diy solar

diy solar

BMS recommendation for 4S 280ah battery

Appreciate the clarification here. My plan has been this exact configuration so was quite nervous reading the earlier posts.

I too am a little confused as to why you would want 2 BMSs on a single 4s - to pull twice the current? Note, still new here and reserve the right to pull the newbie card.
Don't worry, two 4s batteries in parallel is what Steve from Overkill recommended it sounds like there have been plenty of people running this configuration with no problems.

As far as why I could consider this @smoothJoey mostly outlined my use case with one addition I hope to add an additional 280ah battery in the next few years so having the two BMS's already makes that simpler. But mostly it is because I am a noob and the Overkill is easy to understand and highly regarded.

I am planning to use a 200a Daly here is hopeing the lack of documentation doesn't burn me.
 
I Haven't seen a definitive answer on this with respect to the JBD/Overkill BMS: For high current situations, can we bypass this particular model of BMS, altogether? I've thought about a 2p4s or 4s2p configuration and am hinging on the latter (so ignore the particular cell arrangement here, imagine, it could be with 2 BMS for all intents and purposes.

Imagine the black box is all t he DC loads I have. Let's say that a relay is set up so when the batteries detect an overcurrent/low SoC/undervoltage event, they shut off discharge at the FETs. Thus, in the diagram below, also opening a normally-open relay. This would allow the BMS to guard the battery from overdischarge, even if that overdischarge is a result of the inverter drawing too much power, correct?

If I bypass the BMS like in this diagram, does that have any affect on how the BMS evaluates state of charge? If state of charge information is gathered by the wires going to each individual cell (and thus as a compilation of cell voltages) rather than by what charge or discharge current passes through the BMS itself, then state of charge would effectively appear the same, regardless of if a load passes through (and is switcheed directly by) the BMS, correct?


A2sPyVZ.png
 
I must be missing something in looking at your schematic. From the drawing it looks like your Inverter is connected directly to the Battery (albeit thru the NO relay contacts). To me that means the BMS will never see the Discharge current that is being consumed by the Inverter ( the main source of large current loads). That would mean you have lost your "high discharge overcurrent" protection ( - not sure where you charging current is coming from so it might still be going thru the BMS) so the BMS would only "disconnect" based upon individual cell voltages.

Is that your intent?
 
I must be missing something in looking at your schematic. From the drawing it looks like your Inverter is connected directly to the Battery (albeit thru the NO relay contacts). To me that means the BMS will never see the Discharge current that is being consumed by the Inverter ( the main source of large current loads). That would mean you have lost your "high discharge overcurrent" protection ( - not sure where you charging current is coming from so it might still be going thru the BMS) so the BMS would only "disconnect" based upon individual cell voltages.

You do get the "low Vo;tage disconnect" at the set point for low voltage on the cells or at Battery level.
Is that your intent?
One correction to my comments. Your "DC" loads would be seen by the BMS but the BMS would not provide protection for "over current Discharge" unless all your "DC" Loads when above the Discharge current level. You still have not got the BMS to see the major load from the inverter .....
 
I think he wants a BMS, except when he doesn't.
confused3.gif
I've seen this recommended as a means of getting around the issue of passing a ton of amps through the BMS itself. I am surprised you guys are not familiar as it's been mentioned on this forum a bunch, in one form or another, with the JBD BMS, Daly, and others.

But like @WA5IDX was saying, the inability to monitor charge/discharge current is a worry. I wasn't sure if SoC/charge/discharge was calculated by monitoring individual cell voltages. Or, if that was calculated by monitoring the current passing through the BMS itself. If the former is the case, this would work. If the latter is the case, it wouldn't. And if the latter is the case, I suppose it's best to stick with 4s2p (two 4s batteries in parallel, each 4s battery with its own BMS).

The main issue I'm trying to avoid is burning out the FETs of a 120A JBD BMS while passing 100A through it to get a combined 200A to power my largest inverter loads.

