LiFePO4: what voltage?

[kcj]

I converted to LFP batts two years ago. 600AHs in a 6p4s. Principal charge source is solar, with genset, alternators, and shore power to supplement as necessary.

I manage the bank by keeping the cell voltage between the knee walls- 3.45 to 2.9v per cell. Solar controller set to 13.8v, with a 13.2 float. High-low cell delta is less than 30mV when in this range. This delta rises fairly quickly when beyond the range.

I am in the process of testing my cell level OverVoltageProtection. Using a celllog 8, via relays, to open the charge bus contactor. Happy so far with this setup. Some questions came up....

1) when the charge (@60-150A) is cutoff/terminated the per cell voltage quickly drops 0.1v. Is this normal?

2) Which is the more relevant kneewall voltage (for setting the cutoff voltage)—the during charge (higher) or the post charge (lower) voltage?

Thanks in advance for your thoughts
Cheers
John

[john61ct]

Originally Posted by kcj

3.45 to 2.9v per cell

is good, not just resting but while actively cycling

> 13.2 float

Better to isolate if that's for long periods.

> 1) when the charge (@60-150A) is cutoff/terminated the per cell voltage quickly drops 0.1v. Is this normal?

Yes, in fact as they rest isolated will drop further, or a little discharge will speed that up.

> 2) Which is the more relevant kneewall voltage (for setting the cutoff voltage)—the during charge (higher) or the post charge (lower) voltage?

At the top the cutoff should be a little higher than you expect to get.

Say 3.47V , and if needed can back the bank charge level from 13.80V down to 13.78 or even lower if needed,

play it by ear, not losing any significant capacity.

That delta will need to be a little higher if you bottom balanced initially.

[nebster]

Originally Posted by john61ct

> 13.2 float

Better to isolate if that's for long periods.

I'll have to continue to disagree here, because I think this is unsubstantiated rumor. There is scant evidence that low float voltages have any deleterious impact on LFP.

On the other hand, isolating your battery every time it's charged up can have a substantial impact on your system's design and/or how you live on your boat.

So, I would say, no, it is objectively not "better," for many users in many scenarios.

[nebster]

Originally Posted by kcj

I am in the process of testing my cell level OverVoltageProtection. Using a celllog 8, via relays, to open the charge bus contactor. Happy so far with this setup. Some questions came up....

So, to be clear, are we talking about establishing an emergency high voltage disconnect threshold, or is this for a day-to-day charge termination threshold?

1) when the charge (@60-150A) is cutoff/terminated the per cell voltage quickly drops 0.1v. Is this normal?

Sure. It'll actually drop somewhat different amounts depending on its SOC at the time and the rate you were charging prior to stopping. It can hard to see those differences with a precision of one-tenth of a volt, though.

2) Which is the more relevant kneewall voltage (for setting the cutoff voltage)—the during charge (higher) or the post charge (lower) voltage?

You'll want to use the during-charge voltages.

Is your pack bottom-balanced or top-balanced? (It sounds kind of like you just "mid-balanced," a.k.a didn't balance at all, which might be okay but is sub-optimal.) Are you using an active balancing BMS?

Is your charge bus separate from your load bus?

Will you be generally present when the pack is in use/charging, or does this thing have to stay safe when unattended for long periods?

[travellerw]

Originally Posted by nebster

I'll have to continue to disagree here, because I think this is unsubstantiated rumor. There is scant evidence that low float voltages have any deleterious impact on LFP.

On the other hand, isolating your battery every time it's charged up can have a substantial impact on your system's design and/or how you live on your boat.

So, I would say, no, it is objectively not "better," for many users in many scenarios.

I agree.. That is not an option as a true live aboard.. 13.2v or 13.3v is what I find most boats using as float (AT THE BATTERY TERMINALS, not at the solar controller).

[nebster]

Originally Posted by travellerw

AT THE BATTERY TERMINALS, not at the solar controller

+1, everything needs to be at the terminals. And preferably calibrated/offset-adjusted against a reference-grade voltmeter at the start, since readings do vary a bit across devices.

[kcj]

Thanks for insights!

