Too many options in my mind, but... any good?

[Arbol]

The BMS is set to an industry standard so that you get the optimum between battery service life and capacity. Why do you want to change it? do you only part-charge your phone?

I am not so sure vendors try to optimize the relationship life vs capacity. I believe (but I could be wrong) they try to maximize capacity, no matter what with life. Many (most?) buyers want powerful e-bikes, I do not know if many of them are asking questions about life.

Also em3ev sells chargers with a switch to charge up to 90% of SOC, not 100%, he argues life is significantly extended.

All this suggests me that it makes sense to build a big battery for an e-bike, with cheap 18650 cells, which allows to remain in the good SOC area for longer, allowing a longer life. This leads me to think building metal boxes inside a frame makes sense.

 

[Arbol]

Obviously some sense is needed about when to recharge, it would be silly if to recharge after only a tenth of the capacity is used on a very short local trip.

But it would be equally silly not to recharge after a third of the capacity is used and a subsequent ride might take the battery down to very low content. That also stresses the battery.

On most e-bikes with most owners we are only speaking of 30 miles or less range, so there isn't much tolerance for avoiding recharging anyway. On the many e-bikes with 20 miles or less range a high proportion of rides will require a following recharge.

With a small battery, it may happen that for every trip, the biker just plugs in the battery into the charger when he gets home. This would lead to many hours with the batteries having 4.2V, which is bad.

Instead, having a big capacity battery probably leads to less frequent charging, so the batteries remain at 4.2V much less often, which is good.

 

[flecc]

That's only if the battery BMS/charger combination pushed the battery to 42 volts. Those I've had only reach 41 volts full charge and from what I've seen in member's posts that is much more common than 42 volts.

In other words the design is safe for the battery to be fully charged without severely stressing it. Equally e-bikes usually cut out at about 32 volts low charge, once again by design to protect, since the battery could be discharged more. For most users the gain in life from trying to improve on this built-in protection is probably minimal.

Playing about with charging routines I think a bit like trying to get the maximum possible mpg from one's car, only possible with a lot more hassle and sometimes even spoiling the pleasure of using the vehicle.

 

[Arbol]

That's only if the battery BMS/charger combination pushed the battery to 42 volts. Those I've had only reach 41 volts full charge and from what I've seen in member's posts that is much more common than 42 volts.

In other words the design is safe for the battery to be fully charged without severely stressing it. Equally e-bikes usually cuts out at about 32 volts low charge, once again by design to protect, since the battery could be discharged more. For most users the gain in life from trying to improve on this built-in protection is probably minimal.

Playing about with charging routines I think a bit like trying to get the maximum possible mpg from one's car, only possible with a lot more hassle and sometimes even spoiling the pleasure of using the vehicle.

That's a fair point.

However, my point is different: my point is that for a DIYer, it makes sense to overbuild the battery, with many cells (maybe cheap ones).

The reason is:

1. More capacity outright; this is a good reason already
2. Charging less often, which leads to less time at the dangerous 4.2V area
3. More time at non-extreme SOC levels
4. It takes more days to finish a cycle, and a big part of cell life is the amount of cycles, so mechanically the battery lasts longer
5. The biker can do its usual trips without bothering about capacity (the battery has more capacity than before, even though the biker does not use the full capacity anymore; both effects should approximately cancel out each other), while remaining at safe SOC levels all time time

Of course, this has its own problems: price (but 18650 cells are cheap already), building complexity (it is not easy to build a battery from scratch, at least for me ... granted, I am not knowledgeable enough), location (there is no other way than putting a big battery inside the triangle frame, and this requires either polycarbonate style or directly soldering steel or alloy).

 

[Deleted member 4366]

there is no simple indication about the state of charge (SOC) of Lithium ion batteries, but one can assume that when the battery charger LED goes green, SOC is 100% and when the BMS cuts out, it's 0%

I wouldn't assume that at all.
Don't forget that if you assume anything, you make an ass out of U and me.

 

[Arbol]

No assumptions, but facts:

- BMSbattery: "Li-Ion Battery Charge Voltage = 4.2V x the number of cells in series"

http://www.bmsbattery.com/alloy-shell/26-alloy-shell-400w-lifepo4li-ionlead-acid-battery-ebike-charger.html

- em3ev: 54.6V for 13s battery (54.6 / 13 = 4.2V)

http://em3ev.com/store/index.php?route=product/product&path=37&product_id=162

 

[Deleted member 4366]

One major problem with not fully charging your battery: The transistors for the bleed resistors will never open, so you won't get any balancing. You'd need a custom BMS. As the battery gradually goes out of balance, you'll get less and less capacity, and then when you try and charge it fully, the high voltage cells will switch off the charging too soon. The bleed resistors can't cope with a big difference. They'll bleed the voltage down to about 41.5 to 41.7v anyway, so the charging to 42v is only for balancing.

 

[trex]

Why is that? I naively assumed the most important, rather than the location on the frame, would be to have both front and rear suspension.

I do not see why behind the seat post may reduce vibrations better than on the down tube, for a hardtail. Can you please provide the argument for a seat post battery and against one on the down tube? I would like to think about this.

The problem with Chinese bottle batteries is poor design.
The fixing to the downtube is also not great, the bottle can easily wobble from side to side. Contact may be lost under severe vibrations, you can even see sparks coming out of the two flimsy spring loaded contact pins.
The square bottle type may be better, time will tell.
Batteries behind the seat post are held by all sides as opposed to two M5 on the bottle, limiting wobbling from side to side. The output connector (of behind seat post battery) is arranged vertically, also keeps good contact against vertical jolts.
Rear rack batteries are generally very stable but can limit how much you can carry in your panier.

