What lead-acid?

[oil pan 4]

Put the LiFePO4 battery on Anderson quick disconnects and your are all set.
I did go to maine this winter and did not have any starting issues.

[Cobb]

Thank god for anderson connectors. How about using 2 prius sub packs? Great way to recycle batteries when a pack poops out. You can supply yourself and like 50 friends an alternative starter battery.

[redpoint5]

Originally Posted by RedDevil

ditch the crap battery (not literally ) and do LiFePO4...

I'm just about there myself. I have a 4s 4.2Ah LiFePO4 battery connected to 6s 350 farad capacitors and will begin testing replacement of the lead-acid battery in the Camry soon. I'm putting a small current limiting resistor in series with the negative lead of the battery pack to limit the charge current since the pack is only rated for 2C charge. Unfortunately it also limits the discharge to roughly the same amperage even though it's rated to 30C. The capacitors alone should be able to supply cranking power though.

Total weight is 1Kg or about 2.2 lbs.

The car I'm testing on has a relatively high parasitic drain of 60mA, so I can only safely get 2 days of the car sitting parked before I have to be concerned about discharging the battery too far.

Originally Posted by oil pan 4

Put the LiFePO4 battery on Anderson quick disconnects and your are all set.
I did go to maine this winter and did not have any starting issues.

Just ordered my Anderson power pole's and will connect by "battery" with them. The only downside is the measly 45A rating. There are higher rated ones, but the cost and size go up substantially. For this reason, I'm running large ring terminals on the capacitors but going with the Anderson's for everything else. It will be nice being able to quickly disconnect the LiFePO4 to periodically run a balance charge on it.

[Cobb]

Id like to do something like this, but my car also draws 60ma when "off" and it can easily kill a small battery in a week if not driven. I guess the addition of a solar panel to the hood would help prevent that? Then got the worry of over charging the lifepo cells or over volting the electrical system with the caps.

[redpoint5]

Originally Posted by Cobb

Id like to do something like this, but my car also draws 60ma when "off" and it can easily kill a small battery in a week if not driven. I guess the addition of a solar panel to the hood would help prevent that? Then got the worry of over charging the lifepo cells or over volting the electrical system with the caps.

A solar panel in the 2-4 watt range would probably be fine for lead-acid, but I wouldn't use it for LiFePO4 or capacitors. However, I do have balancing LEDs placed in parallel with each of the 6 cells of the Supercap, and those could bleed 120mA of excess current once the voltage was sufficiently high. The voltage required to bleed off that much current might still be too high for a LiFEPO4 though. Perhaps a small solar panel in the 1-1.5 watt range would be fine?

I don't know what I'll do about the insufficient capacity issue, but I have to do something because the car often sits more than 2 days. Now that the weather is nice, I ride Lafawnda instead of driving the car.

The aftermarket radio and alarm is likely contributing to the high draw, but even if I cut that in half, I'd still only get about 4 days of sitting time.

I have ordered 2 current meters and plan to measure every aspect of energy consumption in the charge/discharge cycle. Unfortunately the meters themselves consume 7mA, so that further shortens the amount of time the car can sit.

If I have to buy more batteries to run in parallel, I might end up getting enough capacity that an alternator kill switch becomes a feasible option.

[RedDevil]

I was thinking solar plus LiFePO4 plus 4 of these:
EV-Power | Cell Balancing Module (3.60V- 1.7A)

That (top balancing the cells) should keep it in check. As a bonus, it will also top balance the cells when the alternator is running

[Daox]

Depending on your alternator's output voltage, I don't think you need to worry about top balancing at all. Even at 14.5V, you have 3.625V per cell. That isn't even fully charged. This is a good thing because you don't have to worry about overcharging the cells, and it prolongs their life. The downside is you have reduced capacity because they aren't fully charged. This likely isn't an issue unless you're using tiny cells.

[oil pan 4]

4 cells shouldn't need a balancer. But I bet it wont hurt.

Anderson connectors are rated for continuous use. On a low duty cycle say for something like a vehicle starting application you can safely put double the rated amps through it.
I used double anderson connectors in the suburban. I mainly just needed an anderson connector that came with a pin that could accept 2ga cable.
I have a 200 or 250 amp anderson in the camaro, it uses 4ga wire.

[RedDevil]

LiFePO4 cells are sensitive to overcharging, they will typically stay well below 3.6 Volt during the charge (if not on a very high current) and only climb when really full, and climb rapidly; which does harm the battery.
It does not start wasting excess charge, it starts killing itself at about 4.0 Volt. Some say even at 3.85 Volt.

Other type of Lithium batteries do see the voltage rise gradually with the SOC.
LiFePO4 is the odd one out; it hugs 3.4 Volt throughout most of the range:


(Taken from http://www.nature.com/nmat/journal/v.../nmat2730.html.)
Apparently even LiFePO4 has a small charge/discharge voltage gap; just a few % even on higher charges. But you can also see how stable the voltage is throughout the range. Ideal f.i. for a parallel pack to a hybrid battery. Now you know why I'm interested )

If one cell is slightly fuller than the others, it is in danger of getting overcharged when it has reached 100% SOC.
14.5 Volt minus (3.4 times 3) makes 4.3 Volt and kills a cell. Better not risk that.

[Daox]

4 cells at 14.5V (which is high for most alternators) gives you 3.625V per cell which is almost perfect. I do agree at that voltage, it will charge them nearly full. But, most alternator don't put out a full 14.5V, so its likely going to be charged to a lower SOC. The closer you get to 100%, the easier it is to damage the cells via overcharging as you've said.

The only thing you should have to do is charge up each cell individually to top balance them. Then, assemble the pack and you should be good. Unless the internal cell resistance is horribly off from each other, your cells will stay well balanced over time and you shouldn't have any problems. Otherwise, just charge each cell up individually once a month or something.

I'm not saying a top balancer hurts, I just don't see it as being necessary in this application. There just seems to be too many issues with cheap balancers and failures which end up killing your cells.