Coments and objective criticism needed

[aKO]

Would someone knowledgable please look at the circuit and tell me if its safe or has flaws or needs moifying before i turn on and possibly find out the hard way .

The Lithium battery is sufficient to cover our normal consumption between the hours of 6pm and 10am , when we have bad weather and the main batteries are down to 50% , we use 70% to 85% of the Lithium batteries capasity . The reason for this set up is that until now i have to start a small Honda EU22i generator and run it at night for about 3 hours but even tough the generator starts after only 1 or 2 pulls im finding even that a struggle leaving me breathless , the wife cant start the generator but she can turn a switch . Only had 4 occasions so far when we needed to use the Backup battery but i have been manualy removing the positive input to the Inverter from the main system Busbar and attaching it to the Lithium battery and the reverse the folowing morning . Until we reconect the Inverter to them main systems BusBar the MPPT charges the Main battteries as normal .

I keep noticing continuity between places i would not expecy to see it such as between the terminals on the changeover switch that are not selected and between the input to the switch and the inverter positive when the switch is in the OFF position and also between the BusBars , the switch is new sand functioning fine and i have bought another to check and also when its not in the circuit there is no conuity where there shouldnt be . Removing the Lithium battery makes no difference and i am positive theres not a short anywhere .

 

[Cmiller]

I keep noticing continuity between places i would not expecy to see it such as between the terminals on the changeover switch that are not selected and between the input to the switch and the inverter positive when the switch is in the OFF position and also between the BusBars , the switch is new sand functioning fine and i have bought another to check and also when its not in the circuit there is no conuity where there shouldnt be . Removing the Lithium battery makes no difference and i am positive theres not a short anywhere .

If you are checking continuity using a multimeter, you are probably getting continuity through the batteries themselves. Your negatives in this setup are common, therefore you could have one tester lead on the Lithium battery positive and the other tester lead on the lead acid battery positive and be seeing continuity. With the way you have this wired (per your diagram) you will very likely see continuity between pretty well any of your DC lines. I wouldn't be too concerned about that.

Do make sure though, that you have fuses in your positives! Especially coming right off of your batteries! If the 2 banks were to somehow get connected up while the lead acid were very low in charge and the Lithium were fully charged, you would have some pretty high currents flowing for a little while!

One question that I have though is, if that extra lithium battery can get you through when you run behind, why don't you just add that amount of storage to your main battery bank? I think long term that would be your better solution for a few reasons. For one, your solar would then be able to charge all of your batteries and you wouldn't be dependent on your AC charger. And then the other thing is, that lithium battery will have some self discharge over time, and that gives you the added chance of switching over to it, only to discover that it isn't completely fully charged anymore, and potentially falling back to needing the generator.

Just my thoughts on the matter.

 

[aKO]

Thank you i really o appreciate you taking the time to read and more importantly pass on your observations/thoughts which i have taken note off . In general i now feel more confident about making the final conections , the continuity i was seing in so many places was a concern and although i did wonder if it could have somthing to do with having power in the lines when i check i wasnt 100 convinced of it being the cause .

The points you mention i think i have already anticipated , there are no fuses in the drawing but there is in the system on the positive lines near the batteries as well as 2 pole isolation switches on the PV input . The change over acts as anisolation switch for the Lithium and there is a seperate single pole one for the AGMs . T changeover switch should make sure that one battery is disconected before the other is conected so that both can never be conected at the same time .

I take your point about adding batteries but i wanted to get away from Lead Acid and start to use Lithium , the lead acid although working perfectly are two and a half years old and from everything i have read adding to them wouldnt be recomended , i added a 100 amp/hr Lithium , i would have to add 200 Amp/hrs of AGMs and belive it or not the Lithium cost less , i also like the idea of opertunist charging while Lead Acid need to be fully charged regularly to delay ageing , its not an issue with Lithium , on a bad day they can charge 60% or 70% and be safe to use while it wouldnd do Lead Acid any good to do so .

I have charged the Lithium with the AC charger as well as the DC to DC charger off the main bank , in fact i might turn the charger on around mid day when the AGMs are in absorption and strangling the input . I swap the inverter input from the main to the Lithium once a week so that it also gets cycled and it can be kept at 100% as i read that for longer storage it should be charged inly up to about 80 although that would stil be sufficient it would mean it was getting cucled lower than i would like , i stil have the mndset of usind Lead Acid .

 

[Cmiller]

I take your point about adding batteries but i wanted to get away from Lead Acid and start to use Lithium , the lead acid although working perfectly are two and a half years old and from everything i have read adding to them wouldnt be recomended , i added a 100 amp/hr Lithium , i would have to add 200 Amp/hrs of AGMs and belive it or not the Lithium cost less , i also like the idea of opertunist charging while Lead Acid need to be fully charged regularly to delay ageing , its not an issue with Lithium , on a bad day they can charge 60% or 70% and be safe to use while it wouldnd do Lead Acid any good to do so .

That makes perfect sense. You can use the AGMs this way until they reach end of life, and then expand to fully lithium, or whatever you decide at that point! You are correct about not adding to the AGMs if they are a few years old. I have seen people do that and often the new batteries will actually wear out before the oldest ones, due to seemingly getting worked harder. I have come to the conclusion that it is because the newer ones have less resistance (no sulfation, etc.), and therefore they are "stronger" at the start, so they get overworked compared to the old ones.

I have charged the Lithium with the AC charger as well as the DC to DC charger off the main bank , in fact i might turn the charger on around mid day when the AGMs are in absorption and strangling the input . I swap the inverter input from the main to the Lithium once a week so that it also gets cycled and it can be kept at 100% as i read that for longer storage it should be charged inly up to about 80 although that would stil be sufficient it would mean it was getting cucled lower than i would like , i stil have the mndset of usind Lead Acid .

Sounds like you have a good plan! I love the idea of the opportunity charging using excess solar power when the AGMs are in absorption! Great solution!