LiFePO4 cells charge with CC/CV just like other lithium cells… except to 3.6V max.
Ah so would need some sort of CC/CV IC on it then…hmmm
Only problem…that 2A max charge rate…might NEVER recharge the battery back up, unless it was used as a “balance” charger setup and charging 2A per each cell for the 12vdc pack, also I would think it would need a way to disconnect the LiFePO4 direct connect from the charging system when the car was running, maybe a reverse relay or something like that.
no audio, so not exactly sure what this is about
this is the lasersaber guys build w/ the LiFePO4 integral battery
I know some people have used bare 4S A123 packs directly in their cars and motorcycles without any problems. There are also places that sell drop-in LiFePO4 replacements. The decent ones have built-in BMS and balancing circuitry.
There used to be a video on Youtube showing a guy in Canada starting his car 20 times in –20C weather using a 4S, 2300 mAh A123 pack.
Yeah I stubled across those video’s too
Found this as well
ah dang, too low an input voltage
I have a bunch of 58F/16V Maxwell supercap packs. They can indeed start cars… even if the battery is pretty dead. A friend had a dead battery and the starter relay would just click when the key was turned. We slapped a cap across the battery and went and had a beer. Came back, hit the key, vroom…
The dead battery can’t supply much current, but the cap can capture what juice is left in the battery and dump it out in a heavy burst… they are rated to do 1000 amps. 12 milliohm ESR.
A couple of threads, where these are talked about, each cap may have as much energy as an eneloop so one could design a circuit to charge them from an almost dead car battery or a small solar panel
I would like to design a circuit that could recharge the caps from 4 eneloop that are kept in a reserve flashlight in the car (or a couple 18650s)
The biggest problem with the supercaps is that you need some way to balance the charge when they are in series. The packs that I have use resistors. Some people use LEDs and diodes. Both of those put a significant parasitic drain on the caps. Maxwell has some packs with active balancing that uses op-amps.
Those 58F caps can hold around 7000 watt-seconds of energy… they were used in huge windmills to supply boost current to the vane pitch motors.
I have used one similar to this for the last few months in a mechanical workshop scenario. It works a treat, but the weak alligator clamps can let it down.
I notice in the trade tool flyers that every man & his dog is coming out with their own version now…
This doesn’t answered the OP question, but I’ve been using the Antigravity Batteries Micro Start XP 1 kit to jump start cars and boats on occasion for the last year. Once charged, it does an amazing job and really works as seen in videos and such. I’ve used it to jumpstart a 750cc motorcycle, 4 cylinder and 6 cylinder cars, and V-8 powered boats. Some without batteries in them at all.
However, my particular one arrived with a large amount of parasitic drain and I have yet to return it to Antigravity to have it looked at or replaced. They do offer a 3 year warranty on it.
After that beer, with almost total certainty both the pack and battery would measure the same voltage.
Volts are like water, they only go down hill, without some booster circuit in the middle the pack would slightly charge the lead acid battery and together they started the car.
OTOH it might be very practical to make a booster circuit that charged a supercap bank from a very weak battery, then started the car from just the supercap pack.
That is what i want
A “trick” I have used several times on a very cold morning in the winter is after having failed to crank a car, to wait about a minute, and try again. Sometimes because it was very cold and the chemicals are not as reactive, a second try a short time later can be successful. I believe that initially the current passing through the battery on the first attempt warms the internals of the battery, so that a short time later, the more reactive chemicals can output more current to start the car.
A big mistake a lot of people make when tring to start a car on a cold morning is to let the starter stay engaged in a dead stall. That will suck the life out of the battery and they will have no chance later.
I can see this working.
The super cap would draw very little from the battery, but when it is time to start the car, in the first instant, the effective internal resistance of the combo would be greatly reduced. This would get the starter motor past the most difficult part of starting the car, just getting the starter motor turning, then an instant later the battery alone would be able to continue cranking the engine until it starts.
Since i don’t think this is posted in this thread yet
Perhaps the best thing would be a 120V power supply, plug it into an outlet, connect to supercap pack, a few mins later attach to car
Simple business opportunity, supercap pack (balanced), with household adapter, dead car battery adapter, and voltmeter 
When you first connect the cap to the battery it sucks the voltage down quite a bit. Like all batteries, the voltage recovers somewhat as the load decreases. Waiting a while for the battery to recover is important here. The cap is able to store that higher voltage into what amounts to a battery with a very low IR and dump it into the starter. A weak lead-acid battery has a rather high IR and can’t supply much current on its own.
but can it supply a total of 10-15Wh, even over a few mins?
The caps that I have can do around 8000 Joules (watt-seconds)…
2.2Wh each?
Lead acid car batteries have two properties we are dancing around without explicitly mentioning; recovery and surface charge.
When the LA battery is drained down, it will recovery a certain amount just sitting. Crank on a car until it barely turns, stop, wait a few minutes, and usually it will crank again for a short period.
When a battery is low it will very rapidly increase voltage when initially charged, but this is a surface charge that does not reflect full capacity of the battery and may last a very short time when cranking.
Did some Google, but I can’t find a graph with real numbers showing current and voltage vs time during start up.
Back in the old days of lead acid electric cars (pre EV1) they used to call this growing amps, if you ran out of juice somewhere you would wait 15-20 mins then you would get a few more Wh and you would try to limp to an outlet.
My point is if your battery is more then just barely dead no amount of waiting will help, its below the threshold that can be recovered to start the car, think winter no start or parasitic drain (light left on) or car not run in a while.
The battery still has some juice, you would have no trouble getting 10-20Wh out of it to recharge caps but if its that low there is no way it can start a car.