# Adding a Lithium Booster Pack to a Lead Acid EV?



## MN Driver (Sep 29, 2009)

I'd imagine wiring them in series could be a bit dangerous. The weaker end of the chain will sag terribly and possibly blow up batteries, likely the lead-acids. Putting them in parallel at the positive and negative connections would allow two battery packs to support eachothers voltage. You'd want to disconnect the packs for charging and use the appropriate number of cells in each pack so that the voltage is such to properly support the load but also to not have a large voltage difference when the two packs are connected to eachother via contactor.

I've thought the same of doing a smaller pack for a car that doesn't need much range and putting a pack of more powerful cells in parallel with the first pack. ..then I realized that a higher voltage pack of the same capacity and smaller Ah gives you better performance at the cell level and counteracts the voltage sag by allowing more sag prior to losing performance, especially with modern controllers such as the Soliton that can handle high pack voltages and limit the motor voltage to a safe level.


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## Ziggythewiz (May 16, 2010)

Definitely add in parallel. I'm doing an experiment with a partial pack of CALBs with floodies. Will be posting initial data tonight.


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## dougingraham (Jul 26, 2011)

LithiumaniacsEVRacing said:


> I have heard this can be done for race day only, not sure how to wire a small (high amps) lithium pack to the lead acid pack? Parallel to end POS/ NEG connections, or in series with the lead acid batteries?


This pretty much has to be parallel. And if you are talking drag racing it seems to me that if the lithium pack is big enough to assist the lead pack it is probably big enough to do the whole job on its own, especially if you get the lead out.

Why does there have to be any lead involved?


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## LithiumaniacsEVRacing (Oct 9, 2010)

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?


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## dougingraham (Jul 26, 2011)

LithiumaniacsEVRacing said:


> 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.


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## Caps18 (Jun 8, 2008)

What if you had two different packs, one Lithium, one Lead Acid?

The Lithium would get you the first 10 miles, the lead acid would get you further. The thing is that the Lithium has a lot more cycle life. The Lead Acid is cheaper. The vast majority of my trips are under 10 miles.

(I am looking into this new type of lead acid battery now that is lighter weight, but still has 10% of the cycles of LiFePO4. But it won't cost as much...maybe)


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## Ziggythewiz (May 16, 2010)

For drag racing, I don't think the AH matters beyond providing for a reasonable C rate. The lithium will need to be able to handle over half the load, depending on the IRs of the specific chemistries.

It's just a temporary setup though, once you feel the lithium boost you'll want to get the dead...er lead weight out ASAP. Since the lithium has to carry most of the load, why not all? The booster pack will cost more than half of a full lithium pack.


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