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BMS help

While I’m upgrading to 24v I have a quandary of what to do with 3 small panels. They are 18v each. One of them is partially shaded for about 3 hours a day. I’m wondering whether to:

1. Just put two of them in series and use the other one camping or something. This would give 36v

2. Put all 3 in series and hope the other two don’t get dragged down too much. Giving 54v average.

My other panels are 45v. They were on their own charger but I want to put them all on one charger now I’m upgrading to 24v.

Any thoughts on what might be best to do?

Thanks.
 
Ampster said:
I have a 3P16S pac with one BMS. I only monitor each group of three which are in parallel.It works for me and I do not consider it a short cut.
Click to expand...
Hi Ampster, Yes that is fine if you are not new to Lithium and have matched cells from a reputable seller but it can be a lottery if you don’t. Too many sellers around selling cells that have not been matched and when you are buying from a person who is so far down the chain from the manufacture then the risks are greater. For the additional small cost I wouldn’t direct the OP down that path but maybe he will take your advice and I wish him the best whichever way he goes.
 
Tim Tim said:
that is fine if you are not new to Lithium and have matched cells from a reputable seller but it can be a lottery if you don’t. Too many sellers around selling cells that have not been matched and when you are buying from a person who is so far down the chain from the manufacture then the risks are greate
Click to expand...
My cells were not matched and were from three different vendors who very well could have been far down from the manufacturer. The sellers were reputable however. You did not explain the risks of putting cells like mine in parallel. That is the purpose of forums like these, to provide useful information to readers. Anybody can say the sky is falling. A meaningful risk management discussion would describe the risks and methods of mitigation. What has been your experience that suggests a more complex arrangement has fewer risks or can manage those risks better?
 
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Ampster said:
My cells were not matched and were from three different vendors who very well could have been far down from the manufacturer. The sellers were reputable however. You did not explain the risks of putting cells like mine in parallel. That is the purpose of forums like these, to provide useful information to readers. Anybody can say the sky is falling. A meaningful risk management discussion would describe the risks and methods of mitigation. What has been your experience that suggests a more complex arrangement has fewer risks or can manage those risks better?
Click to expand...

I know very little about lithium batteries, only what I’ve gleaned from about 4 hours of YouTube and some online reading, but (and this is a question not a statement) isn’t it more logical to have every cell monitored rather than a pair or set of cells? What if the weakness or strength of one cell is concealed by the others and it gets over charged or over discharged?
 
slowbutsure said:
isn’t it more logical to have every cell monitored rather than a pair or set of cells? What if the weakness or strength of one cell is concealed by the others and it gets over charged or over discharged?
Click to expand...
it would seem that way logically until you consider that the physics of paralleling cells means thst all parallel cells will be at the same voltage. Thus it is impossible for one cell to be overcharged or over discharged without the other buddy cells being the same voltage. In that case the BMS will detect that condition just like it would a single cell and shut down the pack.

EV manufacturers like Tesla use multiple cells in parallel. Tesla uses over fifty cells in parallel. Older Large format prismatics actually contain a number of pouch cells in parallel inside the case.
 
slowbutsure said:
Thanks. I’m getting these Eve cells:

www.ebay.co.uk

Lifepo4 12v 300ah. CaTL cells grade A. Available For Immediate collection | eBay

Find many great new & used options and get the best deals for Lifepo4 12v 300ah. CaTL cells grade A. Available For Immediate collection at the best online prices at eBay! Free delivery for many products.
www.ebay.co.uk

I will be collecting in person (4 hour round trip ugh). The seller has given me the data sheets and seems above board. The ones from China you mention would not be far from £600 per 4 cells with shipping cost and import tax. less for 8 cells. But the time for them looks like atleast a couple months. A bit long for me as I’m doing this project now.

