Using Diodes???


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bwolten

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Sep 13, 2020
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Little Help please...
If you look at the attached File.. I have 2 Battery banks, I would like to use a make before break circuit with the relays that would parallel the battery banks while switching between them.. The batteries will be at different states of charge. I would like to avoid the in rush of current between the battery banks as the batteries try to balance for that split second...

Question is would a Stud mount diode , schottky diode or some type of diode configuration achieve this???
If not any ideas how to tackle this, if possible???

Thanks Brad
 

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cak

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The short answer is yes you could do this but you probably don't want to.

The long answer is that: the Diodes would also block you charging the batteries through that cable, It will be hard and expensive to get diodes that can handle the voltage and Amperage that you want, There will be efficiency loss from the forward voltage drop over the diode.

Since it seams like you want to frequently connect and disconnect these two battery frequently. If you don't have to do that then my recommendation would be to directly tie the batteries together while the charge is similar and then not plan to seperate them unless for maintenance in which case you would just have to repeat the process of equalizing the battery voltage ether with a separate charger or by connecting them with a resister. In my system I have a breaker for each sub battery pack but don't regularly disconnect any of them and when I do I have to make sure the voltages are similar before reconnecting. If you need that regular re-connecting then a few possible options that come to mind are all dependent on your system looking like it is "smart". If your relays are PWM capable you could use that to slowly connect them while letting them equalize charge. Alternatively you could use the smarts to only connect them once they are close and intelligently connect the correct battery depending on if things are in a charging or discharging state. To elaborate on that thought with an example if Battery A is 26VDC adn Battery B is 23VDC when you are running a load off the system only Battery A would be enabled until it's voltage dropped to 23VDC at which point they would both become enabled. In that same scenario if you started charging the system then Battery B would be the only one connected untill it caught up with the 26VDC of Battery A and then both could be connected. The main trade off I see with this method other than needing the smarts is that each battery on its own would need to be capible of handling the full current of whatever your use and charging plans are.
 

OffGridInTheCity

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Agree with @cak.

I didn't get why you want to separate / rejoin / separate your parallel batteries - maybe you could explain more?

As an operational Tip... I have 7 separate batteries in parallel in my Powerwall. I've created a window of time to able to work on the by coordinating the daily inverter (low) cut-off which leaves the battery bank ~ 3.6v/pack and the use of my iCharger X8 Storage discharge feature as a final step of working on packs - which leaves the packs at 3.6v. No special balance issues or current rush etc.

Both the battery bank low and iCharger X8 discharge stop are customizable to any voltage. My point is that by coordinating battery build processes with battery bank operations, you could create a natural window to work on things smoothly :)
 
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Korishan

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To minimize inrush current you want to use resistors and a relay. Power goes through the resistors until voltages balance out between the two sides and then the relay kicks on to bypass the resistor.
Mosfets would also work for this. A Mosfet would have the added benefit of being able to limit current, too. But you would need to make sure to monitor voltages on both sides of the switch and change the gate of the Fet over time.
 

bwolten

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That is some Great Stuff,, I am just learning so everything helps...

Maybe a little more information on my part will help...
The 2 middle bottom squares are Solar Charge Controllers, I have 1 for each battery.. So charging through the cable might not be an issue??
I have 1 solar array to a combiner box, out of the combiner box I have 2 circuit breakers. 1 circuit breaker going to the batteries SCC.. When Bat 1 is charged, I shut the breaker off for bat 1, which puts the SCC to sleep and turn on the breaker for bat 2 to charge...

question from above:
I didn't get why you want to separate / rejoin / separate your parallel batteries - maybe you could explain more?

Chemistry of batteries, different Capacity, Age difference of batteries..
I basically only want to parallel the battery for a few milliseconds to swap between them to maintain power to the loads..
Reason for using the make before break circuit... With what ever means to prevent the inrush current between batts

Currently I use a off/1/both/2 switch for each battery, with a 25 ohm resistor through post 1 for precharge...

Thanks again!! if more information is needed I will try to explain what I am trying to accomplish better..
 

ajw22

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[different] Chemistry of batteries
Like AGM, and flooded lead acid, each requiring different charge voltages? Or LiIon / LiFePO4?
Are you planning to use something like an Arduino to automate the switching?
Details would help to find better options, or rule out others.


In any case, the circuit looks fine, but be careful when choosing the parts.

Relay: make sure it is rated for DC, and for your voltage and current. Otherwise, the contacts inside can sustain an arc bridge and melt/burn the unit down. Or fuse the contacts and fail to open. Or overheat in general.

Diode: again, make sure it's rated higher than your max voltage and current. Best to go significantly higher in current rating to reduce heating. Schottky type would be best, as they have a lower "forward voltage" of 0.2V~0.4V, reducing the energy lost to the diode.
 

Redpacket

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That is some Great Stuff,, I am just learning so everything helps...

Maybe a little more information on my part will help...
The 2 middle bottom squares are Solar Charge Controllers, I have 1 for each battery.. So charging through the cable might not be an issue??
I have 1 solar array to a combiner box, out of the combiner box I have 2 circuit breakers. 1 circuit breaker going to the batteries SCC.. When Bat 1 is charged, I shut the breaker off for bat 1, which puts the SCC to sleep and turn on the breaker for bat 2 to charge...

question from above:
I didn't get why you want to separate / rejoin / separate your parallel batteries - maybe you could explain more?

