It can be done, 4S would be the safer appoach with a protection disconnect for too low, as alternator float voltage can be as high at 16V, likely using power tool cells, simply to make up the 120A draw of the average starter motor,
There are even people that have retrofitted car batteries with a super capacitor for the cranking amps and a 9AH lead acid battery for the hold over.
I'd have to agree with @daromer re LiFePO4 being more suited. Winston make cases with 4 cells which can be dropped into a vehicle. Provided they are not over charged they shouldn't go out of balance or suffer cell damage. A 40ah Winston should be sufficient for most vehicles. These cells would be sturdy enough for use in a vehicle.
Alternatively you could build your own battery from individual cells. This would mean you could check cell balance or even install a balancing device or a cell monitor.
I've wanted to do this, but don't have enough cells for that yet. Yes would need high current LiFePo4 cells, and some form of busbar internal wiring.
When selecting cells the CCA rating of the original becomes the 10s rating of the new lithium Ion cells.
For example, to replace 400CCA car battery, you could use A123 26650 cells* that do 50A normally and 120A for 10 seconds. 4 in series to get 440CCA, 4 in parallel to get 12V, 16 cells total. Downside is you only have 10Ah, so perhaps add more parallel cells till it's the same size as the original battery.
I've seen dedicated LiFePO4 replacements for lead acid batteries, that have inbuilt electronics. Those were done as ~8Ah SLA replacements, I'm sure somewhere is a proper car battery version.
* these cells are floating around ebay.com in packs of 90 or 100.
Ive easily started many of dead flat car with a 5s pack 4p so 6amp at 18 ish v and they handle it well [obviously not for ever and it puts a ton of strain on them] id love to see a 3s or 4s diy job if anyone comes across one
As said you can buy Li packs for cars today already from the shelf. They are LifePo4 and 4s. They contain BMS and all ready to go. I dont have links on this computer but one guy had some for sale on the forum that you could use for it.
The store i get some of my bats for have several of them on their shelves. Using normal Laptop cells im not sure about.. Yes it would work but for instance my cars are Diesels that need HUGE amount of draw at start... We are talking about several 100A. Of course just bursts.
But yes if we take new cells that do 30A.. Lets say they do 20A.. I would need 3s40P atleast for getting somewhere. But doable yes.
LifePo4 have a larger voltage range and can handle over voltage better than the other. They can go from 2.6-4.2 in worst case and a 4s pack of those are great. (I hope my numbers was correct but feel free to correct me)
I think this is a job for all the cells of low capacity that are not good enough for power walls , they can still put out 3 Amps .. that 's 10W per cell .... 100 cells delivers enough for starting (1Kw ) ... weighs 5kg with the plastic box , that's a quarter the weight of a 20kg lead acid battery ...if you have plenty of these bad cells use 200 , It must outlive and out perform a new lead acid battery ... Still not sure whether to use 3 or 4 banks ???? 3x3.7V = 11.1V ...4 x 3.7V = 14.8V
100 cells each with a 1AHr capacity will give a 25AHr (4bank) battery or 33AHr (3 bank) battery ... I don't think a car battery needs too many AHrs , just so long as it can start the engine.
Yep, here's a video of the SmartBattery. At least they break it all down so you could use it as a template to build your own. Although, you'd have to add in a few extra safety features since we don't have the custom cells they have; which have everything encased.
Just a word of caution: the "environment" in cars is way more hostile to batteries than the one in powersheds. What I'm talking about are temperatures under the hood in range from - 30 to + 60 Celsius even at the mostly mild climate where I live and very real possibility of output of the alternator becoming erratic, not mentioning the spikes/brownouts trying to wreck the control circuits.
So, sophisticated electronics must be used, to avoid the catastrophic results.
I was dreaming for something like, but it was a short dream after I imagined even the part of what such a battery must withstand.
It's not a job for recycled cells and not a task for first or second powerbank build.
Yes its my video and that's why i talked about them the other day. They "Could" work. The ones i tested above is though not made for starting cars due to max current is 150A continues and max burst 300.
Depending on car it may work..
Secondly those only have a slave built in and need an external BMS to control them.
Its funny how google/youtube and other log what we do and write and next time sugest strange stuff... Imagine what would happen if they would see our thoughs....