Lithium battery power is available, usable and safe today in 2011. It already has been used in cars, models, hobby stuff, bikes, non mobility scooters etc for years now. They are used in everything from kids toys, to full sized electric cars and even aircraft. And laptops and phones, as well as hobby equipment like model aircraft and helicopters such as mine HERE! 

Although more expensive to initially purchase, it is actually massively cheaper over the long term as well as MUCH better performing!  Even without its huge range, speed, reliability and fast charge benefits.  Contrary to many "experts opinions" its already as cheap, Ah for Ah today compared to "good" lead (GEL) based batteries.

Higher speed is possible too at least if used with the correct matched controllers/motors taking advantage of the added power and low resistance.

Safe. These are not the same as Lithium Ion batteries in your laptop or the LiPo (Lithium Polymer) batteries used in my helicopter and are fire, and damage "safe" in that they will not burn or explode if misused, overcharged, shorted out, or physically damaged.

It allows the weight of the powerchairs battery to be halved, (with more than double the energy stored) which in itself increases both range and ramp climbing capabilities and improves soft ground operation. And its kinder to carpets, floors, vehicles, and tyres too. 

3000 watt hours of stored energy fits in the same space as 2, normal "Group 24" wheelchair batteries!  That's around THREE TIMES as much usable power, (at the 1 hour rate) as a pair of "Group 24" lead (AGM / Gel) batteries. At around HALF THE WEIGHT!  Or if you prefer, equal usable power, to SIX normal large gel batteries with the weight on just one! Think about this a bit...

That means 15 MPH and 45 Miles+ range. (Actually its a true 60 Miles if you were to use the manufacturers UNREALISTIC tennis court test! (ISO standard test) as they do...

While costing more initially they last around 2000 cycles compared to 500 maximum at 80 percent DOD compared to the BEST quality Gel battery available.  And when you consider that Ah for Ah they are not *that* much more expensive (if at all from some sources). And each Ah takes you about 30 to 50 percent further than a Lead based battery equivalent Ah.  Due to the Peukert value. And battery internal resistance.

HERE IS AN IMPORTANT OFTEN OVER LOOKED FACT...  We know LiFePO4 batteries can do easily the quoted 2000 80% deep cycles. Many have been tested. This  means 4 times the battery life of QUALITY gel batteries if charged daily.  

But here's the bit most people miss. We probably only need to charge these every 3 to 4 days or so... Since they are 3x bigger electrically in use. So in real terms it means 2000 CYCLES x 3 days. Or a realistically 6,000 days before battery replacement. For a heavy daily user.  6000 days is around 16 years!!!  And then they will be "down" to 80 percent capacity level.  So STILL twice as good as a brand new set of gel/lead batteries are ANYWAY!!!

The days of Lead batteries are totally finished. This makes them MASSIVELY cheaper long term than lead based batteries are, as we will likely never need to replace them during a 10 year powerchair lifespan. Most serious powerchair users eat a set of gel or AGM lead deep cycle batteries every year to 18 months. I know I do.

But here's the real reason they are more expensive to BUY:   You can actually fit about 1.8x to 2x the AMOUNT of Amp Hours (GIVING around 3X the RANGE) of battery into the same space your old lead ones came out!  And you SHOULD! This is where the advantage is! This means that you are actually buying double or three times the battery. Like buying 4 or 6 Lead batteries. This is what doubles or triples your range. And your cost, because:
 

So ideally you need to FILL your battery compartment! As much battery Ah crammed in as humanly possible to increase the amount of Ah storage capacity that you have. But that costs real money!!! And do not do this! See MONOBLOC batteries here Again this is what the manufacturers will try to fit. Its "easy" and lazy.

With Lithium Ion batteries like LiFePO4 you do actually get about 1.4 to1.5 times the range from the lithium battery (per same Ah) that you get from Lead batteries due to fact that Lead batteries are rated at the ridiculously slow discharge 20 hour rate. At the 5 hour rate they are about 1/3rd less. That suits a five hour continuous roll in a straight line. But that's not what we do. Ramps, grass, steep hills, accelerating takes power at the 1 hour rate or much more in big short bursts. So your 70Ah lead batteries are really only going to give you half to two thirds of the rated capacity.

