Dr Bob

Yes Pete, I understand that, but what else? What other precautions is he taking?

You may have said that but you also said (see below) I repeat again, this at best misleading and at worst DANGEROUS. If you are homebrewing a system, you need to be very careful if you are taking LiFePO4s up to 100% (or 99%). Once you are in the voltage knee (95%+) then you are in territory where you need to be in control of what is going on. Mark, you blindly tell the world that they are safe and happy in that regime. For newbies who dont understand, that is DANGEROUS. How do you know you are not at 100% in one cell. You look at other forums and people will report their cell balance at 3.7V, 3.5V, 3.4V and 3.4V. Is that safe? Of course it isn't. Only someone who didnt have a clue, or a fool would operate like that. You say in one breath that you charge to 90% then in another you say to almost full. Almost full to most uninititated is 99%. In no posts do you say keep away from 100%. Homebrewing an Li bank needs thought and consideration of what can go wrong and actions that keep the system safe. By buying a 'one stop shop' from a vendor it is different - they have responsibility - but homebrewing is different, you have responsibility. I have read a previous post of your on another thread (which recieved 4 greenies - so it must be right???????????????) which suggests to me that you dont really understand the main issue with Li's. Can you remind us of how you avoid overcharging? I may have got it wrong and you do fully understand and if so, sorry I mentioned it.

One of the advantages of having the hybrid system of 400Ahr Li's in parallel with 400Ahrs of LAs is that you can isolate the Li's for 30 mins to get a rested voltage. I used to do this regularly to be able to draw the graph of Voltage vs Ahrs out (after resetting the Ahr counter on the last charge). Once you understand this relationship, you can then look at the rested voltage vs actual voltage under load. My system seems to register a drop of 0.010V per 1A of current going out, so if I am running off the Li bank and the voltage is 13.05V and there is 5A load, then I know the rested voltage will be circa 13.10V. This relationship seems to work over the 5A to 20A out range and doesnt seem to be affected by temperature. I have not looked to see if it works at higher loads. This of course doesnt work if the solar is active. We have run our new boat without solar for the first year but now have just fitted 500W so now I can only do the 'rested voltage' estimation at night! There is another way of doing it without the hassle of doing any work. If you have a BMV 712 (or similar), then hook it up to one of the victron devices that save all your data to the victron site - ie a venus GX or the new one, or if you are skint, download the victron firmware onto a Rasp Pie and be able to store all your data for less than £90 (the cost of the Pie + cables). Then at the end of a months cruising, download the log that has the actual Volts, the Ahrs used and the current draw/charge as an excel file. Then order the file first on Ahr used, and secondly on Amps. Reject any line that has an Amperage > or < than 0. ie you only have data points where no current was flowing - ie when there was no draw (likely never) or when the draw was equal to the solar going in. Ok, the voltage at A=0 will not be rested as it is likely it was higher just before that reading but you will get a scatter plot of these values if you plot V against Ahrs. Look at the edge of the scatter plot where the voltage for any given Ahr is lower and that is likely to be the most rested point. You will be surprised in a months worth of data how many readings you will get with the Amps very near 0. Checked the results from the first method vs second method and they match exactly. Siiiimples. I use the data output from the victron service (which is totally free) all the time. The Rasp Pie is brilliant.

All good to hear MP. As above, I have just forgotten about balancing. My graph of rested voltage vs Ahrs out is exactly the same now as in summer 2019 although I do need to reset the Ahrs out frequently as there are a few drains not wired into the shunt. To be honest, I just watch the voltage and dont bother with the Ahrs, and never the SOC.

My experience is similar, once top balancing is achieved, it stays that way. For my first 6 months I did worry about balancing and after 9 months finally got them somewhere near balanced - a spread of 50mv over the cells, ie the bottom at 3.45V and the top at 3.5V typically at the end of the charge. It has not changed from that 2 1/2 years on. I dont think about balancing now. It's a non issue. I am not sure if that would be the case of frequently taking the charge to 100%. A lot of the 'information' on the internet suggested high charging could lead to balance instability hence the general accepted norm of charging to 80-90%. Given a number of us have now had Li's on board for 3-4 years and more, has everyone else given up looking at balancing as a non issue?.......but then we were all only charging to 80-90% so it wouldnt be an issue then anyway! (Tom, Simon? .....ignoring Peter as his Valences self balance and cook the dinner as well).

