“Oh, btw...Overkill Solar has an 8s BMS”

[Stewfish]

@Stewfish I'm currently using a 1200w system for a backup powerwall system for my computer and 3d printers. Currently that's 12v and I'm upgrading to a 24v system. I can't integrate with my house wiring due to renting and these work for my purposes of having large capacity backups for critical demands.

Your math kind of hurts my brain a bit so let's try to step back a bit. Let's assume you're cells are 100Ah with a 1C discharge rate or 100A/hour), 1s1p at LiFePo4 is 3.2V. A nominal 8s1p (which means 8 in series 1 parallel) is 24v at 100Ah. . Putting 2x 8s1p systems in series would be 16s1p at 100Ah nominally 48v. 1C discharge rate of 100A/hour or 4800W/hour.

Now to extrapolate to a more real situation, 32x 280Ah cells (often at .5c discharge rate or 140A/hour max per cell) this is a total of 8960Ah of batteries in some combination of cells in series and parallel. Let's say it's 16 series 2 parallel or a nominal 48v 560Ah capacity. This would often be discharged at a Max 280A or .5c discharge rate based on the batteries.

Now you think about the BMS. You want your BMS to be able to cover all the power you are going to draw from that system. When you have sets in parallel that is where you add. In the previous example, a 16s2p, if you used 100A BMS with 2 in parallel, your system could draw up to 200A.

This kind of works but really you should have one BMS to cover the battery bank. The reason you want one BMS is because if you have 2 BMS in parallel,let's say 2x 100A each supporting 16s for a total load of 200A at 48V or 9600W, and one of the BMS' trips due to a low cell, now that entire load needs to be serviced by the one battery and BMS so you are likely to over draw. The ideal solution is to have a bms that is sized for your battery and intended load but if you must putting 2 in parallel to increase draw is a reasonable option. I say one battery and BMS because you don't want to parallel BMS on the same battery bank because then you have the possibility of one BMS getting out of sync with the other BMS when it comes to charging profiles and cut off currents and voltages.

I hope this helps some. In my mind if you have 32 cells and a 16s BMS you would want to have a 16s2p battery bank with a BMS that covers the max current needed up to say a .5C discharge rate to keep the batteries happy.

Awesome that makes sense, right off the bat it makes me wonder

If I have two 3kw MPP all in ones like the 3048 LV
Can I connect one to each 16s but then draw off both to get a combined/stacked 6k from the 32 cells

 

[wings2004]

Awesome that makes sense, right off the bat it makes me wonder

If I have two 3kw MPP all in ones like the 3048 LV
Can I connect one to each 16s but then draw off both to get a combined/stacked 6k from the 32 cells

Certainly can! I'm not sure about the 3048 but for instance the 2424 has a parallel kit where you can tie them together. In fact that would be an alternative good configuration assuming it's parallel in the IC and not at the BMS level. If each IC would be charging and discharging their leg, each would have 1 BMS per 16s battery pack, that seems reasonable. I don't know how nicely the MPPs handle parallel draws but I believe this would be a good configuration. At the end of the day you want all the batteries to be protected and drawing at roughly the same rate, I think this would work but you need to check on the MPP IC to confirm how they handle the parallel loads.

 

[Skippyp]

@Stewfish I'm currently using a 1200w system for a backup powerwall system for my computer and 3d printers. Currently that's 12v and I'm upgrading to a 24v system. I can't integrate with my house wiring due to renting and these work for my purposes of having large capacity backups for critical demands.

Your math kind of hurts my brain a bit so let's try to step back a bit. Let's assume you're cells are 100Ah with a 1C discharge rate or 100A/hour), 1s1p at LiFePo4 is 3.2V. A nominal 8s1p (which means 8 in series 1 parallel) is 24v at 100Ah. . Putting 2x 8s1p systems in series would be 16s1p at 100Ah nominally 48v. 1C discharge rate of 100A/hour or 4800W/hour.

Now to extrapolate to a more real situation, 32x 280Ah cells (often at .5c discharge rate or 140A/hour max per cell) this is a total of 8960Ah of batteries in some combination of cells in series and parallel. Let's say it's 16 series 2 parallel or a nominal 48v 560Ah capacity. This would often be discharged at a Max 280A or .5c discharge rate based on the batteries.

