E-bike battery health
- Thread starter Alba Zeus
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Lithium battery charging is simple compared with the old NiCd/NiMH tech.
It just requires a constant voltage. A fully charged 3.6/3.7V (nominal) lithium cell will be 4.2V and charging higher than this can permanently damage it. Battery pack are made-up of a number of cells in series to reach the required voltage and this is expanded in parallel to increase the total energy (Watt Hours).
Unless you absolutely need the full capacity of a pack, it's always better to charge to a slightly lower voltage, say 4V - 4.1V. This will extend the life of the pack considerably. For storage (more than a few months) I would charge to the nominal voltage and repeat this every few months.
The capacity of the pack will be specified in either Watt Hours (Wh) or Amp Hours (Ah). The former is the total energy in the pack and is the Ah figure multiplied by the nominal voltage. It is safe to charge the pack at 1C, i.e. the 'A' in the Ah figure, but a well-designed pack can be fast charged at much higher currents (5C or even higher).
For instance, a 36V pack with a capacity of 5.6Ah will have an energy capacity of 201.6 Wh. It will be built with 20 individual lithium cells of 2800mAh capacity. 2 parallel sets of 10 cells in series. You could charge it safely by applying a voltage of 42V and a current of 5.6A. The battery management system (BMS) will (or should) take care of charging each individual set of parallel cells and will not allow any to exceed around 4.2V. You could leave it attached to the charger for months without a problem as the BMS will not draw any further current when all the cells are at capacity.
Of course, most packs are supplied with a charger that supplies a fixed voltage/current over which the user has no control. These are usually the Achilles heel of any lithium-powered system and will likely be cheap crap from the Far East rebranded with a sticker.
A good, switched DC supply can often be bought for pretty much the same sort of price as a replacement charger would cost and will allow a wide spectrum of voltage/currents and will permit much more flexibility.
I have been building lithium-ion battery packs for a few year for scuba diving applications. I don't own a e-bike, but if I did I'd almost certainly have had a crack at this too
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ebikeerwidnes
Guru
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One problem I have if knowing how 'full' the battery is
I am fine with the Bpsch system as it has a button on it that I can press and it will light up some leds
but the Carrera system (Bafang) is just a plastic brick with a switch on it - no indicator
clearly I could put it on the bike and check - but even that is a bit dodgy because it tends to go from "all the lights" to "just one light" quite quickly
I presume putting a multimeter between the +ve and -ve terminals would give an indication - but the difference is pretty small
I suppose I shoul try recording it once a week next winter and see what happens
I am fine with the Bpsch system as it has a button on it that I can press and it will light up some leds
but the Carrera system (Bafang) is just a plastic brick with a switch on it - no indicator
clearly I could put it on the bike and check - but even that is a bit dodgy because it tends to go from "all the lights" to "just one light" quite quickly
I presume putting a multimeter between the +ve and -ve terminals would give an indication - but the difference is pretty small
I suppose I shoul try recording it once a week next winter and see what happens
As I alluded-to above, a fully-charged pack will be at 4.2V/cell. For a 36V pack, that's 42V. For a 72V pack, that's 84V etc.
However, even when not at full capacity, the voltage can be at or around maximum but the pack can still accept a trickle charge so may only be at 85-90% full. A voltmeter will only show the potential. Under a load, the voltage will drop very quickly towards nominal, around which it will sit for most of the discharge.
You could put an ammeter inline and when the charge current has dropped to virtually zero, the pack will be full. If you were using a switched DC supply for charging, the current will always be displayed.
The current being applied by the charger will drop dramatically as the pack becomes full. A near-empty pack will take the full current that the charger can supply but this falls-off a cliff as the pack fills-up. It's not linear. You'd imagine that a 5Ah pack charged at five amps would be full after one hour but it might only be 85% and the remaining capacity might take many hours longer as the current drops, eventually to <0.1A or undetectable.
As I already said, if you don't absolutely need the full charge, then don't charge the pack fully. Charging to only 85-90% most of the time could help the battery pack last for many more charges.
youngoldbloke
The older I get, the faster I used to be ...
I wonder if the BMS has a Bluetooth module?
I'm currently building a replacement pack for one of my underwater scooters with such a system so I can monitor the pack's health.
OP
OP
Alba Zeus
Über Member
Yeah I think moving forward for me I will only take the bike to 100% power when I know I'll need the full power. I noticed the other day when I ran the battery to 0% that after it dropped below about 15% it was barely giving me any assist even on Level 3.
Ill follow the 20-80 rule when the bikes not is use.
I tend to run my motor maps at 100% and 99% of the time I'm in L2 with the odd foray into L3 when on a particularly sharp incline. Orbea state their battery will take you circa 40 miles but is that on L3 at 100% or L1 at 100%. Appreciate there is variables but in general?
youngoldbloke
The older I get, the faster I used to be ...
You are right - below 20% there is very little assistance left in the battery. For me. I estimate 2% of battery per mile absolute best case - level 1 only, above cut off as much as possible, in reality for most rides 40 - 45 miles is a safer assumption. That's why I bought a range extender power bank. 70 - 80 miles is now a realistic figure.
That's not how I read it. I think it means make sure it's charged to at least 80% BEFORE storing, then check it doesn't fall below 10% whilst IN storage.
Lithium cells in good conditions can be stored for a lot longer than older technologies before needing a top-up. Buy a cheap voltmeter and just check that the pack isn't falling below 3V per cell. For a 36V pack (10S) that's 30V, for a 52V pack (14S) that's 42V etc.
A good BMS will attempt to prevent individual parallel cell groups falling below a minimum voltage but it can't prevent damage once all groups have fallen below the minimum. So just top-up to the storage voltage periodically (the nominal pack voltage is fine).
Of course, the real question here is: why are the packs being put into storage? Just keep riding the bike!!
Yes, I read that and addressed it. It doesn't say make sure it STAYS above 80% charge; it says make sure it never drops below 10%.
youngoldbloke
The older I get, the faster I used to be ...
It's not clear - you could interpret it either way.
That's why I elaborated on it.
However, I'll try again...
To store a lithium battery pack: charge to around 80% and store the pack somewhere where it won't be subjected to extremes of temperature or shock and periodically make sure that the charge hasn't fallen anywhere close to 10%. Lithium packs don't lose charge very quickly and this would take ages for a pack in good shape. If you're storing the pack for that long, consider selling the bike as you're obviously not using it!
Clearly, the 80% and 10% won't be very easy for most people to check. So, to make things simpler, charge the pack to somewhere around its nominal voltage and ensure it doesn't fall lower than 3V per cell (explained above, but a 36V pack would be made-up of 10 groups of cells in series (10s) - so that's 30V minimum. A 14s pack should fall no lower than 42V etc.)
OR: just check it every day and keep it at 80% - your call. I know which option I'd take.
youngoldbloke
The older I get, the faster I used to be ...
Thank you - but I don't know how I would check the voltage of the Mahle internal battery. They use a unique charging port. However battery levels can easily be checked using their app.
I detect a somewhat censorious tone to your comment regarding bike usage and sale - when my bike was left fully charged I had every intention of using it within the next 12 hours - before I could do that I was laid low for the next 6 weeks, almost hospitalized, by a severe Covid-like viral infection. I would normally be riding throughout the year, all seasons. Sometimes one's activities are affected by circumstances beyond one's control
