New batteries, voltage drop

[pas63]

My boat is pretty simple, LED interior/running lights, auto pilot, plotter, fridge, lava head with electric pump. I've had a pair of 6v deep cycle that supported all this well. Recently my 7-8 year old batteries died. I recently replaced them with a pair of 6v west marine brand (no jokes, my daughter got worked there, got the discount).

From the house bank to ground/selector switch the I have 10' of 1/0 cable (new, replacing #4). I have a 12" 4/0 cable between the batteries (new, replacing #4). From the selector switch to the breaker panel is about 10' of #6 I believe. From the breaker about 12-15' of 10 ga. All this worked flawlessly for years and with the upgraded feeder cables I'd expect better.


Everythis seems to work fine, except..... The damn fridge (Frigaboat) compressor will not start if other things are on. Even cabin lights (LED) on and the stereo seems to drop the voltage down to 10V. I checked the specific gravity which is maxed (I forget the exact number), battery voltage seems to be 12.7v at full charge with all loads off. When I look at the battery under charge (the charger is also new) it seems to hit 13.8 max.

[Fuss]

Load test the new batteries, disconnect each battery before load testing. Then if ok move onto checking new cabling. It worked ok before ... the only changes are new batts and cabling right.

[pas63]

I agree, it all worked before and all that changed was batteries and new cables. Copper is copper. I can't see any reason larger cables would be a negative impact. I also had issues before replacing the cables which is what led to the battery replacement in the first place. The new batteries are my suspect but can the batteries max out the SG in a hydrometer and still be bad?

[Stu Jackson]

Have you considered checking the voltage at the fridge? Could be that the power supply TO the fridge is the culprit.

[Fuss]

Yes they can

[skipmac]

You say battery voltage under load is 10V. Where are you measuring that? At the electric panel, the fridge or at the battery?

If you're getting 10V at the battery then you have a battery problem. Either the batteries aren't charged or they're defective.

[pas63]

I have a bluesea 360 panel with a systems monitor which is where I am reading the voltage. I also used a MM and checked at the batteries which was close, as well as checking voltage at lights (easy access ) in the head. I did check the voltage at the voltage at the fridge after the new batteries but before the cable upgrade and it was down around 10v. I have not had a chance to today.

The batteries are only a month or 2 old and have mostly been on battery charger as I have not gotten away from the dock lately. Looks like I am pulling out batteries.

[skipmac]

Originally Posted by pas63

I have a bluesea 360 panel with a systems monitor which is where I am reading the voltage. I also used a MM and checked at the batteries which was close, as well as checking voltage at lights (easy access ) in the head. I did check the voltage at the voltage at the fridge after the new batteries but before the cable upgrade and it was down around 10v. I have not had a chance to today.

The batteries are only a month or 2 old and have mostly been on battery charger as I have not gotten away from the dock lately. Looks like I am pulling out batteries.

If you are measuring 10V right on the battery terminals and you aren't pulling a really big load like 50-100 amps at the time then the batteries are not charged or they're defective.

[Compass790]

Everythis seems to work fine, except..... The damn fridge (Frigaboat) compressor will not start if other things are on. Even cabin lights (LED) on and the stereo seems to drop the voltage down to 10V. I checked the specific gravity which is maxed (I forget the exact number), battery voltage seems to be 12.7v at full charge with all loads off. When I look at the battery under charge (the charger is also new) it seems to hit 13.8 max.

If your battery only hits 13.8v under charge something is a bit awry with your charger.
Our 225amp pair of 6v deep cycles will go to 14.8v under bulk charging.
It sounds like your charger is going straight to float. Assuming the batteries are the same capacity as your old ones I'd suggest maybe they aren't being charged properly.
Whoops just read the bit you have checked them with hydrometer. I dont know if they can be good on hydrometer & still bad.

[pas63]

skipmac I don't have anything that pulls 100 amps. Even 10 amps for 1 device is a stretch. I think the fridge might hit 8 amps and other than bilge pumps or the pump for the electric head everything else is minimal. Compass790, not ruling out the charger but it's brand new, set for 3 stage. I don't have options to set the voltages but it is definitely charging, verified when turned off for a bit then back on.

