# Lithium for starting battery



## jehan12413 (Feb 4, 2010)

My brother is replacing the starting battery in his race car to save weight. He has found some lithium batteries for motorcycles that have the correct capacity and CCA rating. My question is how do these batteries hold up with a charging sys that is designed for lead acid? I am assuming that the 13.8v charging sys (3.45v per cell)will be ok to maintain the battery without a BMS. Anyone have any comments? Thanks


----------



## Ziggythewiz (May 16, 2010)

They'll need something to keep them from overcharging. Lead can accept constant input and gas off any excess, but lithium will overcharge and die.


----------



## Siwastaja (Aug 1, 2012)

Actually, lithium cells _do_ have a safe constant voltage rating that can be applied all the time. It is the same as the open-circuit voltage for the desired SOC. This SOC should be lower than 100% for increased life, but for LiFePO4, I think it may be even near 100%. So somewhere around 3.4V, it should be OK to keep it for extended periods.

Now, this is for a single cell. For multiple cell, a simple shunting BMS would be great.

But apparently, Winston and possibly some others produce "12V" 4-cell blocks without any kind of BMS as a direct replacement for 12V lead acid batteries. Those are carefully top-balanced. It actually _may_ work in a 4-cell system given careful balance in the start and enough leeway in voltage, mostly because it's only 4 cells... Just one cell rising in voltage would show in the pack voltage and the pack would stop taking current. But I'd think that 13.8V would be _absolute maximum_ for any extended period of time. I'd use 13.5V...


----------



## soccerdad (Dec 18, 2012)

In general it's a great idea on a race car for weight savings or allowing the weight to be relocated somewhere more useful. 

I would try it without the BMS and check what happens at the individual cells using a voltmeter, to confirm they will stay in balance over time. Without a BMS, it is important to manage the charging system voltage when the batteries are at 100% SOC. It depends on the battery manufacturer what the maximum charging voltage is, but let's say it's 4.00 volts max per cell. If the unloaded automotive charging system voltage goes 16V DC or higher, I think the LIFEPO4 cells will not last long. If under 16V, the charging current into the batteries should go near 0 amps as they charge to 100% SOC and the cell life should be okay. If the charging system produces too much voltage, I would consider a hack to add a high current diode and heatsink to the alternator output wire. Each conventional diode will drop the voltage about 0.7V. Maybe there is a better way that somebody knows of but this should work okay. You could source the diodes from a junk alternator. 

On the flipside, if there is a constant current drain (like leaving the lights on) and the cells go to zero or below 2V for any length of time, I expect they will be damaged. They don't bounce back like lead acid. This may leave you high and dry on race day if you forget and leave something energized. Maybe have a spare just in case.


----------



## Elithion (Oct 6, 2009)

jehan12413 said:


> My question is how do these batteries hold up with a charging system that is designed for lead acid?


In general, just fine. If the cells are top balanced, a LA charger is pretty much ideal for a Li-ion battery. At 13.5 V, that divides nicely as 3.375 V per LiFePO4 cell.

The problem may be at the _other _end: over-discharge. If these starter batteries have no internal protector (they just have Li-ion cells by themselves), nothing will prevent the user from cranking the engine too much, and allowing a cell voltage to drop below 2 V. At that point most Li-ion cells are toast. Little will prevent a cell from reversing.

Thomas Cook of Lithiumstart knows what he's doing in this field. Talk to him.


----------



## kennybobby (Aug 10, 2012)

*What kind of race car...*



jehan12413 said:


> My brother is replacing the starting battery in his race car to save weight.


still has an alternator on it? Use a big enough lithium pack to run without an alternator and save more weight and charge the battery in the pits. That's what motorcycle racers do.


----------



## Sunking (Aug 10, 2009)

I am confused. Race cars typically do not have alternators to save on weight and use the smallest battery possible to last through a race.


----------



## jehan12413 (Feb 4, 2010)

Thanks for the replies, even though these batteries are supposed to be drop in replacements I was a little concerned that the charging sys will not keep it near full charge due to the low charging voltage. It's going in a spec miata which has a charging sys.


----------



## kennybobby (Aug 10, 2012)

jehan12413 said:


> Thanks for the replies, even though these batteries are supposed to be drop in replacements I was a little concerned that the charging sys will not keep it near full charge due to the low charging voltage. It's going in a spec miata which has a charging sys.


If it is the stock alternator and charging system then it will likely overcharge and damage the Li pack. 

Measure the voltage at the lead battery while the car is running at idle and 3-4-5 krpm--it will likely run up to 14.4 or higher depending upon the battery's SOC.


----------



## Sunking (Aug 10, 2009)

Seems to me the logical solution is to get rid of the alternator. It is not needed in a race car.


----------



## frodus (Apr 12, 2008)

jehan12413 said:


> ThIt's going in a spec miata ....


IIRC Spec class doesn't let you disable/remove the alternator.....



From SCCA
http://cms.scca.com/documents/Club Forms/2010 Tech Forms/sm-jan.pdf



> The alternator may be OEM equivalent. The alternator
> drive pulley must be stock.* The alternator must not be*
> *disabled in any way*. Spark plugs and spark plug wires may
> be substituted. Ignition timing is unrestricted within stock​adjustment capability.


