# Converting a lead-acid EV to LiFePO4s . . . how?



## speculawyer (Feb 10, 2009)

I'm interested in converting an existing lead-acid based EV to Li-Ions. This will increase range and get rid of the annoying battery anxiety and maintenance associated with lead-acid. (Damn that sulfation & dead cells!)

So . . . any tips on how one would do this?

Obviously I would have to replace the existing charger with a charger for the Li-Ions. But other than that if I replaced the lead-acids with a Li-Ion pack with the same voltage, would the system work fine? What else would I have to do?

Edit: Actually, I don't think I would have to replace the charger. Lead-acid chargers are generally slower, so the lead-acid charger would probably work fine . . . it would just take longer to charger than if I got a faster LiFePO4 charger.

The existing state-of-charge indicator from the lead-acid system would obviously give a nonsensical read-out but I could ignore that and have some sort of new read-out system for the Li-Ions.

Edit: Here too, it seems fine as is. As long as I design a proper matching battery pack, the existing fuel guage should give a pretty good read-out. Perhaps it won't be as linear but the empty reading occurs at the right time to prevent any damage to the LiFePO4s. It would be at around 2.8Volts (assuming the cells are well-balanced).


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## bblocher (Jul 30, 2008)

speculawyer said:


> I'm interested in converting an existing lead-acid based EV to Li-Ions. This will increase range and get rid of the annoying battery anxiety and maintenance associated with lead-acid. (Damn that sulfation & dead cells!)
> 
> So . . . any tips on how one would do this?
> 
> ...


1) Things to consider would be sizing. If you can get cells that will nicely fit into the original racks then that would be a plus. Otherwise you'll need to make new battery racks.

2) As you mentioned already the charger (unless one that can be reprogrammed from LA to Lithium) is worthless and needs to be replaced.

3) Even if the vehicle already has a per cell tracking system it will need to be expanded. Each cell must be monitored for low and high voltage conditions. Preferred the BMS can talk to your charger and shut it down for high voltage conditions when your pack isn't well balanced.

4) Most, or maybe all, current BMS systems are only monitoring systems and don't balance the cells. You'll either have to do this manually as needed or buy or design a cell balancing system.

5) When sizing your new pack keep in mind the current draw the vehicle uses and don't undersize the new pack. You'll greatly extend the life of the pack by keeping your C rating lower. That is the amount of amps you pull from the pack. 3C seems to be the common lithium recommended continuous rating. So for 90Ah cells you can only safely pull a continous 270A. You'll notice, however, they rate the cycle life on only a .3C rating.

That's about all I can think of off the top of my head.


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## ClintK (Apr 27, 2008)

Lithiums have a much larger operating range than lead. Nominal voltage of 3.2V but their range from discharged to fully charged is 2.5 to 4.25V respectively.

120V nominal would be nearly 160V fully charged, 93.75 discharged. Make sure your DC-DC converter / motor / other devices can handle that larger operating range.


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## bblocher (Jul 30, 2008)

ClintK said:


> Lithiums have a much larger operating range than lead. Nominal voltage of 3.2V but their range from discharged to fully charged is 2.5 to 4.25V respectively.
> 
> 120V nominal would be nearly 160V fully charged, 93.75 discharged. Make sure your DC-DC converter / motor / other devices can handle that larger operating range.


Ah yes good point! However many are recommending to not go the max ranges on the cells. To play it safe use 2.8v - 3.8v which will narrow that voltage range a bit if you stick to it.


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## speculawyer (Feb 10, 2009)

OK . . . so it sounds like I would need:

1) The batteries themselves. (Thundersky LFPs probably since they are cheap), 

2) Some type of battery management system that does balancing. This system from EV-Power in Australia looks good. 
http://www.evpower.com.au/-Thundersky-Battery-Balancing-System-.html
Any others I should look at?

3) Some type of charging system.

4) Some type of Voltage read-out system so I know how much power I have left. 


I don't think the physical size is an issue since this would be replacing existing big bulky lead-acid batteries. 

The existing system is an NEV so I don't think I have to worry about drawing too many amps.


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## speculawyer (Feb 10, 2009)

Could this battery balancing circuit be used with Thundersky LFPs?
http://www.astroflight.com/index.php?main_page=product_info&cPath=1&products_id=20
I know it says Lithium Polymer but the electrons don't really care what kind of anode or cathode is being used. Perhaps it is too small?

