I have been thinking of this concept for some time but do not know if its doable.
The skateboard currently runs on 2- 12 volt 8 amp hour sealed lead acid battery’s.
My thoughts are finding some cheap laptop battery’s disassembling them and making a battery pack or packs of Lithium cells to fill in location of current 12 volt packs to hopefully extend range and run time by a large margin.
I am confident I have the skills to tear apart laptop cells and re solder into new configuration.
How would I configure the packs?
How would I charge them?
What kind of protection would I build into the system?
How much more performance could be expected filling in the space with Lithium?
I am not looking to build it faster necessarily just wanting more range.
I would go with LiFePO4 battery packs
Need to know the current draw from the battery currently
They usually have MUCH better current flow rate and are a bit more resilient than standard Li Ion batteries…
A fellow built a solar rechargeable power wheels for his kids…they probably pull as much power as those razor scooters
http://www.atbatt.com/scooter-batteries/b/razor/m/e300.asp
Looking at the standard Lead acid Razor battery
Length 6 inches
Width 2.6 inches
Height 3.7 inches
This battery is “close” to that size
http://www.batteryspace.com/lifepo4-battery-12v-9ah-108wh-18a-rate-with-led-indicator-replace-sla-12v-7ah-lighter-weight-and-higher-capacity——un38-3-passed.aspx
151.2mm (5.95”) x 65.5mm (2.6”) x 95.5mm (3.76”)
They ain’t cheap, but they are alot more resilient than lead acid and can recharge a BUNCH of times (these have protection circuits on the batteries to prevent over drawing/discharging the batteries as well)
Smart charger Smart Charger (3.0 A) for 12.8V (4 cells) LiFePO4 Battery Pack, 110-240VAC, UL / CE listed
I don’t think you would get anywhere the AH rating using laptop batteries as you would from LiFePO4…and not nearly as safe either
A scooter would be hard on laptop cells. I do not like the sound of that setup. LiFePO4 is an option, but does have some downsides including a high internal resistance I think. OTOH if you are running the stock brushed motor setup you probably don’t care about a high IR. Actually, I don’t like the specs of that big LiFePO4 pack. Charge current is too low at 4.5A. I’d want to be able to do a “zip charge”. Maybe look into building a pack based on A123 cells.
fast charging is VERY stressful on battery packs
They are designed to be trickle charged…then dump that power very fast
I’m all out of ideas
Unless you wanted to buy a PCM, then build a 4S6P pack or something
I think the best information for what you’re trying to do will be found on the RC Forums. They work with multi-cell packs all the time. They will also discuss hobby chargers, best battery chemistry and battery matching.
pflexpro is right. You’ll get much better info on a RC forum or an e-bike / e-vehicle forum.
@WarHawk-AVG - you should checkout all the work EveryDayFlier did over on RCGroups dispelling just that notion about A123 M1 cells specifically. Zip charging at over 10c is pretty doable for those cells 
5-10min charge times is like living in a dream world. The problem of course is energy density is not so good… would you rather have regular slow-charge LiPo with an 8 mile range or be able to fast-charge but only have a 4 mile range?
A bit left-field but if you want high drain and fast charging, could you fashion something from supercaps? Downside would be less useful discharge curve and maybe issues with physical dimensions.
Otherwise, RC lipo packs would be a better option than laptop pulls IMO. Just make sure to use a voltage monitor alarm to prevent over-discharge.
I am not concerned about how fast the battery’s charge over night is fine
I would just like to make the run time increase by a large margin and try to do it economically
Packing that compartment full of pulled laptop battery’s for 50 or 100 dollars
sounded great to me but sounds like the consensuses is that’s not a good idea?
you have an idea how many 18650’s will fit in there?
what is your current draw using lead-acid?
does your motor controller have a low-voltage cutoff?
