Of Service to you, Professional Engineering Opinion on AGM vrs Lithium

[chrisboat]

Hi Terry,
So you saying that using 4 lithium in parallel even with method C is not really a good idea.
I definitely dont want a single lithium battery on my boat. I like the idea of more that one in case of failure.

So what is really the best solution?

Chris

[T1 Terry]

Originally Posted by chrisboat

Hi Terry,
So you saying that using 4 lithium in parallel even with method C is not really a good idea.
I definitely dont want a single lithium battery on my boat. I like the idea of more that one in case of failure.

So what is really the best solution?

Chris

Any chemistry battery linked in parallel is risking loosing all the batteries due to one battery failing and dragging the other batteries flat without you realising there was a problem.
By building the battery as a single unit, any one cell showing signs of failure will become apparently quickly if the cell voltages are monitored. If a cell group was seen to be steadily dropping voltage and becoming further and further out of balance, this would indicate a problem within this cell group. By simply disconnecting one cell at a time and leaving it for an hr or so, it will become clear which cell is pulling the group voltage low and that cell would need to be disconnected from the group and replaced when convenient. No total battery failure, just a drop in capacity until the failed cell can be replaced.
To me, that is the best solution, one battery built from smaller capacity cells in parallel providing redundancy if needed yet retaining the original battery set up. No need to drag out one battery and replace it with another or switch between batteries while trying to sort out what went wrong. Better to have a battery that indicates something is going south than one that suddenly dies when you least expected it or had the spare time to sort the problem.

T1 Terry

[hzcruiser]

Originally Posted by more

if you don't install proper active balancing (5-10A) battery doesn't last one season, a month maybe

Where did you get that misinformation from, I wonder?


Lots of people here, including me, have LFPs on board and they don't drift apart unless, as mentioned, there is a faulty cell. And this, as rare as it is, can be detected and mitigated without losing the rest of the batt capacity.

[T1 Terry]

As HZ Cruiser has said, out of balance is rare with a 4 cell in series 12v battery when using quality cells when the law of averages is put into play by linking multiple smaller capacity cells in parallel to create one cell the capacity you need, then link those in series. If you ever do need to rebalance for what ever reason, get a single cell 3.6v charger, 4 amps max is plenty, and charge the lowest cell till it reaches the same voltage as the highest cell. Only do this when the high cell is bouncing at the 3.6v mark and the BMS is controlling the charge, and only after the same cell has been regularly hitting the 3.6v charge cut limit, 3 days in a row minimum and more than 150mV differential between the highest and the lowest.
If this problem seems to occur regularly, try dropping the end of boost/bulk/absorption voltage down a bit and make sure the absorption cycle is less than 15 mins, then drops to 13.7v to 13.8v float and see if the problem settles ... only then do you need to go hunting for a bad cell and this will show up as the pack that is always the low one ..... if it changes from one pack to another, it isn't a cell failure problem but rather a charging regime problem


T1 Terry

[more]

Originally Posted by T1 Terry

As HZ Cruiser has said, out of balance is rare with a 4 cell in series 12v battery when using quality cells when the law of averages is put into play by linking multiple smaller capacity cells in parallel to create one cell the capacity you need, then link those in series. If you ever do need to rebalance for what ever reason, get a single cell 3.6v charger, 4 amps max is plenty, and charge the lowest cell till it reaches the same voltage as the highest cell. Only do this when the high cell is bouncing at the 3.6v mark and the BMS is controlling the charge, and only after the same cell has been regularly hitting the 3.6v charge cut limit, 3 days in a row minimum and more than 150mV differential between the highest and the lowest.
If this problem seems to occur regularly, try dropping the end of boost/bulk/absorption voltage down a bit and make sure the absorption cycle is less than 15 mins, then drops to 13.7v to 13.8v float and see if the problem settles ... only then do you need to go hunting for a bad cell and this will show up as the pack that is always the low one ..... if it changes from one pack to another, it isn't a cell failure problem but rather a charging regime problem


T1 Terry

Or work in the car lithium battery test facility.
do you now for example always go in de-Balans cell close to - pool of battery,
do you know is very important grease in bolt/connection for the cell.
do you know cells can go in de-Balans by charging and discharging?

