Help me calculate the range for my wheelchair

[snaker]

Hi,

My wheelchair with 2 x 12V/35Ah new lead acid batteries is able to go far 18km in maximum. That's too short for me. So, I intend to replace lead acid batteries by LiFePO4.

I will buy 2 units of 24V/20Ah LiFePO4 batteries at pingbattery.com. The guys at pingbattery.com will also help me to connect these 2 units into a logical unit of 24V/40Ah.

Because this is the first time I touch LiFePO4. I do not know if 24V/40Ah LiFePO4 could improve the range my wheelchair is able to go?

I see in forums, there are someones who are using wheelchair with LiFePO4 batteries. So, by your real experience on LiFePO4, please help me to estimate the range my wheelchair is able to go with a 24V/40Ah LiFePO4.

Here is some information of my wheelchair:
- 2 x brushed motors, each motor is 24V/320W
- Weight: 35kg (without batteries), 55kg (with lead acid batteries), 45kg (with LiFePO4 batteries).
- Speed: 6-8km/h

[Burgerman]

Ah...

Is meaningless with Lead acid batteries. You will be getting around 50 to 60% of the ACTUAL Ah that your lead batteries can give in use. Depending on a number of things.
But you cant just "fit lithium" without understanding a lot of stuff. Or you will gain little in range, and they will offer a short and probably unreliable lifespan. And cost a fortune.

To give you an idea, my own chair has around 2.5 to 3 times its original range. If you do the job properly, you can expect the same increase.

If you just fit 40Ah lithium in place of your existing 40Ah lead you gain only about a third in range. And you shorten the life of the new lithium because you are working it too hard.
The WHOLE POINT of fitting lithium in place of lead, is because its MORE ENERGY DENSE. Meaning it allows you to fit more Ah into the same space. This also means the battery is worked less hard, and charged less often, and discharged less deeply. Meaning that as well as huge range you also get a much longer service life IF CHARGED CORRECTLY. That means NO BMS and a high C rate pack, and a hobby style cell balancing charger.

You need to be aiming to fit say 60 to 70Ah in place of your 40Ah lead or greater. PING are a bit dubious in quality, and the built in BMS will "cut power" as you need it on a ramp or on a hill. Because it uses a cheap generic built in BMS with will both cause problems in use and shorten the 2000 cycle lifespan to something much less. Most wheelchairs can require 200A cont from a battery on a steep ramp or slope, which with a tiny 20Ah battery means 10C cont rating. Ping cannot do this.

I suggest you read the thread about Lithium batteries that is pinned at the top of the forum.


_______________________________

Ping specs...

•Voltage: 24 Volts (25.6 Volts Nominal)
•Capacity: 20 Amp Hours
•Dimension: 180*105*155 mm / 7.1*4.1*6 inches
•Weight: 4.9 kg / 10.8 lbs
•Charging Voltage: 29-30 Volts
•Charging Current: <8 Amps (<15 Amps with High Rate BMS)
•Rated Discharging Amperage: 20 Amps

You need about 150 You actually get 40A with two of these packs connected together.

•Discharging Cut-off Protection: 40 Amps (65 Amps with High Rate BMS)

You need about 200 this will cut power off at 65 x 2 instantly as you climb a ramp or slope etc.

•Lifecycle of the whole pack: >85% capacity after 1000 cycles. Lifecycle of single cell: >85% capacity after 1500 cycles, >70% capacity after 3000 cycles. (<1C discharge rate and <1C charge rate)

HALF the lifespan you should get, because of the way the BMS charges the battery.

And the capacity is too small. You need to double this. But you cannot do that with ping because they are physically big/wrong shape.

AND...
http://endless-sphere.com/forums/viewto ... 14&t=50511

Ping has a lot of problems, even if you were to fit 4 of these packs.

[robnnorthaustin]

Agreed that a Ping battery is not ideal, Would a large car stereo capacitor across the output of a modified Ping battery with no BMS add enough of a buffer to the large current needs when going up a ramp? I know this question is subjective not knowing how high or long a ramp or the exact current needs but in the referenced question it seems a ping battery is partly current limited by the BMS so I wondered if adding a Capacitor might make a difference.


For those of us that have smaller battery compartments and finding that pure lead batteries are hard to find in U1 and smaller size, we cant fully implement your design using huge pack of Headway cells. So it seems our choice is either replacing our lead with a smaller LifePo04 pack or just using an add on 2 hobby LifePo04 packs or adding on a hacked (BMS removed) ping style battery.

