Upgrade BMS

I have 2x SOK 12v206 in series for 24v, BMS rated max discharge 100 Amp, hybrid MPPPT lv2424 is 2400 watts output. When I run 120v tools (panels disconnected but full charge on SOK's) 9 amps (1080w) & below I can get many hours run time. When I go above 10 amps, the batteries BMS opens circuit on one of the pair (random) & kills the controller as 24v went away. So I can only use appx 1/2 of the controllers rated 2400 Watt output.

Can the BMS be upgraded to 150 amp discharge protected BMS ?? Pro/Cons ?, Any recommended part numbers or types ?, & more importantly, procedure / method to change it out.

ps; thanks Will for all your work over the years & especially the forum so we can all benefit from accumulated knowledge base from all the folks out there !!

Just based on the info provided, It seems like this is a surge current issue.

I assume the '10 amps' you are talking about is on the AC side. That translates into 10A X 120V = 1200W. If the inverter is 90% efficient that is (1200/.9)/25.6V=52.08A on the DC side. At the full 2400W of the inverter, the DC current is (2400/.9)/52.8=104.16A. The SOK batteries are rated for 100A so the system should be able to handle nearly the full 2400W.

James@N-Utah said:

When I run 120v tools....

Click to expand...

This tells me you are starting AC motors.... and the rule of thumb for motors is that the start-up surge can be ~4x the run-time current. The SOK BMS settings are known to be very aggressive at shutting down the battery on an over-current event so they do not handle surges very well. Even the Bluetooth version of the SOK gives you very little control over the overcurrent settings. Consequently, based on the data provided my guess is that the problem is start surges, not the continuous current.

Changing the BMS might be possible, but I do not know what kind of problems with the physical mounting and cabling might be encountered.
If you do decide to change the BMS, be sure the new one can be set to allow for a couple of seconds of the high surge before they shut down.

Even if the batteries are upgraded to handle the surge, you might find that the inverter has a problem handling the surges. The LV2424 is a high-frequency inverter and it can only do a 2x surge (and that for only a fraction of a second)

I don't know what tools you are using, but you might be able to install a soft-start on some larger stationary tools like table saws, band saws, and compressors. The nice thing about this approach is that it reduces the surge on both the inverter and the battery.

Thanks so much for taking the time !! I'm no sparky so greatly appreciate the math as well & it prompted me to review the manual again & it seems to solidify my thoughts that the discharge protection (BMS) was the kill switch.

These SOK's are in the metal cans so I thought if I could get to a higher rated BMS it would allow me a little farther up the "power ladder". I also have respect for the original engineering that led them to 100 amp BMS so I don't know if going higher to 150a would have drawbacks on the cells. I have no feel for how robust the cell makeup of LifePo are & whether they'd tolerate it well or I'm going to sacrifice cell life.

The unit I made is a roll-around so I can take power to where I need it, but do a periodic drive-by & plug into the panels when needed.

Give me your thoughts on the above mod, or, just get to a pair of 24's in parallel that have higher rated BMS's & I'll get more of that 2400w range I can use w/ portable tools

James@N-Utah said:

Thanks so much for taking the time !! I'm no sparky so greatly appreciate the math as well & it prompted me to review the manual again & it seems to solidify my thoughts that the discharge protection (BMS) was the kill switch.

These SOK's are in the metal cans so I thought if I could get to a higher rated BMS it would allow me a little farther up the "power ladder". I also have respect for the original engineering that led them to 100 amp BMS so I don't know if going higher to 150a would have drawbacks on the cells. I have no feel for how robust the cell makeup of LifePo are & whether they'd tolerate it well or I'm going to sacrifice cell life.

The unit I made is a roll-around so I can take power to where I need it, but do a periodic drive-by & plug into the panels when needed.

Give me your thoughts on the above mod, or, just get to a pair of 24's in parallel that have higher rated BMS's & I'll get more of that 2400w range I can use w/ portable tools

Click to expand...

Like I said, the issue appears to be the tool start surge. The SOKs are known to trip on overcurrent at around 120A so going to a 150A BMS will help, but unless it has better accommodations for handling a surge, it may not help much. What BMS are you thinking of changing to?

James@N-Utah said:

I also have respect for the original engineering that led them to 100 amp BMS so I don't know if going higher to 150a would have drawbacks on the cells. I have no feel for how robust the cell makeup of LifePo are & whether they'd tolerate it well or I'm going to sacrifice cell life.

Click to expand...

There are different factors here: Surge Current and continuous current.

If the BMS would allow for a larger & longer surge current, it would not have a significant impact on cell life. From what is described above, the cells are more likely to die from other factors like calendar life than from the surge current.

The continuous current impact is a bit harder to nail down. (What is the max continuous current you expect? As I mentioned before, you can drive nearly the full capability of the inverter with 100A so continuous current should not be a problem.

