I have two lithium 12v 100ah batteries in series and from the bluetooth app I can see they have gotten out of balance.

The one charges up to 100% and then the BMS kicks in and both batteries do not charge any further so the second one only hits 90%.

What is the best strategy? Get a Victron battery balancer or get a 12v charger and separately charge the 90% up and periodically charge them both up separately? 

A separate charger is more useful in general to me for general purpose charging. Also if I do this do i have to disconnect the link between the two batteries on can i leave the batteries as is and just connect the charger to the relevant battery's terminals?

Well, There are a few things that could cause it... but there is an easy test.

If they are in series, just try swinging the batteries around. In most series setups the battery on the negative end tends to charge faster, or visa versa I cannot remember too well right now, and swapping the battery around is a great way to test if this is what's happening. You can swap them around and if you see that it's the order that's causing one to stop at 90%, you will need to get a balancer to make sure you don't run one battery into the ground. You can find a lot of DIY balancing projects out there and there are easy ways to do it.

 

For anyone with the same problem on a set of parallel batteries, make sure you have positive from one battery and negative from another. This tends to help with balancing, but you may see the same charging behavior even with this wiring. If you are still seeing that issue, there is likely an issue with the battery that means it isn't able to recover any more SOC.

Edited November 21, 20223 yr by Psy
Misread the initial post

They are 12v 100ah each and I have them in series for a 24v system

1 minute ago, Lee2 said:

They are 12v 100ah each and I have them in series for a 24v system

Read what @Psy wrote, swop the batteries around, take the one with the negative lead going to your inverter and swop it, so its positive lead goes to the inverter and its negative lead to the other battery and the battery that had its positive lead connected to the inverter connect it to the negative lead of the 1st battery you swopped and its negative to the inverter now. Not sure I'm explaining it correctly, but "Swop the two batteries around" should really convey what you should do and what may sort you out for a while...

Swapping batteries in series around will do absolutely nothing to resolve this. You need a battery balancer.

11 minutes ago, P1000 said:

Swapping batteries in series around will do absolutely nothing to resolve this. You need a battery balancer.

Would using a Victron 12v charger used every now and then also solve the issue? I would rather get a 12v charger for now or should i just bite the bullet...

54 minutes ago, P1000 said:

Swapping batteries in series around will do absolutely nothing to resolve this. You need a battery balancer.

Well I've seen it before and it's what I advise people do when they don't want to get a BMS or Balancer for gels. So if there is something arb about lithiums that I am missing, I really apologize but its the best advice I can offer... Could you perhaps clarify? I know the Axperts do that, where the battery on one end charges faster than the other but if there is something I'm missing I really would appreciate some advice on that?

 

1 hour ago, Lee2 said:

Would using a Victron 12v charger used every now and then also solve the issue? I would rather get a 12v charger for now or should i just bite the bullet...

Charging isn't going to be the greatest idea... yes you could disconnect the battery and charge it up separately but the reality is that you aren't going to charge that battery after every use. Running constantly with one battery not being fully charged with reduce the expected lifespan of the battery. There is loads of info regarding charging/discharging cycles and behavior for lithiums that will help explain but basically rather go with a balancer or make one (Which is cheaper if you are handy with a soldering iron). I was looking into the new lithium gate motor drop-ins and they advise against putting a lithium with BMS on AGM charging since it will only charge up about 90% before the BMS stops the charging, sources below: 

https://www.power-sonic.com/blog/lithium-vs-lead-acid-batteries/

https://www.challengebatteries.com.au/wp-content/uploads/2021/06/Charging-Lithium-Batteries-using-Lead-Acid-Chargers.pdf

https://enerdrive.com.au/2017/11/29/can-i-charge-my-lithium-battery-with-a-lead-acid-charger/#:~:text=You%20can%20use%20a%20lead,cannot%20be%20permanently%20turned%20off

Their should only be one BMS that covers both batteries. Change the BMS.

Edited November 21, 20223 yr by Tinbum

1 hour ago, Tinbum said:

Their should only be one BMS that covers both batteries. Change the BMS.

This is also a great point, there are ways to make the 2 BMS' sync up and auto-balance or you can "make" a "Master BMS" that'll treat each battery as it's own cell

15 minutes ago, Psy said:

This is also a great point, there are ways to make the 2 BMS' sync up and auto-balance or you can "make" a "Master BMS" that'll treat each battery as it's own cell

Unfortunately, as is often common, the OP hasn't posted what the batteries are or the type of BMS. (It may be batteries that have them inbuilt).

1 hour ago, Tinbum said:

Unfortunately, as is often common, the OP hasn't posted what the batteries are or the type of BMS. (It may be batteries that have them inbuilt).

Let's get Lee2 to answer. I have not come across lithium batteries in 12V 100Ah that do not have their own BMS per battery.

As he indicated I will charge each battery up to 14.4V with a charger that has a lithium profile. Then connect the 2 in parallel and leave them this way for 12hrs to self balance.

Without a battery balancer I would use 2 small voltmeters connected to each battery so there is a visual indication if they go out of balance to top up the lower one. I just switch my inverter off when I give 1 battery a bit of charge to balance. No need to even disconnect any lead. At times mine will differ 0.1V and while using them they get to the same voltage again. If the difference remains then I do charge the lower one. My S-100s are now in use for a year. Current no load level after heating my geyser via heat pump due to LS in the picture below.