Now you think about the BMS. You want your BMS to be able to cover all the power you are going to draw from that system. When you have sets in parallel that is where you add. In the previous example, a 16s2p, if you used 100A BMS with 2 in parallel, your system could draw up to 200A.

This kind of works but really you should have one BMS to cover the battery bank. The reason you want one BMS is because if you have 2 BMS in parallel,let's say 2x 100A each supporting 16s for a total load of 200A at 48V or 9600W, and one of the BMS' trips due to a low cell, now that entire load needs to be serviced by the one battery and BMS so you are likely to over draw. The ideal solution is to have a bms that is sized for your battery and intended load but if you must putting 2 in parallel to increase draw is a reasonable option. I say one battery and BMS because you don't want to parallel BMS on the same battery bank because then you have the possibility of one BMS getting out of sync with the other BMS when it comes to charging profiles and cut off currents and voltages.

I hope this helps some. In my mind if you have 32 cells and a 16s BMS you would want to have a 16s2p battery bank with a BMS that covers the max current needed up to say a .5C discharge rate to keep the batteries happy.

Great response. Considering 2x 4s myself. Now reconsidering

 

[pheidingsfelder]

I'm looking to build my first LiFePO4 setup using 8 of the 280ah cells in 24v. I will be running the multiplus 3000 inverter/charger.
Question is what BMS? I'm at 125amp max on the inverter so I guess the overkill BMS is out? I'm thinking I would upgrade down the road and add a second 8 cell setup with it's own BMS wired in parallel. Is this not the best method?

 

[K8MEJ]

I'm looking to build my first LiFePO4 setup using 8 of the 280ah cells in 24v. I will be running the multiplus 3000 inverter/charger.
Question is what BMS? I'm at 125amp max on the inverter so I guess the overkill BMS is out? I'm thinking I would upgrade down the road and add a second 8 cell setup with it's own BMS wired in parallel. Is this not the best method?

Brother, I'm in the same boat - or RV - as you. I, too, am building a system around 8 EVE 280Ah cells and plan to run the Multiplus 3000/24. As complicated as the whole system is, finding a suitable, reliable BMS with the ability to program the BMS via Bluetooth and reliable low-temp cutoff is proving to be the most complicated item. I wanted to run 24v to make the wiring size more manageable, but I'm starting to think a 12v system might just be easier and would also mean I don't need to buy a 24-12v DC-DC converter, too.

I found this one: 1005001270754639.html but I don't know much about it. I think it's from the same OEM that Overkill Solar uses, but I don't really know for sure.

 

[pheidingsfelder]

Brother, I'm in the same boat - or RV - as you. I, too, am building a system around 8 EVE 280Ah cells and plan to run the Multiplus 3000/24. As complicated as the whole system is, finding a suitable, reliable BMS with the ability to program the BMS via Bluetooth and reliable low-temp cutoff is proving to be the most complicated item. I wanted to run 24v to make the wiring size more manageable, but I'm starting to think a 12v system might just be easier and would also mean I don't need to buy a 24-12v DC-DC converter, too.

I found this one: 1005001270754639.html but I don't know much about it. I think it's from the same OEM that Overkill Solar uses, but I don't really know for sure.

Hopefully someone that has built a successful 24v battery will chime in!

 

[nwillitts]

use 2?

 

[K8MEJ]

use 2?

I don't know if you're trolling or not, but I'll bite.

How does that work having two BMS's trying to keep the cells in balance when the individual BMS's don't communicate with other? How about when temperatures hover near the low temp cutoff setting and within the error range of each separate BMS's temperature sensors? In that case, one BMS might want to cutoff charging while the other doesn't? I suppose in that case you could set the low-voltage cutoff temp a bit higher on each BMS to try and prevent that scenario. I've also read that OverKill Solar doesn't recommend doing what you propose, but I haven't asked them directly.

 

[HRTKD]

I think he meant two batteries.

 

[pheidingsfelder]

I think he meant two batteries.

So building two 4S batteries with a 4S BMS then paralleling those together?

 

[HRTKD]

So building two 4S batteries with a 4S BMS then paralleling those together?

Yes, as long as each 4s battery has its own BMS. Two batteries, two BMS.

 

[K8MEJ]

I think he meant two batteries.

