BMS to control battery charger?

[morvolts2]

Chevy(and LG) did a lot of testing for these packs to last the 8 yrs+ they claim to have achieved. Alot of that has to do with how they are charged and discharged. I think a more reasonable window is 39-49v. I have the individual cells low voltage alarms set at 3.3v each(total 39.6) and low voltage drop dead cutoff at 39.2v. I also have a battery "fuel"gauge,battery voltage digital display,and total amperage draw,all in the steering wheel.
So when voltage gets low the "fuel gauge" goes from green to yellow,then red and flashes.
When in the red if it is still in use,the individual cell alarms will go off,which is an audible alarm,If this is an emergency get home situation it will go until the low voltage cutoff shuts it down at 39.2v. All of these safeties are in place to keep the battery packs happy and healthy.Theres lots of info out there to support the practice of not running the upper and lower limits to the ragged edge.
I havent gotten my hands on a dumb 36v accucharge charger to test,using the 3 rd wire as a control,but cant imagine the transformer being much different.

[Murby]

Originally Posted by morvolts2

Chevy(and LG) did a lot of testing for these packs to last the 8 yrs+ they claim to have achieved. Alot of that has to do with how they are charged and discharged. I think a more reasonable window is 39-49v. I have the individual cells low voltage alarms set at 3.3v each(total 39.6) and low voltage drop dead cutoff at 39.2v. I also have a battery "fuel"gauge,battery voltage digital display,and total amperage draw,all in the steering wheel.
So when voltage gets low the "fuel gauge" goes from green to yellow,then red and flashes.
When in the red if it is still in use,the individual cell alarms will go off,which is an audible alarm,If this is an emergency get home situation it will go until the low voltage cutoff shuts it down at 39.2v. All of these safeties are in place to keep the battery packs happy and healthy.Theres lots of info out there to support the practice of not running the upper and lower limits to the ragged edge.
I havent gotten my hands on a dumb 36v accucharge charger to test,using the 3 rd wire as a control,but cant imagine the transformer being much different.

I have one of those little ebay volt meters on a power supply I built.. be careful with them as they're not real accurate like a good fluke meter.

Here's the charge-discharge curves for the Chevy Volt packs, you want to stay inside the knee's at either end. I annotated the curve for per-cell voltages and a 12s pack. This was something I did a few years go.. hope it helps.

[simicrintz]

Thank you both. So if I am understanding your thought process you like to "undervoltage" the charger. I see the concept and am wondering if the pack still gets fully charged using this method (just runs longer but does not reach or exceed maximum voltage doing so)? I build mobile Reverse Osmosis (RO) trailers and build in a buffer to keep the system safe as well, so I think I am following your thought process :)

I have an AH meter in the cart already that has some relays built in that I can use for HV or LV (or something else) and I plan on watching this diligently. I don't trust the "sliding scale" type meters so I will toggle through this meter to keep an eye on things.

[morvolts2]

Originally Posted by Murby

I have one of those little ebay volt meters on a power supply I built.. be careful with them as they're not real accurate like a good fluke meter.

Here's the charge-discharge curves for the Chevy Volt packs, you want to stay inside the knee's at either end. I annotated the curve for per-cell voltages and a 12s pack. This was something I did a few years go.. hope it helps.

noted on the voltage readings
mine is .4v higher than all my Flukes.
The voltages I stated were as per fluke,not the digital display.


the other thing that hasnt been mentioned is if the SOC is at the top and ambient goes up then you risk being over charged.
If you do accidentally overcharge,immediately begin discharge to below the safe level.

[morvolts2]

Originally Posted by simicrintz

Thank you both. So if I am understanding your thought process you like to "undervoltage" the charger. I see the concept and am wondering if the pack still gets fully charged using this method (just runs longer but does not reach or exceed maximum voltage doing so)? I build mobile Reverse Osmosis (RO) trailers and build in a buffer to keep the system safe as well, so I think I am following your thought process :)

I have an AH meter in the cart already that has some relays built in that I can use for HV or LV (or something else) and I plan on watching this diligently. I don't trust the "sliding scale" type meters so I will toggle through this meter to keep an eye on things.

Dont think of the pack not being fully charged,think of it like ,as you say, a safety buffer.
If you need more capacity,add more packs,dont jack up the voltage on existing ones.

[simicrintz]

Originally Posted by morvolts2

Dont think of the pack not being fully charged,think of it like ,as you say, a safety buffer.
If you need more capacity,add more packs,dont jack up the voltage on existing ones.

I am better with water than I am with electricity!

Got it; thank you. Now I gotta go find a "smaller charger......

[morvolts2]

Originally Posted by simicrintz

I am better with water than I am with electricity!

Got it; thank you. Now I gotta go find a "smaller charger......

any reason why you cant use your charger and have the BMS control it?
only minus I see is the BMS needs to be powered up

water and electricity actually have a lot in common as far as modeling goes.
Electricity is hard for most to understand because they cant see it(and hopefully cant feel it).
If you know flow and pressure and can understand simple one line diagrams,youre half way there

[Sergio]

The issue with water and electricity analogy is that too many people make incorrect assumptions on electricity behavior based on their understanding of water flow.

A 36v charger profile would work for your pack as it would allow for full absorption, but using a 48v profile and having the BMS disconnect the charger is a bad idea as the charger would still be in constant current mode when/if the BMS disconnects due to a cell high voltage condition.

[simicrintz]

Thanks fellas. And I will admit to using the same thought pattern on electricity as water and it messes with my brain (and the fact that I can see water does make things easier for me).

Any suggestions on a 36V charger, Sergio? I know that you have built your pack for (substantially) less than I have so I know that you are the master at finding the best deal!

EDIT: And the battery supplier has provided a wire to hook to the charger negative to control the charger and for some reason I want to shy away from that. It may be a nice redundant feature (and I may hook it up anyway) but I would rather have the correct charger in place instead of trying to force something to work.

[Sergio]

I meant use your existing Summit II with a 36v profile unless your model does not support 36v.

You need to disable the "auto-voltage" or whatever they call it and use a fixed 36v profile, Make sure the profile does not have an "equalization" mode.

Just call Lester and ask them for a fixed 36v profile without equalization.

If You already sold your Summit charger just get a lithium specific charger like the one you posted before for 50.4v since you typically can adjust the voltage to a lower value.