Batteries parellel

[Al Skierkiewicz]

Scott,
While not the best way to do this, I believe you are minimizing risk with the procedure you wrote. The issue with fully wiring batteries in parallel comes when they are mismatched in some way. The two batteries can actually discharge each other. If left unattended, this will result in total discharge of both batteries. While lead acid batteries are more tolerant of this fault, other batteries are not. Any search for RV battery isolators should show a variety of methods and products designed to charge and separate electrical loads with multiple batteries.

[Mr V]

Originally Posted by tr6scott

Ok, can someone enlighten me with some data on what we should see/best practices/avoid in the future?

When we do parades, we use a y battery connector, so that we can keep the bot alive as we hot swap batteries. We do not run the robot while in this parallel connection, it is done to keep the cRio alive, and able to continue on without a reboot.

Typical scenario:
Battery starts off at 13.5v and we drive, shoot, pick up balls with our rebound rumble bot. Typically about 10 mins into the parade, battery voltage is around 10v and things get wonky, (that is the technical term, that is really only understood by the students and drivers.) We stop the robot, plug in a 13.5V battery into the secondary connector of the y, unplug the 10v battery, and continue on with the parade. This swap usually takes about 30 seconds total. While we are doing the swap, there is little power being used on the bot, no motors running, just keeping the electronics alive.

We do this 2-3 times in a typical parade that lasts about 30 mins in Oxford. By the end of the parade the motors are hot, and ready for a cool down.

We have done this 2-3 times a year, for the last two years.

Am I risking the lives of the students, and the entire Oxford community?
Should the Y connector be redesigned with some diodes?
Should the Y connector have a some lower amp protection say 40amp automotive fuse for some catastrophic failure of the "keep alive" leg, and only use that leg to keep the bot alive? ie, time for a battery swap, plug in "keep alive" battery into the 40amp protected connector, pull 10v battery, replace with 13.5 volt battery, unplug keep alive battery. A little more cumbersome, but is it significantly safer?

Data please?

No you are not risking the lives of the students and entire community. Think about when you jump start a car, you are connecting a highly discharged battery with a fully discharged battery that is actually being charged. Yes a small spark is created but a lot of that is due to the fact that the one vehicle has its alternator charging at 14.2 volts.

In may RVs you'll find an emergency start button to allow you to either start the vehicle engine from the house battery bank or to start the generator from the engine starting battery should either be too discharged to provide that function on their own.

In marine applications it is common to find a battery switch designed to hot swap batteries it will have positions of off, 1, 1+2, 2 so you can switch from either battery to the other without interrupting power.

In general when batteries are used in parallel applications you do want them to be matched, same brand/model and same initial SOC to prevent the batteries from discharging themselves when left in storage for extended time periods. The key to that sentence is when left in storage for extended periods.