Day one of build

[newguy199]

Hi, I'm fairly new to anything of this sort and have no prior knowledge to what I have picked up today, whilst trying to put together a solar build for my van conversion. This in mind I have been stuck for a while with the problem of safely storing and managing the batteries I have purchased, because of my low budget I got some eve 280ah 3.2v cells from china 2 sets of 4 to be precise. I plan on wiring these 8 in series 2 in parallel to make one battery at 560ah and 12v, I'm not sure this is the best way but I have looked into going up to 24v and my budget wont allow for the changes. To my main issue getting a bms, I have a 1500w inverter and will not likely exceed loads of 130a so what would be my best option for a bms if I should get one at all?

 

[OffGridInTheCity]

Yes - 2p4s would be 560ah @ 12v and is perfectly good

Yes, you need a BMS. Daly is a popular/simple BMS. Here's an example https://www.amazon.com/DALY-LiFePO4-Protection-Balance-Battery/dp/B08LCSRQF8 There are others (not pushing Daly) but all my stuff is 24v/48v so I'm not a 12v BMS person .

This channel has a lot of youtubes with good info for LifePo4 cells. This is just one of the Daly BMS ones... -

 

[newguy199]

Thanks for the response, I've been watching a lot of this guys videos and have found them really helpful. Saw one just after posting this talking about using 8 cells with 4 in parallel and then connecting those in series to get 12v that seems to make more sense and simplifies the BMS so I pulled the trigger and got a 200a 4s 12v Daly. The part that was confusing me was id have 8 cells for the BMS at 12v if I wired in series then parallel I think.

 

[OffGridInTheCity]

You take the 8 cells and group them in pairs of 2. Each pair you wire in parallel. This is called "2p" (2 in parallel). Then you have 4 pairs. The 4 pairs are wired in series. This is called "4s" (4 in series). This makes a 4s2p 12v LifePo4 battery.

Here's a picture... you can physically arrange them any way you wish as long as they are electrically hooked up 4s2p

I've added the BMS sense leads. The idea is the BMS needs to know the voltage of each pair of cells - so there is an overall negative and then 4 wires. If you use a voltmeter with negative on the 'top wire' (in the picture) and then measure the voltage of each wire below the top wire you'll see voltages of 3.2, 6.4, 9.6, 11.8 - as you work down the series. If you measure between any 2 wires, you'll see the voltage (3.2) of that pair

 

[newguy199]

Perfect this is exactly what I was looking for, thankyou. Do I need to use a shunt if I have a smart Daly or am I ok without one?

 

[OffGridInTheCity]

Do I need to use a shunt if I have a smart Daly or am I ok without one?

Are you thinking switch / circuit breaker?

The Daly BMS will cut-off power to/from the battery if the cells get to hi/low, the current gets too high, and perhaps if the temp is out of range. Some BMSs will also act like a circuit breaker and cut-off on a dead short. Not sure if Daly does this - but it might.

But even with the BMS, as a practical matter, it doesn't hurt to have a switch between the battery and the load - to disconnect current between the battery and the load while working on either. A connector (like an Anderson plug) that you plug in/out to connect to the battery is the same function as a switch - connect/disconnect the battery and the load. *This is just practical advice*

Some will even use a circuit breaker as a switch but also provide an extra layer of high-current protection.

 

[newguy199]

So I should only need some sort of switch for when I am doing work to the rest of the system to cut of the current from the battery but other than that this battery can now go straight to my inverter or 12v fuse board?