Have built a 12v LiFePo4 battery

[Peter Brigg]

I have made a 12v battery using 8 Headway cells configured as 4 parallel units of 2 done up in series to get the 12v.
I also bought and installed a low voltage cutout to avoid over discharge.

It is boxed up and works well however the 2 pairs closest to 12v+ are now lower than the other 2 pairs and I am looking for a simpler way of charging these 2 low pairs.

My idea at the moment is to buy a 240v timer and set it on say 30 mins and then charge each of them in turn-I suppose I could charge 2 of them as a 6v unit.

I am using a small 12v battery charger-max current output switchable 3A or 6A

Sounds a bit clumsy however in the past I have seen how easy it is to get distracted and come back hours later to cooked cells.
Is there a problem with using a 12v charger to bring a single cell up to the others or is this a bit heavy duty?

Any ideas re balancing theses cells is appreciated.
Regards
Peter

 

[amberwolf]

First thing I'd do is swap some cells around so that there is more even capacity in each parallel group. So take one of the cells in the pair that's lowest and swap it with one out of the highest group, and so on.

That alone will probably solve your issue.

If it doesn't then there is something else going on.

Also, if that 12V charger is for a lead-acid car battery, I wouldn't use it on the headway cells. Most of that type of charger that I've had experience with hasn't really got a current limiting / charge curve that's compatible with what you've got there; IIRC. Nor do they always have any DC filtering, so the AC from the wall is just applied as half-wave rectified, unfiltered, making a pulsed current into the pack, going from 0v to peak charge voltage output many times per second (depends on your local AC frequency). THey also don't generally have any form of HVC and their voltage with little load on them might be significantly higher than your packs' max allowable voltage. (have had one with 20V+ output no-load)

If yours is more complex then it might be safe to use on the whole pack, but unless it also has a cutoff (HVC) then I wouldn't leave it unattended or you risk damaging the cells or causing a fire.


But even if it is safe to use on the whole pack, I would never use it on anything lower in voltage than that. Definitely not on a single cell or pair in series.

If you're going to buy something anyway, buy a single-cell charger for your chemistry.

 

[e-beach]

When you say lower, how much lower? What are the voltages. Also, how many amps are the Headway cells?

4S LiFePO4 BMS' are inexpensive as well.

Did you ever check the individual cell capacities to try to balance the parallel string capacities?

 

[dogman dan]

Once you start playing around with batteries, it's time to buy an RC charger. One that does at least 6-8 cells at once, at a decent rate like 150w.

Now you can charge anything from one to 8 cells at a time, and balance the pack if it has a balance plug. It will do lithium, lifepo4 lithium, lead, NiCad, nimh.

Mostly I use mine as a single cell charger, for when I need to charge one cell in a pack. you just need a plug you can use to connect to all the cells, or each cell individually, called a balance plug. this is the jst type plug, to mate with the charger balance ports.

So you need the charger, and a 5 wire jst plug with it's wires. If you buy a 5 wire jst extension, it will have both male and female on it, put the female on your pack. The red wire goes to cell 4 positive, the rest follow the cells through the pack, ending with the last wire connected to cell one negative.

 

[Peter Brigg]

The Headway cells are 10amp
At rest the voltages are 3.37 3.39 3.4 and 3.4 volts

When I connect my lead acid battery charger to the battery (briefly) the voltages are 3.68 3.9 3.36 and 3.36 volts.

Why would the first 2 cells rise but not the 3rd & 4th?
Have incl some photos to help explain.
I have put in a fair bit of time putting it together and a reasonable amount of money also.
Looks like I will have to get a LiFePo4 charger perhaps a single cell charger and also use it for balancing purposes.
Can someone recommend a brand of charger?

I have been careful not to overcharge-the low voltage cutout should look after that issue.

I came across an article where it was recommended that all connector plates and terminals should be cleaned of surface oxide layers-say with a wire brush or steel wool. Have done that however it made no difference.

Regards

Peter

 

[Inwo]

Are discharge wires connected nearest to opposite batteries? Ie. Plus to #1 and minus to #4.
Same with charge.

 

[amberwolf]

Why would the first 2 cells rise but not the 3rd & 4th?

bad cells?
high resistance?

I have been careful not to overcharge-the low voltage cutout should look after that issue.

LVC only takes care of overdischarge.

You'd need an HVC to prevent overcharge.

Without a per-cell HVC you can't know if there's any overcharging happening unless you measure them with a voltmeter on each parallel group as they charge, and stay right there watching them until charge is done.

 

[dogman dan]

The higher charging cell has a tad less capacity than the others. If this persists, you may need to use a balancing charger, as I suggested, each cycle.

Using the RC charger will provide your pack with HVC while charging. It's like just the charging portion of a bms.

You will have to add the jst balancing plug to use the balancing function of the charger of course.

Since you have the wrong charger now,, you need to buy a lithium charger. Since you have no bms, a balancing charger is what you need to buy.

 

[e-beach]

On that pack you should be charging to 14.6v. (3.65 per cell) A balance charger or a bms and a charger should be used.

Charging over 3.65v per cell will degrade your cells.

As stated above what does your charging wiring look like.

However you are talking .03v difference between the cells and that is almost nothing. I have had a my DMM read off by that when I don't have a good connection, as if I wasn't pressing the probes well enough to the surface.

Show us your wiring and explain it a bit so we can see what you are doing.

 

[Peter Brigg]

Thanks for those replies-I will ask Dogman Dan a few questions first.

