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Dr Bob

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Everything posted by Dr Bob

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  4. Tesla now use LiFePO4s in some models ie the M3 SR+. They introduced them almost a year ago. Please attempt to be more accurate in what you say.
  5. What on earth is a 'car' lithium? The first adopters on here (Tom, MP and I) are all using LiFePO4s (thunderskys) ex vehicles. Peter is using LiFePO4s ex vehicle from Valance. These are the safest of the Li battery types around (but not fully safe). It is crazy to assume 'car' lithiums cannot be used on a boat - its not where it came from, its the battery type that is important. Too many peeps on here do not have a clue and certainly do not read what the people in the know are posting. I have mentioned likely a dozen times that mine are LiFePO4 yet a few posts ago someone questioned what type of batteries I have. For Christ's sake guys. Get a grip.
  6. Oh dear. Oh dear. How the understanding of safety has fallen on this forum over the last 2 years. Of course your system will work fine while the BMS works. Of course you are satisfied it is great after a few months. A few months of operation is not Proof. The issue is what happens if the BMS fails. Wake up and smell the coffee. It wont be long before someone has a bad fire (likely not caused by the Li bank) but the insurance assessors will hold the owner responsible as he has been negligent in homebrewing his Li bank (even if the Li's had nothing to do with it). Then we will all come under the insurance industries spotlight. We, as a group, have to get this right and clearly you have not. I will keep calling people out when they are negligent as it will certainly come back to bite us in the future. It's not just you though Mark. The other 'enquiry' from an LiFePO4 user - enquiring if taking his Li's to 16V was ok if an individual cell could be taken to nearly 4V - shows that the understanding of these systems is far from what is required. Your peer group also has a lot to answer for in not curbing the urges of the clearly uninformed. As they say on the telly, I'm out. Bye all.
  7. Dendrite formation is one of the routes but dendrites dont form suddenly. They grow slowly over time, possibly each time a cell get to a high voltage. In the link I posted earlier in the thread, I think there are some links to more detail on this. I am aware of one paper where it looked at dendrite growth over successive overcharges - ie dendrites were found after 10 excursions to 3.8V. It also can happen when charging below zero. If I was installing a system, I would be more woried about poor construction or contaminants in a new battery (again see links in the thread I referred to earlier), ie a 5mm length of wire nearly bridging across the anode/cathode. Is a cheap chinese battery going to be safe? Mine were in service in a van before I got them so I am fairly sure there are no 'manufacturing issues'. We are currently also involved in Li battery recycling and working with some of the new emerging gigafactories. One frightening statistic is that the biggest market for Li battery recycling is the waste from the manufacturing process itself. Typically you find 30-40% waste - and that is not just from floor sweepings! Their QC proceedures must be good. I hope they are spotting all the duff ones! Most LiFePO4s sold as cells (ie my Thunderskys) have plugs in the top casing that blow when the temp reaches a certain point and the liquid between the anode and cathode is expelled thus isolating the plates so no route for the energy to short out and heat up. Just the venting of the liquid makes it safer. The problem is that the thermal runaway could be well on its way when the plugs blow. None of the 'extinguishing' chemicals ie Brominated or phosphated species would stop a themal runaway but may help surpress flames around the burning battery.
  8. It was a long time ago! I may be wrong but after I put together my hybrid system of old LA's plus Li's, I posted a thread wondering if you could just have a simple Li battery on top of your LA's and have a very simple and cheap system. I think that is what Biscuits is referring to. Having now had the experience of running the hybrid system, the more simple system could work as in my summary to Rusty above, if it is big enough and you have some redundancy. It may be to expensive though if you have to buy B2Bs etc.
  9. BTW....... Wot are you doin' here? I've only been let out as @TheBiscuits was casting nasturtiums at my theories and there's rumours of a noodle conspiracy being reported.
  10. If an individual cell goes wonky, then one effect will be the capacity of that cell is likely to be different to the other 6 you have in parallel which will bring down the capacity of the 'large 7 cell' unit. When you charge, then the 4 large cells will likely go well out of balance. Here's where I am at my pay grade limit - but would anticipate that 7 cell unit will hit the voltage knee first and likely overcharge if you charge to your normal voltage. I'm sure Nick will have the right answer.
