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GUIDE TO CONVERT A POWER WHEELCHAIR TO LITHIUM BATTERIES

WHY YOU WOULD WANT TO CONVERT TO LITHIUM ION BATTERIES:

a) At LEAST double the range.
b) At LEAST 5X the battery service life.
c) MUCH faster charging.
d) MUCH less weight.
e) MUCH greater reliability.
f) MUCH cheaper over the long term.
g) Greater performance is possible*

WHAT ARE YOU LOOKING FOR:

 

THIS IS A STEP BY STEP DETAILED GUIDE ON HOW TO CONVERT YOUR EXISTING POWERCHAIR FROM THE OLD LEAD BATTERIES IT USES NOW TO HI TECH LITHIUM ION PHOSPHATE.


Its actually quite simple. Not exactly "Plug & Play" though, so requires someone with a brain!  But its really not too difficult.

Don't expect a "simple dumbed down" easy, swap over though to a ready made "equivalent" because as of 2012 there simply isn't one.

At least not one that gives the true huge benefits of lithium batteries.

Lithium is a "system" not a lead brick replacement. And to do the job properly and gain all the massive advantages of Lithium, it never will be.

Regardless of the already available "easy" solutions or others that appear in the future. Yes they may be out there and work in some fashion.

But they will only give partial benefits, and will never be as reliable or as long lived, or offer the true range and performance possible.

 

   
 

 

John WilliamsonTHE PROPER WAY TO DO THIS

This system described here is how I would replace either a pair of group 34 batteries, or a pair of group 24 batteries as used in most hi-end existing power wheelchairs. The difference between group 24 and group 34 is simply one inch (25mm) in height. Group 34 is used where this matters, for e.g. where seat lifts are fitted. They require the extra space.

Now is the best time to MEASURE your battery area. To determine its height.

If there's room for a battery of 6 lithium cells in height (each one is 40mm so total height is 240mm) then great! You should order 72 cells. If there is less than 240mm, but 200 or more, you should order 56 cells and build the group 34 equivalent lithium batteries.

Building a pack using the Lithium Headway 12Ah high rate 20C cells is the best way available today to get the largest capacity, easiest assembly, best possible range and performance and the safest/longest lasting setup possible in a conventional existing powerchair. This may change over time. But right now is the way forwards. Lower C rate cells, like some rectangular "prismatic" cells, do not have a high enough C rating. Or have fragile connections, or are the "wring" physical size/shape to best fit the existing battery compartment. Etc.

There are OTHER ways, other cells, other chargers, other BMS systems, and they are as far as my research and understanding is concerned inferior for lots of complex reasons.

8 cells of lithium ion phosphate gives 28.800v fully charged. So its similar to leads CHARGING voltage. It soon drops fast to the same value as lead at about 26 volts in the first 50 feet of use or over a day or so if left unattended. So if you see an over volt error on initial start up on some powerchair controllers, just discharge the first 0.1 percent and it drops voltage to a much lower safe level.

After that voltage level behaves differently. Voltage doesn't rise and fall under load like lead batteries. Its solid. It pretty much stays at about 3.4v per cell rock solid for the next 20 miles. So there's no accounting for what your wheelchairs battery meter will do. Its not expecting this. So will definitely read very wrong. There's a tiny possibility your controller decides there's a "problem" and gives an error. That's an unknown. The ones I tried were fine.

You must use high C rate cells though. Or they will die soon. The headway cells are 10C, Lithium isn't tolerant to over C rate discharge, even for an instant.  And a powerchair can require 240 amps peak.