“laptop cells” are good for only up to 1C but if you have enough space you could parallel a lot of them and still get a lot of current, like what Tesla does…
if you don’t have enough space then you have to use “power tool cells” e.g. samsung INR or sony VTC
six groups of cells in series give 25.2 volts fully charged. you can balance charge this using a 6s hobby charger
since you’re gonna use laptop pulls then you pack isn’t gonna be matched so you’d have to deal with parallel group/s of cells getting overdischarged earlier than others. so you’d need a BMS (or at least lipo voltage alarm)
Some LiFePO4 cells have a higher internal resistance (comparable to ICR). But others (such as A123 Systems and K2 Energy) are capable of EXTREMELY high current. The way I see it, LiFePO4 is a no-brainer here. First off, the skateboard is designed around lead acid. And four LiFePO4 cells are basically a drop-in replacement for lead acid. Also, LiFePO4 has the advantage over basically all other Li-Ion chemistries (and lead acid) of having a stable voltage over the entire discharge cycle. So it won’t start to feel ‘weak’ as the batteries are discharged. Fast Tech sells pairs of the A123 Systems 18650s for around $5-$6 for a pair, which is pretty cheap. They are apparently pulled from new battery packs. But Although I have a few, I can’t vouch for them being 100% truthful. On the other hand, they ARE pretty cheap. So they’re certainly worth a try.
Actually many Samsung laptop pulls can easily push 5A continuous with no issues…with 6S6P you can really get some oomph out of em…but balancing and whatnot is the real issue. The other chemistry batteries are safer and can push more current but at a loss of capacity…but increase in recharge cycles
LiFePO4 is the solution…but they are not cheap unfortunately
I estimate you could place approx 54 18650’s in the space occupied by the 2- 12 volt battery’s
Can someone tell me how to determine my”current draw” I am guessing we are talking about it being a 12 or 24 volt system?
The skateboard currently just slows down and looses power eventually so I’m guessing there is no low voltage cutoff.
How do we determine run time when comparing battery’s is Amp hour all we look at?
If so 14 ea. of the 18650 LiFePO4 1200mAh would be equal to my current 2 ea 12 volt 8 amp hour ?
““laptop cells” are good for only up to 1C” what does this mean?
” four LiFePO4 cells are basically a drop-in replacement for lead acid” does this mean series of 4 could be charged with the existing charger or that it would equal the same power?
Thanks every one as you can see I am not an electrical wizard but I am very interested in this whole process
laptop cells aren’t designed to push out lots of amps so your cycle life will diminish greatly, not to mention the voltage sag… but if you parallel enough of them you could bring down the current per cell to around 1C and they’d be just fine at that draw
lifepo4 unfortunately is heavier, bulkier, more expensive, and has lower voltage per cell. so it isn’t necessarily the best solution e.g. tesla motors
seems like you got more than enough space in there.
C means capacity so for a 2600mah cell, 1C current draw is 2.6A
no you shouldn’t charge lifepo4 using lead-acid charger
you can determine your current draw by placing an ammeter in series with your battery. you can also guesstimate this from the wattage of the motor e.g. if motor is 250W then your current draw is around 10A
for 10A you need at least four 18650’s in a PARALLEL GROUP. then you need six of these groups in series for 6s4p configuration (total of 24 cells). if you’re using 2600mah cells this already gives you a bit more capacity than your lead-acids. just add more cells to the parallel group to add more capacity.
you might wanna consider buying brand-new cells so simplify things…
here’s a picture of a 6s BMS. it monitors the voltage of each parallel group and cuts off the entire pack if a group falls below thresholds. most BMS’es also take care of balance charging the pack so you could use a simpler charger
if you go with the cheaper lipo alarm route you’d need to use a balance charger. and the alarm doesn’t actually cut off your pack it simply sounds an alarm. hopefully you’ll hear it and stop riding 
Ratings of the battery
1C = 1xCapacity
More explanation on batteries
Simple breakdown of series/parallel batteries and the problem that can pop up if you have a weak cell