Organising salvaged cells in a DIY battery

[Danidl]

Wheeliepete, thanks for your data point. It does help.

Danidl, I have a BMS - one of those cheap Chinese things, but testing showed problems with it, and putting a cheap Chinese brain in charge and in permanent connection to all that power seems more dangerous rather than less. My bike has an instantaneous voltage readout, so I know I'm never dipping into the danger zone. On-load, it never goes below 3.2 volts per bank, and I balance charge it at least twice a week, so I think I'm OK, but if I've missed something big, please let me know.

How do you know ? Your voltmeter presumably reads 36v as the sum of the 10 tiers of cells. That does not mean that there is 3.6 volt per tier. It means an average of 3.6 .. there is a huge difference. If for instance due to capacity limitations one tier got stuck at 3.8 volts then on charging another might go to 4.6 and funny peculiar things will happen.
To burn a house down for the sake of 12 quid on a new bms is poor economics.

 

[ChrisW]

Danidl, OK - I see; thanks for your patience. But the danger you describe arises on charging, but I'm balance charging off the bike, so the danger you mention won't arise. It's not the £12 which prevents me adding the BMS, it's its unreliability as revealed by my testing of it. It doesn't reliably shut down on low voltage, which is a show stopper, and it has a tiny balancing discharge which means that correcting imbalances takes forever, which make attending it impractical. And, more generally, (and this is true for any BMS) since it's always connected to the battery, attending its operation is entirely not possible - you'd have to sleep with it -, so you have to trust it totally to not unleash the apocalyptic house burning you mention - which does loom large in my mind, which I don't think I could ever do.

I hope you're not getting fed up but if you are - just leave me to it. I'm not ignoring advice from those who've been where I haven't and are trying to help me - I'm trying to integrate their advice with my own practical experience and growing understanding of the topic.

 

[Danidl]

Danidl, OK - I see; thanks for your patience. But the danger you describe arises on charging, but I'm balance charging off the bike, so the danger you mention won't arise. It's not the £12 which prevents me adding the BMS, it's its unreliability as revealed by my testing of it. It doesn't reliably shut down on low voltage, which is a show stopper, and it has a tiny balancing discharge which means that correcting imbalances takes forever, which make attending it impractical. And, more generally, (and this is true for any BMS) since it's always connected to the battery, attending its operation is entirely not possible - you'd have to sleep with it -, so you have to trust it totally to not unleash the apocalyptic house burning you mention - which does loom large in my mind, which I don't think I could ever do.

I hope you're not getting fed up but if you are - just leave me to it. I'm not ignoring advice from those who've been where I haven't and are trying to help me - I'm trying to integrate their advice with my own practical experience and growing understanding of the topic.

On your own head be it. The bms is intended to be connected permanently and it trickle adjusts and balances . Why not trust it? . Millions of people do ( no exggeration ). Do you know more than the engineers who designed these chips ? . Would cost concious manufacturers have avoided them if they could save a few pence?
The chemistry of lithium is more complex than that of a lead acid cell and the material is more volitile when there are unintended current flows. A significant fraction of the cell is carbon and organic solvents like ether with low flash points. You have looked at youtube videos on battery construction now look up the ones on li ion cell combustion. It does not matter whether the cell is discharged or full...

 

[ChrisW]

The BMS balances during charging only. Almost always, it does so by draining current from full cells/banks during charging. Most BMSs are made down to a price, and so they can only drain current slowly, maybe 100mA, and so the whole battery charge rate has to be dropped to the drain rate so that full cells are not being topped up faster than they are being drained.

I explained that, during testing, my BMS was unreliable. That seems like an excellent reason not to trust it to me. I have seen it not work very well.

There is an excellent Youtube channel devoted to exposing the horrifically unsafe devices coming out of China on EBay etc. Un-earthed shower heaters, USB charger with mains exposed on the casings, exploding LEDs, etc. Those Chinese product designers do precisely what you say they don't do - they save a few pence at the expense of safety. And I have bought lots of devices from China which were faulty or whose advertising was complete rubbish. And I've faced devious sellers when I've complained. So I don't know why you assume they must all be well-motivated, highly-qualified engineers. Some undoubtedly will be, but that doesn't mean everything coming from China is top notch. I don't need to think I know better than the best to be generally cautious.

And I know that Lithium Ion batteries are inherently more dangerous. That is precisely why I don't want to put a £7 BMS permanently in charge of a big battery unattended.

As far as I know (assumption 1) , a Lithium Ion battery which is not being charged or discharged is safe as long as it isn't heated way up or mechanically assaulted. When it's charging it's on a balance charger which has numerous safety features and I'm attending it. It sits next to me in my office. When I'm riding it, it's discharging. It currently doesn't get hot in use and I don't allow it to over-discharge. As far as I know (assumption 2), worst case is that a cell fails short circuit, and that might cause very high currents. There are three fuses protecting the battery, but if for some reason they do not blow or do not stop thermal runaway, then overheating, and a very energetic event might occur. I believe that that likelihood is very small, as is that of a car accident or a heart attack. And that low probability should be factored into making a reasonable choice. But if it happens, I'll be on the bike, and outside. I'll be aware that something is wrong quickly, and I can get off the bike and take appropriate action.

Of course, it's not always possible to know what you don't know, and perhaps my assumptions are wrong, but they are not faith-based - I've looked into this quite a bit. But perhaps there is another terrifying failure mechanism I'm unaware of but I have not learned of it here.

 

[Danidl]

When you say charged on a balanced charger what do you mean? Does it check the voltage across each tier as it's charging and ensure that each tier remains in synch. If so does it have a voltage sensing lead to each tier?. If so then that is a battery management system , if not you are fooling yourself.

