Eco Battery Charge Meter
This measures state of charge of you Eco Battery. For your Eco Battery to work, you will either need this Charge Meter, or you would have to use part A-4211 . As long as your cart has one of these two parts, your Eco Battery will work as intended.
If you do not have a charge meter, you can also see the status of your Eco Battery's charge via the Eco Battery app!