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

[Big Eats]

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.

I'm not eliminating my 12v native system, nor the 12v 2000w inverter. The 24v system I'm installing (4000w) will be in addition to the existing system with the intention of removing it in the future when I sell the RV. The new install will simply be acting like a 30a shore power connection.

 

[Big Eats]

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.

Good advice. Thank you.

 

[Daddy Tanuki]

jumping into this discussion just a little late as I have some questions myself.

My current system is a 2p16s 48vdc system at my cabin. I am in the process of ordering another 32 cells to double my capacity and the question(s) that are running through my mind are as follows:

is one large pack, 4p16s better for charge/discharge purposes or would two 2p16s be better?
if smaller separate packs are better would I be better off with four (4) 16s packs be better?
batteries are all 200 a/h calbs.

is one setup better than the other in regards to keeping the batteries balanced? I am thinking that having four (4) 200 amp hour banks would lead to some packs charging or discharging at different rates due to differences in resistance between cables, busbar's etc.

the pros and cons cost wise are irrelevant to me in this endeavor as the batteries are more expensive than busbar's, cables or even BMS systems I already have three (3) DALY BMS's and 1 Chargery unit with the digital relays. so I would only need one more if i decided to break it up into four packs. if I go with two packs then I have a spare, if I choose one large pack the Chargery system could handle all of them.

so anybody who has real world experience with large packs have any knowledge to share? seems to me that even LA or AGM large packs would follow the same basic rules in so far as the distribution via busbars/cables, but not having empirical evidence I have come asking advice.

R

Ken

 

[Dynoman]

I don't know enough about 48 volt Multi battery, but FilterGuy has a PDF (see attachment) in Resources on this site that shows a 32 cell 48 Volt battery. It is a Parallel First configuration. If that configuration would work for you it would allow for two 48 volt batteries. This would give redundancy so that if one battery cut out or needed maintenance you would still have a battery online. What I don't know is if two 32 cell 48 volt batteries in Parallel First configuration can be used in parallel? It would seem so, but I don't have any experience with that configuration.

In the question of a 24volt system the two 24 volt batteries would seem to be the better choice than four 12volt batteries.
The reason I say this is if one is using four 12 volt batteries and one of the 12 volt batteries is taken out of the system by a BMS over voltage there would then be a 12 volt battery in parallel with a 24 volt battery. I assume that the BMS in the lone 12 battery would cut out also, but it is a risk.

If using two 24 volt batteries and one of the 24 volt batteries cut out due to a BMS overvoltage there is a 24 volt battery still in the system. This should work fine as long as the single 24 volt battery can still handle the load. One would need to design the system to ensure one 24 volt battery can handle the loads while keeping the BMS current limit at around 80-90%. So if a BMS is rated at 100 amps this would be 80-90 amps for safety reasons. I personally designed my system to around 80%.

 

[Daddy Tanuki]

I don't know enough about 48 volt Multi battery, but FilterGuy has a PDF (see attachment) in Resources on this site that shows a 32 cell 48 Volt battery. It is a Parallel First configuration. If that configuration would work for you it would allow for two 48 volt batteries. This would give redundancy so that if one battery cut out or needed maintenance you would still have a battery online. What I don't know is if two 32 cell 48 volt batteries in Parallel First configuration can be used in parallel? It would seem so, but I don't have any experience with that configuration.

In the question of a 24volt system the two 24 volt batteries would seem to be the better choice than four 12volt batteries.
The reason I say this is if one is using four 12 volt batteries and one of the 12 volt batteries is taken out of the system by a BMS over voltage there would then be a 12 volt battery in parallel with a 24 volt battery. I assume that the BMS in the lone 12 battery would cut out also, but it is a risk.

If using two 24 volt batteries and one of the 24 volt batteries cut out due to a BMS overvoltage there is a 24 volt battery still in the system. This should work fine as long as the single 24 volt battery can still handle the load. One would need to design the system to ensure one 24 volt battery can handle the loads while keeping the BMS current limit at around 80-90%. So if a BMS is rated at 100 amps this would be 80-90 amps for safety reasons. I personally designed my system to around 80%.

Dynoman,

Thanks, he mentioned exactly what I was worried about in both scenarios. For a 4P16S pack you have to worry about the balancing across 4 cells, one could wander and the other three can cover for it. and on the other hand if you wire as a 16S4P then the individual 16S strings might not balance against each other. any clue on what he means by "corner Cases" in reference to cell packs?

Edit, Never mind, quick google search brought up the meaning in electronics. had never heard it called that before.

R

Ken

 

[Dynoman]

Thanks, he mentioned exactly what I was worried about in both scenarios. For a 4P16S pack you have to worry about the balancing across 4 cells, one could wander and the other three can cover for it. and on the other hand if you wire as a 16S4P then the individual 16S strings might not balance against each other. any clue on what he means by "corner Cases" in reference to cell packs?

Edit, Never mind, quick google search brought up the meaning in electronics. had never heard it called that before.

R

Ken

Ken,
I believe It is and engineering term. I found this explanation. Hope it helps.

In engineering, a corner case (or pathological case) involves a problem or situation that occurs only outside of normal operating parameters—specifically one that manifests itself when multiple environmental variables or conditions are simultaneously at extreme levels, even though each parameter is within the specified range for that parameter.

For example, a loudspeaker might distort audio, but only when played at maximum volume, maximum bass, and in a high-humidity environment. Or a computer server may be unreliable, but only with the maximum complement of 64 processors, 512 GB of memory, and 10,000 signed-on users. The investigation of corner cases is of extreme importance as it can provide engineers with valuable insight into how corner case effects can be mitigated.

You may want to PM FilterGuy and ask him questions. He offers helpful insight to others on this forum.