LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[newhaul]

Originally Posted by john61ct

Yes it's a lot of force, seen people woefully fail trying to use even thick plywood.

Even thick aluminium plate can be found reasonably priced at yards in metro areas, not hard to cut.

For those who think effort in extra intra-bank cooling is worthwhile (I don't for House banks), put plates between each cell with short "wings" extending out say 2" past the cell sides (not up in height, stay below the case height!)

to act as heat sinks

and maybe thermostatically switch on computer-style fans at 30°C post temp.

But crazy overkill for a non problem IMO.

Please post a couple of photos of your build with the alluminum plates between the cells like you describe so we have a better idea what you mean and we have a visual ti go off of when building or upfitting our banks.

[Checkswrecks]

Aluminum plates between cells is done in electric vehicles to more evenly distribute the thermal loads. Here's an LG patent for a BMW"

[Checkswrecks]

Originally Posted by Pete7

So what is the concensus on this LFP strapping/case requirement to prevent swelling? is it a real problem that is going to happen or just someones off the cuff idea?

Spreading the thermal load with aluminum or anything else won't make a bit of difference for the usual low C charge and discharge cycles of house batteries in most boats. However, if your C rate is high enough to heat adjacent cells then some type of cooling to broaden the impact on adjacent cells will mean that the interior cells won't degrade ahead of the outer ones. fwiw - Most auto makers do use some type of liquid thermal management between cells.

As for tight pack or not, there's a pro and a con and both of them pertain to creation of leakage at the terminals. The con to tight pack is that the pressure in the can puts more force on the seal at the terminal. The pro to preventing relative movement is that the less the cells can move in relation to each other, the less the buss bars will impart twisting forces on the terminals by the buss bars. fwiw, some black electrical tape is not going to be much restraint in this regard and Will Prowse continually notes that he is just using it for the test he is doing that particular day.

Personally, I'm for allowing cells have enough space to expand and contract with buss bars which are either woven or have big arcs which can bend to provide strain relief.

[john61ct]

Originally Posted by newhaul

your build with the alluminum plates between the cells

Would be pretty silly to go to that trouble when I clearly stated I don't there's a real issue there for House banks.

Just an "out there" design concept from the DIY EV world, but many even there think it's overkill, and that's with batts getting up to 45+ degrees from high C-rates

[Pete7]

Originally Posted by Maine Sail

Manufacturers such as Winston require cell compression/strapping and sell heavy duty aluminum end boards and the strapping for this.

This cell was out of a Mastervolt MLI battery that had no cell compression on the lower bout of the cells, which is exactly where they bulged.

That is a bit worrying, was this battery likely subjected to high C rates of charging current?

Pete

[john61ct]

Swelling still can occur without any specific damage events, just normal usage.

Yes a major event can also cause (more serious) swelling, to the point of causing physical damage.

But even a normal small-percentage swelling should be prevented through the use of compression plates.

[GoneDiving]

Originally Posted by john61ct

Swelling still can occur without any specific damage events, just normal usage.

Yes a major event can also cause (more serious) swelling, to the point of causing physical damage.

But even a normal small-percentage swelling should be prevented through the use of compression plates.

Just asking: What is the role in trying to stop the swelling? In most chemical reactions pressure and containment promote and excite not reduce it.

Many explosives barely burn in the open air but detonate when placed in a bore hole or barrel.

Thanks

[CatNewBee]

Originally Posted by GoneDiving

Just asking: What is the role in trying to stop the swelling? In most chemical reactions pressure and containment promote and excite not reduce it.

Many explosives barely burn in the open air but detonate when placed in a bore hole or barrel.

Thanks

With swelling the tight spaces between the electrodes grow irreversibly, so does the inner resistance, and the performance of the cell decreases.

By compression you keep the pack tight and get a longer life. Even if it expands a little during deep discharge, the compression forces it back to original when charging.

