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Over Sizing a BMS

Al Slitter

Trying to learn something new every day!
Joined
Feb 22, 2021
Messages
85
Location
Thailand
I have an existing Solar System and it has been working well for the past 1 1/2 years. It is a very basic system and was intended to be a test and light useage system in my small shop. The system contains 2-55Watt solar panels in parallel running to a cheap PWM controller then to 130 Amp lead acid battery system.
I of course have MC4 connectors and combiners as well as fuses on the solar and battery positive lines.
I want to upgrade my system now to Lithium Ion Phosphate and am not sure if I will go to 100Ah 12 Volts or 200Ah 12 volt batteries? The question I have is about the BMS to use if I start out with 4 100AH 3.2 batteries can I use a 200 or 300Amp BMS with it without any issues? I want to future protect my system in case I decide to add additional batteries going forward.
Thoughts please.
 
Is this a good time to change to 24v?

Especially if you want to expand in the future 24v is much better than 12v.

Yes you can use a 200 or 300 amp bms with 100ah batteries. Also look at the Eve & Lishen 280ah cells on this site - search Michael Caro.
 
The question I have is about the BMS to use if I start out with 4 100AH 3.2 batteries can I use a 200 or 300Amp BMS with it without any issues?
Having an oversized BMS is not a problem.
Drawing to much current from your battery is.
The lifepo4 batteries that are popular for our application have a max continuous discharge rate of 1c. For a 100ah battery that is 100 amps and 200 amps for a 200ah battery.
Beware that most of the cheap Ebay BMSs have a very optimistic rating.
If its FET based Ebay BMS you should probably de-rate by between .5 to .66.
That means if it says its rated for 300 amps its probably good for 150 to 198 amps continuous.
The thing to know about FET based BMSs is that the fets fail closed so you won't know there is a problem until your cells need protection.
 
Mostly oversizing has the issue that the system has higher losses. So a BMS rated for higher current will have higher losses in use and when in standby. But mostly a BMS has rather low losses and especially when you constantly use the battery these losses are negligible (at least if we believe the datasheets ;-)). But as Joey says: the batteries maybe cannot handle the current. Some BMS you can set the max current. So this you might consider.
And also as Joey says: does a xxxA BMS is really able to handle that load? So maybe we should always oversize. But if we think the BMS manufacturer is too optimistic: should we use it?


Walter
 
I am going to be using a 500amp 24volt BMS on my system but the max draw should only be 5kw, 209amps or thereabouts. Having seen Will use a 250amp BMS and pulling 290amps as the voltage dropped I figured headroom wouldn't be a bad idea and would leave room for expansion. Although the maximum amp draw capability of my battery bank will be in the order of 1120 amps, I will not even see a quarter of that with the loads so should be safe and cool running.
 
The question is always; why did the voltage drop. What was the component causing the voltage drop? Was it in his test the BMS? Because that would mean a very large heat production in the BMS.
 
The question is always; why did the voltage drop. What was the component causing the voltage drop? Was it in his test the BMS? Because that would mean a very large heat production in the BMS.
By Voltage drop I mean load voltage sag from the 4kw inverter he was using to power 2 heat guns in his capacity test. The BMS cooling fan kicked on a few times, but wasn't on permanently. It handled it like a champ but I would be concerned about long term effects.
 
but voltage drop in the DC section or in the AC part?
Well obviously the sag on the DC section as that is the where the current is going through the BMS and changing to keep up with the constant watts on the inverter. ;)

Here is the video so you can see what happened!

 
he has 111mV over the cable to the BMS and about cable+BMS 250mV(so 140mV over the BMS); at 280A that is about 70Watt of losses. of the 70 Watt about is the 40Watt is lost in the BMS

Edit: don't know if noted how much the drop was at the start of the test.
 
I want to thank everyone for there comments and suggestions that where provided here.

I want to keep this short but maybe a bit more information might be helpful?
I am retired and live in Thailand and have done so for 19 years now so funds are fairly limited and I have to go for real solutions for my needs and provide some sort of value.
As mentioned I set up my very basic Solar System about 1 1/2 to 2 years ago and the specifications are listed above. At the present time the system only powers a small night light with a very small draw. I do have connectors in my shop connected directly to the battery for 12 volt service as needed. I do not have a inverter in the shop and while I have no need for one this might be something I might want to try going forward.
As the pricing of Lithium Ion Phosphate batteries keep falling it makes sense to look at a larger battery bank as part of future proofing.
Switching over to a off grid system does not make financial sense as here in Thailand my electrical bill runs at about $30.00 US/month.
When using an air conditioner it rises to about $45.00 US/month.
The environment that I want is for testing and learning by doing and some sort of process to provide lighting to our home when the electricity goes out ( This happens a fair amount ).
I hope this helps and thank you again for all of the great comments.
 
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