//// How hard is it to build a lithium battery and BMS system ? ////

[Andyinchville1]

Hi,

A friend of mine is selling an Antigravity battery that he bought for over $650 !

In researching what the battery is all about , I noted one super good quality of the battery (at least from my perspective) is its light weight (I had been thinking of relocating my battery to the rear passenger side floorboard for better weight distribution and hopefully making the car feel more nimble).

Anyways, by making a lightweight battery that may save me the trouble of figuring out the best way to relocate the stock battery.... plus (in the future) , should I eventually decide to relocate the battery anyways , the car will still be overall lighter and generally speaking lighter is good for fuel economy and performance.

With that , and having had at least some basic knopwledge of battery tech learned from charging scooters ( when that was a thing ) ..... how hard would it be to self build a battery like the Antigravity battery ? Has anybody here ever taken one apart to see its guts?

I suppose I would not need the restart feature just to keep it simple..... A simple lightweight battery without making my wallet lightweight would be great.

I'm sure one could be built for fairly cheap when compared to an off the s help piece.

Thanks for any and all input.

Andrew

 

[CableJockey]

I haven't taken one apart, but I am familiar with the general design: multiple individual cells, LiFePo chemistry (Li-Ion won't hit needed voltages, or survive the extremes of a car environment) , BMS, and wiring/bus bars.

The cost of your proposed battery realistically cannot be reduced much by going DIY.

An individual LiFePo cell has a nominal voltage of 3.2v, and ~3.6v at max charge. This would require a 4S (4-cell) arrangement to produce the 12v range a car wants: 12.8v nominal, and 14.4v max.
However, using only 4 cells in series will have very low amperage capability (20-50 amps constant and upwards of 100A "burst" for a few seconds) So, you will need a series-parallel arrangement.
Say you are trying to match the cranking amps of the oem Lead-acid battery of ~750A (cold). That would equate to 8 sets of 4 cells, so 32 total.
At a cost of ~$10-$15 each, that is already a good portion of an "off-the-shelf" model.
The remaining portion of cost is battery management (BMS).
Lithium batteries are delicate: if the voltage drops too low, they are permanently damaged. Too high, damage and likely fire. Discharge too fast (beyond the designed rating per cell) get tons of heat, damage the cell, and possible fire. Charging them below freezing temperatures causes damage. Etc.
BMS handles all of the above, plus more:
Lithium batteries (LiFePo) require a specific charge curve of constant current (based on cell rating) up to cell voltage of 3.6v, then constant voltage of exactly 3.6v until current flow slows to 1/24th the initial current. At this point all charging must end, or the battery can sustain damage. The above charge curve needs to be managed for each individual cell to make sure one does not overcharge (bad) before others reach full charge: known as balancing.

With all of the above requirements to maintain a safe and long-lived battery, the BMS will need to have mosfets capable of handling the full output capabilities of the battery to manage discharge current in the event of short circuit, shed any extra voltage an alternator might put out above the maximum 14.4v, etc.

A pre-built BMS circuit board having the amperage capabilities, and that is designed to endure constant use, can easily cost in excess of $200...
Unless you have the knowledge to engineer circuitry, it is probably best to avoid trying to build one from scratch.

Then there is cost for some protective casing, wiring, terminals, etc.

Overall savings seems to be about 10-15% compared to the $650 battery you mentioned.

Cost, complexity, and durability are the reasons Lead-acid remains the go-to starter battery in cars.

 

[Pat Dolan]

I think I just learned more about LiFe batteries than I have ever realized I needed to know. Thanks.

 

[Ol'Rattler]

In Bentley's it says that when you replace the battery, there are 2 types. Always replace the battery with the same type. One type is lead/acid (my Jetta) and the other type is???? They don't tell you what the 2 types are..............

 

[Pat Dolan]

In Bentley's it says that when you replace the battery, there are 2 types. Always replace the battery with the same type. One type is lead/acid (my Jetta) and the other type is???? They don't tell you what the 2 types are..............

About the only other thing would be AGM (Absorbed Glass Matt) that is also lead-acid. the other is wet flooded cell of lead-acid type,

 

[CableJockey]

In Bentley's it says that when you replace the battery, there are 2 types. Always replace the battery with the same type. One type is lead/acid (my Jetta) and the other type is???? They don't tell you what the 2 types are..............

It likely means FLA (flooded Lead-acid) and AGM (absorbed glass mat). Both types of battery are lead and acid based, however AGM is typically fully sealed and utilizes hydrogen recombinant methods instead of venting. This makes AGM intolerant of higher charging voltages.
For example: FLA batteries are comfortable with charging voltages from ~14.2v to ~15v, but AGM prefers ~13.9v to ~14.6v (varies by model and manufacturer, but it should give a rough idea of the difference).

Swapping battery types on the same car can result in an over-, under-charge situation and decrease battery life.

 

[MukGyver]

yes lithium batteries need careful maintenance. my motorcycle lithpolymer battery had the PO wired the heated grips toggle directly to the battery not through the ignition and of course I left the toggle on twice. Damage it useless for the most part.

 

[Ol'Rattler]

It likely means FLA (flooded Lead-acid) and AGM (absorbed glass mat). Both types of battery are lead and acid based, however AGM is typically fully sealed and utilizes hydrogen recombinant methods instead of venting. This makes AGM intolerant of higher charging voltages.
For example: FLA batteries are comfortable with charging voltages from ~14.2v to ~15v, but AGM prefers ~13.9v to ~14.6v (varies by model and manufacturer, but it should give a rough idea of the difference).

Swapping battery types on the same car can result in an over-, under-charge situation and decrease battery life.

Oh I remember now. We had an AGM VS Flooded Cell war here on the forum a few years back. My best guess was that you could go to AGM if you also changed the regulator to one that comes on AGM cars.