----------



## jehan12413 (Feb 4, 2010)

kennybobby said:


> If it is the stock alternator and charging system then it will likely overcharge and damage the Li pack.
> 
> Measure the voltage at the lead battery while the car is running at idle and 3-4-5 krpm--it will likely run up to 14.4 or higher depending upon the battery's SOC.


Something doesn't make sense with those numbers, 14.4v is 3.6v per cell and in order to charge the cells to 4.0v where most chargers cut off the charging sys would have to be putting out over 16v. I 'm more concerned with undercharging than overcharging. Guess I'll just have todo some experiments. Removing the alternator is not an option.


----------



## Ziggythewiz (May 16, 2010)

What cells are you looking at? Most LiFePO4 cells now suggest a charge termination voltage of 3.65 and CALB at least says 3.4 for floating, which is what an alternator would be doing.


----------



## frodus (Apr 12, 2008)

jehan12413 said:


> Something doesn't make sense with those numbers, 14.4v is 3.6v per cell and in order to charge the cells to 4.0v where most chargers cut off the charging sys would have to be putting out over 16v. I 'm more concerned with undercharging than overcharging. Guess I'll just have todo some experiments. Removing the alternator is not an option.


Anything over 3.3V will charge the cells. The higher the voltage, the faster the charge, but current WILL flow at a voltage above nominal. That's not really the issue. You could charge the 4-cell pack with 13.6V, but something closer to 14.8V would be better. Just DO NOT GO ABOVE THAT voltage with lifepo4.

I don't know of any chargers that go above about 3.7V/cell, but users can tell the manufacturer of the charger what voltage to charge at. MOST Lifepo4 manufacturers recommend 3.7V or near that. You may be thinking Lithium Polymer, which is indeed above 4V.

So, really, Our issue here is overcharging. Alternator rectifier output voltages may be too high for lifepo4. You really need to check the output voltage of the alternator with battery hooked up. Have a friend rev the engine to about 4-5K RPM and then check the voltage at the battery with a multimeter. Also, check at idle to see if the voltage is above 13.6V.


----------



## jehan12413 (Feb 4, 2010)

Maybe that's why my numbers are off, I was looking at my headway 38120s which are 3.6 after sitting around for a while. And my 18650 charger cuts off at just above 4v. Thanks for the info


----------



## jehan12413 (Feb 4, 2010)

To be honest I was thinking of just making a pack for him to try using some spare headways.


----------



## frodus (Apr 12, 2008)

What kind of 18650's are they though.... they might be lithium cobalt. All Lithium Ion batteries are NOT equal, there's several different types and voltages associated with each.

Lifepo4 (which is what headways are) should NOT be charged above 3.7V. Period.

Are you sure they're 3.6V after resting? Have you verified your meter reads correctly? All of my 180+ cells are right around 3.3V and they've been sitting for months. 3.6V does not sound right. But it might be if you've been using the wrong charger for them.....

Lithium Iron Phosphate chemistry is ~3.3V resting, you may see people say 3.2V nominal as well.

*Don't just make a pack, do the test I told you to do.* You don't want to overcharge. It may damage more than just batteries if you're not careful.


----------



## dougingraham (Jul 26, 2011)

jehan12413 said:


> My brother is replacing the starting battery in his race car to save weight. He has found some lithium batteries for motorcycles that have the correct capacity and CCA rating. My question is how do these batteries hold up with a charging sys that is designed for lead acid? I am assuming that the 13.8v charging sys (3.45v per cell)will be ok to maintain the battery without a BMS. Anyone have any comments? Thanks


I've floated a 4S battery of the 26650 A123 cells (LiFeP04) for 6 months at 13.8V (3.45vpc) to see if anything bad happens to them. And nothing bad happened. Capacity was the same before and after. The current drops to so close to zero (a few micro amps) it doesn't matter so there is no heating of the cells.

So if your charging system actually tries to float the battery at 13.8 I suspect you would be fine. Just to be safe I float my aux battery at 13.6 (3.4 vpc). If however it tries to float them much higher than that I would be a little concerned. My BMW charging system seems to want to keep the 12V battery at 14.4 which would be 3.6 vpc so it probably not all that good an idea. Then again it might not hurt anything at all. None of the manufacturers tell you what will happen in a circumstance like this.


----------



## jehan12413 (Feb 4, 2010)

frodus said:


> What kind of 18650's are they though.... they might be lithium cobalt. All Lithium Ion batteries are NOT equal, there's several different types and voltages associated with each.
> 
> Lifepo4 (which is what headways are) should NOT be charged above 3.7V. Period.
> 
> ...


Travis,
Thanks again I'll check the charging system.


----------



## agniusm (Apr 30, 2012)

I have made myself one out of A123 20AH cells. I have bare cells, no balancing or low voltage cut off. Made it as lead batteries not holding up in winter when -20C. The only concern i have is alternator charging it when it is below zero. It sure decreases cell life but i yet to find out by how much. total weight is little over 2kg. After 2 months of use they are in balance most of the time and sometimes one cell drifts .01V which is nothing to worry about. Here are some photos:


----------

