Battery running flat?

Have you had issues with the boat battery running flat, although the battery is fairly new then read on!

I was recently told about a common design flaw in many electrical boat installations which can significantly affect the performance of your battery. I have, and I’m sure you have as well, heard stories of boat owners with brand new boats/batteries even who have experienced this, and normally it’s put down to that the boat isn’t used frequently enough and you just need to recharge the battery. Obviously there can be a number of reasons, with e.g. small leak currents that slowly drain the battery, but I believe there is another common explanation to a lot of these issues in that the battery actually never gets charged properly!

The design fault is related to the wiring of the generator (alternator) which is the device which is supposed to keep your battery fully charged.

If the negative pole of your generator is simply “grounded” to the engine block, rather than having a separate wire going back to the battery then you have the issue discussed. The engine block, although being an electrical conductor it is a very poor one, i.e. it has a high resistance. Exposure to salt water and corrosion will over time add to the issue.

When the generator (and related charge control electronics) charges the battery it measures the resistance to determine when the battery is fully charged, in order to avoid over charging. In our case there will be a significant resistance in the electrical circuit which will “fool” the generator to believe the battery is fully charged when it in fact is far from it. This is the reason the battery never gets fully charged and then is even more susceptible to minor leak currents etc.

The solution? Installl an additional wire (large diameter isolated copper) from the generator to the negative pole of the battery to bypass the resistance of the engine block.

DISCLAIMER: This instruction is based on my understanding of the issue and solution. DO consult a professional electrician to have the work done if you have any hesitation on your own capabilities doing it. I will take no responsibility with regards to any potential damage to equipment or individuals following this advice.

2 thoughts on “Battery running flat?

  1. The second problem is that when the voltage in the battery reaches the same level as the one in the generator, there will not be much current flow. The generator may be providing 60A, but when the voltage in the battery reaches 14V (may take a few minutes) the charging will only be a few A. It doesn’t matter if you drive until you’re out of petrol and don’t use any at all, the battery will NEVER be fully charged!

  2. RC is the number of minutes a new, fully charged battery at 80 degrees F will sustain a discharge load of 25 amps to a cut-off voltage of 1.75 volts per cell (10.5V on 12V battery). This battery rating measures more of a continuous load on the battery and is a much better indicator of how it will operate bilge pumps. An RC number given in the specification indicates that it is more than just a cranking battery and probably a hybrid starting battery. This is a very useful rating for a boater.

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