Overkill BMS question about over current load

[2Big2B]

Just looking at the controls and displays, it looks like it tops out at 24V. It would probably work fine for top-balancing the cells but would be too low of a voltage for trying to charge the 24V pack.

Having said that... HP made some great product so if you can get it cheap, grab it.

I am wondering if that HP 6286A could be modified to kick out a higher voltage somehow. Of course the meter would have to be recalibrated in such case. Probably a blind alley not worth pursuing. It would make a fine test/power supply for anything less than 24v 10A though, like a 4S or 7S (?) setup...

 

[2Big2B]

Here's more detail on that charger:


Here's some better detail on the charger:

View attachment 49583


I wonder if the charger's voltage could be adjusted down a volt?

I love the "Insurance holder" name for the fuse holder in this diagram!

"Isn't that just Precious!"​

 

[curiouscarbon]

dear @2Big2B neat project.

custom charge termination should be possible with a little bit of DIY arduino. if a charger voltage is fixed at 3.65V/cell, but you want lower configurable.

proposed concept:

put a contactor/relay on the input wire from incoming fixed voltage charger

sense voltage of battery pack

open/disconnect relay when voltage of battery pack meets programmed requirement

won’t necessarily get time for cells to balance doing this. but if the charger is low amps then maybe less pronounced effect.

edit:
another option is to put a current sensor between fix voltage charger and battery/BMS. just read the cell voltages over UART from overkillsolar BMS -> arduino. then open/disconnect charging relay when voltages and charge current match requirements. happy to help with part selection.

e.g.

VoltsPack = uart.read(bms);
AmpsIn = currentSensor.read();

if(VoltsPack>13.8 && AmpsIn<6)
{
  openChargeRelay();
}

something like that could potentially approximate the feature. probably could just read voltage and current from BMS.

 

[2Big2B]

dear @2Big2B neat project.

custom charge termination should be possible with a little bit of DIY arduino. if a charger voltage is fixed at 3.65V/cell, but you want lower configurable.

proposed concept:

put a contactor/relay on the input wire from incoming fixed voltage charger

sense voltage of battery pack

open/disconnect relay when voltage of battery pack meets programmed requirement

won’t necessarily get time for cells to balance doing this. but if the charger is low amps then maybe less pronounced effect.

 

[2Big2B]

What is a "contactor/relay" ? Maybe a DC electric solenoid switch?

How would I use the 3.65 voltage to trigger it?

 

[curiouscarbon]

What is a "contactor/relay" ? Maybe a DC electric solenoid switch?

How would I use the 3.65 voltage to trigger it?

Yea DC solenoid switch, to program disconnect of charger.

If you have a charger that always charges to 29.2V (or 3.65*8), but you want to only charge to say 27.6V (3.45*8). Have small computer watch and disconnect when time is right.

The OverkillSolar BMS has a data output port that reports the voltage of the battery pack, as well as the amperes current going in or out.

The general idea is to program a small microcontroller like arduino to read data from the BMS data port to determine voltage and amps.

Then achieve custom charge end voltage by DC solenoid disconnect charger by program when BMS data indicates it’s time.

As long as the custom voltage is above ~27.4V it should be ok.. 3.425*8.

Hope this helps, cheers

 

[2Big2B]

Yea DC solenoid switch, to program disconnect of charger.

If you have a charger that always charges to 29.2V (or 3.65*8), but you want to only charge to say 27.6V (3.45*8).

The OverkillSolar BMS has a data output port that reports the voltage of the battery pack, as well as the amperes current going in or out.

The general idea is to program a small microcontroller like arduino to read data from the BMS data port to determine voltage and amps.

Then achieve custom charge end voltage by DC solenoid disconnect charger by program when BMS data indicates it’s time.

As long as the custom voltage is above ~27.4V it should be ok.. 3.425*8.

Hope this helps, cheers

But isn't that charge voltage limit already a programmable parameter of the (Overkill) BMS anyway?

I hadn't heard of Arduino before. I Googled it:

People also ask:​

What is the Arduino used for?​

Arduino is an open-source electronics platform based on easy-to-use hardware and software. Arduino boards are able to read inputs - light on a sensor, a finger on a button, or a Twitter message - and turn it into an output - activating a motor, turning on an LED, publishing something online.

- Thanks for that!

 

[curiouscarbon]

But isn't that charge voltage limit already a programmable parameter of the (Overkill) BMS anyway?

It is, and there should be no problem using the BMS charge high voltage limit to terminate a 10A charge.

The relay/solenoid would keep allow the connected loads to be powered at all times assuming configured properly, but I guess that’s not really so important maybe.

 

[2Big2B]

It is, and there should be no problem using the BMS charge high voltage limit to terminate a 10A charge.

The relay/solenoid would keep allow the connected loads to be powered at all times assuming configured properly, but I guess that’s not really so important maybe.

It would be best to let the BMS handle all connections in/out of the battery pack, safety wise. I like the concept though as perhaps a solution to a problem I haven't encountered yet. Thanks!

 

[KSisk]

I am building an 8S 24v 100AH battery pack with an Overkill 8S 24v 100AH BMS for a "Bounder" model power wheelchair. The pack will replace a pair of 100AH 12v Gel cells in series.

The power wheelchair uses two massive 30A (?) electric motors. I just found a mention in the wheelchair's service manual that it can draw up to a 140A load from a stall (motionless) state to a Start (putting it into motion). Otherwise at full throttle the load will be under 60A.

Obviously the original 24v 100A SLA Gel Cell battery configuration didn't need a BMS - just circuit breaker/fuse protection.

My question is, if I am pulling a momentary 140A load through the Overkill 100A BMS, will it see that as a dead short and shut down? Will that be a problem I had not considered? If so, what would be the best work around - bypass the BMS and draw the load directly from the battery pack? Get a bigger BMS?

Naturally I have emailed this question into Overkill support - which is probably overwhelmed, so I am asking here too. I hope is not just a newbie stupid question. So what does a BMS do with a current greater than it is spec'd for - and/or is there a programable parameter I can set to define the maximum current draw as an over ride.

With the power switch on but the chair not moving, the chair should draw less than 120mA. Next, drive the chair on flat level hard terrain. The BOUNDER should draw between 12A and 20A from the batteries at full speed. Chairs geared for slower speeds and with lighter users will tend towards the lower readings. When accelerating, climbing hills or on soft surfaces such as carpets, the power consumption will increase. At stall, the current from the batteries can exceed 150A. When going downhill, current should be less than 10A except when the joystick is released, currents can increase as the chair slows to a stop. When turning on a hard level surface, less than 15A should be required, and less than 25A on indoor/outdoor carpets

Good info about a battery on wheelchairdriver.com

BM3 Lithium Powerchair Battery. A safe LiFePO4 wheelchair!

The latest development in a line of powerchairs resulting in this do everything lithium powered power wheelchair. Off road and indoor capable with huge range and speed...

www.wheelchairdriver.com

But not sure if I agree 100%