BMS parasitic load?

[E-HP]

I've read on a few forum threads about the parasitic load from a BMS that can slowly drain a battery if left unattended for several months. 11 months ago, I had upgraded my battery pack, and put my old, but still viable pack on the shelf in my garage store room. Things got piled up in the room, so it was a hassle getting to it, so I just forgot about it.

We had a 24 hour power outage 2 weeks ago, so I ended up firing up my generator so I wouldn't lose the food in my refrigerator(s) and deep freeze. I was also able to power the TV, although the cable was out, but we were still more comfortable than the rest of the houses around town, but had limited outlets to charge our phones/laptops, etc. (first world problems), so I figured it was time to dust off the old battery and buck converter for the next time. I cleaned out a bunch of junk from the store room and grabbed the pack.

The pack is a UPP 52V 20Ah triangle pack, with no on/off switch. I just measured the pack voltage and was surprised to find it measured 54.8 volts, pretty much what it was when I put it on the shelf! So what's the story with respect to storing a battery pack with BMS long term? I was expecting a near dead battery after reading about the problem, but it looks like I could store this pack for years and not have to worry about it.

 

[john61ct]

Depends on the BMS model.

Some might power down completely?

Others are such a tiny draw...

Was it Full when you stored it?

 

[E-HP]

Depends on the BMS model.

Some might power down completely?

Others are such a tiny draw...

Was it Full when you stored it?

OK, thanks. I must be lucky. It was just under 55V when I put it on the shelf. I was going to drain it to 52V, but was too anxious to start using my new pack, so just put it on the shelf. I figured that would be OK, but maybe not ideal, at around 3.9V per cell.

 

[john61ct]

A bit lower would be better, and cool temps.

Let us know if you ever identify what model BMS

 

[E-HP]

Let us know if you ever identify what model BMS

Since I don't use the pack for my bike anymore, I'm going to open it up to check it out. I'll report back. :thumb:

 

[ZeroEm]

E-HP » Aug 24 2021 8:44pm

OK, thanks. I must be lucky. It was just under 55V when I put it on the shelf. I was going to drain it to 52V, but was too anxious to start using my new pack, so just put it on the shelf. I figured that would be OK, but maybe not ideal, at around 3.9V per cell.

Must be lucky too. Posted about my battery in my room over winter and thought it had drained down but was wrong. Have a pair of batteries for my trike and never needed them both. I swap them trying to keep the charging cycles the same.

The one on the trike (plugged in) slowly discharges thru controller's off switch but the one that sits in my room at 50% SOC the drop is so small don't hardly check it any more. (which is bad) The one i'm using now sit for 6 months. Might be good info if we find out which BMS have the least draw sitting.

 

[john61ct]

IMO a decent battery build should include the balance wires brought to the outside, so without any disassembly,

the BMS is easily removed/replaced, other gear used for testing / maintenance sessions

That way the parasitic draw does not need to be an issue at all, and choices made based on more important factors like

robust build quality, reliability / longevity,
visibility of cell/group visibility
accuracy
adjustability
robust balancing function, etc

 

[ZeroEm]

by john61ct » Aug 25 2021 8:44am

IMO a decent battery build should include the balance wires brought to the outside, so without any disassembly,

the BMS is easily removed/replaced, other gear used for testing / maintenance sessions

That way the parasitic draw does not need to be an issue at all, and choices made based on more important factors like

robust build quality, reliability / longevity,
visibility of cell/group visibility
accuracy
adjustability
robust balancing function, etc

I had dreams of building batteries before the dream was over it had turned into a nightmare with burnt fingers and house on fire. :lol:

Don't think I will ever get into battery building. I do read a lot of your posts and think I understand. At some point I will open up the batteries and check the groups. May end up reducing the packs down to extender packs. I like the idea of bulk charging with accessible balance wires. Don't know if it's feasible to tap in the the balance leads of the BMS to have outside access. I may have it figured out by the time my packs show degradation.

