My battery monitor shows lower voltage when overkill charge path is opened.

[John Frum]

Got the new bms app and decided to fiddle with the administrative enable/disable toggles for charge and discharge.
Baseline: system is totally idle. Inverter is off.

Discharge disable/enable works as expected.
When I open the discharge path my battery monitor turns off.
When I re-enable it the battery monitor shows 26.3 volts same as the android app indicates.

Charge enable/disable gives unexpected results.
When I open the charge path my battery monitor shows 25.8 volts which is a surprise.
When I re-enable it the battery monitor shows 26.3 volts same as the android app indicates.

Can someone else confirm my results please?

 

[Gazoo]

Which side of the BMS is your battery monitor located.? Or are you looking at the BMS voltage in the app?

 

[John Frum]

Its a hall effect sensor sitting on the positive charge lead off of the core system positive busbar.

 

[HRTKD]

If I think about it, I'll try the same with my Victron shunt. It'll be a couple days at least.

 

[Gaze]

Can someone please link the "new bms app" mentioned above.

 

[Just John]

Can someone please link the "new bms app" mentioned above.

Support/Downloads

User Manual User Manual: Download Manual (6.5mb PDF) Simplified 4 Cell Assembly Ultra simple view of a complete 4 cell battery assembly. – Click for full resolution. Bluetooth Mobile Apps iPh…

overkillsolar.com

 

[gtxltd]

Yes you should see lower voltage when you disable the charge FETs. The discharge current will now pass through the charge FETs by way of their intrinsic body diodes. You will see a diode drop worth of difference in voltage. It's important to note that your BMS will have to dissipate the power lost in that drop and at high current this will be considerable. That's why it's best not to disable charge unless there's an unexpected overcharge condition.

 

[Gazoo]

Got the new bms app and decided to fiddle with the administrative enable/disable toggles for charge and discharge.
Baseline: system is totally idle. Inverter is off.

Discharge disable/enable works as expected.
When I open the discharge path my battery monitor turns off.
When I re-enable it the battery monitor shows 26.3 volts same as the android app indicates.

Charge enable/disable gives unexpected results.
When I open the charge path my battery monitor shows 25.8 volts which is a surprise.
When I re-enable it the battery monitor shows 26.3 volts same as the android app indicates.

Can someone else confirm my results please?

I checked this with no load. Voltage at the battery is 25.05 and voltage output of the BMS is the same, 25.05 volts. No change whether charging is enabled or disabled.

 

[John Frum]

I checked this with no load. Voltage at the battery is 25.05 and voltage output of the BMS is the same, 25.05 volts. No change whether charging is enabled or disabled.

Wonder why they behave differently.

 

[Gazoo]

Wonder why they behave differently.

I have no clue. I even checked it twice to make sure. Only difference is you tested at a higher voltage but I don't know if that would make any difference.

 

[gtxltd]

There is a difference between common port vs separate port BMS. My above response pertains to common port. A separate port BMS will not see the voltage difference. I have common port and I believe the OP also has common port.

 

[Gazoo]

There is a difference between common port vs separate port BMS. My above response pertains to common port. A separate port BMS will not see the voltage difference. I have common port and I believe the OP also has common port.

SmoothJoey and I have the same BMS (Overkill) and it's a common port BMS.

 

[gtxltd]

SmoothJoey and I have the same BMS (Overkill) and it's a common port BMS.

Hey Gazoo what are you using to read the voltage? If you are reading the app reported voltage that would explain it.

 

[Gazoo]

Hey Gazoo what are you using to read the voltage? If you are reading the app reported voltage that would explain it.

My DMM. I have not calibrated the BMS but the voltage difference between what the app reads and what I see on my DMM is only a couple of millivolts and my DMM could be off by that much.

The voltage the BMS displays, and the voltage my DMM displays (taken at the output of the BMS) agree with each other whether charge is enabled or disabled, other than than the 2mv I mentioned.

 

[gtxltd]

I have two of those same BMS and I see almost exactly what SmoothJoey sees on both of them. I also know the schematic of the BMS well enough to know that what SmoothJoey is seeing is the expected effect. If you don't see the difference I would wonder if your charge fets are actually turning off?
I also just did a test where I pulled about 30 amps discharge and disabled the charge FETs and the BMS did indeed get warm in just a few minutes. Then I turned charge FETs back on and it cooled back down. This is what you should expect from a common port BMS.

 

[gtxltd]

With the charge FETs disabled the power dissipated in the BMS is the discharge current x the voltage drop (about 0.5V). So if you pull 60 amps discharge with the charge turned off the BMS has to dissipate about 30 watts. You will feel the aluminum plates get warm for sure. With the charge FETs enables the voltage drop is miniscule and so is the power in watts so the BMS stays cool. I'm sure the overkillsolar guy can confirm this.

 

[gtxltd]

I made a crude diagram to show how this works. Inside my BMS (120 Amp) there are 30 FETs. 16 FETs are on one side of the PCB and 14 FETs on the other side. There are 15 discharge FETs in parallel and 15 charge FETs in parallel. My diagram represent one set. You can see how the discharge current still has to pass through the charge FETs even if they are off. The discharge current passes though the disabled charge FET by way of the forward biased body diode. This is why we see a voltage drop of about a half volt when the charge FETs are disabled. Also when the charge FETs are disabled the charge current cannot pass through because in that case the body diode of the charge FET is reverse biased.

 

[Gazoo]

I have an inverter charger hooked up to the cells. When I turn charging off the voltage of the cells drops very slowly. After I parallel top balanced my cells were all 3.5+ volts after sitting for a couple of days. I will turn the unit off and see if anything changes.

 

[gtxltd]

The fact that you have a inverter/charger hooked up is good information. That probably explains why you didnt see a drop before.

 

[Gazoo]

The fact that you have a inverter/charger hooked up is good information. That probably explains why you didnt see a drop before.

Correct. I turned the unit off and tested again. The battery voltage measured by my DMM was 27.05 volts and the BMS output slowly went down to 26.75 volts. The voltage might have went down more if I waited.

So why isn't the voltage different with the unit turned on. The cells voltages do drop.