Chemistry of batteries, different Capacity, Age difference of batteries..
I basically only want to parallel the battery for a few milliseconds to swap between them to maintain power to the loads..
Reason for using the make before break circuit... With what ever means to prevent the inrush current between batts

Currently I use a off/1/both/2 switch for each battery, with a 25 ohm resistor through post 1 for precharge...

Thanks again!! if more information is needed I will try to explain what I am trying to accomplish better..
As said above, connecting 2 separate large 200AHr batteries together when at different charge voltages/SoC is problematic.
If connecting directly by just relay, "large" currents will flow until the batteries equalize charge state - could spike to 100's of amps. Currents would flow more like up to several minutes. This can be dangerous & stressful for cables, batteries & contacts (MOSFETs similarly high stress). Batteries likely to get damaged, contacts weld, etc like already mentioned. It's a bad idea.
At the very least you would want something like a "precharge system" with eg high wattage (eg 100W) resistor approx 0.5 to 5 Ohms in series (depending on pack voltage).
You'd have a resistor in series with each battery and a relay across each resistor.
Battery carrying load, relay = closed, joining battery, relay = open. Current flows for a while vs resistor, batteries get close, 2nd relay closes.

However, from your picture & application description, just having the load supplied via the two diodes would prevent one battery dumping current into the other. Since the chargers are separate, charging is no issue.
You could safely add/disconnect either battery at any charge state.
Only the load would see a step up in voltage if a higher charged battery was connected. No large currents would flow.
Like also said above you need large diodes (depending on load).
This is how some UPS's already operate internally.
 

bwolten

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I have read about the inrush current between batteries when paralleling at different voltages, bad idea as you all have stated...

Are you planning to use something like an Arduino to automate the switching?
Yes!! MircoLogix 1400 PLC with HMI to control the switching..
I also have the Voltage of each battery monitored inside the plc program with Voltage Transducers... So with the loads on bat 2,, bat 1 is charging, when bat 1 gets fully charged, the program will switch loads over to bat 1.. Reason for make before break Relay logic in plc program..

Relays: TEKilovac Ev200AAANA

Diodes: Looks like I am looking for a 200 amp Schottky diode...

My thinking is possible, just unclear which parts to use..

Thanks for all the info,
 

Redpacket

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If you have the diodes as in your diagram, you don't need the relays....
Although if both batteries are present you could code the PLC so that when battery voltages are within max of approx 0.3 to 0.5V, you could close relays across both batteries to load share without the diode losses.
Pro suppliers like mouser, digikey, RS components, etc would have diodes like that.
Depending your need for less losses, you could do this with MOSFETS, but you'd need some electronics to control them via current direction sensing a bit like camping battery isolators work but in reverse.
 

bwolten

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I did look at the Victron battery combiner but realized it was only for 24 volts mine is 48v

I did come across this at Digikey???

FRS200CA120​

Current - Reverse Leakage @ Vr10 mA @ 1200 V
Voltage - Forward (Vf) (Max) @ If1.8 V @ 200 A

Which at 60 amps would be .54v if I did the math right

It is a learning headache!!!
 

ajw22

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FRS200CA120​

Current - Reverse Leakage @ Vr10 mA @ 1200 V
Voltage - Forward (Vf) (Max) @ If1.8 V @ 200 A
Which at 60 amps would be .54v if I did the math right

Unfortunately no. You have to look it up in the ForwardVoltage-vs-Current graph in the datasheet. At 60A, it's around 1.2V, much higher than the typical 0.2V~0.4V of a schottky diode, so not particularly well suited for your use case. Even if you were to use 2 of them in parallel (so 30A each*1), the FV would only drop to 1.0V.

What chemistry/configuration do your batteries have?

*1) current won't split neatly, unless the diodes are thermally coupled.
 

Korishan

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I have 1 solar array to a combiner box, out of the combiner box I have 2 circuit breakers. 1 circuit breaker going to the batteries SCC.. When Bat 1 is charged, I shut the breaker off for bat 1, which puts the SCC to sleep and turn on the breaker for bat 2 to charge...
I'm confused as to why you would want to do this 🤔

Why not just have both batteries connected in parallel, and then have both charger controllers output connected in parallel. Both chargers will charge the banks based on SoC and input voltages. If the Solar array for Charger1 is too low, it just won't do anything and Charge2 will do the bulk work, and vice-versa.
Completely eliminating the need for any kind of switching, smart or otherwise.

Could you explain a bit more as to why you want to do this type of charging of splitting them in the first place? Or did you just not know you could parallel the outputs of the Chargers?
 

bwolten

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Could you explain a bit more as to why you want to do this type of charging of splitting them in the first place? Or did you just not know you could parallel the outputs of the Chargers?
I only have 1 solar array... I have read that it is not a good idea to hook up 2 SCC to 1 solar array.. What would I be gaining if I paralleled the out puts of the chargers???

And No, I didn't know you could parallel the outputs of the chargers...
 

Korishan

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I only have 1 solar array... I have read that it is not a good idea to hook up 2 SCC to 1 solar array.. What would I be gaining if I paralleled the out puts of the chargers???

And No, I didn't know you could parallel the outputs of the chargers...
Ok, then use just 1 charger, and parallel all your batteries together. It would be far healthier on the batteries if they didn't discharge too far anyways, and a slower discharge rate (aka lower amp draw) is also healthier.
There is no need to make things far more complicated. You really don't need to have two battery banks and try to switch between the two.

And if you do install more panels, say in an area that isn't close to the existing one or gets different shading conditions, then yeah, use the 2nd charger and parallel the outputs.
 
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