Switching between batteries

Thread Starter

ocean750

Joined Jul 15, 2020
16
Hi,
I am working on a system that uses two batteries and switches between them.
The small battery is 48DC 50AHr LiFePO4 .
The large battery is 48DC 100AHr LiFePO4 .
There are 3 work modes:
A: small battery is active. Current is up to 30 A .
B: large battery is active. Current is up to 60 A.
C: Idle mode. Current is 10 A.
Switching between batteries will be done only in Idle mode.

In my initial design there are relays to connect or disconnect the relevant battery.
The workflow is:
1.Press on the ON button. U1 will be active.
2.controller activates U2.
3.battery 1 is supplying current at up to 30 A .
4.Working at Idle mode.
5.controller activates U4. Both batteries are connected for a very short time.
6.Controller disconnects U2.
7.Battery 2 is supplying current up to 60 A .
8.Controller closes K1, activates U3 – charger.
9.charger will supply 5 A. that way battery 1 is trickle charged.

With this design I am worried that when both batteries are connected there might be a very high current between them, that may trip the BMS.
So, I was thinking about adding a big capacitor at V_OUT .
And switching by disconnecting both batteries and then activating one of them.
But because I will need a big capacitor, it may also cause the BMS to trip at power ON.

1595340636891.png
 

Thread Starter

ocean750

Joined Jul 15, 2020
16
So, I started thinking about a different design.
The small battery will be a smaller one, maybe 32DC , (it will need to supply more than 30 A)
The large battery can stay the same.
By using a diode, the switching between the batteries should be easier .
Since I never worked with such high currents before I am not sure which design is better or which components to place.
Also, will be happy for comments about the two schematics and about things I have missed, Did not think about.

Thank you

1595340709400.png
 

sghioto

Joined Dec 31, 2017
1,295
For what you have shown I would select circuit #2.
I suppose there are several ways to accomplish what you are after but simpler is usually better.
Circuit #2 should work OK, but you will need a diode rated at 50amps and heatsinked.
Relay U1 will also need to be heavy duty, like a DC power solenoid rated at say 100amps.
A better choice although more costly is to use a solid state relay for U1.
One question how do you plan to charge Bat_2?
Steve G
 

Picbuster

Joined Dec 2, 2013
1,020
So, I started thinking about a different design.
The small battery will be a smaller one, maybe 32DC , (it will need to supply more than 30 A)
The large battery can stay the same.
By using a diode, the switching between the batteries should be easier .
Since I never worked with such high currents before I am not sure which design is better or which components to place.
Also, will be happy for comments about the two schematics and about things I have missed, Did not think about.

Thank you

View attachment 212687
If I see this correct you want to charge small batt with the large one and connect also the load to it.
Advice: replace D1,U1 and U2 with a power mosfet (P channel 200A to accept inrush current / spikes)
for D1 a resistor to GND will do the job.
Use N fet's on the gates when a positive 'On' is mandatory.
place also some capacitors
Fet's internal resistance is extreme low in compare to a diode hence you gain with voltage lost and heat production.

Picbuster
 

Irving

Joined Jan 30, 2016
940
It would help to explain why you need this complex arrangement. Also based on your battery sizes, what it the expected run time? As shown your charging regime won't cope. You cannot trickle charge a LiFePO4 pack like you can a lead-acid battery, they have very different charging regimes (despite the use of a BMS - battery murdering system - inside the brick).
 

KeithWalker

Joined Jul 10, 2017
1,132
Why do you need to switch between two batteries? If you connect both batteries in parallel (matching terminal voltages first of course) You will just have one battery that can drive the load under any condition.
Keith
 
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