On the BMS, what do you think about pairing the cells (2 in paralell) and putting one lead on each pair (and then the pairs in series) to use a 4S BMS? I can get a Daly 4S 150amp very reasonably priced from the same seller.
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A 4s BMS is for a 12 volt system, period. A 8s BMS is for a 24 volt system. If you have 8 cells you can wire them 2p4s or 8s giving you 12 volt or 24 volts, respectively. What is the system voltage you're aiming for?
 
littleharbor2 said:
A 4s BMS is for a 12 volt system, period. A 8s BMS is for a 24 volt system. If you have 8 cells you can wire them 2p4s or 8s giving you 12 volt or 24 volts, respectively. What is the system voltage you're aiming for?
Click to expand...
See above
It's all been sorted
 
timselectric said:
It's all been sorted
Click to expand...
Yes, as far as I can tell, @slowbutsure is going with a 24 volt system with 8 cells. His discussion with me about parelleling cells or packs was theoretical.
Still waiting to hear from @Tim Tim about why he is so adamant about not paralleling cells. I only think it depends on preferences and use case. For an RV there is value in having redundancy and smaller packs to fit in tighter spaces. For a large stationary pack I am more concerned about kWhs going into the pack and rely on the BMS telling me if a group of parallel cells is having an issue. There are good arguments for every use case.

The likelihood of one cell going bad has not been an issue in the ten years I have been messing around with batteries. The exception to that is my first ebike pack ten years ago when I first got the EV grin from the acceleration of an ebike powered by Headways that could discharge at a high rate. I had no BMS then and reversed one cell so by the time I got home there was the strong smell of ether coming from a cell that had probably reversed itself in a series pack. I do not recall if that cell survived but the real lesson was that a BMS was important.
 
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Yeah thanks for all your help. I can see what @Ampster is saying about parallel batteries only needing one (bms lead) with them being forced to have the same voltages. I had a bit of decision paralysis about the BMS so have just made the decision on what would help me sleep at night, which I realise may not be entirely logical but dealing with electrical voodoo like this lithium stuff, I’ll still sleep better! Appreciate all the help.
 
On another issue can anyone help me understand an amp hour issue. I have a Victron shunt. It uses amp hours to tell you what percentage of battery capacity is left. So I put my new cells in 12v configuration they would be 560ah, in 24v it’s 280ah. I understand the watt power is the same but I’m doing 24v what do i enter for amp hours in the shunt so I get an accurate soc percentage? Thanks.
 
If you are doing 24v, why are you building 12v batteries?
Edit: may have miss read the question.
 
timselectric said:
You answered your own question.
280
Click to expand...
timselectric said:
If you are doing 24v, why are you building 12v batteries?
Edit: may have miss read the question.
Click to expand...

12v was just a comparison as I currently have a 12v system. I’m upgrading to a 24v system but don’t understand how the shunt will show the correct soc using amp hours when the same set of batteries could be configured as either 280ah or 560ah (24 & 12v). Does a 24v ah last longer than a 12v ah? If so then I can u Derby and how the soc would be correct. If not, then I have no clue.
 
It's just calculating SOC, based on ah's in and ah's out.
A 24v ah has more power than a 12v ah. (Twice as much)
 
A question on the voltage of the 24v lithium batteries.

Can anyone tell me what the voltage should be at 10% charged, and at 90% charged? I want to make sure I don't go past those levels. And I have an automatic transfer switch that switches over to the grid when a given voltage is reached so I want to make sure I get that right.

Thanks,
 
This is per cell. Multiply by 8 for 8s.
 

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Top balancing! I have a bench power supply coming. For the cells in 24v series what should I set the charge voltage to? And how do you know when they are full? Do they cut off themselves? I’m concerned about overcharging if I leave it on over night. Thanks.
 
The bench power supply should only charge to the set voltage. Then, it just keeps it at that voltage.
For 8s , set voltage for 29.2
If top balancing (all cells in parallel) set voltage to 3.65
 
timselectric said:
The bench power supply should only charge to the set voltage. Then, it just keeps it at that voltage.
For 8s , set voltage for 29.2
If top balancing (all cells in parallel) set voltage to 3.65
Click to expand...
Thanks. Is there any reason why you can’t top balance by putting them in series and then using your charge controller to charge them and use an active balancer abs your bms? I’m guessing new cells shouldn’t be massively out of balance? I’m happy to do paralell and a 3.65 charge too, I’m just thinking the charge controller would be a lot faster.
 
It's generally recommended to do a paralleled top balance, before you use new cells. The bms will do it, but it's a very long process. Unless you have active balancing.
 
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