Have I bought the right BMS?

Matty-Lee · Jan 21, 2021

Hello all,

First post and I know fairly little about the subject of DIY solar but I'm building a camper van and want to build my own 12v LiFePO4 battery for use with solar, all with the help of YouTube.
I've bought the 4 3.2v cells and I'm waiting for them to arrive.
Today the 60A Daly BMS I ordered has arrived and I'm just confused by the Discharge current being 60A and the Charge current being 30A.

I plan to run a 12v system powered by a single 380W solar panel which will feed into a 40A charge controller which will then charge the 12v battery through this BMS.
By my calculations I'd only require a 30A charge controller but I went a little higher for upgrade-ability.
For the same reason I thought I'd go a little higher on the BMS and choose the 60A option but looking at it, is says 30A for charging.

Could someone clear up the difference between the Discharge current and the Charge current and tell me if this BMS is OK to use with the system?

Thank you,

Matt

FilterGuy · Jan 21, 2021

Matt,
You might want to review this resource:

Beginners Summary of BMS Functions, Types and Features

This is a beginner's summary of the common types, functions and features of BMSs. The descriptions do not attempt to list all information of all BMSs, but it is intended to give you a base understanding of BMSs The document is in 6 sections: 1...

diysolarforum.com

Meanwhile:

The Discharge current is the amount of current coming *out* of the battery that the BMS can handle. There is no info in your post about what your loads are so I can't say if the BMS can keep up.
The Charge current is the amount of current going *into* the battery that the BMS can handle. For this type of BMS it is very common for the charge current to be a lot less than the discharge current. For your situation, I would say the BMS is marginal at best. Your solar panel and controller will be able to produce ~30 A and when you upgrade it will be even more.

To make the situation worse, there is a general feeling on the forum that these type of BMSs should be de-rated by ~40% in order to ensure good longevity. By that rule of thumb, the 30A charge should be closer to .6 * 30 = 18A. Note: We are usually talking about de-rating the discharge spec.... You might be able to be a bit more aggressive with charge current, but even if you de-rate by only 20% it still comes out to .8 * 30 = 24A.

BTW: You have a Daly BMS. (and not the Smart version). Those are used a lot and are reasonably well regarded. Some people use them at their full rating.... so you might be able to get away with out de-rating it.

fisherus · Jan 21, 2021

You should be fine with those settings however, you failed to state how you acquired the DALY. I would suggest if you order directly from DALY you go with a 60/60. Your panel won't come close to producing over 10AMPS so you have nothing to worry about even if you paralelled another panel.

FilterGuy · Jan 21, 2021

fisherus said:
Your panel won't come close to producing over 10AMPS so you have nothing to worry about even if you paralleled another panel.

Matty-Lee said:
I plan to run a 12v system powered by a single 380W solar panel

380W/12V = 31.66A Even if I assume the top end of the voltage (14.6V) it comes out to 26A

That seems to be a lot more than 10A. What am I missing?

Matty-Lee · Jan 22, 2021

FilterGuy said:
Matt,
You might want to review this resource:

Beginners Summary of BMS Functions, Types and Features

This is a beginner's summary of the common types, functions and features of BMSs. The descriptions do not attempt to list all information of all BMSs, but it is intended to give you a base understanding of BMSs The document is in 6 sections: 1...

diysolarforum.com

....

Thanks for the great reply. It is helpful.

I bought the BMS from the DALY Official Store on AliExpress and didn't post a link because as a new poster I didn't want to be thought of as a spammer.

I deliberately went for as simple as I could get to try to keep things simple for myself.

I think what I'll do is buy the 80A common port version of this instead which has a 40A charging capability now that I've read the spec info on the listing

. I like the fact that it doesn't have a fan so will be silent.

I had looked at that model before but it was twice the price and the only extra feature it has that I was interested in is the low temp disconnect but after watching this Will Prowse video I decided it wasn't worth paying for.

If you know of any other simple Daly BMS I can get from that store that would be suitable do let me know.
My batteries are 3.2v.

Is the smart version wort getting instead?

Thanks,

Matt

Dzl · Jan 22, 2021

Matty-Lee said:
Thanks for the great reply. It is helpful.

I bought the BMS from the DALY Official Store on AliExpress and didn't post a link because as a new poster I didn't want to be thought of as a spammer.

I deliberately went for as simple as I could get to try to keep things simple for myself.

I think what I'll do is buy the 80A common port version of this instead which has a 40A charging capability now that I've read the spec info on the listing . I like the fact that it doesn't have a fan so will be silent.

I believe the common port versions have symmetrical current ratings. It is the separate port versions that will have lower charge than discharge. At least that is my recollection.

Matty-Lee · Jan 22, 2021

Dzl said:
I believe the common port versions have symmetrical current ratings. It is the separate port versions that will have lower charge than discharge. At least that is my recollection.