 

[Deleted member 4366]

Chris, you can see why people are getting a bit exited:
"The battery I have is a dog's dinner soldered brutally together by a gorilla with a blow torch, from a patchwork of different cells. It has no BMS. The person who sold me my Ebike toldme he bought the battery from a man at a car boot sale. So real pedigree.

I've owned the bike since last April and used it most weekdays and been charging it from a simple power brick until recently"

That paints a different picture from what you're now saying.

 

[ChrisW]

My balance charger is an iMax B6 mini. It is a balance charger. It is not a BMS. It works as all balance chargers do, which is to say that it monitors the voltages of cells within a battery during charging via a balancing lead, so that it can ensure that each cell is charged to the same level. So yes, it has a voltage sensing lead to each tier. This is what the term "balance charger" means.

A BMS is not the same thing. A BMS implements balance charging, but it is permanentlky attched ot the battery, and it implements additional useful functions. Specifically it may:

1. Limit load discharge currents to some safe maximum
2. monitor battery temperature and take action if alarmed
3. Limit over-charging and discharging

These functions are not performed by a balance charger, which is a desktop unit, not permanently attached to the battery.

 

[ChrisW]

d8veh, yes I see what you mean, but there is no contradiction. I've been responding to contributors' comments.

This thread was originally about how I should build my new battery. Specifically how to organise cells with different characteristics.

I didn't get much help on that, but I got lot of other comments. People said I shouldn't build my own battery with laptop batteries. It was criminally unsafe and doomed.

So, in response to that, I described the gorilla/blowtorch/no BMS history of my current battery. It is accurate. And yet in the almost one year I've had it the battery has done nothing wrong, it has maintained reasonable balance and performance and I'm using it today. I've made Youtube videos on all of that. My experience therefore, is that you can get all those things wrong, and still have a useful EBike battery for essentially no money. I recognise that it's not best practice to do it that way, and that there are safety concerns, but I was addressing the fact that things can be far from perfect yet still be entirely acceptable.

I had thought I'd run this old battery into the ground, thinking it was probably already buggered. But then I thought - why not stop the damage I'm doing to it. Start balance charging it. And also dismantle it to see how bad things had got with all that abuse. I did another video on that and the answer was an overall difference between highest and lowest bank of 0.7 Volts. Doesn't sound much, but it will mean that some cells were routinely being driven into overcharging and others were routinely being over-discharged. This is dangerous though I watched it like hawk, and it will have taken its toll on the battery and will be why it is now losing capacity.

That is why I'm now building another one. To be balanced charged always, to 4.q and not 4.2 volts, but it will not have a BMS for the reasons I've described (the one I have doesn't work well during tests, and I don't want to trust a permanently-connected device unattended, though I accept that millions do trust theirs).

So there is no contradiction in any of this, and I don't currently accept that not having a BMS is more dangerous. The battery is fused, and I monitor it for over-discharged, and I balance charge it. Temperature measurement would be nice, and I can easily add that as a separate function to my new BMS-less battery.

 

[Deleted member 4366]

I don't think anybody is trying to tell you what to do. People give advice based on their experience and their own frame of reference. You can choose what you want to do.

I stand by my advice, which is that if you just like messing about and you want to learn about batteries, carry on with your project, but if you want a cheap serviceable ebike battery, buy a ready-made one.

Be aware of the dangers. The biggest danger is a short in the cell-pack, particularly the balance wires. They're thin and easily cut, so you don't want them running over sharp nickel strips. Make sure that everything is double insulated. That's even more important if you're going to mount the battery in a bag or similar where it can move.

 

[ChrisW]

I understand that people are just giving advice in an effort to be helpful, and I've made sure I said that I appreciate that.

When people tell me emphatically not to do something or that the consequences are dire, then I've explored their thinking to see if there is something I need to learn or not.

Human beings tend to confuse exploration for argument, and invest their egos in their opinions, which often gets in the way of simply finding out what's true and what's not.

I have in the past crushed the plug on the end of a balance lead, and shorted out two wires. It took me a few seconds to work out what was going on and it was intensely stressful. I was OCD about turning my back on that battery even though I'd taped apart the exposed ends. I'd keep going back and looking at it - asking myself what might happen that could cause disaster. And I know that disaster could mean a destroyed home and death. So I understand your concerns and I appreciate your motive in making sure your point has landed.

I know you've built a lot of bikes and I suppose that you've sold them, so it's admirable that you take safety so seriously for your customers.

In particular I can see why thin balance leads and nickel strips are a dangerous combination, and I'll keep that in mind - thanks.

My battery pack is in a kind of hammock which bounces around. A really bad idea, in my view. My new battery will slide on a metal rack into its socket on the bike. There will be no need to disturb wiring on the bike or the battery.

 

[Danidl]

Chris, you can see why people are getting a bit exited:
"The battery I have is a dog's dinner soldered brutally together by a gorilla with a blow torch, from a patchwork of different cells. It has no BMS. The person who sold me my Ebike toldme he bought the battery from a man at a car boot sale. So real pedigree.

I've owned the bike since last April and used it most weekdays and been charging it from a simple power brick until recently"

That paints a different picture from what you're now saying.

You are correct DV, I am now rightly exited..... When advice offered after a long career in electronics is ignored it seems the correct course of action...

 

[ChrisW]

Danidl, You didn't mention your long career in electronics, but then I didn't mention mine either. I learned a lot about aircraft control systems but bugger all about EBikes. Perhaps you specialised in them?

If you had presented yourself as an expert, then I might have seen your advice as more than well-intentioned and helpful. But you seem not to know what the difference between an BMS and balance charging is, which seemed like a highly relevant context within which to consider your advice.

Even so, I listened to your advice, I thanked you for it, and I made my own decisions.

Where in there do you find rightful cause for indignation?