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Design review for Hybred FLA / LFP system

danphillips

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Oct 29, 2019
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Please give this design a review for any mistakes / issues. I have 12 CALB cells (3p4s) to add to my existing FLA cells.

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Quoted from www.zwerfcat.nl

While discharging, the lithium batteries stay above 13.0 Volt until they are almost empty. The lithium voltage is higher than the voltage of a lead-acid batttery under load, so the lead-acid battery will hardly deliver any current, if anything at all.

While charging, the voltage quickly rises to about 13.4 Volt, a voltage where the lithium batteries absorb all the available current but a too low voltage for lead-acid batteries to meaningfully charge. So the lithium batteries take up all the current until the BMS takes them offline, and only then the voltage rises enough to charge the lead-acid batteries.
 
I don't see any reason why the LFP should cut out. FLA needs to go to at least 14.4V, and LFP enjoys a full charge there as well. Charging LFP to 13.4 won't get them fully charged.

You'll need a float of 13.6V or the LFP will discharge floating the FLA.

Put the LFP in parallel with the 12V, limit peak voltage to 14.4V, set float to 13.6V, set low cut-off to 11.6V.

BMS and top balancing mandatory.
 
I don't see any reason why the LFP should cut out. FLA needs to go to at least 14.4V, and LFP enjoys a full charge there as well. Charging LFP to 13.4 won't get them fully charged.

You'll need a float of 13.6V or the LFP will discharge floating the FLA.

Put the LFP in parallel with the 12V, limit peak voltage to 14.4V, set float to 13.6V, set low cut-off to 11.6V.

BMS and top balancing mandatory.
So you're saying no LVD or HVD contactors? Essentially, you'd just use the BMS for balancing and monitoring?
 
Yes but I was responding to your comment. The article says the bms takes the LFPs offline and you said "I don't see any reason why the LFP should cut out". So I was asking you for clarification.
Did you read the article?
 
The two batteries can be charged together properly. The quoted text is not true in most cases unless the FLA/AGM needs to be charged above 14.6V.
 
The two batteries can be charged together properly. The quoted text is not true in most cases unless the FLA/AGM needs to be charged above 14.6V.
Right or if for any other reason the banks' voltages come out of sync. Like say if the BMS took the LFPs offline and the charger continued to charge the FLAs up to 14.6V. In either case, wouldn't you then need to rebalance them before being able to continue on as normal. Does it seem like I'm on the right page here? lol I'm just trying to get my head around this. I like simple but this seems so simple it's complicated haha
 
It's more simple than it is complicated. That article essentially complicates it unnecessarily.

I'm not sure I follow you about "rebalance them"
 
Both battery banks would have to be "balanced" (at the same voltage) before being connected. So every time there is a voltage difference between the LFP and FLA banks, wouldn't they have to be rebalanced?
 
If either one goes offline and charging or discharging continues the voltages will become out of sync.
So, you can't just connect them again without making sure they are the same voltage.
If the BMS took LFP down it would probably be dangerous it the recovery turned it back on.
 
If either one goes offline and charging or discharging continues the voltages will become out of sync.
So, you can't just connect them again without making sure they are the same voltage.
If the BMS took LFP down it would probably be dangerous it the recovery turned it back on.
Yes, this is what was trying to say. Someone else on Youtube was suggesting the use of a resistor between the two banks to prevent the LFP from sucking too much, too fast from the FLA but he didn't elaborate on the size of resistor.
 
Ah. Part of the reason the system should be design that they're never disconnected. Better yet to not do it.

Important you read the whole article:

 
Ah. Part of the reason the system should be design that they're never disconnected. Better yet to not do it.

Important you read the whole article:

But I'm not talking about disconnecting them, I'm talking about when the BMS takes the LFP's offline. Doesn't that, in effect, disconnect them from the FLA's?
 
In practice, that shouldn't happen because one has designed the system to operate within cell limits. Even if it does, merely set the reconnect voltage to 12.8V as they'll never settle to that naturally, and the reconnect process should be done manually.
 
In practice, that shouldn't happen because one has designed the system to operate within cell limits. Even if it does, merely set the reconnect voltage to 12.8V as they'll never settle to that naturally, and the reconnect process should be done manually.
So then is the author's (Zwerfcat) opensource BMS a requirement because it can see the FLA bank?
 
Both battery banks would have to be "balanced" (at the same voltage) before being connected. So every time there is a voltage difference between the LFP and FLA banks, wouldn't they have to be rebalanced?
Just use a charge source on the FLA to bring it up to the rested LFP voltage.
 
In Clark’s video -
he explains how he runs LA & lipo4 At the same time

but for those of us that are not full time cruising, we need to be able to disconnect.
 
In Clark’s video -
he explains how he runs LA & lipo4 At the same time

but for those of us that are not full time cruising, we need to be able to disconnect.
Yes, well we are full time aboard and remote. My system was operating okay up until the other day when my Stirling B2B charger failed. I was never really happy with it anyway, as it wasn't charging the LFP's at a high enough rate.
So at this point, I need to be able to reconfigure my system with what I have, as I have no way of getting a replacement for my BB1260.
I will have a look at this video.
Thanks
 
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