With an RC charger-what does RC stand for.
And the pack supplies HVC- what does that stand for?
I now realise that the battery is set up clumsily-rather than useless outlets for 3v 6v & 9v a simple jst plug is all that is needed to charge and balance cells.
In other words the charger I need balances and charges the cells-simple-I see some on eBay that handle up to 6 cells-I only need to handle 4.

Re the question about wiring-it is pretty simple-on the recommendation of the supplier I wired in a Zeva LVC12 low voltage cutout. This device has a reset button to allow a few seconds of override to use the battery in an emergency if it falls below 11.5v. It also recommends that large loads such as starting an outboard motor not be routed through the cutout. There is a blue indicator light to indicate that the LVC is operational.

On the panel of the battery I have outlets for neg then 3v 6v 9v 12v via the LVC and 12v not via the LVC plus 2 cigarette lighter sockets for 12 v via the LVC

Am currently looking at LiFePo4 balance chargers-thanks again for your help.
Regards

Peter

 

[Voltron]

RC= remote control... the charger isn't remote control, but most RC stuff uses lipo batteries. They usually have setting for different series counts, and different battery chemistry types.


HVC= high voltage cutoff... telling the BMS to stop charging at the right voltage for the chemistry of the battery.. ditto LVC, low voltage cutoff.

 

[amberwolf]

And the pack supplies HVC- what does that stand for?

High Voltage Cutoff. (when pack is fully charged). A BMS would do this.

Re the question about wiring-it is pretty simple-on the recommendation of the supplier I wired in a Zeva LVC12 low voltage cutout.

That is only a pack-level LVC, so it does not detect if a cell in the pack runs low, only if the total pack voltage does. So you could have a cell in the pack that is lower capacity than others and has already run out and is way below it's safe LVC, already being damaged, while others are still high enough that the whole pack is still above the Zeva's LVC so it still allows the pack to try to supply power (further draining the already-too-low cell).

As long as all your cells are equally capable and new, this is not a problem.

But when your cells are not equally capable (even if new) then it leaves the chance of damaging cells running them too low like that; the worse the difference between cells is, the more likely this is to happen if you often use the pack's full capacity.

As for the wiring, you need any load you use on the pack to run thru the LVC (or BMS) so that if the pack is too low then it will not let the load drain the pack any further.

Or you need to monitor the status light(s) on the LVC (or BMS) as frequently as possible when the pack is nearing it's empty point, so you can manually cut it off before it gets too low.

 

[maanebedotten]

How does the pack deliver 3, 6 and 9v? Does it have dc-dc converter or does it just tap between the cells?

 

[Peter Brigg]

Thanks for that info-yes the individual cells need watching and now I realise that an LVC does not necessarily protect the cells-fortunately all cells are new-no mix and match so that may not be a problem.
Re the question of how I get 3v 6v 9v and so on-just by tapping between the cells.

Today I bought a balance charger-a Volt 680ac multi function charger 89 aussie dollars. From a reputable dealer however the instructions make your eyes water-confusing-conflicting and so on.
Time and patience will win out (I hope)
Regards

Peter

 

[dingoEsride]

as Dogman says a Rd C or cell charger, this one is 48v is split into 4 using jst connectors, but can only charge two alternate cell groups, these chargers and similar are cheap but limited to 8 amps and depends also on your supply voltage(12v)amps, I use this way mostly for maintenance and use a bms otherwise

 

[dogman dan]

Sorry I got carried away with the abbreviations. The RC stuff, drones etc, use batteries that have no bms. So they build the charging part of the battery management system into the charger.

FWIW,, I don't think your cells are that bad. They look pretty balanced at nearly full charge. But one is fuller than the other, so it goes to 3.68 or whatever quicker than the others. At that point, the RC charger would stop charging, and start discharging that one high cell, then start charging again later. This does require the balance plug to be added to your pack.

Again,, you could also just put a light bulb on that one fuller cell, empty it manually, then put your charger back on the whole pack. Then that cell will not be more full than the others, and the others will fill, and at the end of your charge you should have all the cells to 3.5v, and none of them over 3.65v.

Ideally, get a lithium charger. A lead charger will continue to trickle, and that is not good for lithium batteries.

 

[maanebedotten]

I get 3v 6v 9v and so on-just by tapping between the cells.

This will cause pack to be unbalanced when these taps are used. Pack lvc will not be enough, you need cell-level lvc to protect a pack like this.

 

[amberwolf]

fortunately all cells are new-no mix and match so that may not be a problem.

It *is* already a problem in that the cells are not performing the same. Otherwise you wouldn't get different voltages while charging.

Even if they are new it doesn't mean they are all from the same batch, or the same calendar age since manufacture, so they dont' have the same characteristics, and you need to test each individual cell and record their characteristics, then match them up so you have a balance of different cells throughout the pack (if there are enough opposites to balance out the differences).

DrkAngel has a thread about building 18650 packs from used laptop cells that shows how to do this. Other people have similar threads. Even though yours are not 18650 laptop cells nor used, you still need to do this to get a balanced pack that works consistently without as much monitoring and messing around with it all the time.

I get 3v 6v 9v and so on-just by tapping between the cells.

This will cause pack to be unbalanced when these taps are used. Pack lvc will not be enough, you need cell-level lvc to protect a pack like this.

And a cell-level HVC during charging, unless a balancing charger is used in balancing mode that has wires that run to each of those taps, plus the positive and negative mains of the pack.