BMS help

[slowbutsure]

Hi

I’m going to change out my lead acid battery bank for lithium. I will be getting 8 x 3.2v cells and putting two lots of 4 in series and then both lots of 4 in paralell to have 12v 560ah lithium battery bank. That’s the hope.
However I cannot find an 8s 12v BMS. All the 8S look to be 24v or higher. So I can think of 3 possible solutions. Can anyone comment on them and give advice please?

1. Can you get an 8s 12v BMS (I’m in the U.K.). A link would be great!

2. My rather expensive (for me) inverter is 12v. So if you can’t get a 12v bms and I have to make the battery bank 24v, how feasible is it to get a 24 to 12v transformer thing to run the inverter, without being really inefficient?

3. Am I over thinking it? Can you use 2 x 12v bms on the same battery bank?

Thanks.

 

[timselectric]

8s means 8 cells in series.
3.2v x 8 = 25.6v
That's a 24v battery.
For 12v you will need a 4s BMS.

 

[slowbutsure]

Thanks. how does that work with 8 cells at 12v if the bms only has 4 leads?

 

[littleharbor2]

You either wire the cells 2p4s or get two 4s BMS.

 

[timselectric]

Two BMS's

 

[littleharbor2]

In other words parallel 2 cells at a time and treat each pair as a single battery.

 

[timselectric]

While 2p4s will save you the cost of a BMS.
You will also lose the redundancy of 2 packs.
If one cell fails, you have no battery.
With 2 packs, if one cell fails you still have half of the capacity.

 

[slowbutsure]

In other words parallel 2 cells at a time and treat each pair as a single battery.

Thanks, I hadn’t thought of that.

 

[slowbutsure]

You either wire the cells 2p4s or get two 4s BMS.

So will a 12v bms be happy with 4 x 6v cells in terms of connecting each “cell” for balancing? Does it matter to the bms that the cell is 6.4v and not 3.2v? Thanks.

 

[timselectric]

You don't add the voltage, in parallel.
It will still be 3.2v.

 

[740GLE]

You connect two cells in parallel.

If you aren’t aware of how to connect things in parallel, vs series, I think it’s time to start doing more research before going any further.

What size is your 12v inverter? That inverter will determine what sort of BMS you need.

What other 12v loads are you supplying?

 

[slowbutsure]

You don't add the voltage, in parallel.
It will still be 3.2v.

Ah I did actually know that as I have set up my led acid batteries on paralell. Tiredness fizzles the brain.

 

[slowbutsure]

You connect two cells in parallel.

If you aren’t aware of how to connect things in parallel, vs series, I think it’s time to start doing more research before going any further.

What size is your 12v inverter? That inverter will determine what sort of BMS you need.

What other 12v loads are you supplying?

Yeah I do understand about series and paralell, I set my existing system in parallel. It’s been a long day! My inverter is 3500 / 7000. Apart from a light I’m not running anything else in 12v. My only reason for staying with 12v is that I do not want to buy another inverter.

 

[740GLE]

So with that size inverter 12v you’re going to need two +150a BMS, and you’ll want to create two batteries vs paralleling the cells to one BMS. So that’s what another $100-150? Vs just one BMS at 24v.

What size cabling do you have for existing inverter? Also what fusing do you have left over from your FLA set up? Most likely it’s a good time to double check that.

 

[slowbutsure]

So with that size inverter 12v you’re going to need two +150a BMS, and you’ll want to create two batteries vs paralleling the cells to one BMS. So that’s what another $100-150? Vs just one BMS at 24v.

What size cabling do you have for existing inverter? Also what fusing do you have left over from your FLA set up? Most likely it’s a good time to double check that.

The inverter is over sized for what I need. I had a 1000w inverter which was just about doing it. I upgraded to the 3500 for comfort. It’s not even breaking a sweat with the load I have on it. So I’m not sure, in my case, I need two 150 amp BMS? If I did need two $150 is a lot less than a new inverter. My current inverter was $440. (I’m in the U.K. so not in dollars but just showing for consistency).

 

[slowbutsure]

So with that size inverter 12v you’re going to need two +150a BMS, and you’ll want to create two batteries vs paralleling the cells to one BMS. So that’s what another $100-150? Vs just one BMS at 24v.

What size cabling do you have for existing inverter? Also what fusing do you have left over from your FLA set up? Most likely it’s a good time to double check that.

Can you talk me through how it works using two BMW’s? Each 12v battery (4 cells in each) having its bms, but still being in the system as one battery (both in series). So each bms covering 4 cells. Is there anything more to it than that? Thanks.

 

[740GLE]

You’d build two batteries with 4 cells and a BMS, then you’d connect them in parallel, just like how you have your FLA set up.

Now sure why you are mentioning both in series? Again this makes me question your understanding of your system and how series parallel works.

Can you draw your current system schematic? Unless you are using 6v cells no part of your system should have series connected FLA batteries.

 

[slowbutsure]

Lithium is new to me and I’m getting my head around a group of cells making up a battery. I have seen a 12v 250 amp Daly which would probably do.

My reason for changing batteries is I have 6 x 130ah lead acid making a total of 390 usable ash’s. I’m only using 128 over night but the voltage drops to 12.2 which disturbs me as that is meant to be 50% charge. I’m concerned for the winter. So I’m attracted to Lithium because of the more consistent volts when discharging and also the faster charge times. Though the price of lithium is eye watering!

 

[Tim Tim]

Lithium is new to me and I’m getting my head around a group of cells making up a battery. I have seen a 12v 250 amp Daly which would probably do.

My reason for changing batteries is I have 6 x 130ah lead acid making a total of 390 usable ash’s. I’m only using 128 over night but the voltage drops to 12.2 which disturbs me as that is meant to be 50% charge. I’m concerned for the winter. So I’m attracted to Lithium because of the more consistent volts when discharging and also the faster charge times. Though the price of lithium is eye watering!

Ok you are building exactly what I have built two 280Ah batteries. You need a 4S BMS for each bank of cells. The size of the BMS will depend on your current draw and being in the UK and I being an ex pat probably the largest appliance you will use is a 2400W. So divide by 12v gives you 200Amps from your batteries but allowing inefficiencies and a margin then 240 amps max but between the two batteries, each bank of 4 cells is a battery, therefore 120 amps from each one. Daly do a 150 amp 4S smart BMS so buy two of those. I actually bought 200 amp BMS’s.

I’m in Australia but will follow you and help you set it up and believe me it isn’t as complicated as you may think.

OK first step. Have you purchased your cells and if so do who from and do you have them yet. If you haven‘t then you you can buy from a recommended seller on here and also buy your BMS’s with your cells and you save on shipping.

Also remember that leaving your inverter switched on it be using standby current of a few amps and over 24 hour hours that can be significant with the size of the inverter you have.

 

[littleharbor2]

Lithium is new to me and I’m getting my head around a group of cells making up a battery. I have seen a 12v 250 amp Daly which would probably do.

My reason for changing batteries is I have 6 x 130ah lead acid making a total of 390 usable ash’s. I’m only using 128 over night but the voltage drops to 12.2 which disturbs me as that is meant to be 50% charge. I’m concerned for the winter. So I’m attracted to Lithium because of the more consistent volts when discharging and also the faster charge times. Though the price of lithium is eye watering!

If you do a DIY battery bank you might find LiFePo4 isn't as eye watering as you might think. I built a 460 Ah @24 volt bank to run my home for about $2,000. A bank of quality AGM's with same amount of useable power could easily cost more and give a much shorter lifespan.