New e bikes with small ah batteries
- Thread starter Swinman
- Start date
That's where you need the test discharge curves for the cell type from somewhere like lygte-info.dk. A fair bit lower than 31% I think.
In real life, cutoff is usually triggered a bit sooner, when sag first takes the voltage as seen by the bms lvc or controller, whichever is set lower, below the set point.
Usually on the last hill...
Again, I have no idea what the battery protection level expressed as a percentage is on ebike batteries, but on my phone for example I get a warning at 15%... so if it's 15% on a 10ah battery, I'd need to buy a 11.76471ah battery to get 10ah. And my 19.2ah battery only has a usable 16.32ah.
Voltage is much easier to measure than capacity used, so that'll be why it is the preferred method.
You wouldn't want to go below 31V as this will significantly reduce the life of the battery. Also, there is not a lot of capacity left below 31V (see curves below). The main reason for the poor real world capacity performance of small batteries is that a high current drain rate per cell is needed at full power compared to a larger battery pack. This is what bleeds away capacity of a battery, you lose a lot when you pull high currents.
Have a look at this plot from our Danish friend at lygate-info.dk:
The energy you get out of the cell is the area under the curve (VoltsxAh). When you take out current at just 0.2 A, you get a lot of energy out. Take current out at 7A, you get a lot less. This is a Samsung high quality cell. Cheaper cells are likely to perform even worse at high currents. Some cannot take anything like a 7A continuous current draw.
Have a look at this plot from our Danish friend at lygate-info.dk:
The energy you get out of the cell is the area under the curve (VoltsxAh). When you take out current at just 0.2 A, you get a lot of energy out. Take current out at 7A, you get a lot less. This is a Samsung high quality cell. Cheaper cells are likely to perform even worse at high currents. Some cannot take anything like a 7A continuous current draw.
Let me clarify about remaining capacity at 31V...
When I said "there is not a lot of capacity left below 31V", I meant when measuring the battery after it had rested for a while and no load was applied. So if you hooked up a battery to your bike that had been discharged to 31V, and that was the voltage you measured when the battery was at rest, there would not be a lot of capacity left.
If you are pulling a big current, the voltage will drop a lot from the open circuit voltage. When it hits 31V, there is still a reasonable amount of capacity left, but you can't get it out without driving the battery voltage even lower, and in the process tripping out battery protection measures.
If you look at the yellow shaded parts on the plot, the situation on the right shows a low current draw and there is not much capacity left (small shaded area) below 31V.
On the left we have a large current draw per cell and there is significant capacity left which cannot be accessed at that current level (large shaded area).
So if you only have two or three cells in parallel in your pack, the current draw per cell will be large and you will be in the situation of the curve on the left.
When I said "there is not a lot of capacity left below 31V", I meant when measuring the battery after it had rested for a while and no load was applied. So if you hooked up a battery to your bike that had been discharged to 31V, and that was the voltage you measured when the battery was at rest, there would not be a lot of capacity left.
If you are pulling a big current, the voltage will drop a lot from the open circuit voltage. When it hits 31V, there is still a reasonable amount of capacity left, but you can't get it out without driving the battery voltage even lower, and in the process tripping out battery protection measures.
If you look at the yellow shaded parts on the plot, the situation on the right shows a low current draw and there is not much capacity left (small shaded area) below 31V.
On the left we have a large current draw per cell and there is significant capacity left which cannot be accessed at that current level (large shaded area).
So if you only have two or three cells in parallel in your pack, the current draw per cell will be large and you will be in the situation of the curve on the left.
It is all a bit of a compromise! Tempting to set LVC higher, say 32 or even 33V, but the inaccessible capacity if it hits 33V under heavy load is best part of 50%. Perhaps 32 is a good choice.
@guerney's 19.2Ah on LG MH-1 cells is I think 6P, so unlikely cell currents ever exceed 3A, so 32V would be useful extra protection without too much loss of usable capacity.
Yes, once you have bought the bike, but if you are thinking of buying, it's worth knowing - rather than being disappointed.
I may well do that, because the 31v is useless for steep hills, and the battery does get hot at that point cylcing home on low battery. I bought the biggest battery the seller had in the UK, because I wanted a bit of futureproofing - laptop batteries lose capacity with use, and I still need decent range a few years from now... if it doesn't burst into flames before then.
Hmmm... Very interesting - non-linearity all over the shop! Not a straightforward matter, is expresing LVC as battery pack capacity percentage...
A big old capacitor then? Buffer sounds a great idea, it'd prevent the battery getting stressed and hot?
Get a power meter and connect it in between your battery and the controller. Then you can see in Wh etc what you have used.
not felt any heat from the batts but the charger gets stupid hot when its charging.
May be of interest...
What current level is your controller?
I was thinking that I should do that the other day, do you happen to have any suggestions of the kind of product I'd need to buy? Basically, I stop well before 31V on most rides, but it would be intersting to find out what watts I am using when, or have used.
Cool! I'll have a good read later. I've left it as I bought it, 15A - I did temporarily change it to 20A, which the controller threw a little wobbly at, but it changed on the second attempt... but I want longer battery lifespan and didn't need the extra power, so it's just 15A.
Thanks! I'll buy one. I've missed using my new soldering superpower. I'll have to figure out how to mount it to the handlebar - it's very crowded with two headlights, a backward facing torch, a cellphone holder, horn, horn switch, headlight switch and DPC-18 display.
Last edited:
Related Articles
-
MTF Enterprises announces acquisition of EMU Electric Bikes- Started by: Pedelecs
-
Wisper 806T folding bike wins Which? ‘Best Buy’- Started by: Pedelecs
-
Sustrans calls for protected cycle lanes- Started by: Pedelecs
-
Amazon launch their first UK e-cargo micromobility hub- Started by: Pedelecs
