Austincrx New Reader
7/3/23 12:27 p.m.

I have a car that I run sprint races in, 45 minutes max run time.  The rules are such that we're all really close on power, the front half of the field is within about 10 hp of eachother.  Right now everyone runs without alternators to free up a few parasitic hp.  I'm trying to do the same.  Short of just throwing batteries in the car and lapping until it dies, I am trying in vein to find some solid info on this subject online.  It's not commonly discussed, as you might imagine.

Issue #1 - lead acit batteries will slowly die, causing weird problems if you're running around on track and the battery amperage gets too low.  This easiest solutions are - A. slap a bigger battery in it (heavy, heavy) or B. run Lithium batteries.

Issue #2 (from Option B above) - Lithium Ion or LiFePo4 batteries lack the high amperage draw that a starter motor requires, meaning they can't start the car easily, but they should be able to keep it running.  Lead acid batteries are able to dissipate that huge amperage load required to start the vehicle.  How does one get around this SAFELY?


My thoughts are this - run a standard car battery and a LiFePo4 battery in parallel to give me starting capability (lead acid battery) and 60 minutes of normal operation time (LiFePO4 battery).  This would be the cheapest solution and also I think the most useful.  The battery will have to be charged on a regular basis, but that's not an issue when you're running 2 sessions per day max - 20-25 min quali and 30 min race or a single 45 min race.  

My real question is: will the lithium ion battery be at risk of damage by running it in parallel with a lead-acid battery?  In theory it shouldn't cause any problems because the Lead-Acid battery can dissipate it's amperage directly to the starter motor not going 'through' the Li battery thanks to the parallel wiring, but Li batteries are not 'normal' and when one ignites due to over heating or arcing, that's a bad situation and you don't want to be part of it.

I've seen competitor setups and they range from 2 enormous Ultima batteries to 2 Li batteries and a small Lead-acid battery in parallel to a single large Li 'starter' battery.  



It gets better - I also don't have a lot of time to test this out, with only 1 track day available between now and the big event (solely due to my personal availability and track day availability).  Running at idle doesn't transfer to a race situation, the amperage used will be very different.  I want to have this and another alteration completed before I head to the track to test them out.  I can always revert back to the standard alternator setup, but testing at a race event is not what I should be doing.



Thank you for your help!

APEowner GRM+ Memberand UltraDork
7/3/23 12:38 p.m.

What we do with Formula Fords is plug a lead acid battery into the car in parallel with the Lithium Ion batteries when we start it and then unplug and run on the Lithium Ion batteries during the race.

Austincrx New Reader
7/3/23 1:31 p.m.

In reply to APEowner :

Thank you, that is good to know.  What size Li batteries do you run (in amp-hour capacity)?

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
7/4/23 9:16 p.m.

45 minutes with no alternator?


If you use an MSD, you may find that you make more power with an alternator than the alternator draws.  MSDs are pretty sensitive to voltage, they don't like being undervolted too much.

Tom Suddard
Tom Suddard GRM+ Memberand Director of Marketing & Digital Assets
7/5/23 6:56 a.m.

Can you wire the alternator to only charge when not under WOT?

Mgbgt_1970 New Reader
7/5/23 12:31 p.m.

In reply to Tom Suddard :

Can you wire the alt to only charge when braking?

Alt shop should be able to set one for you

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
7/5/23 12:42 p.m.

The main power suck from an alternator isn't the charging load, it's the inertia.  Let's say 10 amps constant load (reasonable if a battery can last ~60 minutes), that is 140 watts, which is less than a quarter of one horsepower even after you figure in efficiency losses.


This bears out with testing done by an IT racer weighing the benefits of underdrive pullies.  Removing the belt entirely netted one horsepower, which may as well have been measurement noise.


Depending on your ignition system, you may lose 5-10% horsepower running at 12v versus 14v.

Our Preferred Partners