Back to lead

[emilevirus]

I had put some cheapo Lifepo4 100ah in my govt provided chair. I needed them for something else so I put back the batteries they provide which is pow-r-surge gel (BM says they're rebranded MK). I ride about 10km around town (1h non stop). When I had my Lifepo4, my ZXD was putting about 10Ah back so on a full charge they lasted more than a week. Now with lead, I come back after my ride and they're flat. Chair barely moves. ZXD puts back around 30Ah. How?! I get lead is less efficient but that's 3x more power.

[Burgerman]

You only get around HALF the energy out of a lead battery. Peukert. At the 1 hour rate maybe even less. The rest goes in wasted energy due to resistance, chemical reactions, etc and so you waste a load.

And when you recharge you also waste a lot of pwer too. In the same way. So you always put back more than you take out.

But theres another additional important thing that you dont consider.
Your lithium battery starts at maybe 25.6V. And its still much higher than lead as you roll all of the time. Lead on the other hand starts at about 26V straight ff charge, naturally drops to around 25V after maybe half a mile. Measured OFF load. But it drops to 18 to 22V as you are rolling along when loaded. Lithium doesent much change.

So lets say your motors needs 14V and 30A to keep rolling. With a LEAD battery that is coming from a battery that is loaded up. At this point your battery real voltage will be about 22V. Because of peukert and resistance. With lithium its still 25.5V. What difference does that make?

14V x 30A is 420 watts.
Lithium... 420 watts divided by 25.5V estimated is a 16.5A battery draw.
Lead... same 420W needed to roll right? But now 420W div by 22V battery is 19.1 Amps from that battery. Thats on top of the peukert and resistance losses during discharge and same again on recharge. You are pulling another 3Ah in 1 hour which is 10% extra all by itself.

Then theres the fact that your drop in lithium is around 5x lighter! You are not driving around carting 2 lead truck batteries with you...

And the rest is / can only be your non scientific battery discharge test method. Some days you use more than others...

[emilevirus]

The fact that we can empty 100lb bricks in 1h is insane. My chair isn't the heaviest either. No actuators at all. Just a basic chair. I guess once you go Lithium you can't go back. With lead I go through snow and it barely moves. With Lithium it barely slows down. I don't know how I did before, but now I'd be nervous leaving for a day with leads. Habits change I guess. Back then it was programmed like a grandma now everything is set to 100% so that must not help either.

[shirley_hkg]

Pair of lead are far insufficient to power modern heavy chairs, so we made power banks for wheelchair, to help one in ad hoc situations.

https://www.wheelchairdriver.com/board/ ... 2&start=60

[Burgerman]

Lead battery capacity at 1h rate. (your trip into town)


With lead he faster you discharge it the more energy is wasted and the less capacity it has.
The same happens on charge too. Worse, as you re-charge a sealed battery they make gas. Thhe more chared it becomes, the more gets wasted. That takes energy to split the water to hydrogen/oxyg. They then recombine this which takes more energy... When a battery is almost full it spends another 8 hours with a small current where 99% of it is wasted. But must still be done. Unless you want instead to stop charge early and have 12 hours at float to add to the charge.

So look at this discharge graph. It shows the difference in capacity between a slow "20h rate" battery. And 1 hour.

[Burgerman]

Now that means at an AVERAGE constant rate 1 hour rate, discharge you get 0.5 or 50% of ts capacity.

But we dont take current at a constant rate. We take big bursts. Then lower amounts. During the big bursts, so when we accelerate, turn in place etc with those heavy lead batteries that voltage may drop well below the point shown on the graph. Its possible if you give it some, that you can reach 3C. Thats a really low voltage below which the chair will deliver good power. It drops below 18V. And during thise big bursts you have higher peukert levels too. Further reducing actual range.

As I said many times. Once used to lithium, lead is just a waste of time. Its not just range. But weight. And charge speed and efficiency and a couple of decades of lifespan. And its CHEAPER today as well!

Lets put this differently.
Lithium batteries are all rated at the 1C rate. So a 100Ah battery is 100Ah at 100 amp discharge rate for 1 hour. And it doesent much matter or change anything if you do that over 20 hours ts still 100Ah out.

Lead batteries are not rated at the 1 hour rate. If they were the typical 100Ah (@ 20h rate) lead battery would be a 35 to 45Ah battery...

But in a chair that does 10kph, covers 10km, in 1 hour then that 1 hour rate matters!!!