If you have a very good BMS system then you could go up to 100% but the issue is that once you get there, you are in the region where things can quickly go wrong. Manufacturing defects, denrites, contamination means that batteries are not infallable. Having a home brew system where you regularly get up to that level needs a very well thought out protection scheme. TheBiscuits is spot on when he talks about bare 3.2V cells. It's even worse when talking about cheap Chinese cells of unknown origin. In an out of balance homebrew system, there is a safety risk at 99% charge or even 95% if it is well out of balance. .....and you say it yourself, why go to 100% if you want them work for a long time. I am surprised that in the 3 1/2 years since putting mine in, there has been no degradation in the performance - and even better, the balance has not changed at all. I have not had to re-balance in the last 2 1/2 years - probably because I dont run them up to 100% normally (only twice a year to stretch them!). The problem is that some manufacturers say - yes its fine to go to 100% - and people like MarkH2159 take that to mean it is fine for everyone else and try to convince everyone else of that fact. Home brewers need to make sure they are doing things safely.

The comments that some uniformed people make on here is mindblowing. Where do you get that information from? At best it is misleading and at worst DANGEROUS! If you had said 'mostly safe and happy under 90%' then I would agree. See the following link to a thread posted 3 years ago. At that time there was certianly not enough data to support your claim. Our company has been working on the thermal runaway issues since then (and including LiFePO4 cells) helping develop a pre theremal runaway warning technique. I consider myself a technical expert on this type of battery chemistry, even if I cant design the electical system to control alternator charge. I would recommend MarkH2159 read that thread and the associated links. Anyone considering installing a home brew system of LiFePO4s needs to do a safety assessment on what they are installing and the starting position for that is to as much as possible keep away from 100% charging. I use 90% as my upper limit over which the auto disconnect works and my 'operating proceedure' says only go to 100% when I am personally in manual control of the voltage and current. Keeping LiFePO4s at 90% or below is very very unlikely to present an ignition hazard on well trusted cells (it is not going to happen on a canal boat). The risk is much higher at 99%. There is very little point in taking these to 99%. The whole idea is that you dont need to fully charge. Why risk safety when thermal runaways are a risk?

Yes they are safe but it looks like some people are not listening. Mark, lithium's - whatever the chemistry - are not ever happy at slightly under 100% full. It is not a good idea to charge to that level all the time and keep them there. The company I am a director of does a lot of Li battery testing - to destruction - and are engaged in a number of Eu consortium programmes on air transport of lithium batteries. The power in a 100% charged Li battery is huge. That is when problems happen. Yes, you can go to 100% now and again but that is not the way to treat them, and they certianly shouldnt be left full. I never charge my car to 100% and certainly never leave it above 90% - that is what the manufacture says....for a reason. We test them. We know! Trust me...I'm a muppet doctor.

When I turned up in their offices (I made an appointment before), I was taken into their lab to watch the technician test the two reconditioned units. After 60 secs on each he pronounced that they were in good working order but he knew these were the old design and hence suffered from overheating and recommended I take new ones to overcome the problem. They openly admitted the problem and said the new design was to make the overheating better. They really shouldnt be selling the reconditioned units - of which they seem to have very many - as fit for purpose. They overheat and fail. The new design is better but still gets too hot. Mine do the same as yours and drop power now and again but not that much - but I just dont want to 'burn' 30A of my charge power - so given up using them. Good idea but not that well implemented. I reckon the best way to use a Sterling device is find a 2nd hand AtoB. My old one was brilliant. Sterling could design one easily to do what we need at a fraction of the price of what they charge for a new one. I'd keep an eye out for used ones for sale.

I never use the AH counted in. The Ah on mine drifts as some of the current is not detected by the shunt, - part of the BMS and the cell voltage monitor. I just rely on the total bank voltage. It is set to disconnect automatically at 13.9v at which point the current is down to sub 50A and I have lots of LA to take the turn off. However, I usually turn it off manually at around 13.8V so there is room left for a bit of solar to go in, and as it is off, the alternator voltage goes up to 14.2V+ so makes sure the LA's are topped up. 13.8V is circa 80-90%. You dont have to be accurate. Dont take these voltages as gospel! They are the voltages I see on my system and have 4 years of watching the voltage change. Your voltages might be different. To work out what voltage relates to 80-90% charge, I spend a few hours with the system on charge - at the typical charge rate (important you do this test at the amps normaly used - so either run the engine or do it on shore power with the charger at the same Amps). Write down the time and the amps going in every 10 mins or so. Likely you will see the amps in drop slightlyas the voltage rises from 13.2V to 13.6V. At some stage the amps will start dropping. On my old boat I charged at say 50A and it would drop slowly to 30A then start downwards rapidly. At this point the voltage starts rising faster (ie from 13.7V) and you then take it up to say 3.5V per cell and stop. You are likely then 95%+ charged. You can then work out from the Ahs in and the voltage rise the typical charge % at the various charging voltage. This is only valid however for that charge current - ie it will be different if you start charging at 90A rather than 50A - you will get to a higher voltage before you see the tail current decay.

