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Old 11-12-2018, 14:34   #46
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Re: Can I parrallel different ah lifepo4

Quote:
Originally Posted by CatNewBee View Post
let the bms do his magic.
Could you possibly state what you mean more specifically, from a functional POV?

> The setup will work as long as the cells are in balance
You mean within a given series pack, right?


> If one of the smaller cells fails by a shortcut
Do you mean "shorting out" as in across terminals, or maybe shorting internally?
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Old 11-12-2018, 14:46   #47
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Re: Can I parrallel different ah lifepo4

Quote:
Originally Posted by john61ct View Post
Could you possibly state what you mean more specifically, from a functional POV?

> The setup will work as long as the cells are in balance
You mean within a given series pack, right?


> If one of the smaller cells fails by a shortcut
Do you mean "shorting out" as in across terminals, or maybe shorting internally?
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Old 12-12-2018, 01:30   #48
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Re: Can I parrallel different ah lifepo4

Quote:
Originally Posted by john61ct View Post
Could you possibly state what you mean more specifically, from a functional POV?

> The setup will work as long as the cells are in balance
You mean within a given series pack, right?


> If one of the smaller cells fails by a shortcut
Do you mean "shorting out" as in across terminals, or maybe shorting internally?
I mean, IF all the cells are healthy with no major issues regarding capacity and resistance, you can parallel cell wise. The paralleled cells will be recognized by the BMS or cell module as a single cell and monitored, the SOC of the parallelled cells will be equal in percent as the voltage is equal and he current is distributed across the cells to a ratio corresponding to their capacity ratio. There will be no overcharge or undercharge.

A single cell failure like capacity drop / high resistance will cause imbalances, so the paralleled cell will have to contribute more current and will wear out quicker and the total capacity will decrease down to the level before paralleling, but the BMS will prevent overchage or low voltage - so still safe to operate.

A single cell short is the other possibility that means low resistance. In this case the energy stored in this cell will discharge over this short - what is considered in the design, but also all the additional energy of the paralleled cells will discharge over this short - what will cause the cell heat up and expand and blow the safety valves, what cannot be considered by the cell dessign. So the reaction may be heftier and more damage tobthe paralleled cells can occure. Also the BMS cannot prevent voltage drop below safe thresholds - it is internal, a disconnect of the loads will not help.

That is one reason when dessigning (massive) parallel - serial configurations to protect each cell by a cell fuse.
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Old 12-12-2018, 08:02   #49
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Re: Can I parrallel different ah lifepo4

OK thanks for clarifying.

I am personally more interested in the scenario of rotating different sized mobile powerpaks in and out for charging with the larger / main bank.

I don't feel comfortable using raw wire gauge as a current limiter, and other than Blue Sea's discontinued CL-series VSR, haven't found suitable OTS gear to do so.

Wear and tear on even HD switches may also be an issue?
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Old 13-12-2018, 11:50   #50
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Re: Can I parrallel different ah lifepo4

M. Bartosh wrote:

...Assuming ideal conductors (i. e. zero resistance) the potential at both batteries' poles will always be identical, and the charger will "see" one single "resulting" battery.

Assuming ideal conductors (i. e. zero resistance) the potential at both batteries' poles will always be identical, and the charger will "see" one single "resulting" battery.

--------
This is physics. I agree. Try connecting one with a skinny wire that limits amps..it will get hot and won't charge as fast. Just like water.
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Old 13-12-2018, 14:00   #51
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Re: Can I parrallel different ah lifepo4

Quote:
Originally Posted by rgleason View Post
M. Bartosh wrote:

...Assuming ideal conductors (i. e. zero resistance) the potential at both batteries' poles will always be identical, and the charger will "see" one single "resulting" battery.

Assuming ideal conductors (i. e. zero resistance) the potential at both batteries' poles will always be identical, and the charger will "see" one single "resulting" battery.

--------
This is physics. I agree. Try connecting one with a skinny wire that limits amps..it will get hot and won't charge as fast. Just like water.
well, the same skinny hot wire issue will also happen with a single battery, so what is the message???
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Old 13-12-2018, 20:44   #52
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Re: Can I parallel different ah lifepo4

"well, the same skinny hot wire issue will also happen with a single battery, so what is the message???"

The question was about paralleling batteries of different AH but the same chemistry.

This example shows one way that two paralleled batteries/banks being charged might have different states of charge and the amperage does not flow to the battery that is not fully charged and will take a longer time to charge.

