LiFePO4 in Parallel but Semi Separate

[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?

[CarlF]

The only issue is that the start battery will be charged using the Lithium charge voltages. The absorbtion voltage will be likely be a little lower and the absorbtion time will be much shorter than recommended for the start battery (especially if they are AGM). If you have an alternator on the start battery side of the FET then this isn't a concern.

But I've been doing this for years and the start batteries continue to show no damage from undercharging. I believe that's because the start battery is never deeply discharged because the engine starts very quickly. Even better the Lithium's bank is almost always between 13.1v and 13.5 (its float) so the start battery is always in float.

My start batteries are not AGM but old fashioned flooded. I did that because those batteries are not picky about charge profile as they come from a time when car charging systems were very crude. They are also cheap. So if the undercharging results in the start battery lasting 5 years instead of 7 it's not a big deal.

[smac999]

They probabbly wouldn't produce much of the amps. especially as the start battery will always be 100% full from the house. would depend on wire lenghs and sizes etc of the fet wiring.

the fet is probaboly only rated for 100-200 amp so that would be the concern before the 400a house bank fuse blew.

you could probably measure the house assist current with a clamp meter while starting

[sailorboy1]

I know for a fact that enough current will go through that FET to start a engine with no start battery and the fuse doesn't blow.

[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?

Been 1.5 weeks and probably 10 engine starts and the above doesn't appear to be an issue in case someone reads thread during research

[CaptainRivet]

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?

The manual stating not to use for start means the BMS current limter=switch off are mosfets which get fried by inrush or surge current.
A starter produces both, surge when starting from 0 typically 3-4 times continous for up to 300ms and inrush when its getting stoped by pull back of the actuator. We are talking about 400-800A for up to 300ms....
Having 4x100A your BMS can do 400A cont and most likly 800A shorttermed which is enough to use for a 2000W starter (tyical for leisure engines below 100hp) that saved your BMS till now...how long??? eg higher surge because of diesel pest and engine having hard time starting could possibly fry it. Would you have only 2x100A the BMS would most likely already been fried by your 2000W starter...

the tpical way this is seperated are by an relay or speration diode like ArgoFet. which means starter runs only of the starter battery but not the house.

[Baronkrak]

Originally Posted by CaptainRivet

Having 4x100A your BMS can do 400A cont

Maybe not.
https://www.centurybatteries.com.au/...ium-Manual.pdf

“ Parallel batteries can only increase the capacity of the battery system and increase the working time of the connected electrical devices. However, the maximum or peak discharge current can only be in accordance with a single battery (refer to the individual product spec sheets). “

[CaptainRivet]

Originally Posted by Baronkrak

Maybe not.
https://www.centurybatteries.com.au/...ium-Manual.pdf

“ Parallel batteries can only increase the capacity of the battery system and increase the working time of the connected electrical devices. However, the maximum or peak discharge current can only be in accordance with a single battery (refer to the individual product spec sheets). “

that is specific for this drop in.
in general if you parallel 2 batterie you split current in half, with 4 a quarter. But as i said maybe yes or no as he have to check this for his drop ins. and that it di work now is no guarantee that it will work in the future, especially if an event comes up where the engine hassles to start and current goes up.

[sailorboy1]

it works for any battery, it may not be a perfect balance, but it still works

[CaptainRivet]

Originally Posted by sailorboy1

it works for any battery, it may not be a perfect balance, but it still works

nothing to do with balance. the problem are not the cells, the problem are the mosfets in your drop in BMS which potentially cannot withstand big surge or inrush current that a starter creates. It's stated in the manual so you need to figure out why...

THe question is now how your drop in and their BMS internally are set up. There are BMS like the centaury posted were you don't get double current when paralleling, just 0% more discharge and peak current draw on the BMS but you get double capacity.

As its stated not to use as starter battery so your BMS has limitations here, so you need to figure out which and if 4 BMSs in the 4 batteries in parallel can share this surge load or not. your cells can, your BMS???

and that it did work till know doesn't mean you are safe and not frying your BMS=dead drop in...

[CaptainRivet]

Originally Posted by Baronkrak

Maybe not.
https://www.centurybatteries.com.au/...ium-Manual.pdf

“ Parallel batteries can only increase the capacity of the battery system and increase the working time of the connected electrical devices. However, the maximum or peak discharge current can only be in accordance with a single battery (refer to the individual product spec sheets). “

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.

[sailorboy1]

My drop in LFP say they can do 280a/each for 5 sec. But since I had enough concern to ask I went ahead and redid the FET isolator so the house does not feed the start loads.

[mikereed100]

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

[SV Confianza]

Originally Posted by mikereed100

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

We have 500ah battleborn and run a lewmar v5 windlass no problem.

Our setup uses a stay alive relay to parallel the house and the start when the ignition is on. The theory came from Rod from compass marine who said that the start batteries are almost always full, and the charge voltage for the lithium is basically what float voltage is on the start.

There are more technically correct ways to do it, but ours works.

[sailorboy1]

Originally Posted by mikereed100

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

I can not anymore as the windlass is on the start circuit and I separated it from the house now. 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. When we bring our anchor up that windlass probably sees 10 "starts"