For overcurrent protection @WA5IDX Could I not simply add a fuse or breaker? That handles overcurrent, and the individual cell monitoring by the BMS handles undervoltage, right? My only concern now is if the JBD BMS would, as I said earlier, not be able to monitor SoC accurately since the current consumed by the inverter is not passing through the BMS itself.
 
I think current is measured by what goes through the BMS. The FET based BMS doesn't seem to be a good fit for the relay-based cutoff. If you want relay-based cutoff, get a BMS that is designed specifically for that.

If I hadn't gone with the Overkill Solar BMS, then I would have given the Electrodacus system a hard look.
 
Will does utilize that "method" but notice he does not mention charge and discharge protection.

Steve, at Overkill, has mentioned several times that the Overkill BMS will operate on a continous basisc at rated capacity.

Heat is one of the major factors in destroying a BMS so if you want additional "safety" just add "cooling" via additional heat sink and/or forced air cooling.

Otherwise, I agree with HRTKD that a Heltec, Electrodacus, etc. Would be a better fit.

I'm adding more heat sink and a muffin fan that comes on (115v powered by inverter) when the inverter switches on (easy - pesey) to my two Overkill BMS jic.......
 
I have three of the Overkill BMS incorporated into my large off-grid home system, built with Outback Power Inverters and CC‘s. But I don’t pass any charge, or discharge, current through the BMS for several reasons:
1. Even though it is rated for 100-amps, the heat it dissipates is wasted energy.
2. Outback (and others) caution against disconnecting the load from the CC while solar panels are connected.
2. I could not bear the aesthetics of inserting those MOSFET’s and “puny” 10-gauge wires into my lovely battery interconnects of 2/0 copper welding cable.

instead, a BMS alarm trip will de-energize a small SPDT control relay, one for each BMS, that requires only a tiny hold-in current. On BMS alarm trip, the small SPDT control relay will energize a larger 3PDT relay that has three functions:
1. Activate the inverter remote STOP contacts.
2. Properly shut down the charge controller.
3. Energize a high-capacity normally-closed Gigavac solenoid that disconnects the breaker panel that distributes DC power through my home.

In the event of any BMS alarm trip, from any of the three battery packs, everything shuts down. That might seem inconvenient, but since I (so far) never get a BMS alarm trip, it is really no trouble at all, and I have no concern. This was $120 of added cost for parts, but is worth it to me as a way to manage a large system.

There are many ways to skin this cat. Circumstances vary.
 
Last edited:
Johncfil - That's a good way to handle "most" of the safety features but I'm still not clear as to how you get the BMS to see an "overcurrent condition ( charge or discharge).

How do you handle those two "safety" conditions?
 
Johncfil - That's a good way to handle "most" of the safety features but I'm still not clear as to how you get the BMS to see an "overcurrent condition ( charge or discharge).

How do you handle those two "safety" conditions?
Good question .... I do not rely on the BMS for this function. My Outback Power monitoring system already had that function, plus kWh and amp-hour metering, built in. Plus my sources and loads are very conservatively rated, so that I probably will never get close to over-current In either direction.

So far, I have not perceived any disadvantage arise out of not being able to monitor amp-hours in/out of individual battery packs. The BMS BT app makes it simple to monitor the individual cell voltages regularly, and I’m confident that those cell voltages will foretell any developing problems.

i think that the closest I would come to a problem would be if two of my three battery packed were tripped off line by their respective circuit breakers. Then the remaining battery pack would see the full load/charge. But it is also protected by a circuit breaker that should trip before the battery pack could be damaged by over-current. My main desire is to protect individual LiFePO4 cells from over-charge, and over-discharge.

I would be interested to hear any other suggestions of weakness w.r.t my solution. I might have overlooked something
 
Last edited:
Johncfil - That's a good way to handle "most" of the safety features but I'm still not clear as to how you get the BMS to see an "overcurrent condition ( charge or discharge).