Yes, totally agree. You must isolate the Li's on 80-90% charge.

Pete, watch out on these reconditioned units. I bought 2 of the 60A ones at the cheap price when I got the new boat and installed them. Big problem as they overheated very easily - they take in 60A and give out 45A in power and 15A in heat. They both ran for around 30 mins then gave up as the temp was too high - the heatsinks got to +60°C. I rang Sterling and they were incredibly helpful. I took the two units over to their shop down the M5 and went into their lab. The guy tested them and said they were fine but they have been superceeded with better cooling (heat sink and ventilation) and said they were likely to overheat. They upgraded them to new ones and knocked 20% off the new price. No problems since. They still use 15A to heat the boat but at least they have better ventilation. Make sure you locate the B2B where it can get a good supply of cool air.

Tony, Each boat is different and sometimes you hit lucky. On my old boat, I had fitted an Sterling AtoB and that worked well to control current and turn it off when the voltage was right for the lithiums. The thing was that the Li's were inside under the bed so were 5 Metres away from the inverter and connections to the 12 v system. This had 2 benefits. It restricted flow so the voltage drop was significant viz 0.3V. This meant the AtoB was seeing 14.2V but the Li's only 13.9V so the AtoB turned down the charging when it saw 14.2V. That system was on a 90A alternator so I usually charged at 60A down to 20A. This new boat has a similar location inside for the Li's so a similar voltage drop originally on 50mm cable but less now on 90mm cable. The BtoB's do work but burn 20% of the available charge. The alternator is now 240A but the charge never exceeds 120A so no problem at all. My 50mm cable never got hot so not a concern.

Well its certainly a battery switch that is motorised. The 701-md rings a bell. They are a bit pricey but I am on too low a pay grade to fit a latching relay. The 701-md is more a muppet level piece of kit.

Yes, that's the one No, it looks like that is mine. No problems at all with mine.

Yes, that is the reason I have upgraded to 90mm cable now. If I pull 150A (Mrs B's nesspresso machine), then 80% of the power comes from the Li's and 20% from the LAs. As the overall voltage drops to 12.7V (resting voltage) - very rarely - a 150A drain will likely come from both sets of batteries equally. Its far better now with the 90mm cable. I tend not to run high current devices without the engine running though so its really only the coffee machine in the morning where I see this issue. 90mm cable is bloody expensive!!! The reasons I went for a hybrid system were: - At the time Li's were not proven (2018) and I wanted a way to go back -Mrs Bob didnt want the 12V system disconnected for days (cf MP) -I wanted to be able to take them with me when we upgraded the boat -I wanted a simple control system to stop charging when full without the need to do surgery on the alternator. It has worked out very well and I would recommend it to anyone who doesnt want to rip out an old system.

Hi Mike, I am using the motorised BMP switch. It has worked great since installed with no hiccups. It is triggered by the BMV712 relay when it gets to 13.9V but I usually see the voltage rise and isolate from an easy to get to manual isolator (I have a voltage display visible from the steering position). It also as a simple BMS box that monitors indivdual cell voltage so will isolate the Li's via the same BMP switch if cell voltage exceeds 3.8v on any of the cells. It really is very simple.

Hi Nick! I spent the last 12 month writing that reply. It is actually quite simple really.

BtoB's are very inefficient and turn so much power into heat. My 2 units only ever put out a max of 90A (each rated at 60 A input). I've given up with mine.

I dont see that as an issue. When my Li's are full, they isolate (or are isolated) so only the LA's take charge. Both sets will take charge when they want it. On the LA's the tail current will drop to very low and the Li as they get over 90% need to be isolated (or a BMS recognises it and limits charge). For our application it is simpler to just isolate. Worked great for me for 4 years.

That's not what I see. Typically my LA's are 95% full and the Li's 30% full when I turn on the engine. The LA's fill up as fast with or without the Li''s connected. The tail current drops at the same rate. Both sets of batteis suck in power at their 'independent' rates.

