Replacing the 12 volt lead acid battery with a lighter lithium battery with double the Ah.

[PhilPen]

I very recently, 5th August 2024, replaced the 12V battery in my 2016 Chevy Spark EV with a LiLife LiFePO4 battery. The car has been run through 2 charge cycles (from 10 bars down to 3 bars) and so far I have encountered no problems. First I had to relocate the terminals on the battery to the appropriate side (used large flange nuts JB-Welded to the battery, holding screw in brass terminals). I also had to remove the battery tray which likely removed another 5 lbs; plus added non-slip tape made for stair edges to the base of the battery and to the cross-member on which the battery now sits. I also had to do a little wire extension (there was a large ground wire in among the positive leads running from the battery to the frame (likely only
needed on the gas powered sparks for cranking power, so I could probably have just cut this out.... but didn't just in case it was needed. This short length ground lead limited the movement of the wire bundle so by extending the ground wire 15 cm the positive wire bundle now could reach the battery positive easily. After the removal of the battery tray I made my own hold down using a CamJam 700 lb hold down strap (It runs through the cross-member on which the battery sits) (see photo). I also had to carefully trim some bits of under-hood plastic to make it easy to place the larger battery in the car with a Dremel. I removed around 1" from the plastic at the front of the car, and probably 1/2" at the firewall end. I then carefully ground these cut surfaces smooth, so the actually look 'factory'. Thus I got the larger but lighter (22 lbs versus 33 lb for the AC-Delco lead acid the weight) to fit nicely. The removal of the battery tray etc saved another 5-ish lbs off the weight. The very sophisticated BMS system built into the LiLife battery, should be able to handle any potential charging issues. Now I have a LiLife battery with double the energy capacity (100 amp hr versus 50) battery that fits. I can replace the lithium battery with a factory lead acid in a minute or so if required. I'm hoping the high-end BMS in the lithium cell should allow it to function/charge OK, in my car without any problems, anyway so far so good (two charge cycles completed).

 

[Infinion]

Now I have a LiLife battery with double the energy capacity (100 amp hr versus 50) battery that fits

It's more likely more significant than that, since AGM can handle 80% depth of discharge without significant loss to its lifecycle and capacity degredation.
That means that a brand new 50Ah AGM would effectively have 40Ah, wheras the LFP battery can comfortably manage 100% DoD with many thousands more charge cycles

But there's a big problem

The very sophisticated BMS system built into the LiLife battery, should be able to handle any potential charging issues. [...] I'm hoping the high-end BMS in the lithium cell should allow it to function/charge OK, in my car without any problems, anyway so far so good (two charge cycles completed).

I watched @LithiumSolar 's review and @WillProwse more recent charge, test, and teardown of this battery on YT and noticed a few potential problems you will encounter.

50 seconds in in the Will Prowse test, he charges at 134A and the battery turns itself off. The Spark EV's 14V APM will normally charge its AGM at 125A so when this battery charges, it will also turn itself off. You may not have noticed this yet. If you shut off the car during an overcurrent shutdown, the whole car might go dead for a short while until the battery recovers, dont know how long that takes.

Second problem is winter temperatures. Charge temperature is 0℃ to 50℃ / 32℉ to 122℉ and LithiumSolar and WillProwse both show the battery shutting itself off if the internal sensor finds itself at or below freezing temperatures. Website says it won't recover until it reaches (41℉/5℃). So in the winter, your capacity is really going matter for you as you will not be able to charge at all below freezing during drives and charging. You'll need a heated garage to charge in, or put it in an insulated box with some kind of electrical warmers to keep the battery above freezing.

Or you bring it in to charge, if it only takes a minute to take out that should be doable for you.

 

[PhilPen]

