Battery charging question

Davidasailor26 · Sep 6, 2020

When you checked batteries 1&2 were you still running the engine? If not you’ll need to check the voltages of the batteries when the engine is running to understand better what’s going on.

I presume you have the usual Beneteau battery switch arrangement with a house switch and engine switch, and probably with a jumper in between so whichever switch is on powers everything from the selected bank?

BigEasy · Sep 6, 2020

How old are the two batteries that comprise the house bank?

Benny17441 · Sep 6, 2020

When a battery is being charged and you take a voltage reading it will be higher than the actual voltage. By the same token when a battery is being discharged by an appliance or fixture (load) and you take a voltage reading it will be lower than the actual voltage. If you want a reading closer to the actual voltage it must be taken with the battery at rest (no charge, no loads) for at least two hours. This could account for the readings you have been getting. It is hard to tell about the loss of battery water as it is dependent on ambient temperatures, use and the charging unit. Figure out how your Battery Switches are directing the alternator charge, whether it goes solely to the starting batteries or it also charges the House batteries. Benetau has a unique battery switches system whose setup I cannot advice you about. If you need to to test the alternator all you need is a voltmeter. Normal output voltage should be in a range of 13.5 to 14.4 volts. A fully charged battery at rest should have a voltage of 12.67V. There is a reason why they recommend all batteries to be about the same capacity and age and that is because and older and weaker battery can fool the alternator or charger into behaving as if all batteries needed more charge which could result in one battery being overcharged and hence consuming more water. Try charging the starter battery solely from the alternator and the house bank solely by the inboard 120V charger to see if it still needs more water. They sell a cheap battery load tester ($15) at Harbor Freight and that tool can help you identify a weak battery that may be causing problems within your banks.

SycloneDriver · Sep 7, 2020

What year is your boat?
When you say 12 volts at the panel, is that 12.0? Even 12.9 would be too low with a working alternator, I'm just trying to understand the problem more clearly.
The panel on my current boat reads about 0.2 too low. Not a big deal but 0.2 at the lower end is a lot.
My 2008 Beneteau has a battery isolator inline with the alternator output but my 1988 Beneteau did not.

SycloneDriver · Sep 7, 2020

My boat is not exactly the same as yours but its similar. The batteries in front of the engine are the house bank, the battery on the port side is the engine battery. The previous owner didn't even know there was a third battery.

SycloneDriver · Sep 7, 2020

Virtual beer? I'll have a real one, my fridge is only feet away.
Thanks

Stu Jackson · Sep 7, 2020

Wouldn't how they were wired be more important than the location(s)?

Ron20324 · Sep 7, 2020

Weasel said:
Ok before I go up to the boat maybe someone can answer this question. When I bought my boat, 2004 Oceanis331 it had two batteries. One was in front of the engine and the other was on the port side aft of the engine under aft berth. We have 2 cabin setup. It would seem to me that the aft battery was for the engine due to proximity, with the other for house power.correct?

SOunds right, but you can call them whatever you like. Just be sure to label them somehow so others can follow your rational- or lack thereof

SycloneDriver · Sep 8, 2020

Stu Jackson said:
Wouldn't how they were wired be more important than the location(s)?

Absolutely

JohnShannon · Sep 9, 2020

I would expect 1/2 hour of motoring that the batts would be at 13+ volts. Sounds like maybe your alternator isn't charging the house batteries at least.

Maine Sail · Sep 10, 2020

Weasel said:
Hi guys and gals, hope you’all are enjoying the Labor Day Holiday. I have a question that has been bothering me for the past few months and I thought I would run it by the best panel of experts on the planet. (Sorry I don’t have any photos.)
I have 3 deep cycle batteries on my boat. Two are dedicated to the house, one for engine starting. At the dock under 120VAC power, with the battery charger operating I can go to the battery volt meter on the 12v panel and batt 1&2 read approx. 14 volts.
The only thing that stays on is the refrigeration unit.
If I go out for the day and motor back to my slip, approx 1/2 hour trip, and check the meter for batt 1&2 and it shows 12 volts. Question 1 is why would they read only 12 volts. Isn’t that kinda low ? Second question, the newest of the 3 batteries is just a year old, and should be the engine start battery. It is the most convenient one to check, and I usually find I need to add water to keep it topped off. The other two DieHards never need water. One more thing, the new battery in the engine compartment replaced one of the DieHards that went dry and essentially died. (Reason for replacement). Batteries are 27m. Is it possible that the diode on the alternator has crapped out? How do I check the diode?
I’m a mechanical engineer and i am bumfuzzed by this. Any assistance would be deeply appreciated.

Regards,

If this is one of the analog Beneteau volt meters please do not rely on it for any level of accuracy. They are notorious for their lack of accuracy after time in the marine environment. We routinely measure them being more than 1.0V off a NIST calibrated Fluke. Keep in mind that the difference between a full battery and 50% SoC is far less than 1.0V.

Absorption Charging = 14.4V +
Float Charging = 13.2V to 13.8V
Resting voltage (no charge, no discharge for 12-24+ hours-temp dependent) of a fully charged lead acid Battery 12.72V +
12.0V = Less than 50% SoC, but you'll need an accurate volt meter to determine this

Sears does not make batteries, they re-sticker them, and their flooded marine batteries are typically re-stickered automotive batteries and not at all a "deep-cycle" product. If you use actual deep-cycle batteries, made by an actual battery manufacturer (Trojan, US Battery, Crown etc.), with a good quality marine UL charger and externally regulated alternator, both with temp compensation, you'll have much better luck. Alternatively you can just keep throwing in-expensive batteries at it and accept the short life.

How to Murder Batteries in Half a Year (LINK)

What is a Deep Cycle Battery? (LINK)