That's my fall-back position. I'd prefer 24v to keep the wire gauge down but I might not have much choice. Thanks.

 

[HRTKD]

How long is the run where you actually care about wire gauge? In my case, the batteries and inverter are sitting 15" apart. 30" of 4/0 cable isn't going to break the bank.

Yes, there are other reasons to go 24v. Primary in my mind is the MPPT controller. But in an RV, especially a fifth wheel or motorhome, there are some 12v devices with very large amp draws that have to be accounted for.

 

[pheidingsfelder]

How long is the run where you actually care about wire gauge? In my case, the batteries and inverter are sitting 15" apart. 30" of 4/0 cable isn't going to break the bank.

Yes, there are other reasons to go 24v. Primary in my mind is the MPPT controller. But in an RV, especially a fifth wheel or motorhome, there are some 12v devices with very large amp draws that have to be accounted for.

Would it be advisable to build the two 4S batteries with separate BMS's then putting the two 12v banks in series for 24v? That's what I'm leaning towards if I can't find a decent 8s BMS for a single 8s 24v bank

 

[HRTKD]

Would it be advisable to build the two 4S batteries with separate BMS's then putting the two 12v banks in series for 24v? That's what I'm leaning towards if I can't find a decent 8s BMS for a single 8s 24v bank

If you put two 4s batteries in series, you still have only 120 amps of BMS rating. If they are in parallel then you get 240 amps of BMS capacity.

 

[nwillitts]

I think he meant two batteries.

yes,this.it is what i do.i have 2 sets of 24 using overkill batteries and BMS's.to power my 2000 watt inverter-the load splits in half.
am now building the same thing but in 12 volt to increase amp output.

 

[K8MEJ]

How long is the run where you actually care about wire gauge? In my case, the batteries and inverter are sitting 15" apart. 30" of 4/0 cable isn't going to break the bank.

Yes, there are other reasons to go 24v. Primary in my mind is the MPPT controller. But in an RV, especially a fifth wheel or motorhome, there are some 12v devices with very large amp draws that have to be accounted for.

Hi Jim. In my case they will also be close together, within a few feet. It's not so much the cost of the cabling, it's the ease of routing wires more than anything else and that may be the least of my problems in actuality. I would need a pair of large gauge wires that run from the under the bed in the front of my TT to about 2/3 back where the existing power center is. But even that isn't too expensive. In the end, this may turn out to the be the better option anyway because it eliminates the need for a 24v-12v DC-DC converter, which itself could cause other issues with my ham radios if it emits RFI. And it means I don't have to maintain a separate 12v for the breakaway system and tongue jack or need a DC-DC converter for those large loads.

Now that I've typed all of this, I'm starting to think maybe I should give up my plan for a 24v system

 

[MrM1]

Here is what appears to be an 8s with a 200amp discharge rating from JBD. Would this be the same as the Overkill solar 8s 100amp except in a 200amp version? If you buy direct from AliExpress do you get the harness and temp prob?

116.4US $ |7S 8S 13S 14S 16S 20S 24S li ion or Lifepo4 Battery PCB board with 120A 200A constant current for auto car 48V 72V Battery|Integrated Circuits| - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

 

[Cdkipp]

Here is what appears to be an 8s with a 200amp discharge rating from JBD. Would this be the same as the Overkill solar 8s 100amp except in a 200amp version? If you buy direct from AliExpress do you get the harness and temp prob?

116.4US $ |7S 8S 13S 14S 16S 20S 24S li ion or Lifepo4 Battery PCB board with 120A 200A constant current for auto car 48V 72V Battery|Integrated Circuits| - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

This one does not have the Bluetooth so I wonder about changing parameters.

 

[HRTKD]

Here is what appears to be an 8s with a 200amp discharge rating from JBD. Would this be the same as the Overkill solar 8s 100amp except in a 200amp version? If you buy direct from AliExpress do you get the harness and temp prob?

116.4US $ |7S 8S 13S 14S 16S 20S 24S li ion or Lifepo4 Battery PCB board with 120A 200A constant current for auto car 48V 72V Battery|Integrated Circuits| - AliExpress

Smarter Shopping, Better Living! Aliexpress.com

www.aliexpress.com

There was some other feature missing from the higher amp version as I recall. I would very carefully compare the specs before buying.