[john61ct]

The case being allowed to deform too much allows the expansion of the cell layers to the point they separate too much, increasing resistance and reducing lifespan.

Even further then internal delamination, real damage in the short term.

Apparently causal factors, not just associated with charging factors, but the sitting at high SoC which we all know is to be avoided anyway, but to some extent inherent in many usage patterns.

[tanglewood]

CALB makes no mention of compression. Who other than Winston does?

And is this a height C thing?

[newhaul]

Big question i have just recieved my battery gauge and my big question is put the shunt before or after the bms ?
Before = battery side.

[CatNewBee]

Originally Posted by tanglewood

CALB makes no mention of compression. Who other than Winston does?

And is this a height C thing?

No, it is a deep discharge thing, when you leave your cells at low soc and they self discharge further, the bms disconnect cant save them because it is comming not from other external loads. It can also happen on overcharging and overheating.

[CatNewBee]

Originally Posted by newhaul

Big question i have just recieved my battery gauge and my big question is put the shunt before or after the bms ?
Before = battery side.

The bms sits directly on the cells, the shunt goes after on the pack negative connection, otherwise the bms would read a wrong value on the one cell connected to the shunt and may do funny things trying to equalise it or even recognised it as faulty.

[newhaul]

Originally Posted by CatNewBee

The bms sits directly on the cells, the shunt goes after on the pack negative connection, otherwise the bms would read a wrong value on the one cell connected to the shunt and may do funny things trying to equalise it or even recognised it as faulty.

ok that is just what I was asking about after the bms it is .

I have been usingg Lfp for several years but never had a battery monitor before always just used the reading on my solar controller .

This unit with the 350 amp shunt
https://www.ebay.com/itm/Battery-Mon...edirect=mobile
I will post my evaluation of it

[evm1024]

Originally Posted by newhaul

ok that is just what I was asking about after the bms it is .

I have been usingg Lfp for several years but never had a battery monitor before always just used the reading on my solar controller .

This unit with the 350 amp shunt
https://www.ebay.com/itm/Battery-Mon...edirect=mobile
I will post my evaluation of it

Just to amplify on this consider this chain:

PosBus--4thcell--3rdcell--2ndcell--1stccell--BMSneg--Shunt--NegBus

The BMS needs to get as accurate voltages of the Battery's cells as it can. It uses those voltages to do protection decisions and balancing.

The shunt shown (not a hall effect shunt) is a quite precise power resistor. As a resistor it has a voltage drop that is in proportion to the current running through the shunt (ohms law: current times resistance = voltage).

Victron uses a 50 mV per 500 amps shunt typically so at 50 amps there would be a voltage drop of 5 mV across the shunt. In other words the voltage drop across the shunt varies based on the current through it.

If you were to place the shunt between the BMS and the 1 St cell then the cell voltage that the BMS measures would vary based on the current. That would not be good. It might not be very bad but it could lead to some odd happenings.

Kirchhoff's current law tells us that the current is the same anywhere in the chain (current into a node = current out of the node). In terms of current measurement it does not matter where we put the shunt in the "chain".

However, battery monitors also measure voltage. They measure voltage between the battery positive and one side or the other of the shunt.

If they measure between the battery positive and the negative bus they will have the (current induced) voltage drop of the shunt changing the measured voltage based on the current. If they measure the voltage between the battery positive and the battery side of the shunt they will be more accurate and not have the voltage change depending on the current.

Many battery monitors also monitor the start battery voltage. The shunt induced voltage inaccuracy of the start battery is opposite to the house bank. If the start battery is measured between the positive and the negative bus side of the shunt then the start battery voltage will be more accurate - just the opposite of the house bank.

However - the voltage accuracy as measured by the battery monitor is moot. Typically a BM only displays to 2 digits 0.XX and a 0.005 mV (at 50 amps) difference is not very meaningful. It is well hidden by the battery monitors display precision and the actual accuracy spec.