Battery not fully charging

[pears_91]

Hi

I wonder if anyone may help me here.


I have recently replaced some cells in my e-bike battery. The cells are all the same. LG MJ1's

I did buy them in separate batches .. they look identical. I made the battery pack and replaced the BMS which was broken.

Now I have changed the battery just like I normally would, all of the new cells are only charging to 3.6V. compared to all of the original cells which charge to 4.15V.

When the battery was new all cells would charge to approximately 4.2v. giving me the 42v fully charged battery pack. As it should be for 36v battery.

Currently the whole pack charges to 39v. Then my charger light goes green. Asif all is fully charged. But the pack is made up of approx half new cells all 3.6v. and old cells 4.15.

Sorry I don't think I explain in the most simple way but I hope someone can understand and maybe help me.

 

[pears_91]

Ok

I have discovered it needs to be balanced.

I not need to find a way to do this

I have seen a guy on YouTube using a light bulb.. to drain the cells that have higher charge.

Any suggestions would be appreciated.

I'm thinking I could get a 4.2v charger and just charge them all individually? If such charger exist. I guess it should.

 

[pears_91]

I have found an RC car charger that runs off USB.

I'm quite happy because I guess this will fix a problem I have been having.

My battery keeps cutting out. When it still appears to have charge. But I guess the BMS is cutting it out. Because some of the cells have reached the limit. Even though it shows still having charge. .

 

[Ghost1951]

@ Pears_91

IT sounds very much like you have diagnosed the problem properly. The cell groups are not balanced and the BMS is shutting down when it detects one of the cell groups is at the low voltage cut off point which is usually around 3.1 volts. When this happens, maybe half of your cells are still able to produce current because they are at a higher voltage than the ones which caused the power to be cut.

When you charge the battery, the BMS stops all charging once one cell group reaches 4.2 volts, so you will never get the low cells properly charged by doing the normal series charging with the normal charger, and the out of balance state will get worse as you cycle the battery.

If you have the patience - it will take quite a while, you could bring the low voltage cells to the same level as the ones which are charged using a 4.2 volt lithium charger.I have done that in the past. It did take quite a while, but it does work.

Another alternative is to lower the voltage of the more charged cells by connecting each group to a suitable load. Possible loads might be a high wattage security light bulb, or an electric kettle element, or a shower element, but you would need to work out what current would be drawn so you don't overload the cells you are draining by demanding too much power from them. You can use Ohm's Law to work that out yourself if you have a multi meter to measure the resistance of the load and the voltages you are working with. I have not done this myself so I can't advise more. I am sure you will take care about the heat that will be generated by draining the cells like this. A considerable amount of energy will be turned into heat.......

There are better qualified people than me here who will no doubt come along and give their view.

One more thing - if you have replaced cells in an older battery it is very likely that the new cells will have more capacity than the older ones and so your refurbished battery will likely get out of balance in future, because the new cells are going to still have a lot of capacity left when the older ones are running down. I suppose frequently re-charging the battery before it is well down might help avoid this, but I am not so sure about that.

EDIT:

There are plenty of lithium chargers at 4.2 volts. They are pretty cheap - some of them are at least, but those ones will take a good while to raise voltage very far in blocks of parallel connected batteries. From memory of a couple of years ago - I think it took me about twenty hours to raise one group from 3.5 volts to 4.2 volts in my 13 Ahr battery.

 

[guerney]

Why don't ebike battery BMS manufacturers design them for when cell groups are widely out of balance, as can happen as cell packs age, or have cells replaced with ones charging and discharging very differently, so that the cell pack still is balanced effectively? Maybe some do, but I haven't seen them. Strikes me that ebike batteries would be a little safer if they did.

 

[Ghost1951]

Why don't ebike battery BMS manufacturers design them for when cell groups are widely out of balance, as can happen as cell packs age, or have cells replaced with ones charging and discharging very differently, so that the cell pack still is balanced effectively? Maybe some do, but I haven't seen them. Strikes me that ebike batteries would be a little safer if they did.

I suppose it is all about cost. Top balancing is obviously cheaper to accomplish. Who buys the more expensive battery when they are looking for a new one? Some do most likely, but the BMS makers are selling to a very price sensitive market mostly.

You could charge each cell group individually if cost and complexity was no problem. Series charging is always going to eventually lead to a balance problem. The Op will probably find that happening often because there is not much chance that a battery with new and old cells mixed is going to stay balanced for long unless he does regular, lengthy balance charges with the green light on.

 

[pears_91]

Hi @Ghost1951

Thanks for confirming my understanding, and advice on how to fix.. it seems like a bit of a tricky one.

I have also heard of some kind of charger that can perhaps perform balancing.. I might look into one of those as if my life carries on in this direction it might be a useful tool to have.

I guess there quicker and more automated reliable way of doing it.. I think many of the cells I replaced may not have actually needed replacing in hind sight.. there was just a balancing problem all along.

And the battery is not actually all that old..maybe 1 year but only 50-60 cycles. So I hope shouldn't be too much issue with cells being out of wack.

