BMS for homebuilt lithium ion pack.

[Cerebus]

Am I right in thinking that a BMS will stop over charging of 18650 lithium ion batteries?

For example, if I made a pack containing three cells each of 4.2v max in series, used a BMS, can I connect to a small solar panel (no larger than A3 sheet of paper, don’t have output specs with me) and not worry about overcharging?

Would the BMS cope with an even larger solar panel, again specs not to hand (possibly 50W from memory) but they are old solar panels not the new type (mono crystalline or poly crystalline? Can’t remember which are the newer tech. Mine are the old blue solar panels with random etching affects ).

If not I would have to think again and implement VSRelays etc

 

[Sea Change]

There are different types of BMS.
The simplest use MOSFETs to directly switch on or off the current going on and out. JK, JBD, and Daly are popular examples.
There are other types of BMS which trigger an external contactor, and can therefore handle much higher current as you simply choose the appropriate relay.

My JBD BMS is rated up to 200A at 12v. It cost about £65 a couple of years ago. I'm sure a simple, cheap MOSFET BMS will suit your needs perfectly.

 

[Cerebus]

There are different types of BMS.
The simplest use MOSFETs to directly switch on or off the current going on and out. JK, JBD, and Daly are popular examples.
There are other types of BMS which trigger an external contactor, and can therefore handle much higher current as you simply choose the appropriate relay.

My JBD BMS is rated up to 200A at 12v. It cost about £65 a couple of years ago. I'm sure a simple, cheap MOSFET BMS will suit your needs perfectly.

The BMS I have seen are Ebay types for pennies so definitely not the high amp capacity ones you mentioned.

The draw from the battery pack would be low.

So thank you.

 

[Sea Change]

The BMS I have seen are Ebay types for pennies so definitely not the high amp capacity ones you mentioned.

The draw from the battery pack would be low.

So thank you.

Do they give your a spec for max current etc?

I should also have said, it's important to get the exact right voltage. BMSs are designed for different cell chemistries and configurations.

You'll want a 3S (three cells in series).

 

[Cerebus]

Do they give your a spec for max current etc?

I should also have said, it's important to get the exact right voltage. BMSs are designed for different cell chemistries and configurations.

You'll want a 3S (three cells in series).

No they don’t but I am using for very low current.

Reminded me I must fit a fuse. It is a standalone project.

Yes it is 3S with the cells in series.

Thank you.

 

[William_H]

I use one of the cheap Chinese BMS (3S) for 12 x 18650 batteries charging it off a 12v LA battery. No problems. Certainly I would think your low powered PV panel should be fine. On the other hand I bought a so called solar charger at one stage for one 18650. It had a 5.1 volt zener diode across the input as the BMS was designed for 5v input. I guess with solar you just can not predict how much power will be pushing the BMS.
Your 5 or 10w panel will produce 18v no load and probably pull down to 12v under the load of charging 18650s at 1/4 to 1/2 amp. no problem. However 18v pushing from a 50w panel could push 2.5 amps into 18650s or leave the BMS trying to hold back the excess voltage from the panel when not loaded.
Perhaps if you want to use the 50w panel fit a regulator to bring voltage down to about 13v so giving BMS a chance to control the power. (But giving more power available when light is low). No the cheap BMS do not give limits on input voltage or current so be careful. ol'will

 

[Cerebus]

I use one of the cheap Chinese BMS (3S) for 12 x 18650 batteries charging it off a 12v LA battery. No problems. Certainly I would think your low powered PV panel should be fine. On the other hand I bought a so called solar charger at one stage for one 18650. It had a 5.1 volt zener diode across the input as the BMS was designed for 5v input. I guess with solar you just can not predict how much power will be pushing the BMS.
Your 5 or 10w panel will produce 18v no load and probably pull down to 12v under the load of charging 18650s at 1/4 to 1/2 amp. no problem. However 18v pushing from a 50w panel could push 2.5 amps into 18650s or leave the BMS trying to hold back the excess voltage from the panel when not loaded.
Perhaps if you want to use the 50w panel fit a regulator to bring voltage down to about 13v so giving BMS a chance to control the power. (But giving more power available when light is low). No the cheap BMS do not give limits on input voltage or current so be careful. ol'will

Very helpful thank you.

As to your last sentence, I certainly was wrong because I thought I could hook up a 10W solar panel (as you correctly guessed my A4 sized one is I think) and thought the BMS would stop charging when the 3 cells in series were fully charged.

I thought it offered protection and I am glad you have corrected me.

Does that mean the purpose of these very cheap BMS is to simply balance the voltage and current to each of the three cells?

 

[Sea Change]

I don't know specifically about the type of BMS you're looking at, but the three I have personally used have all given the option of choosing when to turn charge or discharge on and off, based on cell or pack voltage, current, and temperature.

If it didn't do this, it would just be a balancing circuit. Which is a function generally built in to a BMS.

 

[William_H]

Very helpful thank you.

As to your last sentence, I certainly was wrong because I thought I could hook up a 10W solar panel (as you correctly guessed my A4 sized one is I think) and thought the BMS would stop charging when the 3 cells in series were fully charged.

I thought it offered protection and I am glad you have corrected me.

Does that mean the purpose of these very cheap BMS is to simply balance the voltage and current to each of the three cells?

No BMS will cut off charge when cells are full. What concerns me is a large panel might have the ability to provide higher voltage and current than the BMS can handle. Hence on a small panel the voltage will be pulled down by the load to near 13v but a large panel might be able to maintain a higher voltage 18v at a higher current in to cells beyond the capability of the BMS. As I said I don't know of any actual limitations of the BMS so best be safe. 10watt panel might be ok in UK sun but I would not go larger and feel more comfortable with smaller. ol'will

 

[Sea Change]

No BMS will cut off charge when cells are full.

Sorry but that's incorrect. You can program the BMS to take the battery offline whenever certain parameters are exceeded. Cell vintage, pack voltage, current, temperature.

It's not a great idea to be relying on this because many charge controllers will throw a hissy fit if they are suddenly disconnected from the battery and have nowhere to put all that power coming in from the solar panel.

What concerns me is a large panel might have the ability to provide higher voltage and current than the BMS can handle. Hence on a small panel the voltage will be pulled down by the load to near 13v but a large panel might be able to maintain a higher voltage 18v at a higher current in to cells beyond the capability of the BMS. As I said I don't know of any actual limitations of the BMS so best be safe. 10watt panel might be ok in UK sun but I would not go larger and feel more comfortable with smaller. ol'will

I'm not sure what would happen if you tried running without a charge controller. Like you say, the initial voltage could be too high.
The Electrodacus BMS is specifically designed to allow the use of 12v (nom) panels without a charge controller. It's a bit of a niche product though.

A Victron 10A MPPT controller is £45. It is dead easy to use, efficient, has Bluetooth for monitoring and adjusting settings, and has protection against accidental/sudden battery disconnect. That's pretty good value, IMHO.

 

[William_H]

These are the kind of charge controllers OP is asking about 3s Lithiumcharge Controller for sale | eBay ol'will