  11. I have never ever recommended 14.0V. That to me is too high at my 90A charging rate. I disconnect at 13.8V with the emergency cut off at 13.9V to give me lee-way if my cells ever drift out of balance (which I do look at monthly). I have Winston cells so also have yttrium in. What is the point of going over 80% SoC? If its about capacity then put bigger cells in. If newbies cant afford the investment then stick to LA's.
  12. In summary, it will work if: - the Li bank is big enough for 24 hrs use ie 100Ahrs likely - if not you deplete your LA's so sulphation issues - you spend money on some levels of redundancy (or have control of charge sources ie alternator controler/solar charger), but audible alarms are cheap. That is for the 'top up' type hybrid system. Not to be confused by my parallel hybrid system (400Ahrs of LA and 400 Ahrs of Li) which is now tried and tested for nearly 4 years. It cost me circa £1500 to install. It will last at least another 4 years (maybe a lot more) which when you include the cost saving in reduced running the engine (fuel and servicing) is a big saving over replacing 400-500 Ahrs of LA's every 4 years.....but you need to understand how to homebrew a system. It's not obvious from this thread that the average level of knowledge is high enough to have a reasonable level of safety.
  13. Certainly not. This is the problem on this thread. Peeps (including Alistair) do not understand the basics here. The 16V is not what the supplier is recommending/talking about/referring to. You are totally ignoring cell balance. If the bank was fully in balance, yes you could have 16V but that will NOT be the case. Once you get in the voltage knee then you are likely to have a wide range of voltages. In a vendor sourced system than they may have a reasonable balance so maybe 4.0V, 3.9V, 3.6V 3.6V - so a total of 15.1V but in a homebrew system you could be at 4.0V on one cell and the other 3 not even at 3.5V - total 14.5V max. You could then be driving that high cell to destruction. That is the issue about homebrew systems. Too many peeps on here think of a high level cell voltage and multiply by 4. WRONG. So Alistar.....you do not understand correctly!
  14. Fully agree with your comments on correlating charge voltage with SoC. I frequently charge at 90A with my 2 B2B's and 45A with one B2B's. The voltage when I terminate is very different. My 13.8V target is with the 90A charge so the 45A would be 13.6-13.7A. In my posts I do reinforce the point that peeps need to work out their own voltages. My 80-90% target is a point before the amps start dropping more quickly and hence well before the voltage knee. My biggest worry here is Mark saying its fine to charge to 95-99% (ie less than 100%) and peeps who dont understand accepting that fact. I am not aware of examples of thermal runaway in LiFePO4 in the real world but we test them and they do fail. Of course the chemistry is safer and there is less energy density but internal shorts via defects/dendrites etc can trigger runaways. It is interesting in our work supporting the EU inititive of transporting Li cells on passenger planes, the relevant bodies made no differentiation between LiFePO4 and Li-ion cells (ie the regulations on transport will be the same from both). It's also interesting that all this air transport work is assuming the cells in transport are not on charge!!!! The relevant bodies see the risk of thermal runaway on just charged batteries rather than during the charging process. I dont know where the legislation will end up but may be set so the batteries will have to be less than 30% charged and in approved 'safe' packaging boxes. Also, we dont hear about many Li-ion batteries causing problems, likely as most vendors have well designed safety systems.
  15. Not that sort of Chemist Duh! ......but Auntie Waitrose's answer was pretty spot on. Trying to avoid the trivialities of this forum.
  16. Me? I'm a Chemist. We employ engineers to do the maths.
  17. Tell me about it! I bet Auntie Waitrose will be adding them to her list to auction in a few days. Hope there are a few trees still standing in Welsh Wales.
  18. Your are right, they are a mixture of cheap and expensive Chinese circuits which is exactly why I have multiple layers of redundancy. The beauty of my set up is that I can just isolate the Li's and return the boat to LA only when the sprogs take over on board. It's not starting. Its already gone!
  19. Maths was never my strong point. I lied. It was three..
  20. Think about it. Your set up is Step 1, charge to 80-90% not 100% Step 2, solar controller backs its power off when it gets to target current Step 3, the battery manufacturers own BMS can isolate/balance in an emergency That is 2 steps better than charge to 14.0V and ask the cheapo chinese board to isolate as a routine. At least I'm getting there after Mrs Bob
  21. Young? Young.......................? I hit the big two score and ten in April! Have you got the Noodles yet?
  22. Spot on. LA's are also dangerous, just in a different way. Hydrogen Sulphide is a killer. Not that may peeps put LA's under their beds though.
