Not as cheap as lead acid certainly. Looking here as a kind of ballpark:
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's roughly $200 for an e-bike pack (I wouldn't buy batteries from them because shipping from China is annoying). I really don't know what sort of thing you would want for a scooter - rather depends on the range you need I suppose.
Also I don't know what the charge current would be. For a lithium pack it would have its own BMS but you'd need to replace the charger. A basic 36V pack charger might be 42V 2A which isn't a lot - fancier chargers do 5A.
On basic e-bikes there's a speed limit but no software acceleration limit. Acceleration is just limited by how much current you can pull out of the battery, and hence doing a lead acid -> lithium upgrade makes a difference.
:-)
I wonder how much of that is the batteries? I admit if the machine isn't designed to be lightweight then changing the batteries might not help much, OTOH Mum likely doesn't weigh even 125kg so there's plenty of scope for weight reduction if the batteries are most of that.
It's all a collection of tradeoffs - not having worked on one I can't say for definite how they all land.
You can nearly completely discharge a lithium battery whereas you wouldn't want to with a lead acid. As you say you can maintain a higher current for longer with a lithium.
Lead acid still beats Lithium on cost, but the gap is closing, given the lithium's real discharge capacity and the number of charge/discharge cycles they can endure.
I also wouldn't want to use a lithium unless you used a compatible discharge/charge controller.
If you're buying a lithium *pack* it'll likely have a BMS inside. If you're strapping together bare cells, or using LiPo packs from R/C modelling that don't have protection and have a habit of catching fire, that's your own problem.
So in general you don't buy a 'discharge/charge controller' as a separate item - you buy a pack that comes with internal BMS and it'll cut the power if it's unhappy about anything. LiFePO4 is more forgiving in itself if you have a crash and puncture the battery, which is not something the BMS can help with.
(it is worth bearing in mind the current limit on the BMS though - eg it's easy to take 30A on an e-bike when accelerating. A scooter might need more)
A mate is currently in the position of needed a new pack for his 36V eBike so we have been looking at that.
The one we have here has 2 x 45Ah sealed AGM batteries in it.
~5A or less I think.
Yes ... and they normally set the peak charge voltage.
Yeah. I think they don't push them hard because ... the lower current chargers are cheaper <g>, the battery is likely to not get as hot, the BMS might easier manage the charge balance ... they can get away with a cheaper BMS. ;-)
I thought I was reading the specs of one (at least) where that was a settable parameter?
Typically on the stock / basic units yes. We even had that on the RC electric cars I was racing years and years ago to prevent wheel spin (so a crude firm of traction control).
Sure, on an eBike / eScooter / basic mobility scooter (possibly).
I think I have the spec somewhere so could answer that for you but all I know is even a single battery is f'ing heavy (and I'm used to lugging them about).
She's tall and not a slim as she could be but not 'large' as such. ;-)
I've worked on a few now, both ours and for others. The speedo on the last bigger one is a Rhino something and I was amazed at the range of settings it had (and looked into getting the software but it was very expensive).
Even going along some of the narrower pavements round here (with regular crossovers to houses) Mum was expressing some discomfort re how much the scooter was leaning (and I reassured her she was perfectly safe).
The track is a compromise between stability and restriction re doorways etc.
Also check the tyres some of those seem to be rip off prices if my friends are anything to go by. Most disability specific stuff seems to attract a couple of 0s on its price for some reason.
Also, and don't take this the wrong way, before anyone uses it get that persons eyes looked at for macular degeneration too. 60mph, that would be stupid, at the moment there are calls from pedestrian associations to limit their speed automatically to much less than they can do now due to injuries. Brian
With a lot of battery types, you don't want an individual cell to be reverse biased. That means, if one of the cells in a pack is the first to completely discharge, fine, it drops to zero volts. If you had a technical device to detect that condition, you could "milk" the maximum amount of power from the pack. When one cell equals zero, disconnect pack.
Instead, the terminal voltage of the entire pack is monitored, in the hope that no cell has yet reached the knee voltage. At the knee voltage, the cell will rapidly drop in forward voltage, and soon, will end up reverse biased, and damage to the cell starts to occur at that point.
When the pack drops below the knee voltage, the chargers are designed to refuse to charge them.
Making up numbers, say for example, a 14V pack had a knee voltage of around 9V. Then, you'd have a monitoring circuit on the load, which, when it detected pack depletion to 9V, would disconnect the load. And would have to do so, with less than microamp leakage currents flowing. The user would release the trigger on the item, and in addition to battery management, now the load is disconnected.
If the pack then sat "at 0%" for a month or so, it might fall from 9V to 8V. You reach for the charger and plug it in, but the charger, on seeing a terminal voltage of 8V, refuses to put current into it. It does this, because the charger assumed one of the cells got reverse biased, metallic lithium has plated out, and that cell has become a fire hazard.
That's why there is battery management on both ends. The amount of charge the pack holds, is "pushing it". The pack might comfortably and completely safely, be charged to 80%. The definition of 100%, is a compromise value. If you charged the pack to 105%, maybe it would only last 100 cycles. If you charge it to 80% and then downwards, maybe it lasts 500 cycles. Then on the other end, the monitoring is based on measuring the entire pack, rather than monitoring each cell as an individual. If individual cells could be monitored, then more of the available charge could be used (but probably not enough for a heroic improvement). The 9V value is based on experience, defining a voltage value that cuts off the load, before any one cell hits zero volts.
When packs only have the one cell, reverse bias is not possible. This is why digital camera chargers, are only too happy to charge their one-cell pack, all the way from
0 volts at terminals. This is different than a laptop, where maybe the presentation of the 8V value, does not "please" the charger. And it is refusing to charge, because of the fire risk. The one-cell pack then, is a special case, where one of the safety issues is not present, and a more generous operating policy is possible.
Now, if they wanted, they *could* have put all the cells in parallel, and used a boost converter design, to lift the
3.7V @ megaamps to 14V output. This would give the desirable properties of the digital camera charging situation ("zero is OK"), with the power capabilities of N cells in a pack. But nobody does that. Using a power converter, especially a boost with a ratio like that, would not be particularly efficient, and you might lose 50% of the energy as heat. Thus, we are stuck with the crappy properties, the conservative 0% definition, and the possibility of damage at 100% charge. And that's why the charger voltage measurement capability, must be so good. It's to ensure the 100% definition is accurate.
The lithium is also temperature dependent, and should be brought to room temperature before charging. If you brought a pack out of the car in winter, you should allow it to warm up for a while, before attempting to charge it. The cells have better charge acceptance at room temp. Lead acid has much better low temperature behavior (a fully charged lead acid, the electrolyte freezes at around -55C). In BEVs, the lithium pack may be equipped with heating/cooling loop, and when charging current is applied, first the power goes into the heater, if the pack needs to be heated back to room temp. Later in the charge cycle, it might require some cooling. Other consumer goods, neglect all these optimizations. And the user has to do the best they can.
If they are anything like car ones. Li-Ion are about 3 times the price of equivalent good brand lead acid. And on cars, they don't have a three times longer warranty. They are of course lighter and smaller for the same capacity. So may give a slightly better range.
Managed to jump a bit of juice from a powerchair we have with a weak but not dead battery, and the control panel is responding. Not enough juice to whirr the motors, but I feel confident enough to splash out on a pair of new batteries.
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