External BMS?

[Username1]

I was looking at "Chargery" chargers, and saw they also have BMS units that hook up to their charger. Does this mean it's possible to run an ebike battery without any BMS permanently hooked up to the battery?

It seems all production ebike batteries use an internal BMS. Could you build your own pack without a BMS and use an external BMS when charging, or are there other reasons you'd want a BMS always connected?

 

[DanGT86]

I am doing something very similar to what you are asking. I use an RC lipo balance charger when I charge so the cells are balanced at full charge.

I run them without a bms on the bike. I also never run them close to empty. If I am going to flirt with empty I connect cheap lipo alarms to them.

As long as you have a wire to each cell (called balance leads) you can run an external bms or balancer or alarm.

If I was making an ebike for a non experienced user I would just get a BMS in the pack.

For me as a tinkerer I'm constantly moving portions of my pack around on different bikes. My packs are all 36v modules that I switch between 36 and 72v.

If the pack is dedicated to one bike then its probably easiest to just install a BMS once and use a regular charger.

 

[Username1]

Yeah if you use the external Chargery BMS you need balancing leads coming from your battery.

I guess having the BMS in the battery case is best since you only have positive and negative wires leaving the case. This is especially important for swappable batteries, because having to connect multiple balancing leads every time would be very annoying. Even integrating BMS features into the controller would require connecting all the leads every time you remove the battery.

 

[DogDipstick]

I build all my batteries and run the bms external with a 25 pin disconnect int eh balance lines. I use a 3$ gold pin for every one of those 50 pins in the 2x connectors. Cannon gold plated copper barrel crimps in AMP backshells.

Yes, you have a few more things sticking out the battery... Pos and NEg and the 21 pins for the 20s battery cell level voltage sensing. I use 20g wire with a 5A max throughput and apply load up to 1A design current contin. However my cells ar very well behaved and rarley need a balance.. Like once every 6 months or so... they never get out of order. Very good cell.

Why carry round a BMS. Lol. I do not limit output current at all.

 

[john61ct]

BMS is just an arbitrary collection of functionality, maybe balancing is included, maybe protective only. Besides voltage and current, temperature can be important.

With alternative devices to achieve your goals, and the required knowledge, you do not ever "need" a BMS.

They often are just there as a marketing checklist item designed to make customers think the pack is protected, without having to acquire detailed knowledge,

and cheap ones often do more harm than good.

Yes you should accurately know the per-cell/group voltages when diagnosing / monitoring / checking imbalance and uneven wear level as the pack wears out.

Actual balancing is usually much better done by a dedicated active balancer as a periodic maintenance routine.

Charge termination should be based on per-cell/group voltages, but many chargers work that way anyway.

SoC guages are very rarely accurate, a cheap dedicated shunt based wattmeter does a better job.

LVC while riding is controversial, a good controller / CAv3 handles some aspects of that.

and so on...

 

[john61ct]

I build all my batteries and run the bms external with a 25 pin disconnect int eh balance lines.

This is the way.

Even if you use a BMS while discharging, it should be easily removable, often needs replacing long before the cells get too worn.

And all the devices mentioned above need easy access to the balance leads

 

[john61ct]

require connecting all the leads every time you remove the battery.

There are plug harnesses to facilitate that.

But if you want a pos/neg termination only for the discharge cycle, that's fine, but no way around access to balance leads if you want to properly care for your pack.

Most are designed so opening up the case is not easy to do frequently.

 

[eMark]

I was looking at "Chargery" chargers, and saw they have BMS units that hook up to their charger.

Chargery BMS reviews use words like "bad", "scam", "pitfalls". More typical of RC hobbyists that abuse their Lipo batteries with cells that become imbalanced ... so they buy a RC Chargery BMS thinkng it's the remedy.

However my cells ar very well behaved and rarley need a balance.. Like once every 6 months or so... they never get out of order. Very good cell.

Why carry round a BMS. Lol.

The difference between more than a few RC Lipo Hobbyists and DD is like the difference between Night and Day.

I do not limit output current at all.

That's because you know what your equipment and battery can handle without abuse. More than a few RC hobbyists might likewise say of their RC Lipos (e.g. RC Turnigy 20Ah 12C-24C Lipo), "I do not limit output current at all", and then blame the manufacturer (misleading performance claim) for defective cells when their RC Lipo puffs/swells, lol.

 

[DogDipstick]

DogDipstick,
Would you please share some pictures ....

Yes certainly. I will update this post. When I get to sit down at the computer . I am actually casing a battery today: permanan5tly, without a BMS; but with high quality connectors, to the outside of the case, only.

 

[john61ct]

LiPo indeed do require specialised care protocols, and are very fire risky even if cared for perfectly by experts.

The RC and racing communities usually sees them as consumables, when used at high C-rates, recycling after only a dozen cycles is not unusual, getting 100 cycles can be considered unrealistic.

So yes most ebike users must either avoid LiPo packs, or learn how to use them and care for them much differently.

But just because a user "abuses" their packs from your POV does not justify casting aspersions on the products used in the context if those preferences.

Chargery makes fine products, very frequently recommended by well respected members in dozens of forums over many many years.

That of course does not mean all of those products are suitable for any given use case.