Pete, your problem is that you believe too much written on here. The 'Dr Bob' concept works fine – but I dont think I would do what you are asking. Of course you can use Lithiums as a top up tank but you have to watch what you are doing. I've been doing it for 3 years now ….or is it 4? I know what I am doing. It has worked extremely well since I installed the Li's back when only Simon and Tom had them on board. I had them on my old boat, Kharis and then switched them over to my new boat 18 months ago. On Kharis I had circa 250 Ahrs of workable Li capacity paralleled to 6 cheapo LA ( 3 sealed – now 5 years old and 3 wet – now 6 years old). They were in this config for 2 years with us using the system year round. We sold Kharis in mid 2020, unbolted the Li's and BMS leaving the original set up in place and those original 6 LA's are still going strong. No apparent degradation due to undercharging. The current owners havent replaced any batteries yet. I transferred the Li's to my new Aqualine and bolted them up in a similar manner but with 2 Sterling B to B's in circuit, paralleled to 4 tubular gel LA batteries. The boat is out and about all summer but has spent the last 2 winters on shore power. Again I see zero deterioration in the LA's. There is no doubt the way I use them works. I have not compromised any warranties. I swopped them from boat to boat ...and I will take them to the next one if we change again. After 4 years I see no degradation at all in the Li's. I operate the system between 13.25V and 12.80V when on discharge. I get circa 200 Ahr out of the Li's from 80% to 30%. Our current bote is very power hungry and typically I am using 170 Ahrs in 24 hrs. When out and about, we charge every day for 1-2 hours. We have a 240A alternator which typically puts out 110A at the start of charge decaying to 50A as we get to 80% full. Originally I took the decision not to put solar on the roof as the alternator was so big, but last summer I bit the bullet and put 500W of solar on so we dont need to run the engine as much. The key to running the paralleled Li's is not to discharge the LA's much. They are there in reserve if you need them but normally I will be charging as the Li's reach a lower limit of 12.75V so the LA's never see more than 5% discharge. The charging voltage on the Li's up to 80% is typically circa up to 13.8V on the Li's and that is 13.9V on the LA's (Li's have longer wire runs). 13.9V is fine for the last 5% of LA charge BUT ONLY IF THE DISCHARGE IS LIGHT. If you discharge the system down to say 12.3V then the LA's will be down to 50-60% and the final 10% of recharge HAS to be at 14.2V+ otherwise they will sulphate. On the few occasions we have stayed put for days without running the engine, I have charged the Li's in a few hours then when isolated, the alternator is then pushing 14.4V into the LA's to do the final charge. Yes it takes a few hours but it stops the LA's from damage. The important bit here then is to size your additional Li bank to around your daily usage so you normally keep the LA's pretty full. This way you get the benefit of always being able to get charge into the LAs – ie you don't waste valuable solar. The problem with your question is that a 20 or 40 Ahr Li means that you will always be digging into the LA reserve so you need 14V+ to finish off the LA charge. My experience would suggest having at least 100Ahr of usable Li capacity in a paralleled system. In the case of a 20/40 Ahr Li system, both the Li and LA will charge if depleted so loading the alternator (!) and likely the Li will get full first (as it is only 20-40 Ahr), then when isolated the alternator will be putting out the 14V+ the LA needs. It therefore seems pointless to have Li's as the benefit is that when you finish your day engine running, you still have space in your Li's as they dont need to be full – so you can take full benefit of your solar. I would take with a pinch of salt much of what is said on here about how to control power into Li's when charging. Every boat is different and control can be done in many different ways. On Kharis, we had a Sterling AtoB as our alternator voltage was too low (13.9V max). The AtoB controlled the alternator output and so limited the temperature. I later realised the 5 metre run of 50mm battery cable was helping curb too much power. All I had was a high voltage isolation switch to isolate the Li bank when I got to 13.8V. On the new Aqualine I bought 2 * 60A Sterling BtoBs as I did not want to mess with the warranty on the new boat (alternator, existing LA's and the electrics in general). To be honest, those BtoB's were a waste of time. The Li's are again 5 metres from the main 12v system and I started only with 50mm cable. The boat is wired with 2*90mm cable as the alternator is a 240A with a 5kW inverter. I have recently upgraded the Li cables to 1*90mm but never see more than 120 A going into the Li's – although when we've been parked for a couple of days without running the engine and the LA's are down at 12.6V, I see 120A on the Li's and 50A on the LA's when turning on the engine. I now dont bother with the BtoB's and just let the alternator do all the charging – starting at 110A and dropping to 40A ish as it reaches 80-90% when the auto isolate operates. Most of the time I actually isolate the Li's myself as I know when they are getting up to full charge. My alternator never gets over 70°C even in high ambients. Bottom line is that choice of cabling helps control alternator temperature. So, the 'Dr Bob' paralleling method does work but you need a reasonable size Li battery – and I would say that is about the amount of power you use in a day. Any smaller and you will discharge your LAs too deep and require 14V+ to get them back – which sort of destroys the point of having Li's. You say you have taken the plunge with Li's. How have you configured them?