Originally Posted by LithiumaniacsEVRacing
Thanks guys, this idea is for a good friend drag racing with lead acid batteries. Does the size of the booster pack AH/V matter?
Unlike what they say, Size does matter. It is pretty straightforward. The nominal pack voltage should be close to the same. Example:
A nominal LiFePo4 cell voltage is considered to be 3.2V. A nominal lead acid cell is considered to be 2.0V. A 96V lead acid pack consists of 48 cells (eight 12V batteries). That same 96V pack would consist of 30 LiFePo4 cells. The just off the charge voltage after a little resting is between 2.1 and 2.2volts per cell for Lead Acid. It is about 3.4 volts for LiFePo4. So your 48 cell Lead Acid Battery would be between 100.8 and 105.6 and fresh off the charger as high as 115.2. The LiFePo4 would be about 102V and fresh off the charge as high as 108. It shouldn't be any problem just tying them together after their separate charges. The voltages should equalize without any drama.
For LiPo cells like the Haiyin the nominal voltage is considered to be 3.7 and the fully charged voltage is 4.2. This means a 26S pack giving a nominal voltage of 96.2 and a full charge voltage of 109.2 volts.
The capacity of the Lithium type cells needs to be at least enough to boost up the voltage or there is no point at all. Your goal is to get higher voltage to the motor for that blast down the track. You probably want it to support at least half the current so size the pack depth in AH to match whatever half the full power current is. Obviously it would be best to just replace the Lead with Lithium completely and save all that mass you no longer need to accelerate. The at least half is a guess. It might be you need 2/3 or 3/4 to not overpower the Lithium cells. If you are pulling 2000 amps I would at a minimum size the lithium pack to handle 1000 amps. It really depends on how much the lead batteries sag under the load. If you know the internal resistance figures for the batteries you could make a better guesstimate.
If you are required to have a lead acid pack and the lithium is just a booster my temptation would be to use the smallest possible lead pack they will let you get away with and make the Lithium booster do all the work. You know, something like 300 volts of 2AH lead acid UPS batteries and 300 volts of 80AH worth of A123 cells. A 94S4P pack in other words. I have never played with Headways but you might have to go to 100AH of headways to get to 2000A from what I have read.
I hope that helps.