12v 200ah connect in series.

[Divad]

Hi, I needed a battery in a hurry to replace my failing 12v 100ah FLA and bought this one as it was close and available. 12v 200ah battery

I now want to make a 200ah 24v system with an all in one, in the battery specs they say that it can be put in series with a max voltage of 30v,

"Maximum In-series voltage 30.4V"

Great but further down the page they say

"Can be connected in parallel & series for larger capacity (no higher than 24v is recommended, and a secondary 24v BMS is recommended for even charging and discharging/balancing of series connected batteries). "

Would the secondary 24v BMS be external or would I need to replace the original BMS's with 2x 24v one's

Thanx Dave

 

[gnubie]

It would be external in a similar fashion to lead acid battery balancers.

The issue they are covering is the batteries having different actual capacities due to age or straight out paper specs. During charging the differences between the batteries can result in higher voltage to one battery and lower to the other. If that voltage goes up too high the BMS in the affected battery will disconnect, halting charging and potentially dumping any loads you might be powering.

How old, in terms of charge / discharge cycles and to what depth of discharge (all figures approximate OK) is the existing lifepo4?

 

[Divad]

Thanx for your reply @gnubie the existing battery is 3 months old and very lightly used, so maybe 10 cycle max. I have used it to power a small 40 ltr engle 12v camping fridge and cpl of LED strip lights. I have 3 x 250w solar panels in parallel running through a 12v 30A PWM SCC. There are 6 more panels to install. I will be fitting one of these. Victron all in one
It is in a holiday cabin that will be our permanent residence soon as I have recently retired.

I intend on getting the next one in the next 3 mts. Both will be the same brand and spec.

Thanx Dave

 

[gnubie]

When you finally do get the new one, put a few similar cycles on it then bring both up to full charge separately then put in series and into normal service. During the first few charges keep an eye on the individual batteries with a multimeter to make sure the voltage on them is similar. If they do start to get out of step, say 1 volt, IMO you will need an external BMS to keep them in line. Early in the charge cycle I wouldn't expect them to be much out of sync as they will both have comparable charge resistance. Towards the end where the voltage starts to rise is where I'd expect any problems to show.

20 cycles is nothing for a lithium's overall life span, but new cells like to be cycled a bit before they settle into their long term capacity rating.

 

[Divad]

Thanx again @gnubie that sounds like a plan. Ill try to remember to update when it all happens. You have been a great help.

Thanx Dave