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DIY Lifepo4 battery box disconnect: BMS positive too?

meetyg

Solar Enthusiast
Joined
Jun 4, 2021
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Hi.
I'm building a DIY Lifepo4 battery in a battery box, like this:
20211229_082232.jpg

I will be putting a DC breaker on the top of the battery box, mainly as a disconnect switch, so that when doing maintenance or disconnecting the battery, the terminals won't have voltage (positive side).
My question is, should the BMS positive be also connected to this switch, so that BMS won't have voltage either (just for additional safety) ?

What happens when you disconnect the positive lead of the JBD BMS ?
Is it a good idea or will it just confuse the BMS when powering off and then on?
Thanks.
 
I haven't wired anything yet, as I'm currently just capacity testing the individual cells.
But in theory, I was thinking of connecting the last balance lead of the JBD to main disconnect switch that will be installed one the lid of the battery box.

In hindsight, maybe that's a stupid idea, because the BMS will still see voltages from the other cells, and probably go wacko because the last cell is disconnected.

So now my question is about using the onboard "discharge disconnect" of the JBD.
It's there to enable/disable discharge by using a simple switch.

But would that still protect from a short if there battery terminals were to accidentally be shorted out?
 
But would that still protect from a short if there battery terminals were to accidentally be shorted out?
If you get a short between cell(?) terminals, there is no way for the BMS to help.

What is your concern or problem you are trying to solve or prevent?

The BMS is, by design, a safety disconnect that is connected directly to a battery.
 
Sorry for not being clear:
As I mentioned, I'm building a battery, putting the cells inside these boxes:
20211214_140454.jpg
As these will be used in my dad's mobility scooter, I want to add a battery disconnect switch on top of lid, so that if there is maintenance to be done on the scooter, or if the battery needs to be disconnected/removed for transportation, the battery can be safely disconnected with a flick of a switch.

I want the terminals on the top (the lid) not to have voltage/current, so if someone were to make short with the battery wires, nothing will happen (given that they turned the battery OFF).

My plan was to just connect a dc breaker (panel mount) on the lid, which would disconnect the battery main positive (coming from last cell in the series) from the positive terminal (on the lid of the battery box).
But my question was, is that enough, or should I somehow disconnect the BMS too?
 
The BMS has a sense leads to each cell. There is a positive to each cell. You would have to disconnect all the sense wires to stop any small drain to the battery.
The BMS controls by switching the negative connection to the battery. Switching the positive cable 'off' will do what you want. But I would use a heavy duty battery switch, not the CB. You need a Class T fuse to protect the circuit fed by a LiFePo battery.
 
I don’t know if there are circuit breakers that can work as a switch, there are however circuit breakers that can’t, failing before too long.
 
There are CBs that can be used as switches. I don't know if they are bidirectional for HD charging/discharging circuits. I know CBs are not very good for the extreme power available from a LiFePo battery with a short circuit.
 
In my situation, the switch will be under very little load, when switched off manually.
 
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