Battery bms shutoff

[Pat911]

Hi Walter,

That is correct, there's hysteresis between low voltage cutting off and turning back on, and as you correctly state, if you have access to the BMS parameters you can fudge things to get home. But that isn't what has happened in 621gears situation. He is tripping on high current. Some BMS's will come back on after a delay whereas others require connection to a charger to take them out of an overcurrent trip condition.

The "Jump Starter" circuit described will not have enough juice to bring you out of an undervoltage condition, well not unless you have a large enough booster battery to put enough charge into the battery to bring the voltage up. It can, however, kick a BMS that has tripped on high current back into life as all that is required is a voltage of around 3v higher than the current battery voltage to instantly switch it back on. You can then go on your merry way, avoiding conditions that caused the trip in the first place.

Cheers
Pat.

[621gears]

Like I posted earlier. My issue was faulty firmware from Navitas. Still waiting for the engineers to send me a revised update. The jump starter works great! Just have to make sure to unplug the cable from the jump start battery or it will drain out, even though I have a cutoff switch. Pat, do you know why the cutoff switch does not prevent the JS battery from draining out?

[Pat911]

I’m assuming you mean you’re leaving the boost converter connected to the jump battery but the jump battery is being drained even though the boost converters output is not connected to the cart battery. This would be because the boost converter has a parasitic current drain of its own, even with nothing connected to it. It will be quite low but it will drain the boost battery over time.

You can leave the boost converter connected to the cart battery and put the isolator on the input side if you like.

Cheers
Pat.

[621gears]

Originally Posted by Pat911

I’m assuming you mean you’re leaving the boost converter connected to the jump battery but the jump battery is being drained even though the boost converters output is not connected to the cart battery. This would be because the boost converter has a parasitic current drain of its own, even with nothing connected to it. It will be quite low but it will drain the boost battery over time.

You can leave the boost converter connected to the cart battery and put the isolator on the input side if you like.

Cheers
Pat.

The switch is between the jump battery and the booster. Still drains the jump battery. I don't get it.

[Pat911]

Now that is very strange. Are you switching the positive or negative, not that it really matters........Unless you're switching negative and the booster has a common ground, which it probably has, and there's some kind feedback going on. Maybe you can try a 2 pole switch that switches both the positive and negative. Also, make sure whatever switch you use can handle the input side current, 3A or so if you've set the output current to 0.5A.

Cheers
Pat.

[621gears]

Originally Posted by Pat911

Now that is very strange. Are you switching the positive or negative, not that it really matters........Unless you're switching negative and the booster has a common ground, which it probably has, and there's some kind feedback going on. Maybe you can try a 2 pole switch that switches both the positive and negative. Also, make sure whatever switch you use can handle the input side current, 3A or so if you've set the output current to 0.5A.

Cheers
Pat.

I don't remember. I take a look at it. Thanks!

[shoomba]

Here are some pics of the "BMS Jump Starter" I whipped up with info from Pat. I found some DC power connector to bare wire things that made it easy to unplug/plug everything together.

I had some 18 awg lying around so just used that. Ive got a connection coming off the battery I leave connected, that I can connect to the output of the booster. And then I found a battery bank that had a DC connector output that I connect to the input of the booster.

I originally planned to build it all in a waterproof box and just flip a switch but it was hard to find a box that would fit next to the battery and hold everything. Plus my issue is pretty intermittent and random so I don't see myself having to need to use this so often, hopefully anyway. So I ended up just getting a waterproof tackle box to store the jumper and throw the battery just in a front cubby. If I ever need it shouldn't take more than a minute or two to just plug everything together and hopefully be on my way.

Haven't really tested it yet as I don't know how to purposefully trip my BMS but I did switch it on and the BMS app for my battery showed the current and voltage basically invert and change so its doing something. Seems ridiculous that companies selling this drop in systems dont have a better way to reset a BMS. It leaves you stranded with what is otherwise a running cart. Just reset the BMS and slow it down is a lot better than having to get a tow to an outlet to get back up and running.

[Pat911]

Very well done! As others can see now, it really is a simple device.

[621gears]

Originally Posted by Pat911

Good to hear the concept works well. What current did you ultimately set the output to and what did you use as the input power source? Pictures would be nice too :)

Also, I believe you’re using far more than 10% of your battery if you’re getting 15-20mi. I doubt you’ll get 150-200mi on a full charge. The Navitas will be looking at battery voltage to shut off, not state of charge and SOC cannot be reliably estimated with voltage. Your SOC could be anywhere when the Navitas decides it’s time to shut down.

Cheers
Pat.

You are right about me not getting 150-200 mi on a full charge. My battery meter was showing 85% when the batteries were fully drained. By my calculations I was getting about 30 miles on 15%, 2 miles every 1%, in reality, I was getting 2 miles every 6.66%. Found out the meter was giving a false reading. It was not calibrated correctly for some reason. I calibrated it according to the instructions setting the AH to 108 (6 x 18ah each). At this point, I am waiting for Allied to get back to me on how to get the meter to work right. At 85% it really should show 0%. There was nothing wrong with Navitas firmware!

[Pat911]

30 miles is to be expected from a setup like yours. Let us know how you go recalibrating.