Over voltage charge even by a tiny amount, or under voltage discharge also rapidly destroy lithium batteries. Other than that, only cell balance is an issue. This causes rapid death, as one or more cells rise in voltage super fast at the end of charge to above 3.65v. Literally in seconds. This is where on board BMS systems, and built in ones in monobloc batteries, generally fail. This alone gives lithium's a bad reputation for reliability, and "bad cells"... And this is why I use an intelligent charger that reduces charge rate as the cells are balanced at the end of charge. See
Hyperion here


 

TO BUILD TWO REPLACEMENT LIFEP04 BATTERIES YOU WILL NEED:
 

  • A Hyperion 1420i net3 charger. This is the BEST hobby, lithium (and every other chemistry) charger on the market, and its cheap powerful and computerised! And very accurate. It negates the requirement of expensive and inferior, over complex on board BMS systems, and allows perfect charging and cell balance as well as full monitoring and control. On board BMS systems, used by "other" systems, along with dumb chargers are the main reason for lithium battery failures.
  • 72x Headway 12Ah SAFE cells for a group 24 replacement. (or 56 cells for a group 34 replacement). The difference here is just the total battery Height. Either 240mm or 200mm height. You choose. If you can possibly fit the taller one then do so!  These cells are tested to be fire and explosion SAFE.
  • 72x (56x) Orange cell building blocks. These plug together like Lego! Allowing you to build a couple of batteries any shape you wish. See image at top of page.
  • 136x (108x) flat 2 hole "BUS" bars or connectors to join up the cells. Again see top image.
  • A SERIAL type D connector and plug/socket as used on PCs. For your balance connectors.
  • Anderson battery connectors
  • Some 8 gauge red/black cable.
  • Some small diameter wire and "ends" along with some heat shrink tube to make up a loom for the balance wires.
  • 144 (128) Small M6 dome/round headed Allen screws to replace the cheap cross head screws supplied with the headways.
  • Lots of time. Patience and some simple hand tools, crimp, soldering iron etc.

Many places stock the cells, connectors, building blocks, etc. Mine came from www.evassemble.com and they are fast and efficient.

The lithium battery pack I built for my BM3 powerchair is a little different to the ones on this page, as its ONE piece, plus its 45 volts output, and will not work on a 24v stock powerchair. This further takes advantage of the reduced voltage drop under load of these cells, to gain efficiency and speed by increasing the voltage. This isn't necessary for a stock powerchair. But does make even better use of the technology.

 
 

BUILD BATTERIES

Each group 24 lithium battery comprises of 36 cells.  6 cells tall, 6 cells long. That's 72 cells total.  This shows ONE of the two batteries, as well as all the connections. for the balance connectors and bigger power connectors.

This gives a 108 Ah battery.

Considering that only about 50 to 60 percent of the RATED Ah of a lead acid/gel/AGM battery is actually available and useable, this means a HUGE range increase is possible.

E.G. a 70Ah group 24 powerchair battery has only 35 to 45 Ah actually usable. Where the lithium battery has almost all of its rated 108Ah usable. So DOUBLE the range is easy.

 

 

 

 

 

 

 

 

 

The same thing with the smaller (lower) battery below:

 

These batteries will be exactly 174mm wide x 240mm long and 240mm high for the 108Ah one, and 40mm lower in height for the smaller 84Ah one.

But you have to realise that ALL of this capacity is actually usable. With lead batteries only around half the rated Ah is accessible due to resistance, Peukert voltage drops under load and depth of discharge issues.

 

 

 

 

The SECOND image shows the same thing for the 40mm lower height group 34 battery, (200mm tall), is made from 28 cells. Or 56 cells total.

That's 5 cells tall and still 6 cells long, but with some missing from the top row.

 

The batteries here are arranged in parallel groups, and these groups are then also connected end to end in series. Look at the diagrams.

So the group 24 EQUIVALENT battery above for E.G. has groups of 9 cells connected in parallel. Each group of 9x 12Ah cells is connected with all the negative terminals, and all the positive terminals linked together. So it gives a 108Ah 3.2v "big" single cell.

These parallel groups of 9, are then connected in series. That is positive to negative. There are four of these groups per battery, and EIGHT groups of these "big cells" in total (both batteries) so we get 8x 3.2v = 25.6v in total. 

Each battery separately is a 9p 4s pack. The total battery (both together) is then called a 9p 8s pack. Because its 9 in parallel, and now 8 in series, to give us the required 108Ah at 25.6v.

Each one of the above batteries, behaves exactly like an old 12v lead based battery. That is they are fitted in the same battery space, and only need a sheet of plastic or some other insulator to stop the cell ends touching metal, or each other. But at twice the usable Ah, and at half the weight. And 5 times the service life.