Good to here drop is normal.

The OVP in question is the ‘emergency/backup’ disconnect. But this is the only cell level protection.

the OVP voltage is measured at the terminals. I have been getting advanced courses in charging— while discovering the limitation of traditional chargers. Mine do not have separate voltage sense circuits. Which makes any profile subject to the voltage drop of the circuit. A big deal at higher charging level. Preset profiles are for ‘illustration purposes only’. 😁

The solar is set at 13.8, with float at 13.2. Alternator has a balmar 614 regulator set the same as the solar.

all cells are well balanced up thru 3.45v (delta is only 40-50mV). Do I need to worry about top balancing past that?

[rgleason]

Curious,
No BMS, no pcm balancers?
Do the 4p cells all self-balance effectively?
I thought 3p was the most you should go.
What battery cell, manf, size etc?

[john61ct]

Originally Posted by kcj

Thanks for insights!

Good to here drop is normal.

The OVP in question is the ‘emergency/backup’ disconnect. But this is the only cell level protection.

the OVP voltage is measured at the terminals. I have been getting advanced courses in charging— while discovering the limitation of traditional chargers. Mine do not have separate voltage sense circuits. Which makes any profile subject to the voltage drop of the circuit. A big deal at higher charging level. Preset profiles are for ‘illustration purposes only’. [emoji16]

The solar is set at 13.8, with float at 13.2. Alternator has a balmar 614 regulator set the same as the solar.

all cells are well balanced up thru 3.45v (delta is only 40-50mV). Do I need to worry about top balancing past that?

Not as long as they stay OK at the normal top level you use.

[john61ct]

Originally Posted by rgleason

Curious,
No BMS, no pcm balancers?
Do the 4p cells all self-balance effectively?
I thought 3p was the most you should go.
What battery cell, manf, size etc?

What is a pcm balancer?

Series cells don't self balance, only parallel.

Many people go over 3 strings, many are not aware it can cause balance problems.

[rgleason]

Originally Posted by john61ct

What is a pcm balancer?
Series cells don't self balance, only parallel.

Many people go over 3 strings, many are not aware it can cause balance problems.

PCM "Protection Circuit Module"
sorry for the Acronym daze, perhaps TAIFP (turn about is fair play)?

PCM for Lithium battery pack - Evlithium

Other balancers:

https://www.electriccarpartscompany....tery-Balancers
https://www.electriccarpartscompany....ERS-EQUALIZERS
https://www.electriccarpartscompany....ERS-EQUALIZERS

https://www.ev-power.eu/Battery-Management/?cur=1


Why use balancers https://www.electriccarpartscompany....ime_p_699.html




Thanks for confirming over 3S can cause balance problems.

[nebster]

Originally Posted by rgleason

Do the 4p cells all self-balance effectively?

Where do you see 4p mentioned by the OP?

What do you mean by "self-balance"?

I thought 3p was the most you should go.

Where did you hear that?

Nonsense. Lots and lots of batteries are scaled up by placing more cells in parallel.

[nebster]

Originally Posted by john61ct

Many people go over 3 strings, many are not aware it can cause balance problems.

First of all, it's ambiguous whether rgleason is talking about paralleled cells or paralleled "strings" of cells, first of all. But the OP is almost certainly only talking about the former in his pack, and he almost certainly has only one string.

Second of all, where did you learn this magic limit for paralleled strings of cells? I sure can't find it. If you can't provide a reference, then what is it about 4 strings that creates the problem, in your opinion?

Signed,

Owner of six paralleled strings, with no "balance problems"

[travellerw]

Originally Posted by kcj

all cells are well balanced up thru 3.45v (delta is only 40-50mV). Do I need to worry about top balancing past that?

I definately ballanced my cells to a higher level. The reason is in case of an accident where I actually push the cells into the upper knee. I want to ensure that one cell was not out of balance and pushed WAY beyond.


On my Winstons, I balanced up to 3.8V (very carefully).

[john61ct]

Originally Posted by rgleason

Thanks for confirming over 3S can cause balance problems.

that would be over three **paralleled** strings

If you want 144V, then over 44S is unavoidable