BMS Inception..what to do?

Hello bright minds,

I have a question I have a pack that is just about done its made up of Renogy batteries 1s4P. Each battery is 24V/25AH. Per Renogy's website the batteries have an internal BMS and they "self balance" in a parallel setup. My question is, should I put a 3rd party BMS on it? Or just use a shunt (I have one already both a smart and "dumb" one with the screen). I want to be able to see the state of each battery independently.

I'm not sure if its "ok" to add a BMS to a battery that has a BMS. I would turn off any balancing features on the 3rd party BMS, again merely to have insight into each cell. I haven't looked at the "dumb" shunt that closely yet but I thought (could be totally wrong here) that if the shunt loses power, that all the configuration/settings are lost and have to be re-done. If so that's a problem because I don't want the LCD display of the shunt using up power when I'm not looking at it. I was planning on cutting out a nice square hole for the shunt monitor screen. But again I don't remember what happens when it loses power. I gotta find the manual and look that up, or just go on the website and download the manual..

Thoughts/suggestions/comments/revelations about the Renogy batteries internal BMS (which I have zero insight into or control they are not "smart batteries). How you would go about checking the battery pack status as a whole / each battery from a panel (or Bluetooth app). I don't want to open up the box I'm building for it every time I want to check status, that's lame. The smart shunt is for another project so that one is used up.

Thanks in advance for your sage advice!!

Balance needs access to the cells in the battery. Instead of adding a bms on top why not just add 4 battery monitors? If you get lcd screens where you can turn off the backlight the power draw is tiny.

The shunt is just a chunk of metal in the path of the current that has a known resistance. The monitor is just a voltmeter that reads the voltage and translates that to a SOC. The voltage drop across the shunt is typically 50mv but that can vary by vendor and quality.

Which shunts are you taking about?

cyberfed said:

Hello bright minds,

I have a question I have a pack that is just about done its made up of Renogy batteries 1s4P. Each battery is 24V/25AH. Per Renogy's website the batteries have an internal BMS and they "self balance" in a parallel setup.

Click to expand...

Parallel batteries force each other to be 'the same voltage' - e.g. this is part of the definition of parallel. In other words, each of the 4p batteries will have the same voltage as the others all the time. Each individual battery with it's own BMS will keep the internal cells of balanced / protected.

So you don't actually need to do anything for a 1s4P of 24v batteries (with their own BMSs) to run smoothly.

cyberfed said:

My question is, should I put a 3rd party BMS on it? Or just use a shunt (I have one already both a smart and "dumb" one with the screen). I want to be able to see the state of each battery independently. I'm not sure if its "ok" to add a BMS to a battery that has a BMS.

Click to expand...

It's OK but they don't make 1s4p (where each p = 24v) BMSs.

It sounds like you'd like to know if each 24v battery 'overall' is functioning? If so then a simple volt meter on each one will provide proof of life that the BMS is allowing power in and out. The voltage on each individual battery will be the same as the other 3 if all is well. If the BMS is functioning and has balancing and the 4p batteries are all the same voltage and the batteries are the same capacity - then by definition, the cells in each battery are balanced / all very similar voltage.

If you want to know or 'see' the status of each cell in each battery then you need BMSs on each individual battery that can give you cell level data. Maybe you already have this?.

If you want to know the state of charge, then an overall shunt or a shunt per battery can give you this. However, if all 4 parallel batteries are working then the state of charge will be the same on each one (e.g. this is part of what parallel means).

Ok yeah that makes sense, thank you! Yeah I know by the very nature of parallel they will even out on their own. In this case there's an internal BMS also helping making that happen. I wanted to see the health of each battery, in case one is off, and you gave me a very simple answer, slap a small volt gauge to each of the 4 battery terminals. That will tell me what each one is at. Haha I didn't even think about the fact of a 1S4P BMS not existing!

I have the Renogy "500A battery monitor with shunt" and I have another, "Renogy Battery Shunt 300" aka their version of the a "smart shunt" with Bluetooth and other wizbang features.

The 300 Bluetooth is going to be used for a big 24V pack that is has a lot of AH's. You know I overbuilt this particular system, if I were to listen to Renogy they would have me run cable from each POS/NEG terminal in a line down each battery and then connect the ends to my terminal posts. No fuses, nothing in their pictures. Well I'm not down with that. I'm using a busbar as well as a t-fuse between each battery and one again on the cable that will be the main POS going to my terminal post. I added disconnect switches too for each battery. I'm not a huge fan of Renogy, but to be fair I was blindly new at all this when I bought those batteries.

Get this, the battery says (stamped as part of the label on the battery) max continuous charge 12.5A. Then I looked at their webpage for the battery and the advertised pictures and text said 25A was the max charge. I opened up a ticket with them and I had them confirm with engineering 25A is correct. Talk about confusing and lame. I also didn't pay attention the max continuous discharge is 25A, which is a bummer but fortunately I don't have heavy amp pulling devices I'm using these for, but still!

Thanks all! About to post another question unrelated to this build.
Cheers!