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Interesting blog making the case for Li-ion


WotEver

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2 hours ago, rusty69 said:

Interesting that he hasn't bought any! 

I understand he’s doing so but wanted to get all his ducks lined up first - particularly the very important BMS. 

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Lithium batteries in theory allow you to use 100% of their capacity.

I thought that the discussion in another thread concluded that this is well wide of the mark in real life and that 70% down to 30% was more like it, making lithiums no better than lead acids in that regard?

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1 hour ago, BruceinSanity said:

I thought that the discussion in another thread concluded that this is well wide of the mark in real life and that 70% down to 30% was more like it, making lithiums no better than lead acids in that regard?

25% to 80% is often quoted, or even 20% to 85%. The important design consideration is that those limits should be cast in stone and impossible to bypass. 

 

The biggest advantage of Lithium is the charging characteristics; they’ll take a very high charge current (compared to LA) and will keep that up until you stop charging. So instead of 6 to 8 hours to charge a LA bank you can do the same in 2 hours with a Lithium bank. 

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8 hours ago, WotEver said:

I understand he’s doing so but wanted to get all his ducks lined up first - particularly the very important BMS. 

 

12 minutes ago, Athy said:

Has he got three of them on his shi - irt?

Lions, or Ducks? 

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10 hours ago, WotEver said:

I understand he’s doing so but wanted to get all his ducks lined up first - particularly the very important BMS. 

I told you so, Wotever mentioned ducks.

 

My concern with the BMS is how do you switch off the various charge sources? I may be a muppet here understanding what needs to be done but with my bog standard cheapo lead acid system, if my AtoB ex alternator, my battery chargers, or my 2 solar mppts think the battery is 'fullish', they back off the charge voltage and everything is good. If I put in a set of Lithiums with the same 5 charging sources, the BMS would have to stop those sources feeding. Would that involve a load of relays? Could they just be at the battery end or would that wreck the alternator or mppt as no where for the current to go. For solar I guess you could just have a relay upstream of the mppt. Does this mean a whole new set of controllers? I do like the fact that you can charge in a few hours.

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23 minutes ago, Dr Bob said:

I told you so, Wotever mentioned ducks.

 

My concern with the BMS is how do you switch off the various charge sources? I may be a muppet here understanding what needs to be done but with my bog standard cheapo lead acid system, if my AtoB ex alternator, my battery chargers, or my 2 solar mppts think the battery is 'fullish', they back off the charge voltage and everything is good. If I put in a set of Lithiums with the same 5 charging sources, the BMS would have to stop those sources feeding. Would that involve a load of relays? Could they just be at the battery end or would that wreck the alternator or mppt as no where for the current to go. For solar I guess you could just have a relay upstream of the mppt. Does this mean a whole new set of controllers? I do like the fact that you can charge in a few hours.

Depends on the BMS.  Victron's BMS turns off the MTTP using the remote cable and the same with the load relay.   The alternator is wired via the starter battery still, so basically it disconnects the battery link.   Victron do another BMS thats on the negative side that links the batteries and charging stuff rather than remote devices, but it's limited to 200amp and 100amp alternator.

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35 minutes ago, Dr Bob said:

I told you so, Wotever mentioned ducks.

 

My concern with the BMS is how do you switch off the various charge sources? I may be a muppet here understanding what needs to be done but with my bog standard cheapo lead acid system, if my AtoB ex alternator, my battery chargers, or my 2 solar mppts think the battery is 'fullish', they back off the charge voltage and everything is good. If I put in a set of Lithiums with the same 5 charging sources, the BMS would have to stop those sources feeding. Would that involve a load of relays? Could they just be at the battery end or would that wreck the alternator or mppt as no where for the current to go. For solar I guess you could just have a relay upstream of the mppt. Does this mean a whole new set of controllers? I do like the fact that you can charge in a few hours.

Lots of ways as Robbo touched on. If you have a dedicated domestic alternator then cut the field current. For Solar cut the panel feed. For a mains charger cut the mains. Yes, lots of relays, and this is the point; a Lithium installation has to be thought of as a complete system, not just a replacement battery. Cell balancing is another important aspect, which is generally cared for by having a high enough (but not too high!) charging voltage. 

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10 minutes ago, WotEver said:

Cell balancing is another important aspect, which is generally cared for by having a high enough (but not too high!) charging voltage. 

 

That's not my understanding. Cell balancing according to the American bod's blog/article is done on the bench prior to installation by accurately matching the charge in each of the cells. Then they stay in balance almost indefinitely. This applies if you have four whacking great single cells in series to make up the nominal 12v. 

 

The other way, having loads of tiny cells in series/parallel grids, I can't see how cell balancing can ever be achieved/maintained. I suspect if one small cell expires that bit of the grid just stops working.

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Just now, Mike the Boilerman said:

Cell balancing according to the American bod's blog/article is done on the bench prior to installation by accurately matching the charge in each of the cells. Then they stay in balance almost indefinitely.

They will only stay in balance if the subsequent charging voltage is high enough. I think it’s called the balancing voltage or something like that. 

 

Iirc (it depends on the manufacturer) it goes something like ‘over 2.45V to keep them balanced but under 2.48V to avoid overcharging’.  

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6 minutes ago, WotEver said:

They will only stay in balance if the subsequent charging voltage is high enough. I think it’s called the balancing voltage or something like that. 

 

Iirc (it depends on the manufacturer) it goes something like ‘over 2.45V to keep them balanced but under 2.48V to avoid overcharging’.  

 

On the other hand, if they are accurately balanced in the first place, it is hard to see how four cells in series can ever go out of balance. The discharge and charge currents are by definition precisely the same, so if the voltages are matched accurately enough, they must stay the same too. 

 

Intuitively I'd expect some drift over time, but the amercan bod says in practice this does not happen.

 

 

How cell balancing can be maintained in the grid format banks though is beyond me. Other than by having a BMS for every individual cell and them all communicating with each other.

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3 minutes ago, Mike the Boilerman said:

How cell balancing can be maintained in the grid format banks though is beyond me. Other than by having a BMS for every individual cell and them all communicating with each other.

No idea, I’ve not done enough research. 

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