The only batteries of the size I prefer are from questionable sellers. My next battery for my trike will be in the range of >=72V @30ah. Would want to drop it down to 36V for connecting and while not in use. Don't think you can buy batteries with external balance leads? Do think about a pair of 36V with an external switch from Parallel to series. Not sure all batteries are compatible to being connected in series.

 

[john61ct]

The barrier to assembling from sub packs however you like, is the BMS in each.

So long as the cells' terminations and wiring are accessible you can do what you like.

Having one big BMS is IMO the way to go, and the paralleled subpacks connected at the cell/group level.

I would not try to have a S vs P switch, just different wire harnesses to swap how they are connected.

 

[E-HP]

IMO a decent battery build should include the balance wires brought to the outside, so without any disassembly,

the BMS is easily removed/replaced, other gear used for testing / maintenance sessions

That way the parasitic draw does not need to be an issue at all, and choices made based on more important factors like

robust build quality, reliability / longevity,
visibility of cell/group visibility
accuracy
adjustability
robust balancing function, etc

I'm with you, if you mean an ideal pack. Do those packs exist, without having one custom built? I think for me, ideal is to have the "dumb/simple" aspects, like LVC and overcurrent, handled by one component, and the balancing handled separately, either by the access to the balancing wires, or internally with a balancing circuit, with a decent balancing charge rate.

You could swap between using an active balancer that doesn't need the battery to be fully charged to work, or use an external charging unit. I love my newish lipo charger, since it has a 2A balancing current, and you can actually see it working (fast), so something like that that handles 14S or 20S, etc.

 

[ZeroEm]

That would be a battery I would want. then you could disconnect the BMS, no Parasitic load for storage. Would not need to charge to 100% to balance.

 

[Chalo]

IMO a decent battery build should include the balance wires brought to the outside, so without any disassembly,

When I have to replace a BMS (and rewire the multi-pin plug, because they never seem to use the same kind) I like to relocate the BMS to a more accessible location when feasible. For the janky pedicab batteries I usually deal with, that most often means it's still plastered down under some tape.

I think the ideal situation would be the BMS thermally adhered to an aluminum or copper panel in the outer casing of the battery, with all the wires inside the case. I have yet to exercise this option.

 

[E-HP]

I think the ideal situation would be the BMS thermally adhered to an aluminum or copper panel in the outer casing of the battery, with all the wires inside the case. I have yet to exercise this option.

Is the idea behind the aluminum/copper panel, to dissipate heat?

 

[Chalo]

I think the ideal situation would be the BMS thermally adhered to an aluminum or copper panel in the outer casing of the battery, with all the wires inside the case. I have yet to exercise this option.

Is the idea behind the aluminum/copper panel, to dissipate heat?

That's the idea. I know some of the dead BMSes I replace were killed by accidental short circuits, but I reckon some of them succumbed to overheating.

 

[E-HP]

So back on the parasitic loads, is that load being drawn from the main pack +/- connections, or from the individual balance wires? If you want to "remove" the BMS from the circuit, from the parasitic load perspective, do you need to completely remove the whole unit, or can you switch off something at a lower voltage level, like switching the ground wire of the balance wire harness?

 

[Chalo]

So back on the parasitic loads, is that load being drawn from the main pack +/- connections, or from the individual balance wires? If you want to "remove" the BMS from the circuit, from the parasitic load perspective, do you need to completely remove the whole unit, or can you switch off something at a lower voltage level, like switching the ground wire of the balance wire harness?

My impression based on postmortems is that some BMSes run their operational power from one or two cells at one end of the pack. As they sit unattended, the cells in the one or two groups gradually discharge while the others rest with only self-discharge to drain them. Eventually, the affected cells fall to damaging low voltage levels.

Other BMSes use the whole pack voltage. Not only does this tend to keep the pack in better balance, but it provides a much deeper well of energy for the BMS to run on. My anecdotal evidence suggests that most commercial (as in non-hobbyist) packs use this regime. Prolonged storage without recharging could potentially kill a battery with this kind of BMS, though.

