Revov 200ah 12v x2 in Series - Balancing issue

[Gremlin]

I have a Victron Easysolar 24v off grid setup and recently replaced my Lead acid batteries for Revov 200ah 12v. I was in touch with the Revov tech support prior to installation and they recommended that I use a victron battery balancer, which is fine (although I was surprised because the Revov has an internal BMS)  because I had one with the 200ah lead acids.

So firstly, I used a multimeter to check voltages on both batteries and was surprised to see out the box they were both exactly 14.1v. So in theory, balanced.

I connected them in the afternoon so they were on load overnight. Remember I'm off grid so no option of additional charge. In the morning when they hit float the victron battery balancer warned of high voltage on the lower battery with numerous alarms. The Revov app showed alarms for cell balancing in one battery and a discrepancy of 0.4 volts between batteries. The high voltage battery was at 100% charge and the lower at about 97%. 

 

The settings I user defined on the Victron MPPT are Bulk(Adsorption) 28v and Float 27.2v (both of these are 0.2v below the Revov stated max). In the absence on ANY information regarding the charging algorithm I stuck with the Victron default for LiFePO4 of 2hr fixed absorption duration . 

Unfortunately I am not on site at the moment and will be back on the weekend so don't know what the battery status will be. However I have a couple questions:

1. Will the batteries overtime balance?? They are not overused during the week with nominal load. 

2. Is the Victron battery balancer required or even a good idea (not withstanding the Revov advice received).

3. If the batteries don't reach equal voltage, is this a major problem? Should I disconnect and revert back to my old lead acids.

4. Any suggestion to get the batteries balanced between them and also the cells internally?

5. What is a recommended time to be on absorption? Is a fixed period of 2hrs too long?  Or could I use the Adaptive Adsorption setting for LiFePO4 batteries. 

 

I had hopped that this would have been a drop in exercise but it seems not. Any suggestions would be welcomed. Also since I am not there permanently I cant really monitor the system. 

 

 

 

[Gremlin]

Ok - so got some feedback from my questions from Revov so positng them here for anyone with the same problem: 

1. Yes the batteries will balance over time the more they are cycled.
2. The battery balancer is not required but does help to keep the batteries balanced on the top end when almost fully charged.
3. Each battery has slightly different internal resistance, this can be mitigated by having the cable lengths the same and as short as possible.
4. If they become way out of sync then by connecting the batteries in parallel and leaving them for a 24 hour period will allow the cells to share the energy between them.
5. Recommended absorption time is 1 hour.

[stefan44]

good day Gremlin, I am in agreement on what Revov told you,

although I dont have revov batteries, I still employ 12v 200ah freedomwon batteries in paralel to feed a 24v inverter, and they are charged with a Victron smart solar 100/50 and a BMV 702 to monitor the batteries so the system is similar. 

I do not have a balancer between them, just voltage measurement via the BMV for both batteries. 

now during charge and discharge the voltage between the batts differs between 0.0V and 0.3V, it fluctuates the whole time, but as soon as the batteries "rest" meaning no charge or discharge they tend to always settle  between 0.0v and 0.0##v difference. 

they are connected 2p2s for a 24v 400ah battery, the deepest I have discharged the bank was 392ah and the individual batts measured between 12.1v, 12.1v, 12.2v, 12.1v

with no bms cut off.

my setting is as follows, Float 27.15v

                                        Absorption 27.3v

                                         Absorption time 3hrs and tail current disabled.

regarding point number 4 on your post, when I received the batteries I connected them all 4 in parallel and charged them with a PSU (5 amps) set to 14.4v for a week and a couple of days to balance the batts.

point number 3, I cannot stress this enough, cable lengths MUST be equal and as short as possible and thick. Make sure that the bridge between the batteries is also as short and thick as possible. this point number 3 will have the most significant effect over time on a well balanced system. 

after I typed this reply and thought of this system and analysed the data on the VRM portal , It became clear to me that I actually should invest in a 24v balancer, rather sooner than later. as my batteries do start to drift apart over time. 

Are my settings  the "correct" ones?, I dont know, I just know it works for me.

 

[Gremlin]

Hi Stefaan

 

Thanks for the info. I see your Absorption and Float are quire close - .15v vs my .8v

 

My cables are 35mm2 and each 1.2m in length. I think this is ok with a bridge of 200mm. I think you will need two balancers; one for each pair of batteries in series.  Do you know the purpose of Tail current Disabled?

I think I need to invest in a GX (?) so that I can use a VRM.