Battery Charging - What Am I Missing?

[Cbjroms]

The batteries on my Merry Fisher 645 (one year old but new to me) have been on charge for most of the winter but are still testing as partly charged. I suspected battery failure but have had both professionally checked and they are good (but need charging). So I now assume that the problem lies somewhere in the charging circuit and have been trying to understand the way in which it works.

The charger is a CTEK M100 and sitting next to it is a Blue Sea SI-ACR. Both batteries (ie Engine & House) have their own isolator switch. The shore power has been on and connected (but both isolator switches at OFF) until last night but the engine gauge showed the voltage in the Engine battery to be 11.8V this afternoon and the green LED on the Blue Sea SI-ACR was flashing (under voltage lockout?). So I removed both batteries from the boat had them tested - condition is good but charge was low. I refitted the Engine battery and switched-on shore power. The CTEK power light came on but nothing else happened until I turned the Engine battery isolator switch to On, at which point the charger started humming, the engine gauge showed 14.6v and the Blue Sea SI-ACR LED stopped flashing. Subsequently switching the isolator to Off made no difference and the Engine battery continued to charge. Having switched-off shore power I fitted the House battery and repeated the process - same thing happened again in that no activity until Engine isolater switch was flicked On/Off. Measuring voltage at both batteries with the charger running shows 14.5V each.

So my conclusion is that although the boat has sat on the drive over the winter with shore power connected and the CTEK On, it has not actually been keeping the batteries charged. But it is charging both batteries now and all that I have done is disconnect the House battery, run the charger on the Engine battery alone, flick the Engine battery isolator switch and then reconnect the House battery. The only thing that makes any sense to me is that the chain of actions has cleared the voltage lockout which was preventing the batteries from charging. Does this make sense?

 

[jrudge]

I have something similar on my Fairline.

The solution was simple.

The charger fitted gives priority to the engine battery not the house battery.

Once the engine battery is charged it will flip and charge house.

The symptoms were that the house batteries were fine when they had been charged by the engine, but would be almost totally flat in the morning if charged by the charger ( on generator mostly noticeable at anchor).

This was apparently quite common. The solution was to flip the priority so house comes first. Based on this it is now fine. I am not sure why the house batteries charged at all given this scenario, but that is what was happening and this was / is a known issue.

To me this is also more logical. The engine batteries dont do much, they start the engine and then get charged by the alternator. They dont need constant topping off during normal use by the charger.

This is not exactly the same as you were seeing and I am not familiar with your charger, but it might give a lead as to the issue

 

[Richard10002]

I've often wondered how a "professional" checks the batteries, and confirms them as good?

Genuine question...