patient lift

[Burgerman]

How many lifts per charge do you get? And what do you weigh?

Considering binning the daft 2.9Ah lead bricks, and replacing with 21 tesla 5Ah 21700 cells. So increasing from 2.9Ah (1.6Ah usable) to 15Ah. So TEN TIMES better! Yes it into fits the same casing.

[Burgerman]

Rough guess at hoist motor current without measuring it. So I can estimate battery usage per lift.
Of course measuring it will be way easier! But I am in bed.

According to physics, it takes 10 joules of energy to lift 1kg 1 meter.
10kg needs then, needs 100 joules...
200KG (my lift max weight) takes 2000 joules of energy. Actually 180kg but for easy numbers...

This presumes that the motor in the hoist is 100% efficient. Its way more likely to be around 75% at very best. Cheap magnets, plain bearings etc and brushed.
It also ignores the losses in the screw thread driving mechanism and gears internally and screw thread mechanism.

So lets say 2000 joules is really plus 50% to counteract the loss.
So now we really need 3000 joules of energy to lift a heavy user 1 meter each time we lift. Maybe more.

To find out how much that is in current at 24V we need to know how long it takes to lift 1 meter. I timed it. 20 seconds roughly.

If I convert this over 20 seconds we get a huge watts! But this is worse case scenario. The user will not be 200KG. And the losses may be lower.

However that means that 150W div by 24V means the battery / motor will pull 6 plus amps. From a 2.9Ah battery! No wonder we dont get many lifts. Thats a 2C+ battery drain. So when the battery is around half charged the voltage will collapse under load.

This is dependent on user weight too.

And really I should measure it.

[shirley_hkg]

We offer spare lift borrowing service for free.
Original 2.2ah tiny SLA batteries are difficult to maintain , especially in storage mode.
Then I was asked to convert stock battery pack with lithium.

When safety comes first with disabled people , I insist LiFePO4.

7.5Ah cells are put inside. First question being asked is , how many lifts per charge.

I asked one to test it, lifting a pair of MK G24 gel battery.

He quit, and said too boring, after 100 lifts.

I use tub side lift everyday. Last week , my obsolete 12Ah pack was running a bit weak. All 8 cells are found below 3v.

It was being used everyday , but was forgotten to recharge for a year.

[Burgerman]

Thats a lot of lifts. But a set of MKs is 45KG. I am 130KG. So thats 3x heavier. And because of peukert thats 1/5th of the lifts. And the stock battery is only 2.9Ah when new. They dont stay new long. Partly because the chargers they ship with hold them at an initial 29.6V and then a permanant slow cooking 27.6V. Stupidly they advise to leave them on charge indefintely.

I am sure they do this to sell as many batts as poss.

So I think I might "get" a dozen lifts. Which is into chair, out of chair for 6 days. But the battery doesent like that. But I recharge every 3 days for 12 to 16 hours after 3 days because I want them to last and lead doesent much like being discharged.

Would be nice to measure Ah out per lift though. Wont be much. But the battery doesent hold much either!

LiFePO4 may be the best way to go but you cannot get 15Ah that way. 7.5? Which cells did you use?

[Burgerman]

When you say "we offer" what are you? A mobility dealer? Or some kind of club? Or a local group of feinds and users?

Why shirley? Thats a womans name at least here! Confuses me.

Trying to figure out what it is that you are/do over in HK.

With stock 2.9Ah lead, and a 130KG user how many lifts would you expect? (I am tempted to test!)

[shirley_hkg]

Sometimes, my wife types for me.

They are some 1cm*8cm*7cm porch cells.

We are a self organised group, founded by SCI patients.

Now we include others , provided they have impaired movement of both hands and legs together.