Hi folks,
I think this should work.
I had a pair of Sealed 100AH Lead acids that provided power in parallel in a camper (giving 200AH). They were fed by a lead acid recharger.
After one of the pair started to lose a cell, it created a cascade failure of both, as I believe they ratcheted each other down to 3V. I’m now rebuilding it
So, I now have two new generic 100AH LiFePO batteries, and I’m embracing the Victron tech, with both a Blue Smart IP22 charger, a pair of SBP-65 smart battery protect units, (and an MPPT100|20 not shown). I would like some feedback on the following design:
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The theory behind it is that if one battery goes under voltage (10V) it will isolate that battery. If the other is still healthy, it should come back and the battery protect circuit should let me limp along.
I specifically chose the (3) model of the charger since it has three separate charging outlets and it allows the battteries to be set up independently rather than in parallel under one charger. Suggestions, improvements or warnings welcome
If they are generic drop in lithium batteries do they not have a BMS built in to each battery that will cut off the output from one battery on.low cell voltage. If they do then there is no need for the Battery Protect devices. Remember the battery protect only stops discharge on low battery voltage, the BMS should do it on cell voltage which occurs before total battery voltage is too low.
I would not use a SBP in this way.
Should 1 battery have a lower voltage, current will flow from the other higher voltage battery, through the SBP into the lower voltage battery.
Not allowed to have reverse current through a SBP.
Choose batteries that have a BMS that monitors cell voltages, batt over/under voltage cutout, and over current protection.
Then should you require a single SBP on the output of the properly paralled batteries ( see Wiring Unlimited ) if you want a LVCO before the batteries BMS trips out.
It is one charger, with one charge algorithm, the output current is split between it’s 3 outputs.
Useful if you charge separate battery banks. (it prevents current flowing from 1 battery to another battery)
Not so useful for parralled batteries.
Thanks Klim. I appreciate what you are saying here. I’ve already bought the generic batteries (without BMS). I’ll stick with them for the interim. However, I’m unsure if it will work like you say:
I think it will work as follows
As they are maintained in parallel, the voltage on both batteries should be the same.
Current will be drawn from both batteries approximately equally.
Should one battery in the pair degrade, the current load will become asymmetric, but remain in the in-out direction as the Voltage on both will decrease to the 10V SBP threshold.
At that point both SBPs will shut the connection to load.
I’m optimistic that the good battery will recover a bit and rise up to a recovery voltage (which I may have to decrease from 13V to 12). Either way, The faulty battery will still remain isolated, and I can then work on remediation.
This will be to recharge the good battery and remove tha faulty one.
As they are maintained in parallel, the voltage on both batteries should be the same.
Current will be drawn from both batteries approximately equally.
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No problems with those 2 points.
You could be unlucky and have a cell short out , then the other battery will be 3 volts higher, and the current will flow into the battery with the shorted cell, adding to the grief within that battery.
Once again, current will flow through the SBP, from the out terminal to the battery terminal. SBP manual.
Hmm,
Interesting. I actually now think they do have BMS (Himassi brand - surprising because of the price).
OK So I need to rethink this. And I need to rely less on storefront checking of a proposed design.
Maybe I’m just paranoid and can benefit from the simpler design (over-engineered did I hear you say?)
Thanks for the advice.
I’m now rebuilding it 
