How to size a BMS?

Fryguy1111

New Member
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.
 

smoothJoey

Red Wing Blackbird
1500 watts * 1.15 inverter efficiency / 24 = 71.875 dc amps
for 12 volts its 143.75 amps.
if you are buying a commodity chinese bms its a good idea to derate by 25-50%
143.875 dc amps / .66 = 217.992424242 bms amps.
I'm sure you can work it out for 24 volts.
 

Fryguy1111

New Member
1500 watts * 1.15 inverter efficiency / 24 = 71.875 dc amps
for 12 volts its 143.75 amps.
if you are buying a commodity chinese bms its a good idea to derate by 25-50%
143.875 dc amps / .66 = 217.992424242 bms amps.
I'm sure you can work it out for 24 volts.
Thank you, smoothJoey. That's exactly what I was looking for.
 

Fryguy1111

New Member
Much appears to be programmable, but I didn't see a low cell cutoff option. I'll have to read some more about this BMS before pulling the trigger. Do you have any BMS recommendations? I'm just looking for something simple :giggle:BMS-250A1.JPG
 

smoothJoey

Red Wing Blackbird
It only has low cell(Over discharge detect) and its set way to low at 2.2 volts.
2.5 volts per cell is dead flat for lifepo4.
2.2 volts is immediate damage or so I'm told.
I would only set it to 2.5 volt for the capacity test.
For normal use I would go 2.9 or 3.0 volts depending on anticipated load.
 

learning all the time

Solar Enthusiast
I'll have to check that out and maybe reach out to Daly to confirm that. Thanks for the heads up on this.
That was my understanding of the Daly BMS. They ask a few questions such as type of battery, amount of cells, ect. Then they program it for you. I was unsure of capability to make adjustments so I ordered this one for myself.
It has bluetooth and can be programmed but is NOT waterproof.
 

Boat Bat

New Member
Hi there,
I'm new so don't kill me for asking stupid questions:geek:

I'm about to change from 4 trojan T105 total 24v at 205ah to 8 cells 280ah LifePo4 that I've ordered on the Ali.
I was planning to buy the Daly 24v 8s 300A BMS but I'm still not sure.

Would it be better to go for the Daly 24v 250A Smart BMS (cheaper than the 300A regular) so I can change the settings?

The load would be a 2000w inverter, 2 24v winch with 90Amp circuit breaker, fridge, nav equipemt and some lights...

Thanks in advance for you support
 

learning all the time

Solar Enthusiast
Hi there,
I'm new so don't kill me for asking stupid questions:geek:

I'm about to change from 4 trojan T105 total 24v at 205ah to 8 cells 280ah LifePo4 that I've ordered on the Ali.
I was planning to buy the Daly 24v 8s 300A BMS but I'm still not sure.

Would it be better to go for the Daly 24v 250A Smart BMS (cheaper than the 300A regular) so I can change the settings?

The load would be a 2000w inverter, 2 24v winch with 90Amp circuit breaker, fridge, nav equipemt and some lights...

Thanks in advance for you support
Only two stupid questions available. Those not asked and those repeatedly asked a different way to seek a specific answer. lol

Probably lots of opinions on this but it sounds like that winch would be your big draw item? I have a 12v system so not comaring apples to apples I can tell you 250a covers way more than I need. Running my lights, stereo and inverter/charger I use around 7 amps. My fridge is 30amps on 110ac or 5amps on 12v with propane. I average 12 to 15amps with my tv and whatnot. I went with a bigger bms so I could run my microwave at 100amps, my vacuum cleaner 110 amps, etc. etc. I could run my ac at 70amps but 560amps battery I would probably run out of power just as the ac is starting to really work.

Looking at your list you should be fine at 250a and with a "smart" bms you can change a lot of settings.

Do some math on your equipment and double check you will be well within range of your needs. Hope this helps
 

Boat Bat

New Member
Thanks for your answer Learnig all the time,

As soon as I asked the seller of the Daly on Ali if the 250A would do aswell, they just answered.

'OK. had Better use the 300A.Thanks
OK. had Better use the 300A.Thanks'

And moved forward with shipping the 300A (regular) BMS.

Stupid of my side not to do propper research before ordering.

So I don't think that I can change the settings on this one.
It will probably cut out below 2.5v witch I don't like.

Has there been a discussion on a good Battery cell monitor?

Cheers
 

Dzl

Unofficial Forum Librarian
Staff member
Moderator
So I don't think that I can change the settings on this one.
It will probably cut out below 2.5v witch I don't like.
I wouldn't worry too much about that, so long as your battery bank is properly sized to your loads, and/or you have layers of protection (such as your inverters low voltage disconnect) you shouldn't need to rely on your BMSes low voltage disconnect too often or at all, and the occasional dip down to ~2.5V or so is no big deal.

Has there been a discussion on a good Battery cell monitor?
Victron is the popular top of the line choice (Victron BMV or Victron Smartshunt), Aili is the popular cheaper shameless knockoff.
 

learning all the time

Solar Enthusiast
I have the victon 500a smart shunt which is where I get all my numbers but they are expensive. Also, I am looking into my numbers cause my amp draw seems really high and I should be popping fuses like crazy but I am not.

Yes, that Aili monitor is very popular and people seem to like it. This is the link I was going to purchase but went with a smart shunt so I can see everything by my phone. https://www.amazon.com/AiLi-Voltmet...60578&sprefix=aili+battery+mon,aps,199&sr=8-4

Adding to what DZI said, I would think you would be able to change parameters on all most all Daly products. Can you put a link up for which one your getting? Should have a port you can at least hook up your computer I would believe.
 

Dzl

Unofficial Forum Librarian
Staff member
Moderator
Adding to what DZI said, I would think you would be able to change parameters on all most all Daly products. Can you put a link up for which one your getting? Should have a port you can at least hook up your computer I would believe.
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.
 

Dzl

Unofficial Forum Librarian
Staff member
Moderator
The problem is that a fet based bms tends to failed closed.
The typical way to discover this failure is by damaging one or more cells.
Even worse with a statically configured one with dangerously low or high defaults.
Good points, though would apply to the whole category more or less (FET based BMSes) right? Other than the last point at least. Layered protection / voltage limits + a properly oversized BMS would mitigate this to a degree.

Relevant primers for anyone not clear on what we are referring to when we discuss FET based BMS, or unclear what is meant by layered protection.
 

smoothJoey

Red Wing Blackbird
Good points, though would apply to the whole category more or less (FET based BMSes) right?
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.
 
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