What do I need for an 8s 12v diy battery?

the4machin3 · Sep 4, 2021

I just ordered 8 EVE cells from aliexpress (I think it will work out to 12v 400AH), and I’m trying to get the other parts I need before they get here. I was hoping to arrange the 8 cells to have one battery with 12 volts because I have a 12 volt inverter. Can I make one battery? Or do I need 2 BMS’s and make 2 4s batteries? Also, what other components do I need to keep them balanced and working properly? I’ve searched the forums, and I’m sure this is discussed but it’s hard to know what to search to find them. Thanks in advance! (I already have solar panels and a charge controller.)

pollenface · Sep 4, 2021

8s makes 24v

You can either go 2p4s with one 4s BMS or make 2x 4s batteries with 2 4s BMSs in parallel

mikefitz · Sep 4, 2021

You can arrange the cells as a single 12v battery with one BMS or two 12v batteries in parallel, each with its own BMS.

The decision may be influenced by the current requirement of your 12v inverter, For example using the popular Overkill BMS, you are limited to an absolute current of 120 amps with one BMS. Using the two 12v batteries in parallel , each with a BMS, gives you up to 240 amps as a maximum.
( note it's often considered good practice to operate somewhat below the maximum ratings).

Mike

John Frum · Sep 4, 2021

mikefitz said:
You can arrange the cells as a single 12v battery with one BMS or two 12v batteries in parallel, each with its own BMS.

The decision may be influenced by the current requirement of your 12v inverter, For example using the popular Overkill BMS, you are limited to an absolute current of 120 amps with one BMS. Using the two 12v batteries in parallel , each with a BMS, gives you up to 240 amps as a maximum.
( note it's often considered good practice to operate somewhat below the maximum ratings).

Usually things that are rated for 120 continuous amps are rated for 150 amps fault current.
120 continuous amps / .8 fuse headroom = 150 amps fault amps.
Does "absolute" map to continuous, fault or other?

the4machin3 · Sep 4, 2021

pollenface said:
8s makes 24v

You can either go 2p4s with one 4s BMS or make 2x 4s batteries with 2 4s BMSs in parallel

The 2p4s I think is my plan. What else do I need to monitor SOC and balancing and such?

rplarry · Sep 5, 2021

To monitor state of charge you will need a BMS with a way to see what is going on internally as in a built in blue tooth or something similar. For top balancing you will need a constant current/constant voltage charger and connect your cells in series and charge them to 3.65v max and let them charge until there are 0 amps going into the battery bank. When I top balanced mine the lowest amp draw I saw was .1 amp going into the batteries and that lasted more than 24 hours so I turned off the charger and called it good.
The Overkill BMS is a good one and has a Bluetooth option and really good customer support. Much better than trying to get support from China.

the4machin3 · Sep 5, 2021

rplarry said:
To monitor state of charge you will need a BMS with a way to see what is going on internally as in a built in blue tooth or something similar. For top balancing you will need a constant current/constant voltage charger and connect your cells in series and charge them to 3.65v max and let them charge until there are 0 amps going into the battery bank. When I top balanced mine the lowest amp draw I saw was .1 amp going into the batteries and that lasted more than 24 hours so I turned off the charger and called it good.
The Overkill BMS is a good one and has a Bluetooth option and really good customer support. Much better than trying to get support from China.

Can you top balance with solar? I’ve seen a lot of the recommendations for a bench top charger with ac power, which I don’t have.

mikefitz · Sep 5, 2021

smoothJoey said:
Does "absolute" map to continuous, fault or other?

Not sure why I typed 'absolute'. Should have said, 'the BMS is rated for a 120 amp continuous current' and avoided being put in the naughty corner.
As far as the default current limit settings in the Overkill BMS are concerned, the actual trip values are:
130 amps for 10 seconds, 440 amps for 640mS and 1780 for 400uS.

I have evaluated this BMS at 100 amps for two hours continuous and 120 amps for 10s of minutes, so it seems to live up to specification.

Mike

DLTooley · Sep 5, 2021

2p4s can work, but for a lot of reasons related to simplicity 4s2p may be better.
I bought the ‘stupid’ 60a/30a Daly BMS and two of those might be a solution.
A single one in combo with the Overkill might give you the best of both worlds, but there is also the complexity of how well they play together.

What do I need for an 8s 12v diy battery?

the4machin3

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pollenface

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mikefitz

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John Frum

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the4machin3

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rplarry

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the4machin3

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mikefitz

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DLTooley

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