 

[Arbol]

The problem with Chinese bottle batteries is poor design.
The fixing to the downtube is also not great, the bottle can easily wobble from side to side. Contact may be lost under severe vibrations, you can even see sparks coming out of the two flimsy spring loaded contact pins.
The square bottle type may be better, time will tell.
Batteries behind the seat post are held by all sides as opposed to two M5 on the bottle, limiting wobbling from side to side. The output connector (of behind seat post battery) is arranged vertically, also keeps good contact against vertical jolts.
Rear rack batteries are generally very stable but can limit how much you can carry in your panier.

Ah, ok, now I understand, thanks. So the problem is not the location on the frame, is that different locations of the frame are more amenable to better or worse holding cages.

But a given battery, say a triangle one, on the bottom of the triangle, so just above the BB, and inside a metal cage covering the full empty triangle space, will suffer the same kind of vibrations than an equivalent capacity battery behind the seat post, right?

 

[Arbol]

One major problem with not fully charging your battery: The transistors for the bleed resistors will never open, so you won't get any balancing. You'd need a custom BMS. As the battery gradually goes out of balance, you'll get less and less capacity, and then when you try and charge it fully, the high voltage cells will switch off the charging too soon. The bleed resistors can't cope with a big difference. They'll bleed the voltage down to about 41.5 to 41.7v anyway, so the charging to 42v is only for balancing.

That is true, at least for bestech BMS:

Over charge detection voltage

4.2V~4.35V(Adjustable)


Over charge release voltage

4.0V~4.2V(Adjustable)


So, if one sets the over charge detection voltage at 4.2V and the over charge release voltage at say 4.1V, it is safe to charge up to the maximum of the charger, since in effective terms, the battery will be charged at 4.1V.

But to do this, one needs to ask bestech. I am not sure what happens with the sub-optimal BMS most people own.

For em3ev BMS for li-ion:

• Overcharge protection Voltage, 4.2V
• Overcharge recovery Voltage, 4.1V

It is the same, then: 4.2V is to balance, but in real terms, the battery is charged up to 4.1V.

Interesting point, d8veh.

 

[Arbol]

Anyway, the recommendation to build a big battery, and charge only scarcely (say when SOC is getting close to 0%), holds: balancing is not necessary to do it every time, only from time to time is OK, AFAIK. Doing it once per cycle (irrespective of how many days it takes to perform a full cycle) should be OK.

 

[mountainsport]

That's only if the battery BMS/charger combination pushed the battery to 42 volts. Those I've had only reach 41 volts full charge and from what I've seen in member's posts that is much more common than 42 volts.

In other words the design is safe for the battery to be fully charged without severely stressing it. Equally e-bikes usually cut out at about 32 volts low charge, once again by design to protect, since the battery could be discharged more. For most users the gain in life from trying to improve on this built-in protection is probably minimal.

Playing about with charging routines I think a bit like trying to get the maximum possible mpg from one's car, only possible with a lot more hassle and sometimes even spoiling the pleasure of using the vehicle.

Which also makes one if not all become paranoid or more confused.

MS.

 

[flecc]

Which also makes one if not all become paranoid or more confused.

MS.

Absolutely! I say buy, use and enjoy. When the time comes, replace.

Formula for a happy tech life!
.

 

[trex]

Ah, ok, now I understand, thanks. So the problem is not the location on the frame, is that different locations of the frame are more amenable to better or worse holding cages.

But a given battery, say a triangle one, on the bottom of the triangle, so just above the BB, and inside a metal cage covering the full empty triangle space, will suffer the same kind of vibrations than an equivalent capacity battery behind the seat post, right?

I am sure yours will be right.

I also think sacko has the best in-frame battery:

 

[Arbol]

Beautiful bike.

 

[mountainsport]

Absolutely! I say buy, use and enjoy. When the time comes, replace.

Formula for a happy tech life!
.

That is so true flecc, but at the same time it also is good to have such discussions to get all of our juices flowing. We all have different system designs plus battery chemistries and we all have different riding styles/techniques, we must not rule this positive out when it comes down to the best deciding factor of battery care.

MS.

 

[flecc]

That is so true flecc, but at the same time it also is good to have such discussions to get all of our juices flowing. We all have different system designs plus battery chemistries and we all have different riding styles/techniques, we must not rule this positive out when it comes down to the best deciding factor of battery care.

MS.

Indeed, each to their own no matter how complicated, as long as that makes them happy.

It's when they start fretting about whether they are doing the best thing that I get concerned. That's not what e-biking is about.

 

[mountainsport]

Indeed, each to their own no matter how complicated, as long as that makes them happy.

It's when they start fretting about whether they are doing the best thing that I get concerned. That's not what e-biking is about.

Now that I've become more paranoid I have ordered over 50 different types of battery cells to play with in my secured purpose build underground shelter.
I will all let you know how I get on with the experiments.

MS.

 

[mountainsport]

Now that I've become more paranoid I have ordered over 50 different types of battery cells to play with in my secured purpose build underground shelter.
I will all let you know how I get on with the experiments.

MS.

BANG!!

As flecc had said lets all be happy.

MS.

 

[Frankie24]

Is true that the best thing you can do is buy and enjoy but sometimes is not that easy, more if is your first purchase.
Best thing in the life is learn by yourself and your mistakes, in that way you will get next time whatever is better for you but thanks to all of you now I know some more things to think about when I get the e-bike. Because all your replies is harder to decide because I think more about it but probably will be harder to complain because I will know more or less what's going on.