 

[Ghost1951]

I don't think it is that tricky as long as you have a meter and some common sense about handling the battery with the cover off. You obviously have that if you managed to replace cells. Did you replace entire groups of just individual cells you had suspicions about?

The only problem about using the charger to raise low groups is that it takes a while. Use a charger which you KNOW will stop at 4.2 volts. An ordinary USB will go higher which is dangerous as I am sure you know.

 

[saneagle]

Why don't ebike battery BMS manufacturers design them for when cell groups are widely out of balance, as can happen as cell packs age, or have cells replaced with ones charging and discharging very differently, so that the cell pack still is balanced effectively? Maybe some do, but I haven't seen them. Strikes me that ebike batteries would be a little safer if they did.

Most BMSs will balance the pack. All you have to do is leave it on the charger for long enough after the green light. Balancing happens at about 100ma, so takes a while.

 

[pears_91]

I don't think it is that tricky as long as you have a meter and some common sense about handling the battery with the cover off. You obviously have that if you managed to replace cells. Did you replace entire groups of just individual cells you had suspicions about?

The only problem about using the charger to raise low groups is that it takes a while. Use a charger which you KNOW will stop at 4.2 volts. An ordinary USB will go higher which is dangerous as I am sure you know.

Yes sorry, tricky probably not the right word. Maybe fiddily or time consuming.

I have seen what seem reasonable balance chargers second hand for under £30.. but they seem to be for li-po battery not li-ion.

And on Amazon there is multiple parallel charging boards.. less than £10 for some of them.

 

[Ghost1951]

Try what Saneagle said just now. He has a lot of experience.

 

[saneagle]

Yes sorry, tricky probably not the right word. Maybe fiddily or time consuming.

I have seen what seem reasonable balance chargers second hand for under £30.. but they seem to be for li-po battery not li-ion.

And on Amazon there is multiple parallel charging boards.. less than £10 for some of them.

Your battery has its own balancing system, so you might as well use it.

 

[pears_91]

Your battery has its own balancing system, so you might as well use it.

Thanks for the info.. I will leave it on charge .

 

[Ghost1951]

Thanks for the info.. I will leave it on charge .

I'd be interested in how you get on with leaving it on a balance charge. Please make sure to let us know what happens. It might help other people to know what happens. I would think that if it does balance you would see voltage close to or equal to 42 volts on that battery.

 

[pears_91]

I'd be interested in how you get on with leaving it on a balance charge. Please make sure to let us know what happens. It might help other people to know what happens. I would think that if it does balance you would see voltage close to or equal to 42 volts on that battery.

No problem yes I will let you know
I appreciate the help on here .

 

[Nealh]

Problem is the new cells should have been charged seperately to the level of the old ones before fitting.
As cells are out of balance best option is to manually raise the new cells /unbanaced cell groups to match the others.
0.6v is quite a way out and the BMS will take ages to try and sort this out if at all.
The BMS will stop balancing once 4.2 v is seen in one of the cell groups esp with that larger imbalance.

One can manually charge the low groups via the multi wire sense wire connector with an adapted 5v phone charger mod. Though one needs to present all the time to keep an eye on the cell voltages and to cease the cell charge by turning off the charger .

 

[pears_91]

Hi

To give a ending to the post

After leaving 2 days it is not charging past the 39v

I remove the battery from charge and in the meantime made a new battery pack that I am using until I fix the other one.. then I will have 2 packs to rotate.

I have purchased a balance charger online for £24 to charge each cell group individually to 4.2v.

There were cheaper options, such as using a light bulb or 5v charger which I have.. but both would need constant monitoring. I feel I can justify this because I am bone idle and will have flexibility such as to charge a car battery which I currently don't have either.. so I think it's a good multi functional tool to have.

 

[saneagle]

Hi

To give a ending to the post

After leaving 2 days it is not charging past the 39v

I remove the battery from charge and in the meantime made a new battery pack that I am using until I fix the other one.. then I will have 2 packs to rotate.

I have purchased a balance charger online for £24 to charge each cell group individually to 4.2v.

There were cheaper options, such as using a light bulb or 5v charger which I have.. but both would need constant monitoring. I feel I can justify this because I am bone idle and will have flexibility such as to charge a car battery which I currently don't have either.. so I think it's a good multi functional tool to have.

It's not easy to use a balance charger to balance a battery because most don't charge cells individually. They charge cells in series through power leads, except chargers like the BC168 that does charge individual cells and is perfect for the job, otherwise one or more single cell chargers would be better.

 

[pears_91]

It's not easy to use a balance charger to balance a battery because most don't charge cells individually. They charge cells in series through power leads, except chargers like the BC168 that does charge individual cells and is perfect for the job, otherwise one or more single cell chargers would be better.

The charger I have bought is Carson x-base 2.0
It does say it can do 1 cell, and covers the voltage range.

 

[saneagle]

The charger I have bought is Carson x-base 2.0
It does say it can do 1 cell, and covers the voltage range.

I don't think it'll work without making up special connectors. Let us know how you get on and any adaptations you make.

It can do 1S, but only through the power leads, so you have to figure out how to connect them to cells or adjacent pins on the multipin connector.

The BC168 works differently. It's basically 6 independent 1S chargers that charge through the balance connector only.