I haven't noticed any problems so far, one car has had a full moth of daily use (5 days of commuting, with a 70 mile round trip to work plus a 10 mile round trip to the gym before charging) each work day, and somewhat lower use at the weekend. While the other car has does lots of short ~10 mile trips and has been charge every time the propulsion battery has gotten down down to ~3 bars (of course no low temperatures yet, but both cars are charge in a garage under the house which has a minimum winter temperature in the garage of 50 F, and the battery's discharge temperature is down to -4 F). I live in Hamden, CT. so I should be OK, as recently the winters have been very mild. However, I could swap the battery at home if circumstances demanded. You can see more specs on the battery here https://www.litime.com/products/12v-100ah-group-24-smart-bluetooth . The battery can discharge 1 second at 500 amps, 10 or 20 seconds at 200 amps (can't remember which), and it has it a maximum of 100 amp charging current with some transient over current tolerances according to the email from their tech people. Since the car maintains the battery at 100% state of charge when in operation, it shouldn't get many long continuous charges at high currents from the DC-DC converter. In addition I'm not sure if other 12 volt accessory loads eat into the 125 A available to charge the battery (radio, fan, lights etc.) when the vehicle is in use. If so under most circumstances it would have less than the Max 125 amp. Anyway its an experiment and so far so good on both cars. I also have a 500 F 16.8 volt ultra-capacitor (weighs only 5 Kg, and it can supply or absorb 1000s of amps) not installed it in any car just used it for a few fun experiments, but such capacitors (even much smaller ones) could be used to buffer any over current surges, or even replace the 12 volt battery entirely.

 

[ArnieB]

I don’t think I would replace th la battery with a li why take the risk . The car does not have a starter and the amp draw is low a garden tractor batter would suffice. I have not heard of sparks having main battery fires ? I am preping to overhaul spark hv batteries. I will keep this sight informed if I can make progress in this endeavor.

 

[Infinion]

I don’t think I would replace th la battery with a li why take the risk . The car does not have a starter and the amp draw is low a garden tractor batter would suffice. I have not heard of sparks having main battery fires ? I am preping to overhaul spark hv batteries. I will keep this sight informed if I can make progress in this endeavor.

One benefit to having a larger battery is the ability to run all the 12V systems beyond the moment the APM stops providing power from the high voltage lithium battery. This is a particularly salient issue once the high voltage traction battery quits under 250V or while the car is off or in service mode. You will continue to get exterior and interior lights, onstar communication, Wi-fi, power steering, 12V power for chargers and small loads, HVAC fans, windows, and door locks. So having a decently sized 12V battery can be instrumental.

In the situation that you use all of the range in a spark EV, the 12V battery will be responsible for power steering, power brakes. So having a battery that can handle these large loads will make maneuvering safer and less straining for the driver.

In addition I'm not sure if other 12 volt accessory loads eat into the 125 A available to charge the battery (radio, fan, lights etc.) when the vehicle is in use. If so under most circumstances it would have less than the Max 125 amp. Anyway its an experiment and so far so good on both cars.

The APM is capable of 165A. It's likely the DC-DC converter is internally current limited once it hits this limit, so it will maintain max current while the voltage begins to drop from > 2kW 12V loads. The hard 125A limit is just to protect the AGM from charging too quickly, but it does account for 75% of the APM output. You can see the negative terminal of the 12V battery passes through a CT to measure current and regulate it for this purpose.

I also have a 500 F 16.8 volt ultra-capacitor (weighs only 5 Kg, and it can supply or absorb 1000s of amps) not installed it in any car just used it for a few fun experiments, but such capacitors (even much smaller ones) could be used to buffer any over current surges, or even replace the 12 volt battery entirely.

Very nice! I also bought supercaps back in 2018 and 19 and doubled capacity to 1000F 16.8V. I don't use it in the Spark, but I have it for other experiments like pulse motors, solar panel power, UPS, etc. Very powerful and very useful for experimentation. The fastest I've charged them is with 120A off my previous car's alternator. I was also able to bump start its manual transmission engine at 7V. Ran very rough for several seconds until it increased past 10V, but charged up to 15V in 30 seconds flat. I upgraded the active balancers to 50W per cell past 2.63V to prevent overcharging when I do huge energy dumps like that.

As far as energy density goes, it's only 45Wh with 3-6kW power capability, and LTO cells are currently equally as robust, able to be discharged to 0 without damage and capable of millions of cycles at freezing temperatures, so I'd really have to consider them against supercaps for future packs unless new developments blow the current limits out of the water. It's just nice to have a technology that doesn't have any particular charging or discharging limits.

One thing ultracapacitor tech really needs is a bi-directional DC-DC converters that can extract all the energy out of the capacitors down to a few volts. Existing 12V systems don't work past 8V, so a booster that can provide minimum operating voltage would complement the technology's already low energy density, and open up the option not to require voltage matching for a system and its series-connected cells for any chemistry, lithium, supercap, lead-acid, etc

For the Spark EV, however, I'd prefer to have at least 200-300Wh capacity at high power capability because of how the car behaves below 3% SoC. EV's don't have it, but a 12V 'trolling motor' would be a great addition for riding the battery down.