  23. Tesla are responsible for their system and I just drive it. If anything goes wrong with the batteries, Tesla will be held responsible (I hope). I trust them to keep me safe. If I was homebrewing, I would take reasonable precautions to ensure I was doing things properly, safely and was not negligent. For insurance purposes as well as liability, it is important.
  24. Of course he needs more! If you buy a system from a vendor, the responsibilty for a working system lies with the vendor. If you homebrew, you are responsible. What has this forum decended to? 2 years ago, it gave out generally safe advice.....but this? I have just spent two days in 2 * 8hr meetings doing a Hazop (a hazard assessment ) on a new piece of kit going into our lab. Basically, identify anything that can go wrong (mainly aimed at safety) and anything with a 'red' outcome - ie one or more dead, you identify sufficient mitigation (even if it is a one in a million chance of the thing happening). An individual homebrewing his own Li's needs to do this himself. In this case with the alternator directly feeding the battery via BMS, the only protection from overcharging is the BMS. What happens if it fails? What happens if it fails as it reaches its high voltage cutoff (at 14.0V in Mark's case). Simple, the batteries will overcharge if the disconnect fails. What else is he (or anyone else using this simplistic system) doing to mitigate over charging? It is downright dangerous if there is no other mitigation. Buying a full system from one company might be ok (but I still wouldnt be happy) but buying individual components ie a cheap chinese BMS and integrating it onto bare cells is not a clever idea on its own. With no mitigation and publically anouncing there are no problems would be seen as negligent in the event you kill your wife/kids/grandkids. Ignorance would be no defence. The discussion on here 2 years ago was all about different levels of protection. My system use 6 levels to avoid overcharging (over the top (?) but I consider myself a technical expert on over charging Li's - and need to be whiter than white -if there was a fire on my boat, I want to avoid prison). Step 1. Aim normally to charge to 80% not 100% (ie 13.8V not 14.0V) to keep well clear of the critical zone Step 2. Have a controllable source of power (ie a B2B or alternator controller) so the power is reduced and cut off before you reach high voltages. Note a recent post by Nick saying his latching relay is his EMERGENCY DISCONNECT, ie you should be reducing power and cutting it off before you activate an emergency disconnect. I use a B2B which monitors its own voltage and backs off power at or before 13.8V. A piece of wire may stop damage to the alternator (I do that as well) but it does not stop overcharging. Step 3. An audible alarm on my BMV 712 which sounds at 13.9V. That has to be cancelled to stop the noise. You then manually disconnect. Step 4, An auto disconnect switch is triggered by the BMV relay at 13.9V to operarate in emergency only. Step 5, An cheap board from amazon (£10) which sounds an alarm at 14V. Step 6 A BMS board monitoring cell volage and activates the auto disconnect switch at a cell voltage of 3.8V Some of the steps are linked, ie the BMV could go wrong so steps 3 and 4 would fail. The auto disconnect switch could go wrong and steps 4 and 6 would fail. In either case I still have 4 steps of protection. To be honest, I am getting fed up of the B2B so often I skip Step 2 and manually disconnect at 13.8V but I am very consious of this action and ensure it happens. In the 3 1/2 years of operation my auto disconnect has only activated once -other than when it was being tested. It appears a number of you are working on the basis of one step is enough - ie a cheap chinese BMS that will isolate the power. Why do you think Sterling insist on their B2B? Yes it is about voltage etc but it is also about safety. With the B2B you have at least two levels of protection. Newbies need to understand this and not be blindly led into unsafe operating practices. Three years ago Tom used an off the shelf alternator controller, MP homebrewed his own controller. Then Nick homebrewed an all singing all dancing very elegent system with control of power and emergency cutoffs - but he knew what was in the box. Pete, you yourself pushed the point of only going to 80% to keep safe, but at least you had a manufacturers BMS/Battery combined. ........and my Tesla. Warnings from Elon everywhere that its not good to go to 100%. It's not just about battery life. It's about safety as well.
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