PROVIDED you do not run them down below their safe voltage limit, (2.5v per cell) OR charge any cell beyond its safe 3.65v limit. This is easy enough, as the voltage stays pretty much constant in use. Then drops off a cliff at the point where the cells are depleted. As SOON AS YOU NOTICE THIS STOP! Go no further. And you will not damage the batteries. Or monitor the battery cells with a hobby battery tester.

The HYPERION charger monitors all cell parallel groups during charge and will not allow OVER VOLTAGE to happen. This is why we need the 9 thinner wires, that go to the Hyperion balance ports. See the Hyperion instructions. They tell the charger to slow down, and then each cell is balanced individually. After charge each cell will be within 3/1000ths of a volt the same!

The BATTERY METER on your mobility device will be totally useless with a lithium battery. The best measure is to see EXACTLY how many Ah is returned during re-charge which is measured by the Hyperion charger. If its under 90 percent of the battery capacity you are safe! It will be because range will be huge.

 

 

ASSEMBLING PACKS

When you order your cells, it pays to get an extra one, and an extra plastic building block and a couple of extra bus bars. You may break or damage a part or may get a bad cell. Its unlikely but not unknown.

 

MATCHING CELLS

The very first thing you need to do when these new lithium cells are received, is to fully charge them in parallel, (with + ends connected together, and - connected together) in groups of say 5 or 10 or whatever is easy to manage. Charge all the cells, as soon as possible, as per below, and then immediately disconnect them after removal from the charger so that each individual cell is loose. 

How? Charge the groups set to LIFE 3.600v as a big single cell with the Hyperion and then allow them to stay connected AFTER charge for say 2 further hours. The charger continues to charge AFTER the end point slowly. The reason for this is that we need to be sure that they are "soaked" at this 3.600 VOLT and FULLY charged. The charger maintains the voltage after the charge ends for a long time and it allows them to fully soak up the last few percent of charge.  Then immediately disconnect every one and put them all in a big box, put it away somewhere for as long as possible, but at LEAST 2 weeks. 6 is better!

We are trying to ascertain which cells SELF DISCHARGE the most. This is the single most important thing that screws up cell balance. Use (discharging) doesn't. Charging doesn't. Self discharge does dramatically...  We need to be patient and wait as long as practicable. At LEAST 14 days!!! Longer is better.

Then, with the Hyperion charger connected to a PC, Set to 12Ah, 3.600 volts, charge every separate cell individually until the charger says done! Read off the EXACT mAh RETURNED to the cell, and using a marker pen add a cell number and the figure. So say: 1 and 0.677mAh. Next cell 2 and 0.456mAh etc, etc,...

We then need to assemble the cells into 8 groups of 9 cells (for the group 24 sized battery) with the same AVERAGE discharge rate. This is easy! Just add up all the Ah and make sure that you have low ones, and high ones in each group that add up to the same amount! As closely as possible. Being good at maths helps. Lots of trial and error with a calculator and sheet of paper works too... My way!

Don't mix these up! You want to end up with 8 boxes with 9 cells in each. Each box should add up to the same (or close) figure...

IMAGES TO BE ADDED...

 

BUILDING THE PACK.

The building blocks are sort of self explanatory. You need to make 2 sides with 6 cells high, 6 cells long for each 12v battery. Or 5 cells high and 6 cells long, with some missing on the top for the smaller group 34 sized pack. See the images at the top of the page. The best way to do this is on a solid surface with a small hammer and a cloth to protect the plastic. They are a very tight fit!

Don't add the actual cells yet!

Study the drawings at the top. You need to make sure you fit them the correct way around, and in the groups that you have already separated them into (above in 8 groups). assemble these laid flat, with the cells stood upright. Again you will need to add all the cells, then tap the side on with a hammer and cloth. DO NOT add the bus bars yet!

TO BE CONTINUED

 

In a month or so I will add more to this page, as I intend to do exactly this and build two batteries for a stock powerchair and will photograph every detail as I go along. A picture is worth a thousand words and all that. But really its simple.

For more info right now, go to the Message Board

 

 

 

 

 

 

 

 

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