Batteries in parallel with different amps on positive and negative

Wolf

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My bms Is only connected to the battery via the bms leads. Other than that it is correct. Separate relays are powered by low voltage from the bms.
While typing this I’m realizing that maybe the power the relays are using could cause amp discrepancies. Maybe I should use a separate power supply for the bms so it doesn’t use the bms leads
Interesting. Would you be so kind and let me know what model BMS you are using?
 

Redpacket

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I hope you've though about the probability of failure of those relays being quite high - switching DC regularly, etc.
Moving to MOSFETS would do two things:
- increase reliability a lot
- use almost no power to drive them
eg apart from actual balancing current, very little power would flow.

Overall though, if your system is working & keeping your cells balanced, that's what matters.
 

Cheap 4-life

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I hope you've though about the probability of failure of those relays being quite high - switching DC regularly, etc.
Moving to MOSFETS would do two things:
- increase reliability a lot
- use almost no power to drive them
eg apart from actual balancing current, very little power would flow.

Overall though, if your system is working & keeping your cells balanced, that's what matters.
I use high voltage high amp EV relays. They are dependable for EVs so should be ok. They are normally open relays that are kept constantly closed by the bms. No switching. The inverters stop producing before LVD. I’ve read that mosfets are more prone to failure. Also isn’t a bms that has all the power going thru them more prone to failure? Both of the relays I’m using (one for charge on pv panels and one for discharge) use only .5kwh in 24hrs. As for balancing I’ve never had to balance the cells they are EV cells that are matched very well by internal resistance.im going to use a separate external power supply (instead of balance leads) to power the bms-relays. That should mitigate some of the power flow on the bms leads
 

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Redpacket

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OK, so since I haven't seen the BMS circuit I'm guessing you are using those (nice looking !) relays to switch off the inverter, etc at low pack voltage. or similar.
Yeah, I wouldn't put the pack output through MOSFETs except for smaller uses, etc tools, e-bikes, etc. I was thinking of MOSFETs for cell by cell balance load switching. But it sounds like you're using the Chargery for that. All good.
 

ajw22

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I use high voltage high amp EV relays. They are dependable for EVs so should be ok. They are normally open relays that are kept constantly closed by the bms. No switching. The inverters stop producing before LVD. [...] use only .5kwh in 24hrs

Zero experience with EV relays, but .5kWh per 24hrs (21W) sounds like quite a lot. Do they get hot?
Again, not sure if it's recommended/required for that particular relay, but there are very simple circuits to significantly reduce operating power requirements of DC relays:
Section "1. Example of CR circuit method", requires just 3 very basic parts.

Essentially, it initially allows full power to get the switch moving. And then reduces the current once the relay has settled in the closed position.
 

Cheap 4-life

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OK, so since I haven't seen the BMS circuit I'm guessing you are using those (nice looking !) relays to switch off the inverter, etc at low pack voltage. or similar.
Yeah, I wouldn't put the pack output through MOSFETs except for smaller uses, etc tools, e-bikes, etc. I was thinking of MOSFETs for cell by cell balance load switching. But it sounds like you're using the Chargery for that. All good.
The inverters have cutoff voltage settings. If for some reason the inverter didn’t shutoff then the bms will stop sending 12v to the relay therefore allowing the discharge relay to open. Same if the chargecontroller somehow didn’t stop charging and overcharged a cell, the charge relay on the pv panels input would no longer receive 12v from the bms and that relay would open.
 
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Cheap 4-life

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Zero experience with EV relays, but .5kWh per 24hrs (21W) sounds like quite a lot. Do they get hot?
Again, not sure if it's recommended/required for that particular relay, but there are very simple circuits to significantly reduce operating power requirements of DC relays:
Section "1. Example of CR circuit method", requires just 3 very basic parts.

Essentially, it initially allows full power to get the switch moving. And then reduces the current once the relay has settled in the closed position.
Combined the relays use .5kwh. So like 10w each. They do get a little warm. I figure there’s loses with a bms like Daly that have all battery power going through the bms. Thanks for the link though
 
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Redpacket

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Typical (non-bypassing) operating current of most BMS like yours (without the relays) should be quite low, a few mA?
 
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