[hzcruiser]

Originally Posted by T1 Terry

As HZ Cruiser has said, out of balance is rare with a 4 cell in series 12v battery when using quality cells when the law of averages is put into play by linking multiple smaller capacity cells in parallel to create one cell the capacity you need, then link those in series. [...]

Parallel linking the cells first, then putting 4 of such blocks in series to get nominal 12V would present an arguably bigger disadvantage IF one cell goes bad: it would still pull down the other cells that are linked in parallel which causes all the batts to deteriorate until it is being noticed by the user.

To my knowledge there is no BMS that can differentiate between an acceptable (or 'normal') balancing current and one caused by a faulty cell.
Hence I'm not applying such a setup nor do I recommend it.

Either way, this is already getting off topic and there is another thread that covers this detail, see below:

https://www.cruisersforum.com/forums...ml#post3285362


I'll add a screenshot to the latter to explain it a bit more.

[T1 Terry]

Originally Posted by hzcruiser

Parallel linking the cells first, then putting 4 of such blocks in series to get nominal 12V would present an arguably bigger disadvantage IF one cell goes bad: it would still pull down the other cells that are linked in parallel which causes all the batts to deteriorate until it is being noticed by the user.

To my knowledge there is no BMS that can differentiate between an acceptable (or 'normal') balancing current and one caused by a faulty cell.
Hence I'm not applying such a setup nor do I recommend it.

Either way, this is already getting off topic and there is another thread that covers this detail, see below:

https://www.cruisersforum.com/forums...ml#post3285362


I'll add a screenshot to the latter to explain it a bit more.

This is the reason why continuous active balancing can be more of a curse than a help, it covers up issues in the battery pack.
The only difference between 4 x 100Ah cells in parallel and 1 x 400Ah cell, is the links between the 100Ah groups are on the outside, not inside the plastic case where you can't do anything about a problem that surfaces.
One bad cell or cell connection a parallel 4 x 100Ah cell x 4 in series (4P4S) will pull the 4 x 100Ah = 400Ah cell down exactly the same as a 400Ah cell with the connections on the inside. The difference is, you can do something about it with the connections on the outside, you can't with the connections on the inside.

T1 Terry

[T1 Terry]

Originally Posted by more

Or work in the car lithium battery test facility.
do you now for example always go in de-Balans cell close to - pool of battery,
do you know is very important grease in bolt/connection for the cell.
do you know cells can go in de-Balans by charging and discharging?

Ummm....? Electric vehicles in mass production do not use LiFeP04 cells, they are not the best weight/capacity cells on the market, just one of the safest. LTO is even worse in the Wh/kg scale, but they are the choice for pacemakers and nerve block stimulators etc in the medical field because they suit that job the best. Horses for courses. LFP, LYP and LTO cells do not go out of balance by charging or discharging .... as long as you stay within the safe parameters. The cells will go out of balance if you push the voltages outside their comfort zone or attempt to hold them on a float charge that is higher than 3.45v per cell, the cell with the lowest internal resistance and/or least capacity, will always suffer voltage run away .... but that is covered in my point about a poor charging regime. Basically, if you have cell balance problems with a 4 cell in series LFP or LYP battery, you are doing something wrong.

T1 Terry

[more]

Originally Posted by T1 Terry

Ummm....? Electric vehicles in mass production do not use LiFeP04 cells, they are not the best weight/capacity cells on the market, just one of the safest. LTO is even worse in the Wh/kg scale, but they are the choice for pacemakers and nerve block stimulators etc in the medical field because they suit that job the best. Horses for courses. LFP, LYP and LTO cells do not go out of balance by charging or discharging .... as long as you stay within the safe parameters. The cells will go out of balance if you push the voltages outside their comfort zone or attempt to hold them on a float charge that is higher than 3.45v per cell, the cell with the lowest internal resistance and/or least capacity, will always suffer voltage run away .... but that is covered in my point about a poor charging regime. Basically, if you have cell balance problems with a 4 cell in series LFP or LYP battery, you are doing something wrong.

T1 Terry

LiFeP04 is still in use for bike, industry DIY project, golf car, and similar

[goboatingnow]

Battery imbalance is very much a function of C charging and discharging. EVs are a very punishing application , boats are typically <1C applications

In this scenario , LiFeP04 cells tend to converge not diverge

Recommendations from one type of usage conditions don’t reliably transfer to other situation