How to size a BMS?

[Dzl]

With a smart bms you can set safe values to test and some of the better ones allow you to administratively open the fet circuit to verify that it opens.
I'll never use a dumb bms again for cells I care about.

Yeah lacking an ability to test or verify without going to excessively high or low voltage would make me uncomfortable.

 

[Dzl]

Personally I prefer the more modular model of the non-fet based BMSes, particularly ones like the SBMS that control chargers/loads directly. But I understand that is not for everyone and not the most simple beginner friendly approach. So I do understand why the FET based BMSes are so popular.

 

[John Frum]

Personally I prefer the more modular model of the non-fet based BMSes, particularly ones like the SBMS that control chargers/loads directly. But I understand that is not for everyone and not the most simple beginner friendly approach. So I do understand why the FET based BMSes are so popular.

My ideal bms would be a programmable bms that just does voltage sense, balancing and instead of fet based current path it just presents 2 potential free switches. 1 for charge and 1 discharge. If your gear has remote switching capability its a no brainer.
If your gear does not support remote switching then put a shunt trip in that particular circuit.

 

[Dzl]

My ideal bms would be a programmable bms that just does voltage sense, balancing and instead of fet based current path it just presents 2 potential free switches. 1 for charge and 1 discharge.

If I understand what you are describing I think the SBMS fits that broad brush stroke. With up to four control lines/switches to be used as you please. Though it may be a bit more complex / feature rich than what you are describing.

 

[John Frum]

If I understand what you are describing I think the SBMS fits that broad brush stroke. With up to four control lines/switches to be used as you please. Though it may be a bit more complex / feature rich than what you are describing.

Nobody will make the one I want because its too scalable.

 

[Roswell Bob]

Hello. I've been digging thru the forum as well as Will's videos, but haven't found a clear description on how to properly size a BMS. I plan on replacing my existing Trojan T-105 lead acid (total 450AH) battery bank with a similar sized DIY lithium battery system. I'm looking to build out a 560AH lithium battery bank (2 - 4 x 280AH cells) battery bank. I currently have a 1500 watt inverter/charger and 500 watts of solar. I know my wife plans on using her kitchen appliances, but I'm not sure what load those pull off-hand.

I'm guessing I need an 8-wire BMS or 2 x 4-wire BMS (or a 4-wire BMS for a 4p2s battery system). I don't need a low temp cut off feature in the BMS as my boat is in Mexico and the average temp in winter is about 55° to 60°.

Is the BMS sizing based on the expected amps drawn thru the inverter?

Thank you.

The appliances may be key to this discussion. Ranges, microwaves, toasters, coffee pots, griddles and similar can blow a hole in your budget. A 1500w inverter will only operate one of those at a time.

 

[learning all the time]

Not sure about a port for custom config, I believe some newer ones may have UART or CAN or something as an option, but the original ("non-smart") Daly BMSes are not user configurable. Its their weakness and strength. No ability to tweak or fine tune, but also zero configuration burden on the user and no settings to screw up which many newbies value. All of their older models (up until this year) were this way as far as I know. This year the Daly Smart BMS was released which is more configurable and has monitoring functionality. Since then they have iterated some, and I'm not sure what all has changed, and my knowledge may not be up to date. But what I can say is that most models of the Daly BMS up until the release of the Daly Smart, are not user configurable, and do not have functional low temp cutoff. I believe the factory defaults are something like HVD of 3.7 or 3.75 and LVD in the ballpark of ~2.2-2.6. The model I just checked has limits of 2.2V and 3.75V, I believe Daly will customize these settings for you, I am not sure if there is a fee or minimum order quantity for custom settings.

Thanks DZI. I was not aware of this but fantastic info for the future if I ever need another one.

 

[keimor85]

How do you connect the high voltage wire(cables) to the plates on the Daly 250A and 300A BMS. I looked and looked but can't find anything?

 

[learning all the time]

If you are upsizing your wires or one came disconnected this is a great video or I am not understanding what you are trying to do.

 

[keimor85]

I'm looking for the three hole terminals (lugs) that will attach to the daly BMS 250A and above units. I found a video that shows them but have not been able to find the terminals by them selves to buy. Do they come with the BMS? I can't find it in the literature.

 

[learning all the time]

Ok I understand now. My Daly came with the already. If you don’t have them you can get them at Amazon or an auto parts store and crimp them on. When I crimp I use a vice and solder. In a pinch I have used pliers and a hammer for a temporary fix. This is not a real good method though. Hope this helps

 

[keimor85]

Yes, it helps. I have not ordered yet but I could not find in Daly literature that the lug came with it.