Thanks, if you take a look at the Common Port spec sheet of 80A one I mentioned above it says they are different. Well at least for these ones.

fisherus · Jan 22, 2021

FilterGuy said:
380W/12V = 31.66A Even if I assume the top end of the voltage (14.6V) it comes out to 26A

That seems to be a lot more than 10A. What am I missing?

Look at the ISC rating on your panel. Production of AMPS by a solar panel is configured using the amount of voltage and amperage it can produce at maximum sunlight levels. I imagine the amount of voltage your 380W panel is rated at maximum (VOC) is under 50V and your VMPP voltage is under 40V. You can't use the 12V figure to divide your wattage by to figure the amount of AMPS your panel puts out. On the back of your panel should be a sticker label showing the VOC, VMPP, ISC and IMP ratings the panel can produce. If it doesn't have the label, look up the information on the internet for your panel.

texskeet · Jan 22, 2021

FilterGuy said:
380W/12V = 31.66A Even if I assume the top end of the voltage (14.6V) it comes out to 26A

That seems to be a lot more than 10A. What am I missing?

There is no 12 v 380 w panel. 50 volts is with no current so nothing is getting to the bms. and if short is in effect the bms is toast any way. the out put is less than 10 amps.

FilterGuy · Jan 22, 2021

fisherus said:
Look at the ISC rating on your panel. Production of AMPS by a solar panel is configured using the amount of voltage and amperage it can produce at maximum sunlight levels. I imagine the amount of voltage your 380W panel is rated at maximum (VOC) is under 50V and your VMPP voltage is under 40V. You can't use the 12V figure to divide your wattage by to figure the amount of AMPS your panel puts out. On the back of your panel should be a sticker label showing the VOC, VMPP, ISC and IMP ratings the panel can produce. If it doesn't have the label, look up the information on the internet for your panel.

We arn't talking about the amps the panels are putting out..... we are talking about the amps going into the battery.

texskeet · Jan 22, 2021

FilterGuy said:
We arn't talking about the amps the panels are putting out..... we are talking about the amps going into the battery.

Where are these amps coming from then.

FilterGuy · Jan 22, 2021

The

texskeet said:
There is no 12 v 380 w panel. 50 volts is with no current so nothing is getting to the bms. and if short is in effect the bms is toast any way. the out put is less than 10 amps.

Guys, it is not about the voltage or current of the panel. It is about the current going into the battery.

He has an MPPT controller that acts as a dc-to-dc converter. As a first approximation the watts going into the controller will equal the watts coming out. If there are 380W going into the MPPT controller, there are approximately 380W coming out. (Yes, there are some efficiency losses. That is why I call it a first order approximation.)

On the battery side of the controller if it is putting out 380W at 12 volts the amps are 380W/12V=31.7A.

texskeet · Jan 22, 2021

Ok This will be my last statement. Where are you getting the 12 v ?

fisherus · Jan 22, 2021

FilterGuy said:
We arn't talking about the amps the panels are putting out..... we are talking about the amps going into the battery.

I understand that however, his statement was regarding the amperage produced by his panel being 31.66A. Better read a little bit better before you criticize someone's answers. It's plainly evident that he thinks that everything is locked into his 12V system voltage. For whatever reason he can't conceptualize beyond the 12 volt his system is rated at.

FilterGuy · Jan 22, 2021

texskeet said:
Ok This will be my last statement. Where are you getting the 12 v ?

The 12 volts is the nominal voltage on the battery side of the MPPT Controller. The actual voltage at the battery will range between 12V and 14.6V and The job of the MPPT controller is to find the best Voltage and Current on the Panel side to maximize power (watts) and then convert that power to the voltage-current pair needed to charge the battery. Ideally, it would do it without any power loss so the power (Watts) coming out of the solar panel is the same as the watts going into the battery.

fisherus said:
his statement was regarding the amperage produced by his panel being 31.66A.

FilterGuy said:
380W/12V = 31.66A Even if I assume the top end of the voltage (14.6V) it comes out to 26A

That seems to be a lot more than 10A. What am I missing?

I was the one that brought up 31.66 A.

This is the original question from the OP.

Matty-Lee said:
Could someone clear up the difference between the Discharge current and the Charge current and tell me if this BMS is OK to use with the system?

He was asking about the BMS.... That is all about the current at the battery. Not at the panel.

And then the discussion took a weird turn.

fisherus said:
You should be fine with those settings however, you failed to state how you acquired the DALY. I would suggest if you order directly from DALY you go with a 60/60. Your panel won't come close to producing over 10AMPS so you have nothing to worry about even if you paralelled another panel.

Since the original question was about the BMS, I assumed the 10 amps was referring to the current through the BMS. Consequently I asked:

FilterGuy said:
What am I missing?

That does not sound like a criticism to me.

