I recently attempted to replace the starting battery (Battery #1) in a temporary dual parallel AGM battery setup. I had removed the starting battery and in the interim was relying on the deep cycle accessory battery (Battery # 2) for starting as well. (Note: I do plan to eventually add a battery isolator for a true starting/accessory battery setup.)

In the course of installing the new Battery #1, I inadvertently neglected to attached the positive cable between Battery #1 and Battery #2. Specifically, the positive terminal of Battery #1 was only connected to the alternator. However, the negative terminal of Battery #1 was connected to the negative terminal of the Battery #2, which in turn was grounded to the engine. The positive terminal of Battery #2 was still connected to the starter, which enabled me to start the car. Shortly after starting, I noticed liquid leaking out of the vent hole of new Battery #1. I immediately disconnected it believing Battery #1 was defective I didn't realize at the time that I forgot to connect the positive side of the Battery #1 to the positive side of Battery #2 - duh! (See picture/diagram)

So my question: Was it user/installer error that caused the leaking (possibly from overcharging) or was it simply a defective battery?

Battery Hook-up Error

  • Welcome to Motor Vehicle Maintenance & Repair! Jan 13 '21 at 23:32
  • 1
    If the batteries are usually connected in parallel, how is one a starter battery and one an accessory battery?
    – HandyHowie
    Jan 14 '21 at 11:08
  • Thanks. Yes you are technically correct. The intention was to have one battery for starting and one for the accessories but I never got around to installing the isolator. So in this case. I was just using it to distinguish the two batteries (i.e. the intended starting battery being battery #1 and the intended accessory battery being #2, that were temporarily being connected in parallel). But my question remains.
    – DTOR13
    Jan 15 '21 at 4:48
  • Was the alternator connected to the starter? Perhaps you should update your question with a clear diagram of what is connected where.
    – Solar Mike
    Jan 15 '21 at 10:39
  • Edited my original post to clarify. Also attached a image/diagram. Thanks.
    – DTOR13
    Jan 15 '21 at 23:07

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.