 48A@11V ish, so I drew around 26Ah of charge from the battery. At this point, the voltage under load was around 10.3V.

I then turned off the blower and high beams, leaving just ignition and low beams on, measured current was around 23A. Left it on for 15 minutes, drawing another ~5-6Ah.

Then I adjusted the jumper cable clamps a bit to get a better connection and attempted to crank the engine. To my surprise, it actually managed to get through one or two compression cycles! I turned all the electrical loads back on to get 45-50A of current again, and found around 2V of drop at the car's battery terminals, so it's no surprise I wasn't able to crank the engine, but I have no doubt the battery would be able to do it with a better electrical connection.

That said, after a few failed cranking attempts, the battery voltage was dropping very quickly, so that was basically all it had left. Interestingly, the open circuit voltage actually recovered to 10.8V within a few minutes, so maybe it actually has a little more juice left. With ignition switched on but nothing else, it reads 10.45V at my multimeter. I actually feel pretty confident that the engine would crank with this voltage, though maybe not at -20 degrees.

I think I'm comfortable believing the usable capacity of this battery is 32Ah, and will remain over 30Ah for probably a few years of use, so my little gamble hasn't been all that stupid. Now to obtain a European gas guzzler to put this battery in...

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Old 01-22-2021, 08:38 PM   #12 (permalink)
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Alright, I connected it to the Porsche (2.7L 12.5 compression ratio) when it was almost empty (11V). The ECU was reading only 10.7V and all kinds of warnings were flashing on the screen.

It had to crank over several times before firing up fully warmed up. Just to see how it would do, I switched the car off after a few seconds, and the BMS shut everything off! I'm still not quite sure why that would happen, since I don't think it was near the low voltage cutoff. After disconnecting a cable to reset the BMS, I tried again, and it started just fine.

I only thought of measuring the charging current after a few minutes (at which point the voltage had risen 0.5V), and got an incredible 76 amps. The ECU was showing 12.3V at 800rpm, but revving the engine up to 2000rpm to get the 180A alternator putting out full power brought the voltage to 12.8V, so I think it's safe to say the battery was happily charging at well over 100A with no issues. Due to the chemistry, it can do this all day long, which is quite incredible!

I think for a car with a big V8 or something, I would want a true 40Ah, maybe 45 just to maintain the ability to start the car when it's near 0%, but this ~3xAh pack seems adequate for most applications. I was thinking of getting a 1st gen 2.5L Boxster as a toy car, this battery will work nicely there.

Last edited by serialk11r; 01-24-2021 at 02:13 AM..
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Old 01-22-2021, 09:02 PM   #13 (permalink)
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Old 01-24-2021, 05:38 PM   #14 (permalink)
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I was doing some reading about lead acid battery charging again and forgot where I read it, but apparently at cold temperatures, more modern cars with AGM batteries will increase the float charge voltage for some reason. I have no clue why this is the case, but apparently the maximum target voltage can be as high as 15.6V?

A paper I read said LTO-NMC cells will suffer permanent damage if charged over 3.0V (that's 15.0V for a 5S pack) because chemistry, so using this on a newer car with a "smart charger" on a cold day would either destroy the battery (if it has a BMS with high voltage cutoff) or trigger the disconnect and probably make the ECU go nuts. All the cars with dumb charging I've seen don't really go over 14.4V ish, which is safe.

For a newer car or a car with a larger engine, I'm starting to think a heated and low temperature cutoff LiFePO4 battery would make more sense, perhaps with a supercapacitor bank near the starter to help with cranking, as 50Ah of LTO starts to get somewhat bulky and heavy. The higher voltage would help with starting as well.

ReLion has a 50Ah heated battery available off the shelf, but its 675 dollar asking price is very steep, and the low current limit means you definitely need the supercapacitors

https://relionbattery.com/products/lithium/rb50-lt

There are also these "Grepow" cells that can charge at 0.2C and -20C with reasonable capacity degradation, which is better than the 0.1C max rate that a normal battery can handle at just 0C:
https://www.grepow.com/page/low-temp...e-battery.html

Last edited by serialk11r; 01-24-2021 at 06:17 PM..
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Old 01-24-2021, 05:57 PM   #15 (permalink)
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I've started using lead acid in winter and lithium in summer.
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Old 01-25-2021, 02:43 AM   #16 (permalink)
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Quote:
Originally Posted by Ecky View Post
I've started using lead acid in winter and lithium in summer.
You sir are being smart, while I'm trying to figure out how to unnecessarily throw money at a battery XD

Any lithium is fine in the winter as long as you keep it topped up (driving everyday is generally sufficient). There's a guy on Spyderchat who has used his 7Ah LFP battery for many winters with no issues (as the car is daily driven).

Let it go down even a little bit though...and the battery is toast if you charge it with the alternator on a cold day. My issue is I have no means to trickle charge or heat the battery, and I don't take my car out every single day, so LTO batteries are attractive since they are like an indestructible supercharged lead acid.

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