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My observations combining lead/acid & lithium

Bossrox

Solar tinkerer
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May 21, 2021
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345
Location
Columbia, SC
Will they work together well? Yes!

I just acquired my 1st 280 a/hr 8s lith pack & have a good 300 a/hr agm bank I didn't want to abandon, so I decided to tie them together to see how well or not they'd work together.

1st surprise was the lith pack took on the total inverter load til the voltage got down to about 25.6v, then the agm's started easing in, & because the agm's began contributing before the lith's bms cut off, it slowed down the the discharge rate of the lith's towards their end of cycle.

Now on the charging side it's a bit trickier, the lith bank robs most of the charger current & knocks down the volts to about 28v thru the entire charge cycle so the agm's charge slower.

Since I only draw no more than 35% off the agm's, they're charged before the lith's anyways, so my concern was, if the charger is in bulk mode when the agm's were fully charged, would they get overcharged? No & or maybe, 'cuz the lith's hold down the chargers voltage to a near steady 28v until the lith's reach saturation so if the agm's are charged, they're getting 4 tenths of a volt more in float than normal. Not sure if that's much of a concerning issue.

But when both banks are charged & the charger goes to float, to get 3.5v to the lith cells, I had to set the float volts to 28v rather than the normal 27.6v. I can't say if having a float 4 tenths of a volts higher than normal is damaging to the agm's & I may be taking a risk of their longevity doing that.

What I really like about this set up is with them tied together you won't get a sudden shutdown of your inverter like if you were on lith only.
 
Why would you try to get the LFP cells to 3.5V for float? 3.4V is preferred.

AGMs rarely like to be floated above 27.6V and doing so shortens their life.
Agree.

I would just stick with the 24V AGM default bulk and float charge voltage settings, say:

Bulk charge 28.2V = 3.525V/cell LiFePO4
Float charge 27.0V = 3.375V/cell LiFePO4

When system drops to float mode, bugger all charge will head the lithium's way, while the lead will just keep sipping away as normal.

It's in my future to do something similar, although I expect I'll only add ~25% of the capacity current lead battery bank in the form of LiFePO4.

I see no point at this stage because I am not daily cycling my battery bank. But if/when that changes, then adding some lithium I reckon is a great idea. I even have the connection/fusing in situ ready to add an extra battery in a jiffy.
 
Could you explain in more detail has to how you are actually coming up with these numbers? For example your statement "lith pack took on the total inverter load til the voltage got down to about 25.6v"?

Also "I only draw no more than 35% off the agm's, they're charged before the lith's anyways".

What numbers are there to support this statement "it slowed down the the discharge rate of the lith's towards their end of cycle"?

Are all these based solely on voltage measurements? Are the two banks physically separated from each other to make measurements?
 
What I have mused about doing, rather than paralleling AGM and LiFePO4, is keeping AGM on battery inverters (Sunny Island) for surge loads, and putting LiFePO4 on separate AC coupled inverters, e.g. Sunny Boy Storage.
What this would do is cycle the LiFePO4 as needed to keep AGM at float, rather than cycling AGM at night and recharging when the sun comes up.
That is the concept, and I think Sunny Boy Storage does UL-1741-SA frequency-watts. But not sure if it responds to frequency shift for both quadrants, so it would source and sink on command.
 
Could you explain in more detail has to how you are actually coming up with these numbers? For example your statement "lith pack took on the total inverter load til the voltage got down to about 25.6v"?

Also "I only draw no more than 35% off the agm's, they're charged before the lith's anyways".

What numbers are there to support this statement "it slowed down the the discharge rate of the lith's towards their end of cycle"?

Are all these based solely on voltage measurements? Are the two banks physically separated from each other to make measurements?
I have current meters on both banks so I can see which bank is transferring how much current. The lith bank supplies 100% of the current to the inverter til it reaches 25.6v then the agm's start easing in. When that starts, it slows down the lith bank discharge 'cuz now both banks are sharing the load & consequently delays the speed the lith bank deteriorates.
 
What I have mused about doing, rather than paralleling AGM and LiFePO4, is keeping AGM on battery inverters (Sunny Island) for surge loads, and putting LiFePO4 on separate AC coupled inverters, e.g. Sunny Boy Storage.
What this would do is cycle the LiFePO4 as needed to keep AGM at float, rather than cycling AGM at night and recharging when the sun comes up.
That is the concept, and I think Sunny Boy Storage does UL-1741-SA frequency-watts. But not sure if it responds to frequency shift for both quadrants, so it would source and sink on command.
Since that 1st post, I've found the charger parameters needed some tweeking for an ideal balance. I've changed the absorption to 28v to keep the lith cells from going over 3.5v. The agm charge time increases as a result but still gets there & the float is at the agm's normal 27.6 which puts it at 3.45v per cell for the lith bank which I'm told is a pretty good value to be at.
Really tho, the best thing about this set up is no sudden kill of the lith bank by the bms. So once the bms does stut down the pack & the agm's are taking the full load, I have my inverter time set to shut down when about 1/3rd of it's capacity has been used.
The agm's take on the full load when the combined banks total voltage drops to about 24v. I have my bms kill the pack when the cells reach 3v.
 
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