Parasitic Load Problem
We have a 2000 Rinker 310 Fiesta Vee that had 5.0 MerCruisers with Bravo 3's. Approximately 3 years ago Mercury replaced the outdrives due to excessive corrosion. After the outdrives were replaced and examined after 45 days, it was noted that corrosion was still occurring. I asked the mechanic to test the MerCathode system (he had been instructed by Mercury that this step was unnecessary). The conclusion of the test was that the system was not working. That problem was corrected and no additional corrosion has since been noted. Ever since the outdrives were replaced and the MerCathode system repaired we cannot keep batteries in the boat. We have 3 batteries (2 starting and 1 house) with a battery switch. The problem has been addressed by local mechanics to no avail. We are aware that the problem can be intermittent (spent the night on the hook and the generator started the next morning - ran down the lake and took a 2 hour swim with only the stereo going and all 3 batteries were dead) and are looking for some help in solving this problem.
Thanks for your help and keep up the great work!
David K. Spencer
Collierville TN
This is an intriguing question. Mercury Marine has been a leader in corrosion analysis and control for decades. They manufactured some of the first impressed current corrosion control systems (that’s what the MerCathode system is, it uses electricity, sort of working in reverse to stray current, to prevent corrosion) for recreational vessels decades ago. Having said that, they have had their share of difficulties with corrosion and the Bravo 3 drives. In your case, however, it sounds as if a malfunctioning MerCathode system was the culprit. Remember, the MerCathode does require current to operate, albeit just a small amount. If your boat’s batteries are not being charged when it’s not in use, this system will eventually deplete them. If, on the other hand, the batteries are going dead overnight, it’s unlikely that the MerCathode system is the cause. If all the batteries are going dead, as you stated, then there must be a common connection between them, a less then desirable state of affairs (the start battery should remain isolated unless you are paralleling it with the house batteries and if so I’d be curious as to why). The bottom line is you have what we in the industry call a parasitic load, something that is insidiously depleting the batteries. This can be tracked down by a savvy marine electrician and his or her multi-meter. First, the batteries should all be load tested to ensure they are sound. Then, with all loads in the condition they are when the batteries are going dead (everything off except the MerCathode, bilge pump and stereo memory perhaps), the multi-meter should be connected between the battery negative terminals and the boat’s negative bus, which will enable it to register any current usage. If it’s indicating more than the milliamp draw imparted by these few devices, then something else is sucking the life out of the batteries and on a vessel this size it shouldn’t be too difficult to track down.
Steve C. D’Antonio, Technical Editor