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Voltage Drop Thru BMS

TGPB

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Aug 16, 2020
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Is it normal to have a voltage drop going thru the BMS"
I have 13.78 volts at battery terminal where BMS is connected and 13.32 volts coming out of BMS. I'm getting similar voltage drops on both batteries.
 
Not speaking from personal experience so take what I say with a grain of salt, but I would expect some voltage drop is normal, but 3.3ish% seems quite high, since that is before accounting for Vdrop in the wiring + any of the other connections and components.
 
Depending on the specs of the mosfet, anywhere from 0.1 to 0.5v forward voltage drop is normal an unavoidable.
Multi-meter voltage across the input and output of the BMS to confirm that.
This is where the heat comes from in high current mosfets, and limits their practicality at higher current.
 
I discovered this with my fluke multimeter. I'm concerned with the voltage sensing relay for the generator. It will come on sooner, which isn't a bad thing. But it will continue charging after the batteries are at the desired voltage because the true battery voltage will be higher than the sensing relay voltage.
I was trying to set this up so the BMS was a redundant part of the charging system.
 
What current was passing when you measured the voltages?

What is the maximum current the BMS can handle?

What size and length of wires are you using?
 
I'm curious on this. Deciding on going with mosfets or contactors for 24V@200A max load 50A nominal.
The entire Daly BMS alone is cheaper than the contactors.
 
What current was passing when you measured the voltages?

What is the maximum current the BMS can handle?

What size and length of wires are you using?

No load on battery

125 amp BBS

3 x 10 ga. into BMS and same coming out. Less than a foot long.
 
Well if there was no load then the current was 0 and thus the Vdrop due to IR is 0 too.

So the voltage drop you measured is clearly not because of losses at high current. Either the BMS is actually open and you measured a ghost voltage, or there's something weird going on.

Can you try putting a load and measure the voltage accross the BMS again?
 
I will try that after I get them put back together. I had an issue when I did the bottom balance test.
 
I had an issue when I did the bottom balance test.
Side issue, but worth stating, if you are using a BMS, you should almost certainly be top balancing not bottom balancing.
 
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Bottom balance is for EV type use where the battery is taken down to low SOC. For house battery use it is better to top balance.
 
Bottom balance is for EV type use where the battery is taken down to low SOC. For house battery use it is better to top balance.
Adding to this, even if you think bottom balance makes sense for your context, if you bottom balance but use a top balancing BMS, your manual balance and your BMS are working against eachother (the BMS will immediately set to work trying its hardest to undo what the manual balance did) at least that is my understanding.
 
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