I realize now the 10A statement was not talking about what the OP was asking about: BMS Charge Current. I am not sure why the conversation was diverted that way but it did not address the OPs question and certainly added confusion to the thread.

fisherus said:
Better read a little bit better before you criticize someone's answers.

Good advice.

Matty-Lee · Jan 22, 2021

If you guys are getting confused imagine how it is for a newbie.

If it helps solve any confusion this is the system I want to link up.

Solar panel
Solar charge controller
BMS (change from the one I originally asked about)
Batteries

Thanks for the posts so far.

Matty-Lee · Jan 22, 2021

fisherus said:
Look at the ISC rating on your panel. Production of AMPS by a solar panel is configured using the amount of voltage and amperage it can produce at maximum sunlight levels. I imagine the amount of voltage your 380W panel is rated at maximum (VOC) is under 50V and your VMPP voltage is under 40V. You can't use the 12V figure to divide your wattage by to figure the amount of AMPS your panel puts out. On the back of your panel should be a sticker label showing the VOC, VMPP, ISC and IMP ratings the panel can produce. If it doesn't have the label, look up the information on the internet for your panel.

Thanks, this is helpful to know.
The ISC rating says 11.47 Amps so if that is the maximum the panel can spit out then it's not going to exceed the maximum of 30 Amps that the original BMS I asked about is rated for? Even if it does go through the charge controller that is rated for 40 Amps.
In that case I may stick with the BMS I now have rather than order the replacement.

MisterSandals · Jan 22, 2021

Matty-Lee said:
The ISC rating says 11.47 Amps so if that is the maximum the panel can spit out then it's not going to exceed the maximum of 30 Amps that the original BMS I asked about is rated for?

As FilterGuy has been saying, the BMS does NOT see the volts and amps of the panel. The panel(s) are hooked to an MPPT SCC which converts the volts and amps (based on its MPPT algorithm and other settings like battery voltage and max charge amps) that charge the battery(s).

The panels produce 380w at "some" voltage/amperage combination and the SCC converts this to some other voltage/amperage combination to charge the battery bank.

It the case above, the panel produces 380w, 11.47a, (unstated voltage but roughly

380/11.47=) 33v
The SCC, converts this to perform a suitable charge (assume ~12-14v, i'll use 12v): 380w, (380/12 =) 31.66a

The BMS is rated to charge at 30a. I am not sure how the Daly handles overage amperage (31.66a). It may release the magic smoke, it may trigger the BMS to cut off the circuit, it may clip to 30a, i dunno.

OP, is THIS what you were wondering about? The conversation has wandered and at times disregarded what an SCC does.

FilterGuy · Jan 22, 2021

Matty-Lee said:
Thanks, this is helpful to know.
The ISC rating says 11.47 Amps so if that is the maximum the panel can spit out then it's not going to exceed the maximum of 30 Amps that the original BMS I asked about is rated for? Even if it does go through the charge controller that is rated for 40 Amps.
In that case I may stick with the BMS I now have rather than order the replacement.

The current at the panel is *NOT* the same as the current at the BMS.

Your statement of 30A charging current in your original post was pretty close.

Matty-Lee said:
By my calculations I'd only require a 30A charge controller but I went a little higher for upgrade-ability.

Your calculations were pretty much correct. Lets walk through this.

Here are the specs of the panel:

Notice the Vmpp is 34.77V at the 10.93A of Impp.
34.77V x 10.93A = 380.04 Watts. (as you would expect).

But the 12 volt battery can't take 34.77 volts! The job of the solar charge controller is to convert the 34.77V at 10.93A to a voltage and current the battery can handle. The output voltage and current of the charge controller will be *different* than the input voltage and current, but the watts in and watts out will remain the same. (Watts in = Watts out. Actually the wattage will be slightly lower on the output due to inefficiencies. The lost wattage shows up as heat in the charge controller).

If the battery is totally discharged, it will be at ~12V. If you have 12V at 380W coming out of the charge controller the current will be 380W/12V= 31.66A.

MisterSandals · Jan 22, 2021

Matty-Lee said:
Even if it does go through the charge controller that is rated for 40 Amps.
In that case I may stick with the BMS I now have rather than order the replacement.

Your BMS is at risk of seeing more than 30a. As stated above, i do not know what happens.

You can throttle your SCC down to something like 25 charging amps which should make your BMS happy (some BMS's do not live up to their rated tolerances).

Have I bought the right BMS?

New Member

Solar Engineering Consultant - EG4 and Consumers

New Member

Solar Engineering Consultant - EG4 and Consumers

New Member

Unofficial Forum Librarian & Perpetual Newbie

New Member

New Member

New Member

Solar Engineering Consultant - EG4 and Consumers

New Member

Solar Engineering Consultant - EG4 and Consumers

New Member

New Member

Solar Engineering Consultant - EG4 and Consumers

New Member

New Member

Participation Medalist

Solar Engineering Consultant - EG4 and Consumers

Participation Medalist

Similar threads