Either way, if you unplug the balance leads you can shut off the BMS. The only other wires connected between the BMS and the battery go to the negative cell terminal, and don't complete a circuit that has a voltage gradient.

 

[john61ct]

Yes the way BMSs convert / draw their power varies very widely, can't really generalize.

And on the cheap end, build quality is often very poor, they are at least as often the cause of packs failing as they are protective.

And as Chalo points out, most pack manufacturers chop and change BMS models, probably just buy whatever's cheapest whenever they run out.

The only BMS function that pertain to the usage / discharge cycle is the LVC, and if the cells are bottom balanced, the controller or CAv3 type device can handle that at pack-level bulk voltage.

HVC is only relevant to charging, if regen is an issue the controller should be handling that as well.

And IMO balancing should be a periodic scheduled maintenance routine, the way 99.9% of BMSs suck at it so much, only suitable for packs that don't need frequent balancing anyway.

And BMSs absolutely are not needed during pack storage, can murder the cells if not checked regularly.

 

[john61ct]

Personally, I'd love to see BMS become a swappable module of the controller, the per-cell voltage sense modules separate from the cutoff circuitry

maybe Vasili will go that way when the Nucular BMS is developed.

The controller should also be part of charge control / conversion, being able to plug into a variety of charge sources on long trips or while touring would be priceless

 

[john61ct]

Do those packs exist, without having one custom built?

Not yet

> balancing handled separately, either by the access to the balancing wires, or internally with a balancing circuit, with a decent balancing charge rate. You could swap between using an active balancer that doesn't need the battery to be fully charged to work

Yes dedicated balancers are the way to go unless you find a (very rare) BMS that does it well, and doesn't cost as much as the whole bike.

> use an external charging unit. I love my newish lipo charger, since it has a 2A balancing current, and you can actually see it working (fast), so something like that that handles 14S or 20S

Hobby chargers are often poor build quality too, and the availability of units designed for 10S or higher has declined a lot in the past decade.

Using a sub-pack building block approach at 6-8S would be the way to go for that approach.

But few balancing chargers are much better than cheap BMS, inaccurate sensors, low rate.

Which pretty much leaves dedicated balancers.

 

[john61ct]

Would not need to charge to 100% to balance.

Yes! the reason adjustability is critical. Start-balance voltage should be set well below your **desired** CV setpoint.

And the balance current rate high enough to complete the process before you would have hit charge-termination normally.

But dedicated balancers do the job completely separately from the charging process

can be used for bottom balancing, or even at the midpoint if desired (as new cells are usually delivered)

 

[ZeroEm]

Prefer the larger batteries, don't like charging every time I ride so my batteries most of the time are between 50-70%. would like to balance them in this area.

Every 3-6 months I charge them to 100%, the rest of the time 80-85%. Only several times have they been down to 40%. Most of my rides are with people so even a 30 mile ride I may use only 200-400w. Do like to go on longer faster rides and use up to 500-1300w.

 

[Chalo]

don't like charging every time I ride so my batteries most of the time are between 50-70%. would like to balance them in this area.

That's what active balancers are good at.

 

[ZeroEm]

Two active balancers, please.

 

[E-HP]

don't like charging every time I ride so my batteries most of the time are between 50-70%. would like to balance them in this area.

That's what active balancers are good at.

I bought a few active balancers that I was going to use on my lipos, but decided not to, since they pretty much stay in balance and my charger can balance them when needed (without fully charging). I'm thinking about testing them on my old pack to see how well they work. I recall when I got them, that they can be daisy chained, so I should be able to cover 14S.
I recall reading one of your posts a while back that you are using an active balancer w/o a BMS. Is it a single unit, or did you daisy chain a few together?

 

[Chalo]

I recall reading one of your posts a while back that you are using an active balancer w/o a BMS. Is it a single unit, or did you daisy chain a few together?

I used a single 12S balancer on a 12S battery.

I'm confident in this system only because the other pieces (charger and controller) do their own quite conservative high and low voltage limiting.