series vs. parallel

[carolinabigfoot]

I am trying to replace the dead batteries in my 24V battery bank with LFP batteries. Can somebody explain to me what difference it makes in practical terms (battery performance, battery life expectancy, reliability) if I connect 2 12V batteries in series vs. connecting 2 24V batteries with lower Ah rating parallel. I have already a few ideas about the question, but reading in the forum makes me think that I could be wrong or that I could overlook something. Thanks!

 

[alcook62]

The primary benefit I see to having two 24v batteries in parallel is the redundancy in your system. If for some unfortunate reason one of the batteries fails, you can still have the power to run critical loads (albeit at only 50% of your previous capacity). Another factor you should be aware of is that many ready-made 12v LFP batteries cannot be run in series due to their BMS not playing together properly. Should you choose to go the 12v-in-series route to achieve your 24v battery, ensure that the batteries you choose are capable of this.

 

[rmaddy]

In the case you describe there is little practical difference. But I would lean toward two 24V batteries in parallel. They are easier to keep balanced, and as stated earlier, if one fails you still have a working system. Just make sure you connect the positive and negative load wires to opposite batteries. Do not put both on one battery.

With two 12V in series (assuming the batteries even support being in series) it is really important that both are at the exact same SOC before connecting in series and you might need to redo that once a year or so if the SOC starts to diverge over time. And if one fails your whole system is down.

But neither setup is really any better with regards to performance. You can lower life expectancy in either case if things are not setup correctly. Two 24V in parallel is a big win for reliability from the point of view of the whole system if one battery fails since you can keep running on the remaining battery (at a diminished capacity of course).

Full disclosure - I have two SOK 12V 206Ah batteries in series because they didn't have 24V batteries at the time. Once I got the two to the same SOC they have been working just fine. If they had the 24V batteries at the time I would have bought those instead.

 

[carolinabigfoot]

In the case you describe there is little practical difference. But I would lean toward two 24V batteries in parallel. They are easier to keep balanced, and as stated earlier, if one fails you still have a working system. Just make sure you connect the positive and negative load wires to opposite batteries. Do not put both on one battery.

With two 12V in series (assuming the batteries even support being in series) it is really important that both are at the exact same SOC before connecting in series and you might need to redo that once a year or so if the SOC starts to diverge over time. And if one fails your whole system is down.

But neither setup is really any better with regards to performance. You can lower life expectancy in either case if things are not setup correctly. Two 24V in parallel is a big win for reliability from the point of view of the whole system if one battery fails since you can keep running on the remaining battery (at a diminished capacity of course).

Full disclosure - I have two SOK 12V 206Ah batteries in series because they didn't have 24V batteries at the time. Once I got the two to the same SOC they have been working just fine. If they had the 24V batteries at the time I would have bought those instead.

What did you do to get them to the same SOC?

 

[rmaddy]

What did you do to get them to the same SOC?

First I wired both 12V batteries in parallel (yes, parallel, not series). I then connected a 12V LiFePO4-friendly battery charger. The positive lead on one battery and the negative lead on the other battery. I then let the charger run as needed to complete the full charge getting the batteries to 100%. When the charge was complete I removed the charger but I left the batteries connected in parallel for the day. This allowed them to settle and remain perfectly balanced at the same SOC.

If your two batteries are starting with very different SOC then you might want to fully charge each one individually first before connecting in parallel and completing the above steps.

The next day I separated the batteries and I then installed them into my electrical system connected in series.

 

[carolinabigfoot]

First I wired both 12V batteries in parallel (yes, parallel, not series). I then connected a 12V LiFePO4-friendly battery charger. The positive lead on one battery and the negative lead on the other battery. I then let the charger run as needed to complete the full charge getting the batteries to 100%. When the charge was complete I removed the charger but I left the batteries connected in parallel for the day. This allowed them to settle and remain perfectly balanced at the same SOC.

If your two batteries are starting with very different SOC then you might want to fully charge each one individually first before connecting in parallel and completing the above steps.

The next day I separated the batteries and I then installed them into my electrical system connected in series.

Cool! Is there a charger you can recommend. I have a NOCO genius 7.2/3.6, meaning the charging current for 12V is 7.2A and for 24 V 3.6A. Low charging current is probably good for battery health, but, obviously, charging will take a pretty long time.

 

[rmaddy]

I bought a Victron Energy Blue Smart IP67 12V/17A charger.

 

[carolinabigfoot]

I bought a Victron Energy Blue Smart IP67 12V/17A charger.

Thanks! You've been very helpful. Hope your off grid adventure is going well.

 

[wattmatters]

This allowed them to settle and remain perfectly balanced at the same SOC.

They would settle at the same voltage. How close that is to the same SOC will depend on a few things. If they are well matched batteries to begin with then they will be close enough.