Personally I am with P1000 that changing the position of each battery makes no difference in charge/discharge of batteries in series.

Edited November 21, 20223 yr by Scorp007

They are Revov First Life 100ah batteries with the XiaoXiang BMS built in

18 minutes ago, Lee2 said:

They are Revov First Life 100ah batteries with the XiaoXiang BMS built in

Do you have a link. I suspect that the ones you have are designed not to be used in series. You need a bms to control both batteries.

1 hour ago, Tinbum said:

Do you have a link. I suspect that the ones you have are designed not to be used in series. You need a bms to control both batteries.

How would you do that without opening the battery. Warranty?

22 minutes ago, Scorp007 said:

How would you do that without opening the battery. Warranty?

Exactly, they are not designed for series use.

If you look on the manufacturer's web site they do batteries for series connection but they have external communication. But I suspect these are not those.

Edited November 21, 20223 yr by Tinbum

If its these https://revov.co.za/products/life/revov-12v/

Interestingly it does say for 12/24v chargers though. I can't see how that will work without communication between them.

 

interesting if you read this last sentence here;

Edited November 21, 20223 yr by Tinbum

20 minutes ago, Tinbum said:

If its these https://revov.co.za/products/life/revov-12v/

Interestingly it does say for 12/24v chargers though. I can't see how that will work without communication between them.

 

interesting if you read this last sentence here;

Very simple. Each battery has its own BMS for balancing between cells. A battery balancer is used to balance up to 4 batteries in series.

What else must you balance?

Why do you say in this case you cannot series if the manufacturer says it's OK?

Edited November 21, 20223 yr by Scorp007

21 minutes ago, Scorp007 said:

Very simple. Each battery has its own BMS for balancing between cells. A battery balancer is used to balance up to 4 batteries in series.

What else must you balance?

Why do you say in this case you cannot series if the manufacturer says it's OK?

That's for Hubble batteries not the OP's batteries!

On the manufactures web site I can't see any mention of a battery balancer or in any instructions. If they recommend them for series they should point out that other external equipment is required.

The OP doesn't have a balancer and the batteries are not getting to 100%. It's not working.

Edited November 21, 20223 yr by Tinbum

They say they can be connected in series up to 24v. Let me contact them and ask

14 hours ago, Lee2 said:

They say they can be connected in series up to 24v. Let me contact them and ask

That says 2nd life batteries, I thought yours were the 1st life. Which are yours exactly?

16 hours ago, Tinbum said:

That says 2nd life batteries, I thought yours were the 1st life. Which are yours exactly?

FWIW the same applies for their new cells. Choose as you wish between new or 2nd life. You are welcome to confirm it with Revov if you won't accept it from forum members showing their replies and info on their Web 

1 hour ago, Scorp007 said:

FWIW the same applies for their new cells. Choose as you wish between new or 2nd life. You are welcome to confirm it with Revov if you won't accept it from forum members showing their replies and info on their Web 

I have said myself that the manufacturer has said they can be used in series, if you care to read my posts properly.

What I am saying is that these clearly do not work as they are. You can't have one BMS shutting down resulting in the current stopping to the other battery.

If a manufacturer is going to say they work in series they must make it clear that other equipment is required. Especially as they require each cycle to reach 100% or the warranty is void!!!!!

 

Edited November 23, 20223 yr by Tinbum

When wiresd in series, one of the batteries will almost always has a slightly higher resistance than the other.

As a result, the one with the higher resistance does not charge quite as quickly as the other one and charging will stop before it reaches fully charged as the lower resistance batteries BMS will stop charging. 

The lower charged batteries's BMS then stops discharging earlier than the fully charged battery as it has a lower voltage. 

The gap in voltages will just keep on getting bigger every time they are charged and the packs need a balancer to ensure that they remain at the same voltage. 

 

Geewiz have a balancer for this specific scenario.

https://www.geewiz.co.za/hubble-battery-balancer/185922-lithium-24v-battery-balancer-equaliser-with-bluetooth-suitable-for-all-battery-types-24v.html

Is not too expensive and the BMS's in the batteries should then kick in and out at the same time and you will be able to get full capacity from the batteries. 

This one is cheaper https://www.geewiz.co.za/hubble-battery-balancer/185924-lithium-24v-battery-balancer-equaliser-suitable-for-all-battery-types-24v.html 

 

 

4 minutes ago, Sc00bs said:

When wiresd in series, one of the batteries will almost always has a slightly higher resistance than the other.

As a result, the one with the higher resistance does not charge quite as quickly as the other one and charging will stop before it reaches fully charged as the lower resistance batteries BMS will stop charging. 

The lower charged batteries's BMS then stops discharging earlier than the fully charged battery as it has a lower voltage. 

The gap in voltages will just keep on getting bigger every time they are charged and the packs need a balancer to ensure that they remain at the same voltage. 

 

Geewiz have a balancer for this specific scenario.

https://www.geewiz.co.za/hubble-battery-balancer/185922-lithium-24v-battery-balancer-equaliser-with-bluetooth-suitable-for-all-battery-types-24v.html

Is not too expensive and the BMS's in the batteries should then kick in and out at the same time and you will be able to get full capacity from the batteries. 

Exactly, They are not like lead acid where the charged battery will still allow current through. (Though from experience, many many years ago, if you take a very small current from a single 12v battery in a 24v system (lead acid) this will result in what you say. Again you have to use a balancer or a 24v- 12v voltage converter).