[Fortytwo]

Disconnect everything from the batteries. Connect the charger to the batteries and charge them. Test the voltage. And connect/flip breaker/insert fuse for one thing at a time.

[Wotname]

Originally Posted by pas63

I have a bluesea 360 panel with a systems monitor which is where I am reading the voltage. I also used a MM and checked at the batteries which was close, as well as checking voltage at lights (easy access ) in the head. I did check the voltage at the voltage at the fridge after the new batteries but before the cable upgrade and it was down around 10v. I have not had a chance to today.

The batteries are only a month or 2 old and have mostly been on battery charger as I have not gotten away from the dock lately. Looks like I am pulling out batteries.

When you used the MM, were you checking the voltage at the battery terminals or at the cable terminals that attach to the battery? Of course it should be the same but if it isn't, there is part of the problem

[Sternwake]

Long ago, my first real battery bank, I had a group 27, 13 months old, that would drop to 10.67 volts under a 3 amp load, and rebound to 12.83v soon as the load was removed.

I was using two other 27's same make and age. Shallow cycles.

The next lasted to 23 months and shorted a cell.

The third was relegated to engine starting duty, never deep cycled, and made it to nearly 7 years of age total.

Those first two battery failures.....My solar controller's float threshold was set incorrectly. Once it got them to 14.7v it dropped to float. No absorption duration. They were being cycled very shallowly. Both times I noticed much poorer voltage retention all of a sudden and when I de parallelled them the bad battery became obvious. The first one I had to apply a load though to see voltage tank.

Several subsequent 12v batteries I cycled would still achieve maximum specific gravity, as capacity declined. Full charge resting voltage was still as new as well.

I judge battery health and performance by voltage retention under load now, then how many amps it can take for how long when well depleted from a high amperage charge source seeking mid 14's, and how long it takes for amps to taper to low levels or the hydrometer to max out, and if available, how many amp hours the charger counted during recharge, if there were no loads running.

YOu might try a deep cycle with light loads, and then use a relatively high amperage charging source which can get them to 14.8v then hold them there for 4 hours, but I suspect one of your batteries has a bad cell which drops out under load. Put a voltmeter across each 6v battery under load. Its likely one stays at 6.4, the other drops to 4.x
If WM does not warranty both, the other is young enough to perhaps Be OK in series with a new one.

Does Daughter's discount eliminate warranty?

Good luck, and try to make sure your charger is capable of getting them to the mid 14v range for a period of time.

[smac999]

10v on the Full battery with small load is bad battery. (Measured at battery)

Measure each battery. If you have 10v. On the bank. If One is 6v and One is 4v. You know the issue.

[skipmac]

Originally Posted by pas63

skipmac I don't have anything that pulls 100 amps. Even 10 amps for 1 device is a stretch. I think the fridge might hit 8 amps and other than bilge pumps or the pump for the electric head everything else is minimal. Compass790, not ruling out the charger but it's brand new, set for 3 stage. I don't have options to set the voltages but it is definitely charging, verified when turned off for a bit then back on.

NOTE: voltages mentioned below all assume electrical loads are switched off and you are measuring voltage at the battery.

Well exactly. So a problem with the battery or charging. Do you have a way to measure amps in/out of the batteries? If the charger is working properly, after the batteries go through bulk and absorption to float stage the voltage and amps will drop. The float voltage when the batteries are fully charged would be in the 13.8V range. The amps when the battery is fully charged should be around 1% of the total amp hour capacity of the bank. Assuming your WM batteries are about 220-230 amp hour capacity then around 2 amps max. This also assumes the charger is working properly. Can you see higher voltage and higher amps when charging the batteries from a low state of charge? This is how to confirm your batteries are fully charged.

If you reach this stage and you still drop to 10 V at the battery when the fridge tries to start then I would suspect the batteries. But you should look at how many amps the fridge is drawing. Could be the compressor is going bad and pulling too much power.