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Old 22-08-2017, 00:05   #91
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Re: Hybrid Battery Bank Idea

As always - it depends on what you are looking for and what you are willing to pay for.

It is true, there are different products out there with various strategies.

Cheep 12V battery blocks with a balancing interface just have an additional connector socket besides + and - to connect to an external LFP charger/balancer interface and balance the cells occasionally using grid power - otherwise the cells are charged using the alternator and drift slowly. This is the "poor man BMS" solution, where the human factor and laziness mainly affects the life span of the battery.

There are top-side balancers out there, some even programmable, where you can setup a voltage level, when the balancer starts working. Mainly to prevent balancing and loss of energy when discharging or sleeping.

There are bottom-side balancers / equilizers, that balance when you discharge below a certain soc to get all cells level for the next recharge, but they are not so common.

There are also more expensive dynamic balancers that balance all the time while charging - they are more sophisticated and expensive devices, they measure the highest and the lowest cell and transfer the charge between them using different methods (switching high cap capacitors to transfer the charge, DC-DC- charger logic switched between the two cells with galvanic isolation, ...) so very little energy will be lost during balancing.

More expensive stuff with better software do this even on the discharge side too. Assume you have a battery pack of 16 cells - 700Ah each and one cell has become defective and can store only 500Ah. This cell will charge faster than the others to full (and will be balanced until all other cells get they full charge), then you discharge that battery. Having dynamic balancing on the discharge side you will get 600Ah or more out of the system (compared to the 400Ah without balancing), even one cell has only 500Ah capacity, because during discharge the capacity of the good cells will charge back the week cell until all cells reach their low level threshold, so you always have maximum usable capacity available.

You will be noticed by the system, that you'll need to replace the one cell soon, however your battery keeps delivering its full potential.

Regarding low temperatures. LiFePO4 should not be charged below 0C, LiFeYPO4 can be charged / discharged down to -45C. This is one reason why people want to use the LiFeYPO4 in live-aboard RV's / boats when living in freezing regions.

I've been not in touch with the Lithionics BMS yet, so I cannot comment on the advantages / disadvantages of it. May be some BMS do harm a Lithium battery if it is poorly designed, however a well designed and configured BMS can get most power out of your investment for a very long lifetime of the battery by looking after state of charge, managing charging currents and detecting cell problems.

A good BMS is a safety device - especially in high capacity / high voltage installations.
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Old 26-12-2018, 11:47   #92
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Re: Hybrid Battery Bank Idea

cross pollination LFP's Charging Voltage and State of Charge - Cruisers & Sailing Forums
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