I've also discovered the world of "Duct Tape Batteries" (so called because they are home-made battery packs created by duct taping together LiFePO4s and supporting circuitry). Enterprising Chinese people create and sell pre-made battery packs on eBay that include LiFePO4 batteries, a battery management system (that seems to do balancing), and a (wimpy) charger. In essence, it would seem that these pre-built duct tape battery packs would give me everything I need already assembled. 

I wonder if I could have them make a special pack for me? I'd want one that is a fair bit bigger than the packs they make which are sized for electric bikes. 

Example of a duct tape battery on eBay


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## ClintK (Apr 27, 2008)

Elite power solutions has some package batteries, BMS, and charger...
http://www.elitepowersolutions.com/products/
I think a couple EVers here have used them.


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## bblocher (Jul 30, 2008)

speculawyer said:


> Could this battery balancing circuit be used with Thundersky LFPs?
> http://www.astroflight.com/index.php?main_page=product_info&cPath=1&products_id=20
> I know it says Lithium Polymer but the electrons don't really care what kind of anode or cathode is being used. Perhaps it is too small?


No the voltages are different for polymer vs lifepo4.

Here is the lifepo4 version.
http://www.astroflight.com/index.php?main_page=product_info&cPath=1&products_id=21

It only shunts 150ma but for smaller cells it would probably do fine.


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## bblocher (Jul 30, 2008)

ClintK said:


> Elite power solutions has some package batteries, BMS, and charger...
> http://www.elitepowersolutions.com/products/
> I think a couple EVers here have used them.


Yeah here is something small close to that ebay link.
http://elitepowersolutions.com/products/product_info.php?cPath=1_8&products_id=82


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## speculawyer (Feb 10, 2009)

bblocher said:


> No the voltages are different for polymer vs lifepo4.
> 
> Here is the lifepo4 version.
> http://www.astroflight.com/index.php?main_page=product_info&cPath=1&products_id=21
> ...


Duh . . . I sure missed that. Thanks. 

I'll probably used the 40Ah or 60Ah cells. Do you think it will work for those?


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## speculawyer (Feb 10, 2009)

bblocher said:


> Yeah here is something small close to that ebay link.
> http://elitepowersolutions.com/products/product_info.php?cPath=1_8&products_id=82


OK, I figured out that I need 72V. I think this charger from Elite would be perfect:
http://elitepowersolutions.com/products/product_info.php?cPath=9&products_id=80
But they mention 24 cells in series. I think that may be too many for a system based on 72V of lead-acid batteries (6 in series). . I'm thinking 23 of the Thundersky LFPs in series. Does that make sense?


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## bblocher (Jul 30, 2008)

speculawyer said:


> Duh . . . I sure missed that. Thanks.
> 
> I'll probably used the 40Ah or 60Ah cells. Do you think it will work for those?


I hear that once balanced these cells stay well balanced unless you go beyond 3C. If that's the the case then they may. With 150ma drain it would take over 6 hours to remove only 1Ah worth of power. However, they say they continue to balance after charging so I guess if it's running 24/7 it should get it balanced eventually and then from that point be able to do it much faster.

Another thing to consider is that you'd need to overlap these balancers to truely balance your pack, that is if they support it without some strange behavior. So they'd effectively only do 5 cells a piece. For example on the first balancer you'd connect cells 1-6, on the seconed balancer the first cell is also connected to cell 6. Cells 6-11 are on balancer two and so forth. This way over time the lowest cell voltage is eventually seen across all balancers. It might then be possible one balancer finishes and turns off too early.

Which brings up my last concern. If these do shutdown, do they turn back on if balancing is needed or do you need to disconnect and reconnect them? Would be nice to plug it in and forget about it but not sure they work that way.


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## speculawyer (Feb 10, 2009)

Elite Power looks like a good place, but they don't seem to have any balancing systems. Their packages are just batteries & charger.
http://elitepowersolutions.com/products/product_info.php?cPath=1_4&products_id=36









And the BMS systems seem to just be Battery *Monitoring *Systems that do no balancing. :-/
http://elitepowersolutions.com/products/product_info.php?cPath=6&products_id=49


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## bblocher (Jul 30, 2008)

Yeah 

I wasn't thrilled with paying that for the BMS that doesn't balance but you do get a one year warranty with them which is void if you don't use their charger and bms (if I understand the warranty correctly).

I'm making my own simple balancing circuits for my build.


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