LiFePO4 in Parallel but Semi Separate

[ThereAndBack]

Originally Posted by mikereed100

Just curious. Are you able to run your windlass off your LFPs?

I run a lot of stuff off our LFP batteries. We have 4 x 100ah@12v Battleborn batteries in parallel.

Regular "heavy" loads:
- Watermaker HP pump ~ 1600w (through inverter)
- Hot water kettle AC current ~ 1800w (through inverter)
- Electric winch on deck ~ 120 amps @ 12v
- Lofrans Project 1000 windlass - 1000+w @ 12v


Hasn't bothered the batteries, we've had them for 5 years now. I don't really worry about anything that is going to pull less than 200 amps at 12v.

[Baronkrak]

Originally Posted by sailorboy1

I would think that if the LFPs aren't supposed to be used as a start battery surely they can not be used on the windlass.

It does not mean that LFPs drop In cannot assist a start battery.

[Baronkrak]

Originally Posted by ThereAndBack

We have 4 x 100ah@12v Battleborn batteries in parallel.

Are these LFP batteries drop In?

[mikereed100]

I don't want to hijack this thread so have started another on this subject here:
https://www.cruisersforum.com/forums...ml#post3730191

[Baronkrak]

Originally Posted by mikereed100

Just curious. Are you able to run your windlass off your LFPs?

Indirectly yes he does.

Originally Posted by sailorboy1

..I would like to keep that as it maintains the start battery charged’'.

By the way, heavy work with a windlass may involve more than LFPs.

[sailorboy1]

Originally Posted by Baronkrak

Indirectly yes he does.

i don't anymore

[ThereAndBack]

Originally Posted by Baronkrak

Are these LFP batteries drop In?

Yes, battleborn 100ah 12v x 4. They are 5 years old and still work fine. I'm a liveaboard and run off solar/generator, no shore power.

[wholybee]

Originally Posted by CaptainRivet

in this case discharge rate of one battery (100AH typically max 100A and peak 200A), even you have 4.
thats an example were your starter creating 300-400A surge for 100ms would fry the BMS in the drop in.

I don't think that is true. I think that language is included to make support easier when people start asking complex questions. And/or, to cover the case if a single BMS cuts out, then the other does see full current.

It is a fact that if you have 2 batteries (each with it's own built in BMS) that the current will be split, maybe not exactly even, but it will be spilt. Each individual BMS will only see about half the total current. There just isn't anything going on (as long as both BMS's are working) that would limit the total safe current to that of a single BMS.

Also, it is generally a myth that starting currents will fry a BMS. (or windlass currents etc.) On very a cheap POS BMS maybe. But FET's are VERY fast. The damage is caused be heat. Heat requires both current and time. A BMS is fast enough to detect excessive current and open the circuit before damaging heat is generated. Real world example, I have tested my BMS with a direct short, several thousand amps. It shut off and protected itself, no damage. So, if a BMS is not capable of starting an engine, the most that should be expected is that it will cutoff due to excessive current, and not start the engine.

[sailorboy1]

Originally Posted by sailorboy1

I am installing 400aH of LFP next week. Each has continuous discharge rating of 100a for a total of 400a (would blow my fuses). The manual says not to use the batteries for a start battery.

My current arrangement is that the house and start batteries are separated by a FET isolator. The wring is arranged so the house can feed into the start, but the start can not feed the house circuit other than "leakage". I would like to keep that as it maintains the start battery charged.

Does this present an issue to the LFP batteries that they share an engine startup?

I decided to do what I should have done at start, ask the battery manufacturer.

They say it is OK so maybe I will change it back to way it was as the windlass ran better that way as it also is off the start battery circuit.

[sailorboy1]

Originally Posted by sailorboy1

I decided to do what I should have done at start, ask the battery manufacturer.

They say it is OK so maybe I will change it back to way it was as the windlass ran better that way as it also is off the start battery circuit.

I pushed back some on their reply and got"

"Yes, like you said. The lithium battery bank will output current if the starting battery starts have power drops. But the starting process will only take few seconds for the starting battery to power, so as long as the starting battery is fully charged, it should be ablt to start the engine just fine. For a 12.8V400Ah battery bank, the maximum continuous output will be 400A max, so maximum power of 5120W. So what is the rated power/current for your windlass? If the amp draw to the battery bank exceeded the maximum current the BMS can handle, the BMS will cut the battery off for protection. Best wishes, Your friend Mike From Power Queen"

[sailorboy1]

So I changed back to my original because the windlass really liked having the house bank to supplement the start circuit. The windlass was very noticeably slower off just a G27 start battery and the engine only supplying a small amount of current while dropping/raising the anchor.

[smac999]

Originally Posted by CaptainRivet

in this case discharge rate of one battery (100AH typically max 100A and peak 200A), even you have 4.
thats an example were your starter creating 300-400A surge for 100ms would fry the BMS in the drop in.

that makes no sense. wire equal lengh jumpers and you can draw the max from each battery. 400 total.

they probabaly just add that disclaimer to protect themselves from bad wiring jobs

[Baronkrak]

Originally Posted by smac999

that makes no sense. wire equal lengh jumpers and you can draw the max from each battery. 400 total. ...
.....

Originally Posted by noelex 77

Also consider that two batteries in parallel will not share the load exactly evenly so you cannot simply double the maximum draw available from a single bms.

So you disagree with the above statement?

[noelex 77]

Originally Posted by Baronkrak

So you disagree with the above statement?

Yes, I do disagree.

Equal length wiring helps, but you can never equalise all factors. For example, the many connections involved especially if you include, as you should do, all the bus bars internal to the battery. Even if you could ensure perfect and uniform connections, they are not likely to stay this way in a salt water environment subject to movement/vibration. There are also other factors such as the changing internal resistance of battery itself.

It is difficult to quantify these effects, but I would not be depending on the total output of the battery bank to simply be the sum of the maximum output each battery is capable of delivering.

Once a single battery reaches the limit where the BMS shuts down, there will be an increase current demand on the remaining batteries producing a cascade failure. This will shut down the whole battery bank. I think some reserve capacity should be incorporated.

How much reserve is needed will be primarily determined by the properties of the BMS. Does the BMS cut out at exactly the specified maximum current (unlikely) or has the battery manufacturer already incorporated a reserve, and if so how much?

This could be determined experimentally for your particular BMS.

[sailorboy1]

Originally Posted by sailorboy1

I am installing 400aH of LFP next week. Each has continuous discharge rating of 100a for a total of 400a (would blow my fuses). The manual says not to use the batteries for a start battery.

My current arrangement is that the house and start batteries are separated by a FET isolator. The wring is arranged so the house can feed into the start, but the start can not feed the house circuit other than "leakage". I would like to keep that as it maintains the start battery charged.

Does this present an issue to the LFP batteries that they share an engine startup?

So I spent 4 months anchored in Bahamas/Florida with this arrangement. Pulled the anchor up MANY times including some aganist wind/current. Never any problem and the windlass worked a lot better in this arrangement than when I decided to rewire so only the start lead acid battery could operate the windlass.

Just in case someone asks a similar question.