[Burgerman]

>>>Agreed that a Ping battery is not ideal, Would a large car stereo capacitor across the output of a modified Ping battery with no BMS add enough of a buffer to the large current needs when going up a ramp?

Well it would need to be as big as a barrel, but theoretically. But not practical.

As for building a smaller lithium battery you could use those small cells that are in Nandols new pack, or say 70Ah Calb, or any small A123 type cells.



http://ev.whitecape.org/insight/A123/A1 ... ies051.jpg



http://www.diytrade.com/china/pd/950053 ... _cell.html

Etc. Lots of options. Take a look! https://www.google.co.uk/search?q=a123& ... fepo4+cell

[snaker]

@Burgerman:

Thank for your valuable info about LiFePO4 batteries. It seems I have to read all 35 pages in the sticky lithium battery thread to get more knowledge.

So, Ping has problems in quality. Could you suggest me some better brands (but not too expensive ) of LiFePO4 batteries?

My wheelchair is a lightweight model. Its battery case is small = 26.0cm x 18.5cm, x 19.0cm (L x W x H). It can only contain 2 units of 24V/20Ah. I have no skills in the electric domain, I am afraid that I would be unable to combine small cell batteries in a safe and correct way.

You said my wheelchair would need a batteries pack which must has 150A discharging and 200A of discharging Cut-off Protection. Is this discharging from heavyweight wheelchair models? Because my wheelchair used 2x12V/35Ah lead acid batteries for years and I think a 12V/35Ah lead acid can't have 150A discharging. I watch on my lead acid batteries' labels and I see the info below.
Initial Current:
- Floating Use: No Limit
- Cycle Use: 12.4A MAX

Does it mean my current lead acid batteries only have 12.4A discharging?

Or is there any way to calculate the discharging Amp requirements based on motor's info (e.g Wattage, Voltage)?

[Burgerman]

My wheelchair is a lightweight model. Its battery case is small = 26.0cm x 18.5cm, x 19.0cm (L x W x H). It can only contain 2 units of 24V/20Ah. I have no skills in the electric domain, I am afraid that I would be unable to combine small cell batteries in a safe and correct way.

Then there's no way to do this. At least no sensible and reliable way with any real benefits. There are no ready to fit easy lithium batteries that are a straight drop in replacement that will work or be any real benefit in a powerchair to date.


See here small lead battery.
Closest lead battery on this list is 18Ah at the 20H rate. It can do 1,200 Amps! Its engine cranking capability is still 460 Amps pulse (5 seconds). http://www.odysseybattery.com/extreme_b ... specs.aspx

Or is there any way to calculate the discharging Amp requirements based on motor's info (e.g Wattage, Voltage)?

No. Motor watts tells you nothing useful.
A stalled or climbing powerchair takes the maximum amps - per motor - that the controller can provide typically. That may be 70 to 120 Amps per motor. So 140 to 220A when under load intermittently.

In order to gain any sensible battery service life in a power chair, not have BMS systems cutting off power, and to not stress cells excessively you need to INCREASE the Ah you fit. And use a high rate low resistance type of cell. That will ensure your investment works, and actually gives improved range and lasts well. If charged correctly...

So, Ping has problems in quality.

Not particularly. The real problem is the Ah is too small. Meaning you overwork the cells, gain little in range, and so shorten cell life. And the BMS built into that box is inadequate for driving. And that the BMS alone also causes damage in the very way it controls charge balance. There are better ways to do this.

[robnnorthaustin]

See here small lead battery.
Closest lead battery on this list is 18Ah at the 20H rate. It can do 1,200 Amps! Its engine cranking capability is still 460 Amps pulse (5 seconds). http://www.odysseybattery.com/extreme_b ... specs.aspx

Or is there any way to calculate the discharging Amp requirements based on motor's info (e.g Wattage, Voltage)?

.
.

Bm, The common size for most of the 18 to 22 amp batteries used in lightweight wheelchairs and scooters here in the US is considered half U1 size or 7.2" x 3" x 6.6" and the closest odyssey battery is the 680 model. I tried unsuccessfully to modify one of my battery boxes to except the larger PC680 and it technically worked but did not look great so I returned batteries. Anyone that buys one of these Great batteries will more then likely have to modify their battery enclosure.

[Burgerman]

I wasn't suggesting fitting one. I was comparing a lead acid amp capability with the lithium battery from ping to show that the lead can make an awful lot more amps size for size.