Lithium's are all rated at a much more realistic 1 hour rate. In fact it doesn't much matter how fast you take out or put back the power, capacity stays the same with high rate Lithium's like I am using. Whereas the actual capacity of a Lead (gel) battery is less than its "rated" value when the power is taken out faster than 20 hours. Lead batteries are useless when power is taken out really fast. Like in short powerful bursts, as we typically do in a powerchair.  So a 70Ah lead battery is actually a much "smaller" battery in real terms (at about 38Ah) at a high rate than a 70Ah Lithium battery is. Which isn't obvious to most. You cannot just compare Ah to Ah...

There is less voltage drop under load with lithium too, so MORE of the available capacity can actually be used up at the end of the charge. So lithium's always give a range increase even with the same Ah capacity. So adding say an extra 50 to 60 percent in Ah capacity actually gives around double or three times the real world usable range.

So just replacing your 70 Ah Gel batteries with say 100 Ah of Lithium batteries (which will fit your battery box easily) will increase the range by around double in actual use. Or possibly a little more. Its hard to say without knowing the exact specs of the chair, the usage pattern, the old batteries exact specification etc. 

But there are no guarantees your existing powerchair controller will not give "errors" if you tried this. With say 8x LiFePO4 cells to give a close approximation to two lead acids at 24v.  But I doubt it will be a problem in most cases  You would need to test. The battery "gauge" definitely wont read correctly though! Just ignore it!

HOW I AM DOING IT:

The cells built in plastic blocks, are all linked with bus bars, and have cell balancing wires attached too!

LETS BE CLEAR. THERE IS NO GOOD PLUG AND PLAY LITHIUM BATTERY / POWERCHAIR SOLUTION. AND ITS UNLIKELY THAT THERE WILL BE.

Lithium's are not simple like the old lead batteries.

If you are looking for direct plug and play lithium's to SWAP your lead batteries for you will not find it. There is no lithium direct replacement battery you can just fit to your existing powerchair. Not yet, And its not likely for a number of reasons.  Life's not that simple. Its a system. Not a battery.  Once you understand how though, its actually surprisingly easy to change to lithium batteries.

Now when I designed the battery compartment of my new BM MK 3 powerchair I allowed for 6 different battery options. 2 different lead Odyssey and 4 Lithium.  Its called flexibility!  and with this controller that I am using ANY VOLTAGE up to 50v is great! The more the better. So any battery type is usable.

More voltage however, used with stock powerchair motors, means more speed without loss of torque or range, less amps on battery cables plus greater overall efficiency. It allows the controller to be effectively an efficient gearbox.  So I gain speed "free" of charge so to speak.

I made / designed the chair so it was both Lead (Gel or AGM) friendly, for both 24v (2x Group 34/24 batteries) and 36v (3x Odyssey PC1200 batteries) as well as two different types of lithium ion cells in both 24 and 36 (or more) volt arrays. I will be using approximately 45v. 

Lets never say I don't think ahead!!!  All of these options will both fit and work properly in my newest powerchair.

Spec Sheet for MK  As used in many hi end wheelchairs.

• Gel MK group 24 Specs:
  
  73.6 Ah at 20 hour rate (only useful for standby lighting etc  - you don't drive at slow constant speed for 20 hours!!!)
  66.0 Ah at 5 Hour rate (Closer to how powerchairs use batteries, but still much too slow rate).
  42 Ah or less (Estimated from Peukert law and resistance) at 1 hour rate. This more accurately reflects the real demands of ramps and thresholds, turning and accelerating. As we use powerchairs.

So at the 1 hour rate a pair of these Group 24 MK batteries gives just 1008 Watt Hours. Compared to 3098 Watt hours for the new lithium pack.  That is 3x the capacity & half the weight!

That means MY HUGE Lithium battery is 3x (THREE TIMES!) better than a set of GOOD Gel batteries by size and usable capacity alone.

Or the Lithium Pack is as good as 6x group 24 MK batteries!  At the same weight as just one!  That means if your chair previously had A 20 mile range, now it has 60+ miles... And you saved 50lb in weight too.

PLEASE READ THIS PART AGAIN!!!

To charge any of the above lithium packs you will need a Lithium "balance" charger that can charge up to the number of cells you need. 8 cells for 24v, 12 cells for 36v, 13 cells in my case for 43v.  Or a BMS system.

That needs a charger that can charge with a CC/CV of 3.600v per cell. Actually 3.600v  is considered a better figure than the manufacturers 3.65v per cell if cell life is important to you.

And the charger needs to be able to balance all the cells during charge. I use the  Hyperion advanced hobby charger for this.  Its not ideal, but I already have several for charging my old LEAD batteries!  And it does advanced graphing, PC control and logging, and works from 12v in my car too...