However under normal circumstances, when one battery has less capacity, and one of the batteries becomes (or is) fully charged, the battery resistance to this battery rises and therefore amps from the charger flow into the other battery which is not yet charged (provided the cables and connections are adequate).
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Old 14-12-2018, 00:39   #53
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Re: Can I parallel different ah lifepo4

Yes, as long as all cells are healthy, no problem. If vou parallel not just the cells, but the whole batteries in a 4S2P configuration, each battery needs its own BMS, because the cells are independent of each other, also battery fuses on the plus poles would be a good idea before paralleling.

You can use the same solenoids for over voltage protection and under voltage protection by both batteries, but will need some logic to combine the signals from both BMS to interface the power switches. Internal battery faults will disconnect the loads or the chargers, but will not protect the batteries from each other in a failure situation (dangerous on cell failures! single cell failure can destroy both batteries)

Another (safer) option would be to have a separate solenoid per battery connected to the respective BMS to fire up independently and isolate a faulty battery string before joining them to parallel. This battery solenoid must than handle both signals (over voltage protection and under voltage protection) of that particular battery BMS, because than the batteries do not have separated buses for charging and discharging any more (cross-charging) - so same combination logic as above would be necessary on each BMS, but with different wiring.
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Old 14-12-2018, 06:54   #54
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Re: Can I parallel different ah lifepo4

The affirmative answers above still apply even if you have no BMS-as-a-thing, but just a DIY collection of components providing whatever level of "BMS functionality" you feel is required for your situation.
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Old 15-12-2018, 16:01   #55
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Re: Can I parallel different ah lifepo4

CNB wrote:
Another (safer) option would be to have a separate solenoid per batteries connected to the respective BMS to fire up independently and isolate a faulty battery string before joining them to parallel.

-----
Yes, kind of like standalone trojan trillium batteries wired in parallel, each with own bms and disconnects.
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Old 13-12-2020, 16:23   #56
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Re: Can I parrallel different ah lifepo4

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Originally Posted by thomm225 View Post
So are you saying the larger and smaller batteries will then charge and discharge at the same rate?

The point is, it's not good to connect batteries of different sizes in parallel

I got away with it for a few years but due to the different sizes, the battery(s) always charged up much faster than they did after I put two of the larger ones in parallel. The larger battery failed a couple years early than the smaller one did, and the larger battery was a year younger

How did it work the last time you tried it?

Please don't extrapolate your particular one-off experience to an over-generalised and incorrect statement.



If you had mentioned from the beginning that this was your experience, it would have shed a different light on it.



The conclusions you draw are not in line with electrical engineering basics. E.g., as mentioned, the charge controller doesn't know how big or how many or how old the batts are, and it doesn't care. It looks at voltages and currents.



Simply stated: The smaller battery cannot magically raise the bus voltage because it's full while the bigger batt gets "left behind" as you said. The larger battery dictates the bus voltage because it has the lower impedance (unless its faulty). The bus voltage only raises when all the batts increase their terminal voltage.
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Old 17-12-2020, 18:11   #57
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Re: Can I parallel different ah lifepo4

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Originally Posted by CatNewBee View Post
Yes, as long as all cells are healthy, no problem. If vou parallel not just the cells, but the whole batteries in a 4S2P configuration, each battery needs its own BMS, because the cells are independent of each other, also battery fuses on the plus poles would be a good idea before paralleling.

You can use the same solenoids for over voltage protection and under voltage protection by both batteries, but will need some logic to combine the signals from both BMS to interface the power switches. Internal battery faults will disconnect the loads or the chargers, but will not protect the batteries from each other in a failure situation (dangerous on cell failures! single cell failure can destroy both batteries)

Another (safer) option would be to have a separate solenoid per battery connected to the respective BMS to fire up independently and isolate a faulty battery string before joining them to parallel. This battery solenoid must than handle both signals (over voltage protection and under voltage protection) of that particular battery BMS, because than the batteries do not have separated buses for charging and discharging any more (cross-charging) - so same combination logic as above would be necessary on each BMS, but with different wiring.

wouldn't the fuse protect the batteries..unless I'm not understanding how
you have them connected. but battery1 plus-fuse1-(system output)-fuse2-battery2 plus. doesn't BMS on each battery protect it from overload.
now let's assume that Battery1 has a cell short(likely or not). now we have a large voltage difference between both bank.shouldn't this blow at least one of the 2 fuses?
am I missing something? I'm trying to move to LFP
my boat already has 2 house banks. I'm just trying to keep for redundancy(not extra ah)


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