Which is better - two 8-cell 24v batts or four 4-cell 12v batts for 24v system?

[Big Eats]

I will be installing 16 270ah cells in my RV as a 24v system. My initial concept was to build two 8-cell 24v batteries and then parallel them. I figured that I'd wind up saving some money on fewer BMSs and less cabling to deal with.

Well I also know that you don't know what you don't know... so it never hurts to ask.

Is there enough benefit to making four batts to justify the cost difference?

 

[jwelter99]

What BMS have you decided on?

 

[Big Eats]

What BMS have you decided on?

Overkill.

 

[jwelter99]

I'd do two 24V packs. Keep it simple.

 

[John Frum]

So the question is 4x 12.8 volt batteries vs 2x 25.6 volt batteries?

A 25.6 volt system is half the amperage for the same power so you can use smaller wires.

If you have 12 volt loads its a vote for a 12 volt system.
Double vote if the 12 volt loads draw substantial current.
If you wish to charge off a 12 volt alternator its another vote for a 12 volt system.
If you wish to run an inverter > ~2200 watts its a vote for a 24 volt system.

 

[jwelter99]

So the question is 4x 12.8 volt batteries vs 2x 25.6 volt batteries?

A 25.6 volt system is half the amperage for the same power so you can use smaller wires.

If you have 12 volt loads its a vote for a 12 volt system.
Double vote if the 12 volt loads draw substantial current.
If you wish to charge off a 12 volt alternator its another vote for a 12 volt system.
If you wish to run an inverter > ~2200 watts its a vote for a 24 volt system.

He clearly states a 24V system in the RV. So the end result is 24V and the question is over 2 parallel 24V batteries or 2p2s of 12v.

 

[John Frum]

He clearly states a 24V system in the RV. So the end result is 24V and the question is over 2 parallel 24V batteries or 2p2s of 12v.

Is there enough benefit to making four batts to justify the cost difference?

Making 4 batteries from 16 cells suggests 12 volts.

 

[jwelter99]

I will be installing 16 270ah cells in my RV as a 24v system. My initial concept was to build two 8-cell 24v batteries and then parallel them......

 

[Big Eats]

I'm sorry if I wasn't clear.

If I build four 4-cell packs - I will series each pair of packs and then parallel them. The goal either way is 24v.

I can see where maneuvering the smaller packs might be nice since these will be in the lower compartment of my bathroom closet - and the larger, heavier packs might be more awkward.

Is there a BMS-type reason to use one method over the other?

 

[Big Eats]

I'd do two 24V packs. Keep it simple.

Noted. Thank you.

 

[John Frum]

I'm sorry if I wasn't clear.

If I build four 4-cell packs - I will series each pair of packs and then parallel them. The goal either way is 24v.

I can see where maneuvering the smaller packs might be nice since these will be in the lower compartment of my bathroom closet - and the larger, heavier packs might be more awkward.

Is there a BMS-type reason to use one method over the other?

16 cells is ~200 pounds so I see your point.
You would also get 120 amps vs 100 amps throughput.
On the downside you would have 2x the number of bmss to fail and incur resistance.

 

[Big Eats]

16 cells is ~200 pounds so I see your point.
You would also get 120 amps vs 100 amps throughput.
On the downside you would have 2x the number of bmss to fail and incur resistance.

Actually while the number of BMSs goes up - so does the throughput. 2 x 24v x 100a = 4800w versus 4 x 12v x 120a = 5760w. Hmm... that is something to consider...

My inverter will be the Samlex EVO 4024 - so I think having the extra BMS capacity of using four of them might outweigh the cost.

 

[Just John]

If you need a 24v system, 8 cells and a single BMS is easier to maintain. You need to keep 4 batteries "balanced" rather than 2. You might try 4 cell packs with large gauge cables connecting them as an 8 cell battery.

 

[HRTKD]

When you put two 120 amp BMS in series, you still only have 120 amps. Yes, it's 20 more amps than the 100 amp 24v BMS, but my preference is to create the voltage of battery needed not put lower voltage batteries in series to build up to what you need.

So my recommendation is 8s2p.

To what smoothJoey is saying, if there are substantial 12v loads in the RV that have high amp requirements, staying 12v is the often the better way to go. But if the existing system is already 24v, then it's a no brainer.

 

[pheidingsfelder]

I had the same decision to make. I wasn't sure about a heavy pack being too difficult to get into my RV. I decided to go with the 24v 'native' setup using only the 2 24v JBD (overkill) BMSes. I figured I could get help lifting them into the RV.

 

[HRTKD]

I had the same decision to make. I wasn't sure about a heavy pack being too difficult to get into my RV. I decided to go with the 24v 'native' setup using only the 2 24v JBD (overkill) BMSes. I figured I could get help lifting them into the RV.

I thought about building my batteries in place in the trailer. But in the end, I built them in the garage with all the bus bars, BMS, compression frame and cables connected and then lugged them into the RV. I was dealing with just 4s batteries though. Knowing what I know now, I would do the same with an 8s battery. Anything bigger, no way!

 

[Big Eats]

When you put two 120 amp BMS in series, you still only have 120 amps. Yes, it's 20 more amps than the 100 amp 24v BMS, but my preference is to create the voltage of battery needed not put lower voltage batteries in series to build up to what you need.

So my recommendation is 8s2p.

To what smoothJoey is saying, if there are substantial 12v loads in the RV that have high amp requirements, staying 12v is the often the better way to go. But if the existing system is already 24v, then it's a no brainer.

Yes the two 120a BMSs in series will only deliver 120 amps... but with two of those packs paralleled - you get 240 amps at 24v... which delivers 5760 watts. The paralleled 8s packs will only deliver 200 amps at 24v... which is only 4800 watts.

The real question is then whether or not I need the extra overhead with the Samlex EVO 4024 inverter if I really load it up (not too likely).

 

[Big Eats]

I had the same decision to make. I wasn't sure about a heavy pack being too difficult to get into my RV. I decided to go with the 24v 'native' setup using only the 2 24v JBD (overkill) BMSes. I figured I could get help lifting them into the RV.

Yes the weight issue IS an issue... getting them in the RV is one thing - getting the heavier, larger pack into its final destination inside the RV is a little trickier. Ultimately - if it's only about the extra installation effort - then going your route makes sense (and keeping the extra money for the other two BMSs in my pocket).

 

[Big Eats]

I thought about building my batteries in place in the trailer. But in the end, I built them in the garage with all the bus bars, BMS, compression frame and cables connected and then lugged them into the RV. I was dealing with just 4s batteries though. Knowing what I know now, I would do the same with an 8s battery. Anything bigger, no way!

That's good to know. Thank you.

 

[HRTKD]

Yes the weight issue IS an issue... getting them in the RV is one thing - getting the heavier, larger pack into its final destination inside the RV is a little trickier. Ultimately - if it's only about the extra installation effort - then going your route makes sense (and keeping the extra money for the other two BMSs in my pocket).

Make sure the location you're going to place the batteries can handle the weight. I put in brackets (attached to the wall) and extra support to make sure I didn't stress anything.