How do you handle those two "safety" conditions?
That is what fuses are for. ;)
 
Not to be picky but with a combined Inverter/Charger that might be difficult since the "feed cables" to/from the Battery Bank to the Inverter/Charger is both Charge and Discharge. The "Discharge" fuse would be a much higher value than the "Charge" fuse.

With a separate Charge and Inverter setup, I could see the fuses working. Simple even if not as "elegant" as the BMS. I'd suggest Circuit breakers however since they can be reset.
 
Not to be picky but with a combined Inverter/Charger that might be difficult since the "feed cables" to/from the Battery Bank to the Inverter/Charger is both Charge and Discharge. The "Discharge" fuse would be a much higher value than the "Charge" fuse.

There is probably no one solution that perfectly fits all systems.
I have a circuit breaker AND a (larger) fuse in series with each battery pack.
Given the huge store of energy, that makes me feel better.
 
Last edited:
Not to be picky but with a combined Inverter/Charger that might be difficult since the "feed cables" to/from the Battery Bank to the Inverter/Charger is both Charge and Discharge. The "Discharge" fuse would be a much higher value than the "Charge" fuse.

With a separate Charge and Inverter setup, I could see the fuses working. Simple even if not as "elegant" as the BMS. I'd suggest Circuit breakers however since they can be reset.
I see your point.
I'm not to worried about a charge source giving more than its rated/configured current.
Maybe I should be.
 
Last edited:
I see your point.
I'm not to worried about a charge source giving more than its rated/configured current.
Maybe I should be.
Could you not just add a fuse/breaker inline between the charge source and the charger/inverter? i.e. if you have a 120VAC input from a RV hookup, have a fuse on the 120VAC before it reaches the charger/inverter? I'd assume if there was a fault (e.g. a short) on the inverter/battery side of things, and you had a fuse for the max discharge current there, that'd still open as such a fault would cause hundreds more amps to flow than what anyone would need as their max discharge current?
 
With a separate Charge and Inverter setup, I could see the fuses working. Simple even if not as "elegant" as the BMS. I'd suggest Circuit breakers however since they can be reset.
Given the three disadvantages that I associate with using a low-cost, unsophisticated, DIY-solar, BMS in the traditional manner, I don’t think they are a very “elegant” solution for large capacity systems. But I understand what you mean.

Unless one is ready to spend a ton of extra money, some compromises are probably required with any solution.

John
K7KEY
 
Last edited:
Could you not just add a fuse/breaker inline between the charge source and the charger/inverter? i.e. if you have a 120VAC input from a RV hookup, have a fuse on the 120VAC before it reaches the charger/inverter?
You do need a fuse/breaker like that, but that would not be sufficient. A proper fusing scheme needs to account for the possibility of a short circuit, or excess current, anywhere in the system.
 
Last edited:
Given the three disadvantages that I associate with using a low-cost, unsophisticated, DIY-solar, BMS in the traditional manner, I don’t think they are a very “elegant” solution for large capacity systems. But I understand what you mean.

Unless one is ready to spend a ton of extra money, some compromises are probably required with any solution.

John
K7KEY
LARGE capacity is relative.......but I whole heartedly agree. I am building a 544 AHr battery ( 2 each 272 AHr Lishen ) each with their own 120A BMS. To me that's a LARGE RV system but a "small" system in any other application. LOL.....
 
I don't have much to offer other than to say thank you to everyone that is giving input on this question.
This board is really my go-to source for finding out cutting edge techniques to solve my solar challenges.
I wanted to tackle the DIY LiFePo for the same reason that I wanted to put brakes on my F150.
It's cost effective and I can do it probably as good or better than the local shop.
Also - I'll know every component of the system.

I just got 4 of the EVE cells and have decided to use the Overkill 120amp 12 Volt 4S unit with M6 studs. It's going into an RV and I will be using it for DC power only - maybe add a 1000 watt inverter to run a laptop or two (not running the A/C, or microwave).
Gotta' start somewhere right ! I will be using a Victron 100/30 Smart Solar, Lynx, and a handful of ground deployed panels.

Keep posting - this is all great info...

Albuquerque Tom
 
Back
Top