Note: 100 A on 100Ah cell is 1C discharge. This is certainly within the capability of LiFePO4 cells, but on the higher end of what some manufacturers recommend. Higher continuous current will cause more heat, and heat can degrade the battery, but without more info on the specific cells it is hard to say. The other factor in this is that the cells are in an enclosed case that will contain the heat of the BMS. This in turn will raise the ambient temp the cells operate in. One good thing is that the metal case is better than plastic for dissipating heat. If you went to a different BMS that would allow the surge but kept the total continuous current down to 100A, there should not be an issue. In fact, a higher current BMS is probably going to generate less heat at 100A than the factory BMS generates.

Also, you may be giving SOK more credit than they are due. They are selling to the value-minded market and 100A is kinda the norm for that market so they are unlikely to spend more on a higher amp BMS even if the system could handle it without harm.

Greatly appreciate your insights & knowledge !! Thanks for surge vs continuous view as they do bear separate consideration also appreciate your view on SOK / cost as they very much are economy segment.

I haven't had a chance to research BMS / specs yet, just thinking find one that's less touchy / more tolerant of high initial voltages / surges, &, fit in the area of the one that's there now, hopefully.

I do notice some battery spec's that show sudden discharge limits of 2 seconds or 10 seconds at an elevated amperage, but they are usually on the more expensive batteries that are better defined on the engineering aspects, but as for the BMS they use, which may be proprietary too them, so may be un-available as a retail part.

Had a crazy idea from a mostly mechanical minded guy, they put additional capacitors on HVAC compressor contactors & on controls systems, sometimes in the motor starting circuits, or, directly on the motors, maybe a small bank of capacitors may act a bit of an energy reservoir that could curb the hit on the BMS some, but I don't know if I'm dreaming or not.

Would appreciate when you may have a moment to steer / introduce me to the BMS world so I can start looking seriously at capabilities & spec's as well as sizes that may be out there, as well, my other thoughts, it would be appreciated very much !!

Also I'm thinking if time issue turns out too much for me, a pair of 24v 150Ah in parallel w/ 150 amp BMS's may let me get more of that 2400w MPPT range than I'm getting know, or hopefully w/ less touchy performance.

Thanks for letting me have some of your time & knowledge !!

James@N-Utah said:

Would appreciate when you may have a moment to steer / introduce me to the BMS world so I can start looking seriously at capabilities & spec's as well as sizes that may be out there, as well, my other thoughts, it would be appreciated very much !!

Click to expand...

The 100A JBD BMS may actually do what you need. It has settings for surge limits as well as short circuit limits. (Overkill sells the JBD). I have not looked at them recently so I don't know if they have good higher amp BMSs.

The JK line of BMSs might also be a good choice, You could put a 200A JK BMS is and set it to tollerate surges.

Thanks so much for your time, insight & knowledge !!

It has been a privilege to have your support & greatly appreciate the recommendations !!

Curious question, the soft start units you referred to earlier, is that a logic controlled / capacitor based smart unit, would be curious to know how much of capacitor bank they use if any & how big the units are ? could you steer me to some examples of soft-start units ?

Here are a few other ideas to fix your issue:

Add an extra 24v battery (or two). This will allow to surge to be spread out over more batteries. Plus it will give much longer life when running tools. You even want to look at a single 24v battery instead of two 12v units. In addition you could get some cells and a bms and DIY your own second battery. This is probably the most cost effective path.

Change out the bms (or new bms for second battery).

There are two main types of bms for switching- FET based and contactor based.

FET based is what you currently have and what all the cheaper BMS’s have (because it’s cheaper). These have an amp limit 100a or 150a. Overkill is also FET based (I have used the 12v overkill bms on a project and I like it).

The second type of bms is a contactor based bms. The bms controls a contactor (very large relay). The advantage here is the battery is only limited by the cells. The BIG disadvantage is cost. These bms’s are expensive plus the other needed equipment (contactor will be more than a fet based bms). The three brands I know of are Batrium, Rec-Bms, and Orion.

I installed a Batrium bms in my MotorHome. That battery has eight 272ah cells in a 12v battery- so a 544ah 12v battery. I wanted a very large 12v battery to keep the boost ability if it was ever needed. (using the lithium battery to help start the big Diesel engine). The contactor I used is a Kilovac EV200AAANA.

I would recommend buying/making an extra 24v battery- that will be the most efficient route.

Good Luck!

James@N-Utah said:

Curious question, the soft start units you referred to earlier, is that a logic controlled / capacitor based smart unit, would be curious to know how much of capacitor bank they use if any & how big the units are ? could you steer me to some examples of soft-start units ?

Click to expand...

I don't know what they have inside.

Do a search for "Soft Start for table saw" or "Soft Start for power tools"

Here is an example"