ChrisW

Hmmm. I watched this video: https://www.pedelecs.co.uk/forum/threads/high-current-charging-protection-without-fuses.34602/ which explains that a crowbar circuit is used to protect a voltage/power-sensitive circuit form over-driving, but it does it by short-circuiting the power supply. Unfortunately, in my application, this would also short circuit the battery bank - which is exactly what I'm trying to avoid. If I have this wrong, I'd appreciate your comments. Either way - thanks again for trying to help me.

Thanks Andy, I'll research the crowbar.

Thanks for the advice vfr.

Hi Daniel, a couple of reasons. Firstly charger working on this principle are very slow becuase the MOSFETs which do the bleeding are whimpy and you can't charge the pack any faster than the highest cell bank can bleed or it would "overtake". Secondly, it wears out batteries, since the charge cycle incorporates discharging. It just seems like an uncomfortable compromise (although my own solution is now looking a bit that way too).

Hi, I am building a LiPo 7S bike battery charger. Rather than taking the conventional approach of charging the bank from its two ends and bleeding individual cell banks if they rise above the average, I want to charge all cell banks individually with a TP4056 or similar charge controller. I am providing isolated/floating 5V for each controller by using 7 Wall wart USB chargers. This means I can connect them in series without shorting anything. This works but - because of the large almost-flat voltage part of the LiPo charge curve - the charge controller is extremely sensitive to the sensed battery voltage and will throttle back the charging current greatly if even a tiny rise in battery voltage is detected. This is of course, doing what it's meant to do. However, if I try to charge through the balance charging leads, which are thin and high resistance, the controller takes forever trickling in charge, because these balance leads drop significant voltage for reasonable charge currents. They were never meant to carry charge current - only to sense voltage at no current - so again, all is as it should be. So I want to replace them with beefier low-resistance cables which can feed higher charging currents without dropping voltage - so the charge controller can sense each cell bank's charge state more accurately and charge as fast as possible. BUT this approach is dangerous. If the charge controller fails short-circuit then a cell bank is shorted out and the thicker wires will not blow so the whole battery could burn up. Fuses would be the obvious approach - a fuse in each charge line. The problem is that these also drop significant voltage so we're back with very slow charging. I suppose I could fit Hall effect sensors and relays and add a microcontroller to sense the current and turn off the relay if current becomes too high (or probably hold ON the relay as long as current is NOT too high) but this adds a great deal of complexity. So my question is - is there an easier/cheaper/better way to provide this protection? Any other comments on this proposal are also welcome. Thanks Chris

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?

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.

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.

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.

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, 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.

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.

Danidl - thanks for your interesting insights. Spanos - the benefit of experience - thanks. Kiwi, neath - more opinion points - thanks. 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 told me 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. Even from that world of theoretical misery the battery has served me well and continues to do so, though it's losing capacity noticeably now, which is why I want to build new one. I don't think it will be worse than the one I have. Online, I see people building powerwalls, and EBike batteries from salvaged cells, and reporting success. Also, I made a spot welder, and I have the cells. They are from unused laptop batteries. And finally, I simply don't have the money to buy a new battery or new cells to build one with., so I'll build the battery and see how it goes. My request for advice was about organising the cells. From what I've heard here and elsewhere, I'll worry less about IR, try to balance the capacity of the series banks by using uneven numbers of cells where necessary, and balance manually. From the other comments you offered outside my question, which I appreciate, I'll expect poor performance and short battery life, but I'll see how it goes anyway. Thanks again.

Hmmm. With a nominal 100 milli Ohms IR, the 10-ish amps needed to drive my 25V system at its rated 250 watts. Across 0.1 Ohm, 10 amps drops 1V and dissipated 10 watts. That's 70 Watts total in the 7P pack. The bike woks fine at 23V That's where my theoretical analysis runs out of steam. I was wondering abut real-world practice. If I mix low and high IR cells in parallel within each cell bank, will they "sort themselves out" or blow themselves up? I guess the voltage in the high IR cells would droop more and so the low IR cells would source more current. Difficult to analyse and of course, ideally, you wouldn't need to because you wouldn't do it at all. You'd use new, balanced cells. BTW these laptop batteries are unused so I'm not clear why I should limit them to 1C if their spec says 3C. Safer? Then there's the matter of measuring IR. I'm using my iMax B6 mini, but I already know it contradicts itself - measuring very different IR values depending on whether you're measuring cells on their own or in a string. If I had an accurate current-measuring device I'd do it manually. I have one on order. Then there's the meticulous versus gung ho camps. I'm in the "can't afford to do it "properly"" camp, but it'd be nice not to set fire to anything. Past experience says the nay sayers should be listened to but not blindly obeyed. Do it anyway, but keep a keen eye out for what they warned you about. What I really need is a sugar mumma to take me to top-of-the-lines-ville.

Thanks again guys.