Can I Connect Dissimilar Batteries in Parallel?

We print our 800 number (888-8-OPTIMA) and e-mail address (info@optimabatteries.com) on every battery we sell. Today's question comes from Ryan H, who asks:

Can I connect my new BLUETOP D31M in parallel with my one-year old (Other brand) Marine 29 series?

We would strongly discourage anyone from connecting batteries in series or parallel applications, if the batteries are not identical in age, size and type. It sounds like your batteries are different in each of those ways. Different brands of batteries can have different charging and discharging characteristics, with some accepting a charge or delivering current faster than others. That can be true even if the batteries are the same size. Different types of batteries (flooded or AGM) also can have different charge/discharge characteristics. When you connect two or more batteries that don't charge and discharge at the same rate, one battery will probably end up overcharged and/or one battery will end up undercharged. Neither is a scenario you want to have happen to your batteries, as it will probably shorten the lifespan of both and could create a potentially create a dangerous situation, if one battery gets severely overcharged.

The same is also true of batteries that are identical in every way, except that one battery is older than the other. As batteries age (or get used), their charge/discharge profile changes. As such, they essentially charge and discharge at a different rate a year later, than they did when they were brand-new. That means you shouldn't connect batteries together that aren't the same age or haven't been used in the same application since they were new, even if they are the exact same make and model. 

Unfortunately, that means when one battery in a bank of two or more batteries needs to be replaced, they should all be replaced at the same time. That doesn't mean the other functioning batteries should be discarded entirely, but they should not be used in an application that has batteries that differ in age, size or type. For some marine and RV applications that use a lot of batteries, it may make sense to isolate a larger bank of nine batteries into three smaller banks of three batteries, instead of connecting them all together. That way, if one battery goes bad, far fewer batteries need to be replaced.